Tach install in 1984 C30 with 6.2L.

**Sandman729** · 11-20-2006, 19:56

I am attempting to install a tachometer intended for a gasoline application (its actually out of a 1981 chevrolet pickup) and get it to work in my truck. Does anyone know if I can use the vss located on the tail section of the
NV4500 transmission I installed in the truck to send a signal to the tachometer? Which of the two wires coming from the vss would be used? One or both? If my vss will work, will the circuit currently featured in the "Tachometer Alternatives for the 6.2L Diesel" article on the diesel page concerning using a gas tach in an early 6.2L application work for me? The circuit pictured in the article is a great idea, but I don't know how to acually make it (I have little experience reading electrical schematics and I don't know which wires connect to what). Any information would be very helpful and appreciated. Thanks.

**moody** · 11-21-2006, 00:23

Dr. Lee credits a GMCTD as the desiner oof the circuit. If you search under his name you should be able to find a step by step building guide for the circuit. if you want I will look around to see if I have a hard copy somewhere.

**moody** · 11-21-2006, 10:51

okay.... gmctd (member 1505) posted on 2003-12-06 15:09
a step by step build guide for the tach driver that I belive you are talking about.

I think you might find a diesel tach out of an old chevy heavy truck that will fight your dash style. if you use a gas tach you will only be using the bottom 1/3rd of the scale. I belive thse engines are held to about 3600rpm and run best at 2000. If you search under Snoman87 he put togeather a nice dash setup using an older dump truck tach and stick on gage faces.

**DmaxMaverick** · 11-21-2006, 10:58

Also......

If you are able to use the VSS signal from your NV4500, which may not be possible (wrong type of signal), it will only be accurate when driving in direct gear, and not at all unless you are moving. It is a vehicle speed sensor, not an engine speed sensor.

**Sandman729** · 11-21-2006, 21:52

I hate to sound like an idiot, but I can't find the post that user "gmctd" posted about the tach install/project I speak of. How do I find it? I searched for all posts made by user gmctd and found nothing (actually I did find one post from user "convert2diesel" about how great gmctd's circuit design worked for him, which was frustrating because I want to try it also). How does one find this post or contact user "gmctd" or "conver2diesel"? Any help is appreciated. Thanks.

**moody** · 11-22-2006, 17:18

Looks like the search just goes back a year and we are past that now. I'll look for a hard copy but it may take awhile for me to send it.

**mhagie** · 11-22-2006, 20:34

If you would like somewhere in the garage I have a schematic to install a tach using a sensor at the harmonic balancer.
I am using the Dr Lee California vac pump w/ pulse generator, the only thing good i know of to come from California emmission crap.
If you are interested pm me and I can send you the schematic by email.
I agree abouit the HD truck instrument cluster w/tach If I could find one thats what I would have in mine.
Merle

**gmctd** · 11-22-2006, 22:46

The '89-'91, and possibly earlier, gassers had quartz speedometers, for use with the VSS and VSSB modules - the speedo can be grafted into the earlier Rallye gage-pak instrument clusters.

The cluster housings are ABS, easily bonded with common ABS cement from your fav hardware emporium.

I grafted the 5000rpm Rallye tach from an '86 305 Blazer and the quartz speedo from a '91 350 Suburban into my '89 GMC dooley inst cluster when I converted it to EFI 6.5TD.

The '88 electric VSS installed in the T400 fed the VSSB, then to the quartz speedo.

The tach needed a little more consideration, requiring a mosfet circuit tied directly to the Crank Position Sensor input at the PCM.

No failure since incept date of 4\00, tho I did recently convert to 4L80E with dual VSS sensors, including PCM to match - reset the VSSB to compensate, for desired continued accuracy.

**neo** · 01-09-2007, 21:18

Did anyone ever find this circuit layout? I would really like to try building and using it. I don't have a tach, but have the 4:10 gears in my 89 V2500 and boy does it sound like it is spinning pretty fast. Going to the Charlotte NC AutoFair/swap in the spring and would like to install by then. Thanks!

**moody** · 01-11-2007, 08:14

Sorry, i'll look this weekend. I'm away right now.

**vr1967** · 01-11-2007, 14:09

Moody,
I would be interested in a copy also. If it is something you need to make a hard copy of, let me know and I'll pay copy cost, postage, etc.

Thanks,

Virgil

**neo** · 01-11-2007, 21:23

Me to. Glad to pay for a copy. Let me know and I can either paypal or send check, your preference.
I did read the really good write-up that uses the speed sensor on the top of the vacuum pump for those engines equiped with one. Mine isn't and thus, I don't have the speed sensor to derive a signal from. I have no problem gluing a (or two if needed) magnets to the flex-plate or somewhere and using a prox sensor to ge tthe base signal.
THanks again!!

