1992 C1500 Speedo, Tach, and Cruse Control stopped working

JustRangerThings · November 7, 2020

As the title states, my speedometer, tachometer, and cruise control all stopped working at the same time, and the check gauges light is on. This happened after the truck sat for about 2 months, and was working perfectly fine when parked. I have already checked the grounds, and cleaned them all up, no change. Any help? I would like to avoid having to buy a new cluster, as my truck only has 64K miles and I dont want to take one apart to set the mileage and risk breaking it. And money is also tight so any help is appreciated!

Brian Lindsey · November 8, 2020

could you have a rodent problem eating wires , that's a normal thing when a truck sits for any length of time

JustRangerThings · November 9, 2020

23 hours ago, Brian Lindsey said:

could you have a rodent problem eating wires , that's a normal thing when a truck sits for any length of time

Its a possiblity, however I have thoroughly checked the wiring (at least in the engine bay) and there doesnt appear to be any damage. I'll look a little closer but i doubt that is the problem.

thx1138v2 · November 10, 2020

Check the wiring near the end of the transmission - the Vehicle Speed Sensor. Trace it back to the harness and into the cab. The speed is a purely calculated number from the tach, the differential ratio, and the tire size. The tach also unergoes some numerical manipulation.

There is one wire that I found on my 1994 that runs through the brake light switch that carries some pulses also so look under the dash by the brake pedal. That's where I had a problem. You wouldn't think the brakes have anything to do with it but it's there to tell the cruise control when to disengage when the brake pedal is actuated.

JustRangerThings · November 10, 2020

2 hours ago, thx1138v2 said:

Check the wiring near the end of the transmission - the Vehicle Speed Sensor. Trace it back to the harness and into the cab. The speed is a purely calculated number from the tach, the differential ratio, and the tire size. The tach also unergoes some numerical manipulation.

There is one wire that I found on my 1994 that runs through the brake light switch that carries some pulses also so look under the dash by the brake pedal. That's where I had a problem. You wouldn't think the brakes have anything to do with it but it's there to tell the cruise control when to disengage when the brake pedal is actuated.

I'll check it out, what should I look for?

thx1138v2 · November 10, 2020

7 hours ago, JustRangerThings said:

I'll check it out, what should I look for?

Typical things: loose connectors, worn or broken wires, worn or frayed insulation. If possible check continuity from one end of each wire to the other.

On my '94 there was one wire that went from the Vehicle Speed Sensor up the transmission, into a wiring harness, through the firewall, and down into the brake light switch, and across into the ECM that is behind the glove box. When I checked it from the VSS to the brake light switch it was OK. But when I checked it from the brake light switch to the ECM it showed an open circuit. It turned out to be a bad circuit through the switch. I replaced the brake light switch and all was good but I'm not sure what you will find.

Edited November 10, 2020 by thx1138v2

JustRangerThings · November 10, 2020

1 hour ago, thx1138v2 said:

Typical things: loose connectors, worn or broken wires, worn or frayed insulation. If possible check continuity from one end of each wire to the other.

On my '94 there was one wire that went from the Vehicle Speed Sensor up the transmission, into a wiring harness, through the firewall, and down into the brake light switch, and across into the ECM that is behind the glove box. When I checked it from the VSS to the brake light switch it was OK. But when I checked it from the brake light switch to the ECM it showed an open circuit. It turned out to be a bad circuit through the switch. I replaced the brake light switch and all was good but I'm not sure what you will find.

And you think this would fix my tach too?

thx1138v2 · November 12, 2020

On 11/10/2020 at 5:58 PM, JustRangerThings said:

And you think this would fix my tach too?

Maybe. You just can't tell until you do the testing. Or you could forego the testing and put a new brake light switch in to see if it helps. They're cheap. Mine was around $12 but that was some time ago so they may be a bit more now. At the time I was digging through the schematic and looking at ECM's, VSS's, etc and since it was the cheapest part in the schematic and easily accessible I thought I would try it. It fixed mine and once I had the old one out I took it apart to see how the internal wiring was done and discovered the cruise control circuit that I wasn't thinking about earlier.

Give it a try and let us know.