Multiple questions...

[meby]

The first question is about the speedometer. The truck I bought came with a broken odometer. The speedometer worked, but the odometer didn't. Mark sent me another used one to replace it with, but now in addition to the odometer not working, the speedometer will not go past 55 mph.

Order of events.

Replaced gauge = odometer still not working.

Replaced vss with a new one = odometer still not working.

(I don't recall if the speedo only went up to 55 mph before the vss or after.)

I'm drawing a blank on what is causing this. Loose wire? Does the signal from the vss run through the computer? Suggestions?

Second question: where is the coolant temp sensor located? Is it the one in the middle of the block, behind and below the injection pump?

Thanks guys.

[Richie O]

Coolant sensor is on the drivers side all the way back. I bought 1 last year for like 70 bucks at NAPA.

[Ace]

The speedometer is grounded through the PCM (computer). That was a step I went through in eliminating the PCM from my truck. A bad ground "might" cause your 55 problem. I'd re-plug the PCM cable with a good shot of WD40 and check grounds. You could try directly grounding the VSS as a last resort, but if it fixed it that would be just basically bypassing the actual problem. That was the last thing I did did before removing the PCM.

[meby]

So what you are saying is that the vss feeds into the pcm and the pcm sends a signal to the gauge?

Does the gauge (the odometer and the speedometer) read the same signal? Meaning, why would the speedometer work, but not the odometer?

[Mark Nixon]

The VSS does NOT feed through the computer on a '90.
There was also NOTHING wrong in the speedo wiring, the old speedo would run 70, no problem.

Meby, that replacement speedo worked in all functions when it left here, BUT, my drill only turns it @ 35 mph.
{edit}I have a wiring set-up to spin a VSS to test the electric speedos{end}
The trip-odometer not working could be the gear got broken from vibration or idiots dropping it from 50 feet.

I'll pull another one out of storage and run it in the hauler for a few miles and ship it out.
I'll make sure it's a'92 or '93 one, as they SEEM to be less problematic.

Mark.

[meby]

"Mark Nixon"]The VSS does NOT feed through the computer on a '90.
There was also NOTHING wrong in the speedo wiring, the old speedo would run 70, no problem.

Yeh the old one ran higher than that.

[Ace]

Oh yeah, I see you got a 90. That starts in 91.5...

[Mark Nixon]

Even then, the speedos bench test the same way, as the 2 versions of speedo head are wired the same way in the cluster.

Black is speedo ground and the black wire side of the VSS and both wires are grounded on the power source.
Red is the power side of the speedo, goes to the positive terminal of the power source.
Green is the signal side on the speedo, white on the VSS to the speedo, it just ties together.

This is the lay out of the pins on the speedo head itself, NOT from the circuit board the speedo plugs into.

Mark.

[Ace]

The VSS still gets it's ground through the PCM on the 91.5-93 trucks. I know this because my truck no longer has a PCM. The only way the speedometer will work without it is if you directly ground the VSS. Go ahead and try it if you don't believe me.

Even then, the speedos bench test the same way, as the 2 versions of speedo head are wired the same way in the cluster.

If that is the case, then it sounds like he has a grounding issue, like I originally suggested.

[Mark Nixon]

I agree, to a point.
If the needle is fluctuating at the indicated 35 mph, then I would also concur to a grounding problem.
While I had the truck there was no such issue and while the odometer was not working for part of that time, the speedo was very accurate and well exceeded 35mph.

I believe I just sent him another bum speedo, so I'll eliminate the possibility of it happening again by thoroughly testing it.

Mark.

[dpuckett]

If the old speedo worked, odo broken, my guess would be the little plastic gears. Replace it with a 92.5-93, and you dont have the gears to break.

Matt- you see the 3.07 axle Mark has?

[meby]

The build date on the truck is Nov, 1989.

The original and the speedo that Mark sent work, but the odometer doesn't on either. The one that Mark sent is the one that only goes up to 55 mph. The needle doesn't fluctuate, at least not so much that it is obvious.

The 92-93 gauge, is it a direct swap?

[meby]

Yeh, I saw the axle. The issue is mostly getting it out here from Nebraska and winter is usually slow on work (and money) for me. This year the jury is still out, I have work right now, but don't have much lined up yet for Jan - Mar. I'm trying to keep expenses to a bare minimum until I figure out what is going to happen after the end of the year. We'll have to see, I guess.

[meby]

Mark, before you send another speedo/odometer, let me run the grounds down. Late Sat. night I took it for a spin and the odometer started working and the speedo ran up as high as it should, but it started fluctuating. I'll let you know what happens.