Okay thank you and do you know any other ways before the kit or how much it’ll cost to get it fixed?
Speedometer Issues
- Thread starter Finn
- Start date
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No idea. Never needed to mess with it.
You can solder in dip switches on the VSSB or signal converter mounted on the cluster. @someotherguy has a nice write up on it somewhere, I lost my bookmark for it.
someotherguy
Truly Awesome
The "programming kit" includes a jumper that plugs in behind the cluster, you then supply 12V to the jumper and it blows all the fuses inside the cluster that are used as switches to configure the speedometer. You then replace the fuses with jumpers that you break off corresponding ones to act as replacement switches for the now-blown fuses. It works fine but it's a hokey arrangement if you ask me. However, it does avoid anyone having to go "inside" the cluster and work with the sensitive electronics, do any soldering, etc.
Yep! Much the same as we modify the 1992-1995 gmt400's DRAC module with a bank (or two, depending on design) of DIP switches, one can modify the 1988-1991 gmt400 gauge cluster. ONLY if you have reasonable electronics soldering experience, though. It's easy work, but if you aren't familiar, please farm it out to someone that does this type of work. Guys that do gaming console "mods", radio/amplifier repairs, stuff like that.
My write-up is on Chevytalk but as I've become used to saying these days, my host provider (godaddy) sucks so my picture links may appear broken. You can either choose to right-click the broken pics and "open in a new tab", or "allow insecure content" on Chevytalk. (Chevytalk is https, and my site on godaddy where I host the pics is http)
Recalibrating 1988-1991 gmt400 speedometer: https://www.chevytalk.org/fusionbb/showtopic.php?tid/258562
I may as well just re-create the write-up here, since I can upload the pics directly. Let's give it a shot in the next reply:
Richard
someotherguy
Truly Awesome
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.
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.
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.)
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.
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.
Continued next reply..
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.
You must be registered for see images attach
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.
You must be registered for see images attach
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.)
You must be registered for see images attach
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.
You must be registered for see images attach
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.
You must be registered for see images attach
Continued next reply..
someotherguy
Truly Awesome
Continued...
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.
Here's GM's supplied chart for use with their recalibrating kit.
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.
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.)
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
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.
You must be registered for see images attach
Here's GM's supplied chart for use with their recalibrating kit.
You must be registered for see images attach
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.
You must be registered for see images attach
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.)
You must be registered for see images attach
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
@df2x4 @kennythewelder this writeup should be a sticky....
Not sure about a sticker, but, it is a grate write up by Richard, @someotherguy. It would make a grate how to install DIP switches in 88 cluster thread, or something like that.
someotherguy
Truly Awesome
Limited audience, to be sure. And, the more stickies at the top of each forum, the farther someone has to scroll down before getting to new posts.
IMO a FAQ post sticky'd that included links to relevant write-ups is a cleaner way of doing things. However, not everyone is hip to what FAQ means, even when you spell it out in the post title.
At any rate, this is how the 88-98 truck section is setup on Chevytalk; I've collected write-ups and helpful posts on various subjects from the members and put them in one spot. It hasn't been maintained in a while but has held up pretty well over the years. In that section we have just two sticky posts - the FAQ, and "members' trucks" post.FAQ - FREQUENTLY ASKED QUESTIONS - PLEASE READ! - Chevy Message Forum - Restoration and Repair Help
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Richard
