Lil Red - Duramax 6600/Allison Conversion/Swap & OBS/NBS Dash Conversion

[arveetek]

Thanks for all the info, Jim! I, for one, would like to see the A/C condenser modification info.

Casey

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Lil Red had been on the road and drive-able for about 12 months by the time the following photos were taken. This means I had driven it through the previous summer (and The 2004 Diesel Page Rendezvous) without air conditioning. It was seriously hot that summer, and I knew that Lil Red simply had to have air conditioning going forward! One summer was enough to tell me I didn't want a repeat.

By the time the truck became drive-able the year before, I had a pretty good idea how I would complete the A/C installation - I just needed to move through the process of modification/installation. And, I could lean on a friend and automotive A/C expert from Tampa, Florida (Gordon Marks) who answered all of my questions and even loaned me some of his hose making tools and supplies during this process.



The above image shows the newly modified hood latch bracket, made to accommodate the added space required by the intercooler and A/C condenser. It was all TIG welded together. Most of the individual metal pieces used here for the hood latch bracket mod/extension were made from rectangular steel tubing.

The long section of rectangular steel tubing was notched to allow it to extend 3-4" up the back side of the hood latch portion - to provide additional structural rigidity. The bottom of the new hood latch bracket (that bolts to the bottom of the radiator core support) includes a flat steel pad large enough (about 3" wide x 3" tall x 1/8" thick) to clamp the bottom part of intercooler to the radiator core support when bolted on. The upper portion of the modified hood latch bracket is likewise used to clamp the top of the intercooler against the top of the radiator core support. A 1/8" thick rubber strip was used wherever the modified hood latch bracket assembly made contact with the intercooler.

Note: Moving the long section of the hood latch bracket a couple of inches farther forward meant the lower/center attachment mount for the grille will also need to be shortened by the same amount. In Lil Red I simply measured how much to remove, cut the plastic grille mount by that amount, then pop-riveted a 1/16" thick aluminum 90-degree angle onto the shortened plastic grille mount to accommodate the new dimension. Then, I drilled a new hole into the rectangular steel tubing on the modified hood latch assembly (with nylon insert, like OEM) to attach the shortened grille mount. Easy...
--------I'm not using an air/ATF cooler - just the cooler inside the radiator. This lightweight truck doesn't need the additional cooling... plus the Allison has a deep-finned pan. On a hot day, the Allison runs at engine temperature around town, but drops some on the highway. If your truck will be used to tow, I'd install a factory ATF cooler in the upper area just to the right of (passenger side of) the hood latch mechanism. There should be enough room there. You'll have to fab some bracketry, but that should be easy for you at this point. Or... you could put a different shaped aftermarket ATF cooler in the area behind the holes in the bumper... Likewise with the smaller power steering cooler.

I bought a brand new factory GM A/C condenser for this project - made for the model year of Lil Red, then modified it to fit. It wasn't that expensive... surprised me.

Incidentally... The new condenser arrived without any paint on it... just raw aluminum. I initially wanted to spray on a nice layer of satin black paint, but in the end I just dusted on a thin layer to make it less visible through the grille. Paint can act as a thermal barrier, and I didn't want to hamper the condenser's ability to reject heat. So, if the paint looks a little thin here, that's why. The steel brackets, on the other hand were removed and properly painted before final assembly. I also used thread locker on the sheet metal screws used to fasten the new brackets to the ends of the condenser.

The 3-axis position of the actual hood latch components couldn't be changed (hood latch, safety catch). The intercooler installation forced the condenser location to be father forward than the original had been. Mounting the condenser farther forward meant the condenser couldn't be as tall as the original. Shortening the condenser required removing the top 4 aluminum cross tubes along with the cooling fins that covered them. What's described here is the best way I discovered to modify the A/C condenser.

Once I knew how I was going to mount the condenser and knew what mods were necessary, I used a 1/16" thick 4" diameter cut-off wheel on an angle grinder to carefully slice through the delicate fins just beneath the 4th cross tube. This worked well, but go slow, the aluminum cross tubes are thin walled and easily damaged. The remaining stub ends of the cross tubes on the far right side (passenger side) of the condenser were left long enough to allow the stub ends to be rolled over and crimped, followed by aluminum solder/welding. The stub ends that you need to solder/weld closed should be left long enough to account for a flub or two (i.e. the need to re-cut square, fold over, crimp and re-weld). The tubing is pretty thin-walled and delicate. I used a propane torch for this, and I practiced for quite a while on the excess tubing that had been removed earlier. You can buy the torch and flux-core aluminum welding rod at Harbor Freight.

