speedo calibration of drac

roy67ss · November 14, 2005

Hi gmtech,

Here's the scenario. My son's truck is a 91 3/4t 4x4 454 4l80 trans. It has the moony gauges which I want to change out to a 91 cluster with tach and gauges. It is plug and play except for the speedo business. I know that GM has a kit to burn the existing chip and add on a new calibrated chip to correct the speedo.

I would rather add on a drac unit with the dip switch mod but, I haven't found that kind of mod anywhere.

I have a few questions of you guys.

1. Does anyone know exactly which of the 8 holes on the back of the cluster that are used to fry the existing calibration and which ones get 12v or ground or what?

2. The wires are all there to go to the external DRAC except for the speedo drive which is internal on this cluster. Is it possible that one of the several unused terminals on the plug is/could be the one for an external speedo signal (circuit #824) from the drac?

3. I presume that the recalibration unit from GM just makes connections between certain holes on the back of the cluster similar to the 7 that are in the drac unit. Does anyone have the sheet or list of which pins are used for the various ratios? In all my searching, I have never found that information.

With the info for questions 1 & 3, I'm sure it would be a simple thing for anyone to recalibrate their speedos when changing tire sizes or rear end gears on 88 - 91 without having to go to GM for their 'special' kit.

Thanks, Roy

GM-Tech · November 14, 2005

Wow, you're really testing my memory on this one. Each of those 8 little holes is connected through a fuse. The factory blows the appropriate fuses to calibrate the speedometer to match what's needed.

When we had to recalibrate for whatever reason, we had a connector that plugged into the back of the cluster, connected to power and ground and it blew all 8, or whatever remained. Then, you get this little black clip (it's not a chip, just jumpers in one clip) and break off the appropriate pins to match the tire size/axle ratio. Then just plug the clip in.

I actually have the chart for which pins to break off for which tire/axle combination you want. And I can get the part number for the kit to pop the remaining fuses and I believe it comes with several of those jumper clips.

I can't help you with modifying some other type of setup to work.

roy67ss · November 14, 2005

Thanks,

I'm going to break down and order the kit. I'd probably be needing a different chart anyway seeing as how we are in the metric world here in Canada and the ratios have to be workable with kilometers and not miles like you guys.

GM-Tech · November 14, 2005

I'd probably be needing a different chart anyway seeing as how we are in the metric world here in Canada and the ratios have to be workable with kilometers and not miles like you guys.

I'm sure even in Canada, a 4.10 axle is a 4.10 axle. Don't see how a ratio would change just because you use the metric system. And tire sizes are metric, so no difference there. Unless you guys express the wheel size in millimeters. Lets see, P275/55R20 would be P275/55R50.8. Hmm, looks weird.

I'm sure the only difference in the speedometer itself is on the face of it with the numbers. They just print 80 kph where 50 mph would be, etc., etc. The needle goes to the same place.

roy67ss · November 17, 2005

You are right, the chart is the same. I just had a problem with the drac I had taken out of a 93 4x4 that had 3.42 gears all its life, and comparing the pins with the chart at this site and thought too much because the chart made it look like it was setup for 4.10 gears. So, I thought maybe metric was the problem. But now I know there is a problem with the chart and I'm trying to figure out where it goes wrong. The calculator works but then indicates pins opposite to the chart. Anyone using it should do a lot of careful checking before using. I'm going to check it with another drac or 2 and be able to make the necessary corrections to it.

Thanks again for the help,