Recalibrating 1988-1991 speedometer / dash cluster

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someotherguy

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(I couldn't find a search result here for this post so I'm copying it over from my old Chevytalk write-up from 2011 - I'll have to break the post into two parts to include all the pictures. Sorry for the super-long book style writing.. I'm trying to give a good understanding of the mod before someone dives in, ya know?)

Modifying the 1988-1991 gauge cluster for DIP switch configuration - this assumes a fairly decent degree of electronics soldering ability; if you don't have this, please find someone else to do this job!

There are write-ups about reconfiguring 1992-1995 fullsize truck DRAC modules to recalibrate the speedometer, including replacing the hardwired jumpers with DIP switches. However, there seems to be a lack of good info on modifying 1988-1991 clusters and a general understanding of how they work. They don't use a separate DRAC module like 1992-1995, but have similar circuitry built directly into the cluster.

Instead of hardwired jumpers, these clusters have a bank of 7 tiny 1/4 amp fuses. The factory calibrates the speedometer by selectively blowing out some of these fuses when the truck is built, based on calculations including the OEM tire size and rear axle ratio. The combination of blown and good fuses operates just like a bank of hardwired jumpers. They're either on, or off.

Later if you needed to recalibrate for a tire size or gear ratio change, you would purchase a kit from GM including an adapter that plugs into the cluster's harness socket. It lets you apply voltage that wipes out all 7 of the fuses so they're open and no longer configuring the speedometer calibration. You would then pick your tire size and gear ratio from a supplied chart. It would tell you which pins to break off a programming clip (basically just a multi-jumper in a single pin strip package) which you would then plug into the socket on the back of the cluster, and you're done.

The problem with that setup is the supplies are obsolete and becoming scarce. Beyond that, once you've broken the pins off the programming clip, if the speedometer is still incorrect (or you change the tire size/axle ratio again later), you'll need to try again with another clip. My goal was to replace this with a simple DIP switch, similar to the way we're modifying the 1992-1995 DRAC modules.

One key thing to remember about a DIP switch-modified 1988-1991 cluster is that the settings chart is NOT the same as the 1992-1995 chart. Trying to use that info will cause you to make up new cuss words. Been there, done that. Following the charts (and "serious math") provided by a certain magazine article will also cause insanity, as they didn't proofread their material – the final number they came up with to use with the chart isn't even listed, due to two bumped decimal positions and a misprint in the figures.

Instead, the GM-supplied chart that doesn't have you doing any math up front; just has a list of tire sizes and gear ratios. The pins it's suggesting to break off the programming clip, you can look at just like DIP switch settings. If it says remove pins 1,4,6 – that means your DIP switch would read 1,4,6 OFF – 2,3,5,7 ON. The downside to this chart is it's limited in the number of different sizes you can pick from, though keep in mind you can use "tire math" websites to compare your size to the listed ones and pick one that is similar in circumference to your tire.

I'm still working on deciphering the erroneous charts and math I've seen so far in hopes that I can provide a more flexible solution for picking settings. For now, hopefully at least the chart will be useful for a fairly decent number of typical gear/tire size changes.

Now on to the modification...

Remove the cluster from the truck; be careful not to damage the gauge cluster trim bezel, and also be sure to disconnect the gear indicator "string" from the shift mechanism on the column before removing the cluster!

Remove metal rear cover (eight 7/32" screws) and take care to not cause damage to circuits with ESD (electrostatic discharge.) In other words ground yourself frequently while working, before touching the circuit board, etc. and if you're in an area with relative low humidity consider wearing a grounding wrist strap. It sounds paranoid, but the amount of static electricity required to cause circuit damage is so low you can't even feel it happen.
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Remove lens and shroud assembly as one unit, no need to separate them unless you wish to take advantage of this opportunity to clean them up.
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Remove speedometer (four 7/32" screws), two round light shrouds, and odometer/tripmeter assembly (two 7/32" screws and pull harness from circuit board - removing the odometer assembly is to prevent damage to the reset button.)
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Here are the seven 1/4 amp fuses used for factory calibration; we'll be removing these as they're a one-shot item, cannot be re-used as once they're blown, they're blown. I checked each one with my meter set on continuity to see which were "on" (not blown) and made dots with a Sharpie on the circuit board next to those. This way I'll always know how it used to be set, instead of possibly losing a handwritten note with the settings.
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Using a soldering iron and a vacuum-type desoldering tool, remove solder from the back side of the fuses, and also the soldered-up (but currently blank) row of pins in between them. Sometimes it helps to melt the old solder by adding a small amount of fresh solder to get it going.
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(continued in next post...)
 