Removing most of the top 4 cross tubes and their fins to allow the condenser to fit, and then soldering/welding the ends of the tubes closed was the most tedious part of this. Honestly, I didn't know if this sort of modification would work when I began the condenser installation. But, in the end I didn't mess up the new condenser, and it installed nicely. Practicing your aluminum soldering/welding on the leftover condenser tubing you removed is essential... dare I say required. It took me a while to learn how to do it without melting the tubing.



The above shows one of the bottom corners of the condenser, and how I made a bracket mount. The bracket is made from 1/16" x 1" wide steel strips bought from a local Ace Hardware store. The mounting bolt hole I used here, at the bottom of the new bracket (sorta behind the wiring loom), was one of the mounting locations for the original condenser. The right/left sides of the new condenser are mounted tight up against the intercooler, so I used a layer of dense double-sided foam tape to separate and cushion the sides of the condenser where each rested against the intercooler. You can sorta see the white strips talked about in these photos. The dense double-sided foam tape I used came from a friend of TheDieselPage.com who owns a local auto body shop - though I only peeled the backing off one side (didn't want it to stick to the intercooler). The tape is made to attach emblems and trim onto cars... I imagine this tape can be sourced through retail outlets that sell auto paint and body supplies.



The upper mounts were constructed as shown above - similar to the bottom mounts. The top most mounting bolt (behind the yellow arrow) was one of the bolt hole mounting locations used by the truck's original A/C condenser. In fact, all four corners of the condenser used the original mounting bolt location.

As mentioned earlier, the A/C system has worked very well since the initial installation/R-134a charge in 2005. It's not needed a recharge, a top off or any other service. In addition to installing a brand new condenser, I also installed a brand new accumulator that mounts onto the firewall, in part because I was converting the original refrigerant used in this truck from R12 to R-134a - I wanted make sure there wasn't any debris or non-compatible compressor oil left in the system. Even on +100 degree days, the system can easily maintain comfort inside the cab.

Because I was adapting a new style A/C compressor to an earlier truck, I needed to make a couple of hose assemblies. This was accomplished by using some new hose lengths and appropriate fittings, assembled using an A/C hose crimping tool. All of these items can be acquired online for not a lot, and Youtube will show you how to use them. Not hard! Contact me via email if you need additional tips.

In the end, the cool factor arrived just in time!
Jim

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A bit hyperbolic perhaps, but this new video asks the question... "Is this the best Duramax Swapped OBS Chevy K1500?" Included here in this video is some basic performance discussion, and you'll learn more about what Lil Red used for performance products when the truck was at the track...

You can help our video channel to grow and reach more people! Please subscribe to our YouTube channel - run your mouse cursor over the little circle truck image in the upper left of the video shown here to see the subscribe button.

I recommend that you create a YouTube account, if you don't already have one. It's easy and free to register at YouTube.com, plus you'll be eligible for free stuff here in The Diesel Page before long. Before long we'll begin offering all member-only online content here at TDP free of charge to those who are subscribed to our YouTube channel. Your YouTube account name will be linked to the free access list. Just send me your YouTube user name using the "Contact Me" link in my signature below to gain access.

TDP's long-term goal is to produce more free content videos. Some of these videos will be derived from the several books we've published. The first book to be transformed into a video series will be the "Duramax Service Guide". This book is, in my opinion, the most valuable book we offer.

Where are we are currently in the Service Guide series? The first two videos in the series have already been published and are now available as videos - an Introduction to the Tools, Supplies and Procedures that are necessary to begin a project like Duramax 6600 LB7 injector replacement, head gasket replacement (coming soon), water pump replacement and more... We hope you'll like them!

The 6.5L Diesel won't be left out. More about the 6.5L Turbo Diesel Blazer as well as segments from the 6.5L books will be featured in video format as often as schedules allow.

Subscribing will ensure you don't miss any new videos. As always, your input, suggestions and advice are all important.