Jeff

**moody** · 01-14-2007, 01:03

I could not find the posts I was looking for in the data base by using the search function. I had a paper copy of the build instructions gmctd gave out to one of the other members to build this tach drive. I do not have a better schematic I can send you.Below I have a typed copy of the build steps.

I know a few of the members have used this to drive factory tachs that were originally installed in heavy chevy trucks. I think snowman87 got his whole instrument panel from a dump truck. I have built three of these and they all worked. One has seen only intermitant use one not yet installed ane on I have lost touch with but it was running fine as a daily driver last I heard.
RIA Colour Code
0=Black, 1=Brown, 2=Red, 3=Orange, 4=Yellow, 5=Green, 6=Blue, 7=violet, 8=Grey, 9=White

Tach drive build instructions.

Transistor placed on its back, plastic side up, legs towards you.
Left leg=base, Centre=collector, right=emitter

Place the 15K resistor (brown, green, orange) on the metal transistor tab folding the leads down along side the body so one is parralel to the base and the other to the emitter lead.

Form a small hook at 90 degrees in the resistor leads placing one on the base and one on the emitter lead.

Attach a black wire to the emittor lead and solder, this will connect it to the ground.

Place the 220K resistor (red, red, yellow) on the transistor body parallel to the left side edge. Form a similar hook in one shortened lead and hook it around the base lead.

Place the 1K(brown, black, red) resistor on the transistor body parallel to the right side edge, form a similar hook in one shortened lead and hook it around the collector lead.

Attach a yellow wire to the collector lead and solder. this will be the tach drive output to the tachometer.

Connect the upper 220K lead to the upper 1K lead. Attach a red wire to these leads and solder. This will connect to the +12 volt power source.

Place the 2.2uf cap between the 220K and the 1K resistors on the transistor body. Form a similar hook in the lower lead, wrap around the base lead and solder all leads. Attach a green wire to the upper lead and solder. This will be the input from the ESS.

Should have one lead from the 15K, one lead from the 220K, one from the 2.2uf on the base lead.

Should have one green wire on the 2.2 cap lead input.
Should have one lead on the 1K and a yellow wire on the collector lead - Tach drive.

Should have one lead on the 15K and a black wire on the emitter lead - ground.

Should have one red wire connecting the 220K and the 1K leads - +12V.

Make sure nothing is touching that should not be, then RTV the assembly to make it sturdy.

Connect.

**neo** · 01-14-2007, 07:59

Great write-up Moody. Thanks. I will try (eventually) to draw out a schematic based on your directions. Once (if) I can do it, then I will post it. During my search, I found a schematic by the good doctor and I believe GMCTD that represented what I believe is the same circuit. It was under the archived "how to's" IIRC. However, one big question: does this only work witht he vacuum pump Engine Speed Sensor? I don't have the luxury of having one on my beast. It is just a plain'ol vacuum pump. If it is, any idea if I can modify it to put like a pick-up on the top to detect rotation? If I did, and I can figure a way to induce a small voltage spike relative to engine RMP, can I then use this circiut to preduce the square wave needed by the tach? Thanks again! Man this is fun. Much rather be doing this than WORK!

**snoman87** · 01-14-2007, 13:17

Originally Posted by moody

I think snowman87 got his whole instrument panel from a dump truck.

I used a tack from a 87' C60 Dump Truck, with white face guage and the tach adapter provided by Moody, worked perfectly and was plug'n'play. One easy option is to get a california emission 6.2 vacuum pump (Bumper to Bumper part# CDN*64-1202, GM Part# 7849303).

**moody** · 01-14-2007, 18:55

credit for the write up goes to the originator gmctd I only get credit for keeping track of it.
All the parts can be found at "radio shack/ curcit city" i will look around for part numbers.
As to what signal it can be hooked to?
Best to check with the originators.

**gmctd** · 01-15-2007, 09:03

What an improvement on that old instrument cluster!

Are those overlays on the steel faces, or are they total replacement plastic light-thru faces?

FYI - You will need a 2v ac signal at 4 pulses per crank revolution - that combo vacuum pump\ESS unit is around 140 schekels at gmpartsdirectdotcom.

Problem is, they stiff ya on the shipping, so don't be surprised if the bottom line is higher than dealer-list price

**Ebrown** · 01-18-2007, 16:17

What is the 6.2 warm idel rpm so I can see if my tack is set right?

**gmctd** · 01-18-2007, 21:11

Depends on the transmission and the idle adjustment screw - you'll need to get it checked with a photo-tach for reference.

**kirt7717** · 10-22-2012, 22:48

i have been on my computer all night looking at different forums and i think you guys answered my search. now before i go buying parts, can someone confirm to me that i have it all figured out correctly. i have a gas engine tach and a kodiak heavy truck gm tach. if i buy the vacuum pump with the engine speed sensor that was just mentioned, can it be simply connected to either one of my 2 tachs? without making the tach drive you guys are talking about. i want to do this the easy way! i just put an 83 6.2 into my 86 chevy half ton in search of awsome fuel milage!

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