Weird check gauges/transmission issue

[jkeaton]

Speedometer pegged at 100. Tachometer reads 0 no matter what. Check gauges light flickering and trans won’t shift. I can manually shift between 2 & 3, but it won’t shift on its own and won’t go into OD or first gear (manually) just started this on my way home from work. Transmission rebuilt 2 years ago, only put about 4000 miles on it since. Fluid level is good. Hoping something like a speed sensor or something? Trans was fine before the gauges started acting up. 1995 K1500. 213,000 miles.

 

[Schurkey]

Old-style air core gauges, or the newer stepper-motor type?

The stepper-motor guages are NOTORIOUS for failure. Fortunately, a simple and inexpensive "fix"--replace the stepper motors by soldering new ones onto the instrument cluster.

My '88 has the old-style pre-stepper-motor gauges. When the speedo quit, the odometer also quit, as did the cruise control, ABS, and the torque converter clutch.

When the speedo came back to life, so did all the other accessories. The speedo and accessories were intermittent for about two years--they'd work sometimes, they'd quit working for a mile or fifty. It was totally random and entirely un-predictable.

I replaced the speedo, with no improvement.

Eventually, I put the original speedometer into a replacement (used) instrument cluster--bingo--works perfectly ever since. The instrument cluster has ICs that take the signal from the vehicle speed sensor, modulates that signal and sends it to the cruise control, speedometer, ECM, etc. With no signal from the VSS, the speedo, ECM, ABS, and cruise control are helpless.

The ECM would control the trans shifting on your vehicle, too. Connect a scan tool, drive the vehicle, and see what the ECM is getting for VSS information.

 

[jkeaton]

Stepper I assume. I have an actron scanner. I will have to check if it shows VSS info.

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[Schurkey]

My scan tool will interact with stepper-motor gauges on my Trailblazer. I can move the needles to the far end of the range at the push of a button.

This is a great way to diagnose stepper motor gauges.

 

[jkeaton]

My scan tool does not read that info. Replaced the VSS, no change. Next is to pull the cluster out an see if the ribbon cable connections got messed up somehow. If that does not fix it, it's going to the recyclers.

 

[Schurkey]

Better option: Find a REAL scan tool.

 

[jkeaton]

I have an actron one that reads and data logs all kinds of stuff, just not to the point of driving stepper motors on an OBD1 vehicle. Thanks for the tip though.

 

[jkeaton]

Scanner told me it was the ignition switch. Not. The journey begins...until I found out I don't own a 4mm external torx bit...lmao

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[jkeaton]

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For those of you who may face this issue, it was the ignition switch. All is good in the GMT400 world.