Thx,
Jim

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Here's an article I wrote for the May 2006 issue of Diesel Power Magazine involving Lil Red, which details the installation of a performance ATS Allison. This transmission has been in Lil Red all this time since. There have been no codes or slips detected. - Jim









See our YouTube channel!

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I used a TH400 auto transmission bolted to the LB7 during the first trial fitting sessions in Lil Red's chassis. The TH400 was lighter than the Allison and a bit smaller - easier to manage when dealing with a minor transmission tunnel clearance puzzle.

As you can see, the earlier Chevy/GMC TH400 transmission bellhousing bolt-pattern is the same as the Allison, and (though not shown) the bolt pattern for the TH400/4L80-E torque converter is the same too.
Jim

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The bellhousing design allowed GM to utilize the Duramax in the van application with the 4L80-E automatic transmission (250-hp/440 lb-ft). And, then there's this...



GM Powertrain's 2000/2001 Pikes Peak C1500 race truck (GMT-800 body style) was also equipped with an LB7 Duramax and a special 4L80-E automatic transmission that had been carefully assembled by GM Powertrain. That race-prepped Duramax 6600 engine had been hand assembled by Isuzu engineers in Japan, and was producing more than 600-hp, or so I was told.

By the way... I came to within a whisker of getting that Pikes Peak GMC from GM Powertrain. The promotional program that that truck was built for was complete by early 2002. In talking to the GM Powertrain promotional team at the time, we began to explore the possibility of The Diesel Page acquiring the truck, and then continue with promotion. Initially, the deal was approved, and I began preparing for the trip to Michigan to get the truck... Unfortunately, the deal fell through. I never did hear what happened to the Pikes Peak truck after that.

That's me in the red shirt standing alongside the truck - at the DMAX engine production facility in Moraine Ohio. In the background you can see a banner given to TheDieselPage.com by DMAX, GM Powertrain and Allison during our 2001 Rendezvous.



Another shot of our visiting DMAX in Moraine Ohio.

Jim

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L5P Duramax Swap Mounts

A question came up recently about whether the swap mounts we offer will fit the newest version of Duramax 6600 - the L5P. Our mounts were designed for the LB7, LLY, LBZ, LML and LMM (2001 up to the 2016 model year engines - essentially all but the L5P which first appeared in the 2017 model year GM trucks). All pre-L5P engines utilize the same engine mounting dimensions and bolt patterns. The L5P block casting, on the other hand, was redesigned to include a new engine mount design and bolt pattern, and the L5P incorporates a completely different oil pan (which may actually be a better fit for the GMT-400). We're working on offering an updated mount design for the L5P/GMT-400 swap (for the 1988-1998 body style), and I hope to have swap mounts for the L5P available later this spring.

In addition, we've seen that through the years, GM Powertrain has added more and more emissions-related components to the top of the engine. The L5P is the pinnacle of that trend. To maintain adequate hood clearance in a GMT-400 body style truck, installing an L5P would require a cowl induction hood and/or a 2" or so body lift. I'll have more information on all this as soon as I get my hands on an L5P. In the meantime... Click HERE to read more about our current mounts or to place an order.

Jim

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Here's a photo that was shot during the fitting/modification process for getting a Dodge Cummins intercooler to work for this Lil Red project - and get it all to fit behind the grille.



My reasoning for widening the inlet/outlet was to position the required-new hole through the radiator core support to just outside the boxed-steel section(s) of the core support. This helped to maintain the structural integrity of the core support and maintain access to the core support frame mounting bolts. The downside though is that the widened inlet/outlet required modifying the park/turn light housings. This also required modifying the aluminum tubing used to connect the intercooler to the engine. In my case, I used 3" aluminum agricultural sprinkler pipe as the stock to extend the factory OE piping.

If doing it all again, I might investigate using an aftermarket intercooler core sized for the existing real estate behind the grille while doing my best to maintain the engine's flow requirements and park/turn light assembly clearance. - Jim

By the way, this modification to the intercooler (on both sides) cost $500 at a metal fab shop, and that was on top of the $200 cost of the salvage eBay Dodge intercooler. A new factory Duramax intercooler at the time was $600 through our local dealership. Decisions... decisions...

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I've been asked recently about the radiator I'm using in Lil Red... I installed a brand new 2001 model year radiator originally produced for a Duramax/Allison powered 2500HD/3500 GM pickup truck. This update describes how that new radiator was installed, and will answer the questions I've received about it. This is an easy topic! By the way, these pics were taken in May 2025.

