Recalibrating 1988-1991 speedometer / dash cluster

[Vettepilot]

Interesting. Thanks guys. In the old days of course, we just swapped out the gear on the cable. Every once in a while, the one inside the trans would be wrong, or bad, which was much more involved.

So, can something change or go bad inside the Drac to make the speedo inaccurate? Something inside the head unit?

Thanks!
Vettepilot

 

[Shwa Kid]

...So, can something change or go bad inside the Drac to make the speedo inaccurate? Something inside the head unit?....

Not like the old school stuff. The 88-91 uses a VSS on the trans which creates a square wave signal/pulses, the two twisted wires go into the back of the gauge cluster, through some circuitry to the speedo. The stepper motor (amongst other components) could fail, but it's not like the old plastic gears that get chewed up or start slipping.

FYI anybody doing a LS swap and wiring these old speedos, I tried using the ECM output, retrofitting a DRAC module, all kinds of stuff and just resorted to teeing into the twisted pair from the trans output VSS (I had a 4L80E), two wires to the ECM/TCM, two to the original speedo wires and it worked perfectly. I should have started with the simple answer.

Man, I must be getting old, I remember the original time this got posted (but I think it was on FSC). I seem to remember a part about chalking your tire and the ground, rolling it one revolution and measuring the distance to get the circumference rather than using the calculators as they don't 100% align. Then also verifying with GPS, which was expensive and fancy at the time.

 

[Vettepilot]

Ok, let me re-phrase my question.

I just got this old 1994 K1500 GMC truck running, and the speedo is showing speeds of nearly double what we're traveling.

What might be wrong and what do I go after? How do I fix it?

Thanks!
Vettepilot

 

[scott2093]

Jumpy speedo sounds like vehicle speed sensor.
Pretty extreme speed differences.
A scanner would tell you what the truck is thinking speed is and may actually be right on. Which would point to cluster.
So scanner to see speed compared to gps would eliminate having to goof with drac for the time being.

Transmission kinda relies on all that speed stuff too so good to get it sorted....

 

[Vettepilot]

You know, when I dropped the fuel tank and changed the fuel pump, I noticed that the last time that was done, they squished a wire bundle between the bracket and the frame. At the time, I thought "well, if the brake/tail lights on this don't work, I'll know why..." But now I'm wodering if speedo wires were not damaged too. Guess I'll start there.

Being a mechanic for over 50 years, I never really believed in the concept of a "Lemon". (Well, except in the case of Fords...) But this truck has me wondering! Man! It's been giving me fits at every turn!

Vettepilot

 

[schaefsh]

Greetings Richard- thanks on behalf of everyone for this amazing writeup and tutorial on the speedo recalibration. I have completed the DIP install for my 88 K1500 but am needing the correct orientation of the switches because a similar result is not listed on the chart offered. Would you be willing and able to provide the "tire math" for the switch locations for my project?

I measured the Circumference and it came in at 107.5"

1988 K1500
BF Goodrich All Terrain KO2
35X12.50R15
GU6 3.42 Gearing
700R4 transmission
NP241c

Thanks in advance!