Speedo Recalibration

[johncarrol]

I changed gears and tire size in my 1991 GMC 6.2 and would like to recalibrate my speedo and get my ABS working. I"ve heard the DRAC is built in my speedo and guage cluster for my year. When I removed the cluster, I noticed there was a place for a DRAC programming clip on the back, but there was no clip. I checked for a DRAC under my dash and inside my ECM box where I was told it could also be,but could not find one. My speedo works but reads about 10mph slow around 55 and my rear ABS never worked since I've owned the truck for about a year now. I will attempt to make a programming clip, but I wonder how my speedo even works at all without one installed. Either the DRAC is located somewhere else or the programming clip was removed at one time but still allows the speedo to work, but not correctly. Does anyone else know?

[john8662]

Be curious how you'll recalibrate this cluster. Knew there was something to it just haven't done it.

I'd be interested in seeing this as it develops. Please document it here.

J

[john8662]

This might explain your lack of a programming clip. Doesn't need one as it's internally calibrated.

http://www.chevytalk.org/fusionbb/showtopic.php?tid/258562/

I haven't had one of the classic body instrument clusters apart, but it might be similar to the style in the GMT 400 trucks.

J

[johncarrol]

First of all, I want to thank you John, for your reply and the information you came up with to help me, and to everyone else for their information I was able to dig up on this subject in other forums. Finding out I could recalibrate my speedo without spending a dime, blew me away. Before deciding to search the internet, I was ready to purchase a device to hopefully wire into my truck without messing anything up, that in the end may or may not have worked. 1991 is a strange year. the electronics are different, possibly because the 4L80E was used for the first time. I was told the device may not work because it was intended for either earlier or later year trucks. This project was starting to become a nightmare. Anyhow, before your reply and link to additional info you gave me, I assumed the 7 fuses were resisters of different values and connecting certain ones via using pins outlined in the chart would change speedo readings at certain speeds. The link you provided answered my question as to how my speedo worked without any programming clip. I checked my board and the fuses that were not blown were at position 3, 4, and 7. I decided to try the configuration closest to my tire size and gearing to start with, and just returned from my first test. I'm happy now my speedo is +-1mph of my GPS. I don't know yet if my rear ABS works, but I will post the answer later. I reprogrammed my speedo differently than the instructions showed because I believe my way was much easier and safer. The chart showed pins intact at position 6, 7, and 8 on the factory reprogramming clip for my setting. I took wire cutters and cut off all of the one side of the fuses on the side of the board they were located, and bent them all out of the way. There is plenty of room for them not to touch anything or each other. I then took 3 approx 1" pieces of wire that would push through the holes from the side the programming clip plugs in leaving approx. 1/8" sticking out that side that will leave most of the length sticking out the other side. I just lightly bent the long ends together and put a dab of solder on them. Sticking out that far, there is still plenty of room so as not to touch anything, and also keeps a hot solder gun away from vital components. Very difficult to ruin the board this way and easy to change configuration if needed. Just pull wires out and start over with new ones. No messing with dip switches and board soldering. It's not like when you finally get it correct you will go back very often and change it. Only one question I have is position 8. None of the configurations on the chart shows pin 8 removed. It was shown intact on a photo of a programming clip. The manual indicates it as a ground.

[john8662]

Cool deal. glad that the same guts are in the 90-91 speedometer as in the GMT400 trucks with the moon-beam gauges (and some needle 91's).

Great leg work. Pictures would have been nice ;)

I think that pin 8 is always connected, seems to be consistent with the DRAC and VSSB modules I've soldered dip switches onto.

[johncarrol]

Sorry, no pics yet. I will in the near future pull the cluster out again and take pics, because I will replace a larger bulb I put in my high beem indicator to notice it more. Now driving at night, it's like a blue headlight shining in my eyes when it's on. As far as the rear ABS: I didn't put the truck on the road until after I changed the gearing, so I didn't know if the rear ABS worked before. Before the speedo fix, if I braked hard, the rear tires would lock and slide wet or dry. Now on dry pavement, they will lock for a split second, then seem to pulse like other vehicles with ABS. I didn't try it on wet pavement yet. The speedo recalibration did change the way the rear brakes act. I will pull the rear drums to make sure everything is OK there. As far as the #8 pin. I ran out of room in my last post. My thought is: if it will work with no clip from the factory, or with dip switches in place of the factory fuses, then I believe it should work without a #8 pin conected. I thought about experimenting, but decided to quit while I was ahead.:)

[johncarrol]

Speedo still works great, but I should have my butt kicked. I wanted to light up my guages more. Especially the odometer and trip meter plus the high beam indicater, so I installed bigger brighter bulbs that would fit inside in those places when I did the speedo recalibration. I was driving two nights ago using my high beams and started to smell plastic burning just before pulling in the driveway. I removed the cluster yesterday thinking I ruined everything and would need to find another diesel cluster which would be almost impossible. I lucked out and caught it in time. There was only minor damage. I installed the correct bulbs, put everything back together, and so far,so good. As promised, the photo shows the inside of the board with one side of the fuses cut off and fuses bent out of the way. Also wires protruding from #6,7,8 where the reprogramming clip comes through from the other side which are soldered together far enough away from the board to hurt anything. I left approx. 1/8" of the wires stick out the back side where the clip inserts.

[john8662]

Cool, thanks for sharing the guts of your instrument cluster!

So, what bulbs did you put in there?

Same size dimensionally but higher wattage?

[johncarrol]

I found the bulbs at an auto parts store years ago. I used them before in a much older cluster to light it up more, which worked. They fit in the same socket, but are much bigger. You can see in the photo where I installed them. They didn't seem to touch anything, but gave off so much more heat, it nearly destroyed everything. Lucky I caught it in time.

[trbankii]

One option for brighter bulbs is to switch to LED - more output at a lesser wattage, so it will stay cooler than higher wattage incandescent bulbs.