I chose to go this route after I measured the radiator in my 2001 GMC 2500HD Duramax powered pickup. That 2001 model year Duramax radiator was the exact same dimension (core height, width & thickness) as the radiator found in a 1994-1998 6.5L diesel powered OBS (Old Body Style - GMT-400) pickup. With just a simple mod (removal of a mounting lug), the new radiator was a direct bolt-in - using 1994-1998 factory OBS rubber mounts, brackets and the 1994-1998 OBS 6.5L diesel fan shroud. Perfect!



You can see here that the 1994-1998 6.5L diesel fan shroud takes a step up in height of about 2" from the top of the core support. Lil Red originally came with a gas 350, so it had a shorter, thinner and narrower radiator and smaller fan shroud (though the truck's radiator core support was essentially the same as that used with the 6.5L diesel). With minor mods (removed a couple of knock-out panels to match the width of the new radiator core), Lil Red's original radiator core support allowed for a direct bolt-in of the larger Duramax (or 6.5L diesel) radiator and the OBS factory 6.5L diesel fan shroud - both (core support and fan shroud) fit the 2001 Duramax radiator perfectly...



The arrow in this photo points to what remains of a rubber isolated mount that was part of the Duramax radiator. I simply cut that mount off. Then, the Duramax radiator was slid into the core support and held by the same factory rubber mounting system used for a 6.5L diesel radiator.



I installed a 2001 model year factory 9-blade composite fan made for the Duramax. However, because the opening in the 6.5L diesel fan shroud was designed for a 19" fan and because in my swap the center of the fan sits about 3/4" above the center of the fan shroud opening, I reduced the diameter of the Duramax's 21" 9-blade composite fan down to a diameter of 19" (1" off every blade).

The fan trimming was super easy. I simply made a pattern using card stock that maintained the fan tip profile, but reduced the length of each blade by exactly 1" using a Dremel cut-off wheel and sanding disc. I was careful, took my time, and the end result was nearly perfect. The blade tips are very lightweight, so there wasn't a concern about a loss in balance over the 9 blades - if you do your best to make each blade tip the same. The modified 19" diameter fan shown here still moves a lot of air. But, if this were an 2500/3500 Duramax powered truck, I may have considered trimming the fan shroud to help maintain the full-diameter fan.

I will say that I continue to be super impressed with the factory OBS 1994-1998 6.5L diesel fan shroud. The shroud looks great and is very well made. Even when bought over the counter at our local GM dealer's parts counter, it was a super good deal... The plastic is thick enough to prevent any sort of distortion from typical use (i.e. leaning on it), and it looks great too.

Swapped trucks using the Allison can't incorporate a factory OBS 6.5L diesel radiator only because the internal radiator ATF cooler and fittings are too small to work with the Allison. If your swap incorporates a ZF-6 speed (or other) manual transmission, I don't see why you couldn't use a factory OBS 6.5L diesel radiator. Both radiators use the same materials (aluminum core with plastic end tanks), same height/width/thickness/inlet/outlet and even appearance. I contend that GM Powertrain began with their 6.5L diesel radiator when developing the cooling system for the Duramax.

And incidentally, I swapped out Lil Red's original vacuum-assisted brake booster and brake master cylinder for a hydraulic brake booster and matching brake master cylinder made for the OBS 6.5L diesel equipped trucks. Yes, the master cylinders are different, depending on whether it's a vacuum-assisted or hydraulic-assisted application. The diesel (hydraulic) brake parts were a direct bolt-in swap. Once that was completed I simply connected the 2001 factory power steering pump produced for a Duramax to the hydraulic brake booster and steering box. The package works as expected. And, I later learned that the OBS trucks equipped with a big-block gas engine will already have the hydraulic brake booster and matching brake master cylinder.

I did not install the auxiliary ATF air/oil transmission cooler normally used in a Duramax/Allison powered pickup truck. Nor did I install a power-steering fluid cooler. If this truck was being used for towing I would have installed both coolers. But, this is a lightweight truck that doesn't stress those systems. Normal summertime transmission temperatures run near or somewhat below engine temps. For example, today with ambient temps approximately 80 degrees F, and after driving the truck for about 30 miles, the engine temp measured 188.4 degrees F and the Allison ATF measured 181.3 degrees F with a scan tool. There hasn't been much of any degradation (smell/color change) of the transmission fluid or PS fluid, so ATF/PS fluid temps must not be out of hand. But, I may add those coolers to those systems someday just out of curiosity.