someotherguy

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DIP switch soldered in and the far right row soldered up although nothing is in it. That step isn't really necessary but it's habit, so I did it.
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Here's GM's supplied chart for use with their recalibrating kit.
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DIP switch set with 1,4,6 OFF – 2,3,5,7 ON – corresponding to settings on the chart for 3.42 gears and a 225/75/15 tire.
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Sharpie warning on the back of the cluster to not attempt using a GM reprogramming kit on it (in case anybody happens to have one) as it will no longer work on this modified cluster (actually, a programming clip WILL still work, though there's no need to zap the fuses anymore since they're gone - just set all the DIP switches to "OFF" for the same effect. However the DIP switch makes it pointless to bother with a programming clip anyway.)
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What I usually do at this point is leave the front of the cluster in its current disassembled state, but with the metal plate reinstalled on the back – install it into the truck, temporarily mount the speedometer by plugging it back into the cluster but leaving the screws out, and go verify the speedometer reading against GPS. If it's accurate, then go ahead and put all the pieces back together and you're done.

Richard
 

someotherguy

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For what it's worth, the 2011-era quote "I'm still working on deciphering the erroneous charts and math I've seen so far in hopes that I can provide a more flexible solution for picking settings" - is lost to the wind as that truck left my possession shortly afterwards and I haven't owned another 1988-1991 truck to experiment with! So until someone has better info to share, we're currently stuck with the factory chart of tire/gear/settings combinations. The stuff I read in a few of those old articles was sketchy at best.

Richard
 

GrimsterGMC

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For what it's worth, the 2011-era quote "I'm still working on deciphering the erroneous charts and math I've seen so far in hopes that I can provide a more flexible solution for picking settings" - is lost to the wind as that truck left my possession shortly afterwards and I haven't owned another 1988-1991 truck to experiment with! So until someone has better info to share, we're currently stuck with the factory chart of tire/gear/settings combinations. The stuff I read in a few of those old articles was sketchy at best.

Richard
When I got my 1988 k1500 it had 3:42 ratio but also 33" tires so I did this mod to correct the speedo. Because the manual didn't have 33" tires as an option I simply combined the diff ratio with the tire circumference to end up with a "meters traveled per revolution of the driveshaft" and compared that to the factory options and came up with a perfectly calibrated speedo. Then I did the same when I changed to 4:11 diffs.
 

Vettepilot

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Sounds like I found the right place here. I have a new to me 1994 GMC K1500 that I am fixing up. It is fully optioned, and has the speedo with tachometer. The tachometer appears to be working right, and the speedo seems to work smoothly, but indicates far too high. On a test drive just now, we were seeing about 55 or 60 while going 30 to 35. (Numbers not exact.) We did notice the speedo needle jump once or twice. Did not have time to test cruise control.

Now it's laid up to fix an oil leak, but what might my speedo problem be??

Thanks!

Vettepilot
 
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someotherguy

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Sounds like I found the right place here. I have a new to me 1994 GMC K1500 that I am fixing up. It is fully optioned, and has the speedo with tachometer. The tachometer appears to be working right, and the speedo seems to work smoothly, but indicates far too high. On a test drive just now, we were seeing about 55 or 60 while going 30 to 35. (Numbers not exact.) We did notice the speedo needle jump once or twice. Did not have time to test cruise control.

Now it's laid up to fix an oil leak, but what might my speedo problem be??

Thanks!

Vettepilot
Right place (the forum itself) but the wrong thread. :) No worries. But this thread is about recalibrating the 1988-1991 speedometer, which is slightly more involved than the 1992-1995. For 1988-1991 the circuitry is built into the cluster and takes a little more surgery. On 1992-1995 you can simply modify the DRAC module which is located under your glovebox compartment. Go check this site out for more info: https://harristuning.com/Tbi/vssb-drac-calibration/

They mention the "old site" had someone suggesting to install DIP switches, but they recommend just moving the jumpers around instead. Meh.. I'll stick with installing DIP switches. It gives you the flexibility to fine-tune until you get it accurate to your liking. It's cheap (cost of the switches is practically nothing), and it's easy if you can do basic electronics soldering.

Richard
 

scott2093

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The tachometer appears to be working right, and the speedo seems to work smoothly, but indicates far too high. On a test drive just now, we were seeing about 55 or 60 while going 30 to 35. (Numbers not exact.) We did notice the speedo needle jump once or twice. Did not have time to test cruise control.
Gear ratio or tire changes require messing with the VSSB/drac,..... the VSS (speed sensor located on the transfer case in 4wd) can get wonky and cause jumpy speedo. If moving jumpers around on the drac doesn't get you there, my guess would be the actual cluster.

Chased my tail with my 93 and was super far off on Harris tables to get anything close to reasonable.
Basically had to input I had 4:10 gears when they're 3:73.... Stressed the driveline shop out for no reason questioning their previous rear end install...lol
Was my cluster in the end...
 
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