I've been digging for years, 'addressing' this issue multiple times over, and so far have had some limited success. Now I'm diving deeper, and if anyone around here happens to be an electrical guru, I'd appreciate a team-up. Maybe we can help bring some instrument clusters back to life. I'm not an electrical engineer, and am more or less exploring the limits of my knowledge already.

Symptoms:
- Intermittent speedometer. Works 99% of the time, unless it's rained or had recent snowmelt, then it will reliably sit at zero until I've driven "a while." A while is completely dependent on HOW wet conditions are - more enduring recent rain, longer drive time before the speedo wakes up. Not much rain, it might spring to life a block or two out of the driveway, long before the engine heats up. If it's been dry conditions, it'll work instantly, 100% of the time. This is very weird to me and may point to corrosion/intermittent ground somewhere farther down the line - I haven't explored much outside the dash cluster for this issue.
- Odometer's been dead for...thousands of miles. Bright side is since the truck doesn't know I'm putting miles on it, I haven't needed an oil change in, like, eight years
- Tach is...mostly good. On rare occasions I'll notice it get a bit unstable and just start jumping around +/-500rpm or so for a few seconds. This seems super intermittent, honestly pretty rare. Rare enough I haven't been able to tie to to any recurring conditions.

What I've done so far:
Full teardown of the dash cluster. As many folks have experienced, I had a handful of burned traces and cooked capacitors. I replaced the two electrolytic caps on the board, I have not replaced any of the SMD ones or the one film cap, or any other components. Hours (and hours) spent ringing things out on many occasions. At this point, I'm confident not traces are broken or anything of the sort. Certainly could be some other component bad on the board. If I knew values for everything on the board, I'd happily replace it all wholesale, but I haven't been able to find a board level schematic to pin down the unknown items, and I imagine the transistor array and the DIP IC are probably unobtainium if either of these prove to be the issue.

Currently trying to reverse engineer things so I can bench test the speedo/odo board instead of constantly pulling the cluster in and out of the truck. I have a cheap PWM generator to stand in for the speed sensor, which is electronic on my truck and not cable driven.

Going by the factory schematic, this board has four electrical connections to the truck:

1) Ground - this connects to pin 10 on connector C on the dash cluster, which routes back to the speed sensor
2) IGN - pin 8, connector D (plus a circuitous route back to multiple other connector pins through several unused dash lights)
3) SE - pin 1, connector D; speed sensor signal
4) 4P - pin 6, connector B. The diagram says this is "From TCCS ECU, Cruise Control ECU, and PPS ECU."

Can anyone shed light on the 4P connection? I can't find any reference to what or where the "PPS ECU" is. I don't have cruise. This connection looks like it goes to the collector of an NPN transistor, with the base being the speed signal. From that, I'm assuming this connection is only used to feed speed information out to the various ECUs and has no impact on functionality of anything in the dash cluster. I'd love to be corrected (or confirmed, if that's the case) on this if anyone knows what they're talking about.

I look forward to the first "have you checked your speed sensor?" reply.

 

Uh, have you checked your sensor? On my '94, it's a 3-wire connection (pwr, ground, signal). I didn't try to test the sensor itself, but once i connected a signal generator to signal and ground the speedometer/odometer worked fine. Replacing the sensor fixed it for, oh, 15 years now.

The sensor is a hall-effect + amp chip. Someone on this site reported cleaning oil out of the sensor restored it to life. The sensor is available from RockAuto and is not that expensive.

 

I figured that went without saying, but I guess I'll state it: the speed sensor has been tested and cleaned, with no impact on the symptoms. Admittedly that was several years ago, but the problem hasn't changed from before checking it out to today.

I'd be shocked if there aren't other downstream issues, like a loose splice point or shoddy ground somewhere. My main curiosity with this post is if someone who actually understands electronics can help me grasp what the fourth connection to this PCB is doing.

My thoughts: speedometer works 99% of the time. Odometer hasn't ticked a tenth in years. The way I'm looking at the schematic, there's only one signal input to the PCB that both the speedometer and odometer are mounted to. To me that says the signal input is not an issue. At the very least, it certainly says the speed sensor is working 100% of the time when the speedometer is working - so I'm ruling out the speed sensor as a possible issue (for nonfunctional odometer specifically...it could still be contributing to the speedo being erratic when wet (ew.) ) from all this:

1) I'm *assuming* the speedo and odo both function from the same speed sensor input; i.e. X pulses per mile creates the appropriate gauge output for each instrument.

What I'm really not sure about is that the "4P" terminal on this PCB goes to the TCCS ECU, Cruise ECU, and PPS ECU. My understanding of TCCS is it's essentially just pre-OBDII OBD, so I don't imagine it actually contributes to vehicle function...more of a 'black box,' but I invite correction on this if I'm wrong. I don't have cruise, so I assume no cruise ECU even exists. The PPS ECU is a mystery to me. What is it? Where is it? is progressive power steering, so not an input. From this:

2) Given that this line goes through the PNP collector, I think that could be running the transistor as a current amplifier, which may be necessary to drive the stepper motor for the odometer. But this is really reaching beyond the limits of my electronics knowledge and I'm kind of force-fitting the little I know into what I'm seeing. Basically, I wonder if the 4P connection is somehow providing the odometer signal?

If (1) is correct, then I have a problem that is isolated to this PCB, and potential root causes are pretty limited.

If (2) is correct, then the PCB may be fine, and there could be a problem with wiring between, or directly with one of the ECUs in question, or any combination of these things.

Edit: Figured out PPS is the progressive power steering ECU, so another item that definitely isn't providing a signal to the speedo or odo.

 

Not a single bored electrical engineer around here?

If I provide +13.4V to the IGN terminal, ground to the ground, and a signal to my SE terminal on this board, I get zero action. That means either the '4P' connection is required in some capacity, or I'm missing something (or somehow totally smoked this PCB along the way).

Does anyone know the 'gearing' on the speed sensor? If I recall correctly, I get four pulses per revolution from the sensor when I was testing it. But how many revs does the speed sensor do for a given distance traveled?