Those who have built project vehicles know that their car/truck is usually never 100% complete - seems there is always something new to add or complete, or something that needs to be updated/modified/improved. After driving Lil Red for 21 years, I have a lot of hindsight to pass along here in this forum thread... Hmmmm... Besides driving it more, what would I do differently if starting over?...

Let know if you have any more questions.

Jim

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Another video is in the works. These photos were taken at Painted Rocks lake here in western Montana.



It's beautiful this time of year, and allows for some good photo opportunities. That's Lil Red rounding the far corner! Next video should be out soon!

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On 2025-06-16 13:11, William D wrote:
> Jim,
> I just recently received the conversion guide in the mail over the
> weekend and have enjoyed reading through all the information listed
> in your Silverado conversion however I am still on the fence as to
> which way to go ( I've got time ) . While I was waiting for my book to
> arrive I have been scanning through the members pages and found the
> "Swapping engines " link in the reference section. I know that I'm
> going to remove the 7.4 big block rat motor but which way to convert
> to diesel is still up for debate for me. I was under the impression
> that the 6.5 Detroit was a boat anchor and should be avoided at all
> costs but the more I dive into the subject the more I'm finding that
> the biggest issues were more caused by Chevrolet engineers rather than
> Detroit and that all of these issues are easily remedied. I'm looking
> for the most straightforward conversion possible without breaking my
> back as I'm 7-8 years away from retirement. I've already dumped a load
> of cash into the drive train of my K2500 and still have a lot of
> upgrades to install so I'm likely not going to sell this rig to look
> for another project truck and I have no interest in financing a new
> diesel rig so I'm very much interested in doing a diesel swap into
> this rig whether it be a 6.5 or 6.6.
>
> Looking at the key points of a 6.5 swap it seems to be comparing the
> 5.7 liter characteristics to that of the 6.5 but I'm curious to know
> what or if any of these compare to the 7.4? Also my rig is equipped
> with the NV4500 manual five speed which I plan on retaining. So if I
> go with the 6.5 swap will I have to upgrade my flywheel and clutch to
> accommodate the higher torque of the Detroit motor? I'm assuming that
> the motor mounts will be the same for both motors and I'll also have
> to change wiring harnesses? Are your books on the 6.5 Detroit
> inclusive to the workings of the motor or are there any articles on
> doing this sort of swap?
---------------------------------------------------------------------------------------

Hi William,

Questions...

1- How nice is the truck? What year truck is it?
2- What is your level of expertise for auto mechanics?
3- Is this a truck you intend to keep?
4- How will you use the truck, or how would you like to use the truck? i.e. tow a big travel trailer several times a year, light towing more often than that (i.e. boat), commuter mostly, or family transportation all the time, etc.
5- The truck's current differential gearing?
6- Swap budget?

Beyond that, a nice Duramax swap will make it easier to find parts and service, and your truck will be more valuable if the swap is nicely done. The 6.5 can be made to be quite capable, but it wasn't designed for the most severe towing duty, and parts/service are becoming harder to find. You'll be the service provider.

For your specific 7.4L equipped truck, the existing radiator, fan shroud and hydraulic brake system can be maintained no matter which engine you go with.

Check out this link for a Youtube video about a 1998 Chevy/LBZ Duramax/ZF-650 swap.
https://www.youtube.com/watch?v=iDr8vhoGJ8Q

The important thing for you here is that he installed a ZF-650 6-speed manual. Some of that manual transmission info might be of interest to you, and I'm sure the truck owner would answer your questions about a manual transmission installation. Beyond that, I'm pretty sure your NV-4500 can be adapted to work, if you keep engine power reasonable. The "650" in ZF-650 is the rated torque limit. The rated torque limit for the NV-4500 is 460 lb/ft. The stock 2001 LB7 Duramax factory rated torque was 520.

I've always compared a properly built 6.5L turbo diesel to a stock 7.4L gas engine in overall performance, but with nearly twice the fuel economy. The Duramax, even with stock programming will perform above that. The Duramax fuel economy will be 11-12 while towing and 18 or a little better while traveling empty at Interstate speeds - depending.

Your 7.4L gas V-8 engine mounts are the same as those used by the 6.5TD. The motor mounts are different for the Duramax. The Conversion Guide shows what they look like. The flywheel/pressure plate/clutch plate are different for the 6.5 and the Duramax, when compared to the 7.4L gas.

And, visit this link to learn more about wiring for the Duramax.
https://www.thedieselpageforums.com/...133#post333133

The books we offer on the 6.5 include all of the diesel-related wiring circuits. It's really not too complicated. Your existing ECM/PCM will be pretty confused, but there are work-arounds for just about everything.

Lastly, if you go with the 6.5, you can install a mechanical fuel injection system. That simplifies the wiring to its most basic form. My recommendation would be to install the mechanical diesel fuel injection system if you decide on the 6.5.

Jim

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On 2025-06-17 11:51, Ronald W wrote:
> Hello, I was wondering if these conversion mounts still allowed the
> use of the stock 4l80 transmission? I intend on keeping the factory
> drivetrain trans back. Will these work for that? Or at least get me
> close enough I can modify these to fit?
>
>
> Ron W
> Carson City NV 89706


Hi Ron,

The mounts are not transmission specific.

A little history... The first in-house prototype Duramax powered GM pickup trucks used the 4L80-E, till the Allison-1000 became available. And, GM built a Pikes Peak race truck in 1999/2000 using a prototype 630-hp Duramax 6600 in a GMC C1500. That truck utilized a built 4L80-E.

In addition, the Duramax 6600 powered passenger/cargo vans in the early 2000's were all were equipped with the 4L80-E. These engines were detuned to 250-hp and 440 lb/ft of torque. That's still really good! So, you could either acquire a G-Van ECM/TCM or have yours programmed using van specs.

And, the Duramax transmission bellhousing bolt pattern is the same as all prior year Chevy/GMC V-8 bolt patterns going back to the 1950s. And, the 6-bolt 4L80-E diesel torque converter will bolt to the stock Duramax 6600 flexplate.

Thanks, Jim

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When Lil Red's swap began back in the 2002-03, I decided to explore the possibility of making Lil Red's original steering column and ignition switch work with the new factory 2001 GM Duramax/Allison electrical system. I did this for a couple of reasons. 1- To help keep the project within budget. A complete salvage yard steering column from a new truck was priced at about $250, less airbag. And 2- I wanted to determine if it was possible to keep the original column, because lowering the cost would make the swap more attractive to more people. Back during this time period, there weren't many of these trucks in the auto salvage yards.

I was able to keep the original column, though there were a couple of compromises. Among that list of compromises was the fact that the 1989 ignition switch(s) didn't have a "Hot in Off, Run and Start" circuit. So I needed to add a separate switch to the ignition switch stack (there are multiple switches in that stack) to make that happen.

The ignition switch solution discussed here worked perfectly for about 20 years and many-many starts - till one day it didn't. As Murphy's Law would predict, I was 250 miles from home with no tools or test equipment when it first "didn't". The symptoms were: Rotate the ignition key to RUN, let the glow plug wait to start light go out, then crank the engine. I did, and the engine started normally, but this time all of the dash gauges/lights suddenly went dark and dead. The engine continued to run, but obviously all was not right. So, I switched the ignition back to Off, took a deep breath, then tried again. No joy... insert sad face... So, at this point an ignition switch problem was high on my short list of possible causes.

The dash instruments were all active when first rotating the ignition key to run, so obviously the cause of the problem wasn't fuse related. Without tools or a multimeter, I did the only thing I could do... I tried it again. This time the gauges danced a little, but remained on. Deep breath released, frown dissipated and I drove the 250 miles home without further difficulty - I didn't shut it off till I got home.

Since the problem first appeared, it continued to randomly pop up when releasing the ignition switch after the engine started. When this occurs, the transmission wouldn't engage and the SES lamp would be present once the dash lit up (ECM/TCM comm errors and others) following one or more attempts at off/run/start key rotations. I assumed there was a problem with at least one element of the ignition switch. Now at home and with access to my multimeter, I probed the ignition fuses when the problem next appeared - to help locate the faulty ignition switch element, if that were the case. My first guess was correct. I had already suspected the additional non-OE "Hot in Off, Run and Start" ignition switch element that I'd added 20 years ago was likely the culprit, which is identified as the switch for IGN-0 (ignition zero in the following diagrams). So, I looked up that diagram to refresh my memory.



The above image shows a portion of the ignition switch circuits. The yellow highlighted trace in this diagram shows the path that power takes from the 12v power bus through the non-OE ignition switch I added (IGN 0). This switch routes power to the IGN 0 fuse found in the fuse panel - located on the left side of the dash (see image farther on). This next image shows where the power goes after passing through the IGN 0 fuse.




My IGN 0 switch was just intermittently flaky, about 1 in 3 key cycles. IGN 0 power is used by the Instrument Panel (that makes sense), by the Theft Deterrent System, and by the ECM/TCM - pointed to by the green arrows in this circuit image.

No wonder the problem set codes. I had considered 20 years ago that the correct operation of the Theft Deterrent system was likely essential for me to get the engine running because I knew I'd need to perform a "Passlock Security system relearn", so I did what was necessary to add an ignition switch element that would deliver power to the circuits discussed here in "Off, Run and Start". Back in the early 2000s, there was no EFI-Live or aftermarket vendors capable of programming around the factory Passlock Security. Nowadays, you can avoid Passlock Security with programming, and the ignition circuit we're discussing here may no longer be necessary if Passlock is done away with.



The IGN 0 fuse can be found in the top right corner of the fuse block. The blue line shows where I added a temporary jumper to the fuse block assembly that supplies 12 volt power to the IGN 0 fuse via the air bag fuse, which itself is "Hot in Run & Start", but not hot in Off. The IGN 0 circuit is still protected by its original 10 amp fuse. This truck doesn't have airbags, so it's perfectly safe to "borrow" power from the air bag fuse. The engine doesn't seem to mind the jumper. No codes are set when it's in use, despite not being powered when the key is in the "Off" position.



The above is a photo of the steering column with the outer dash trim panel removed. The camera lens was held near the end of the turn signal lever for this shot.

Red arrow: Micro Switch
Green arrow: Control rod connecting the ignition keylock barrel to the actual ignition switches located farther down the column.
Yellow arrow: The fabricated plastic block that clamps to the control rod. This can be moved up/down to fine tune the switch activation.

The micro switch shown here illustrates how I solved the problem of not having an ignition switch element that supplied power in "Off, Run & Start". The actual factory 1988-1994 ignition switches are not located near the key lock assembly, but are activated via a long rod that connects the rotating barrel of the key lock assembly to the ignition switch assembly located much farther down the steering column. I simply used that push/pull rod functionality to activate a "micro switch" like that shown here. Aside from mounting the micro switch in position I also fashioned a plastic block that I could clamp onto that control rod, which itself moved up and down the column as the ignition key is rotated. I set the switch to activate when the key lock assembly was in the "Off, Run and Start" position. It always worked perfectly, till it didn't.



I'm in the process now of getting a replacement switch. It'll be easy to replace. As of right now, I don't know whether the problem is due to a faulty switch contact or if there's a mechanical problem with the switch paddle. I won't know which is the cause till I get the dash partially disassembled to see the switch. Whichever it is, it'll not be difficult to fix. I'll update this post once I know for sure.



Back when the Lil Red swap was just beginning, I acquired a factory 2001 ignition lock assembly, which contained the "Passlock Security" components I needed (a resistor code) as well as a factory ignition switch package. I tested that ignition switch to verify that its operation and wire colors matched the wiring diagrams I had - they did. Rotating the key barrel of the ignition lock assembly would spin the toothed gear seen here, hence rotating it through the various key positions (Acc-Lock-Off-Run-Start).

So, all this could have been avoided if I'd had a steering column matching the model year of the drivetrain. Live and learn. But... I sorta like the truck's original column.

PS... because I purposely chose to install as much of the 2001 system wiring, fuse blocks and electrical system as possible, I could use the GM factory diagrams and troubleshooting methodology to locate and repair this problem. In addition, the Conversion Guide includes additional diagrams and info to allow the OBS steering column controls to function/interface with the newer Duramax electrical systems - such as Park/Neutral switch, turn signals, intermittent wipers, windshield washer, horn, and cruise control.

Jim