I need a little help. I have a '92 D250 (2wd) extended cab with auto transmission. I have read all the past posts that I can find on the speedo problems and have tried many of the suggestions, but to no avail. Here are the symptoms:
The speedo began by bouncing around with no apparent pattern as to cause. It would usually jump over to the highest reading and the odometer would be clicking off miles (even while stopped). Somewhere along the line I lost the cruise control and overdrive. I changed out the VSS at the transmission and I spliced in the new plug that came with it. I shrink-wrapped and taped all connections. I also turned the VSS while the square shaft was inserted into the "tube" on the transmission and it clicks fine, so I assumed it was not rounded off in there (do I need to test this more...maybe open it up and check it out inside the transmission?). I checked both wires from the VSS to the computer (PCM) and got good OHM readings. Test drove and had same problems. I then unplugged the PCM and inspected, then filled the plug with dielectric grease. Test drove and the speedo didn't jump around at all.....just dead. Later it jumped over to highest reading for a short while (clicked off 13 miles) and hasn't moved since. I took dash out and speedo connection looked fine. I filled it with grease (as I did all connections in the circuit) and replaced. Nothing. I then, out of desperation, replaced the PCM ($165.00).....Nothing. Ouch. I am planning next to unhook the VSS and spin it with drill. Here are my questions:

If the key is on and I spin VSS with drill, should speedo work? If it doesn't work, but should, I figured I could check the wires at speedo connection with voltmeter and if I get reading while spinning VSS, then that would narrow it down to the actual speedo not working. Am I on the right track?

Is there some "Odometer tampering safeguard" that I don't know about that could cause the problems?

Does anybody know which wires run from the PCM connector to the speedo so I can check them out (which numbers in the connector)?

Sorry this is so long....figured the more you know, the more you could help. Thanks in advance for any help.

Needing overdrive in Arkansas,

uriahpick

 

I don't think the problem is in the speedo - if the VSS was working you'd have OD. I'm working out of a '93 FSM here so bear with me - your wire colors may be different but the system works the same.

Down at the VSS you have 3 wires. White/orange is signal, it goes both to the speedo and pin 47 of the PCM. Black/light blue is sensor return (grounded in the PCM) on pin 4. Tan/yellow is 8 volts out of pin 7. With the key on you should have 8 volts between tan/yellow and black/light blue down at the VSS connector.

The speedo should work if you spin the VSS with a drill. The speedo does not run off the PCM, it gets its signal from a branch of the same white/orange wire off the VSS. It gets B+ and ground from the instrument panel circuit board.

 

Thanks wannadiesel for the reply and the help. On my VSS, there are only 2 wires: the white/orange wire running to pin 47 and speedo, and the black/light blue wire running to pin 4. From my diagram, it looks like there is a tan/yellow wire running from pin 7, but it looks like it runs from the crankshaft position sensor. I will do the drill test at the VSS and see if the speedo works. If so, then the problem must be between the VSS and the pinion gear. I haven't gotten a code for "no VSS signal" yet (#15 I think.....this has had me confused as I was thinking the VSS was working), but it seems I remember now reading in a past post that the PCM is looking for a continuous signal over so many miles (11??) to determine "road load conditions". I don't know if I have actually driven it that far since replacing the PCM. I need to drive it around and see if it will produce a code.
Thanks for the wiring info. I was under the impression that the white/orange wire running to the speedo came after the PCM in the circuit, but from your post I am getting that it branches off before the PCM. Is this correct? I knew I should have posted here BEFORE buying the computer.................Stupid, Stupid, costly mistake.
No cruise control had me baffled and I was trying to figure out the circuits for the 4 wires running to the servo....I didn't think about the VSS killing my cruise until I read your post. With no error code, I had ruled out the VSS........gotta drive it more. It must be in the VSS/transmission end somewhere. Thanks again. I'll keep you posted.

 

Hmmm. They must have changed things in '93, the way yours is actually arranged is the way my FSM shows for everything except auto 2wd's. Make sure the black/light blue wire is a good ground, and see if your VSS will generate a current if you spin it with a drill.

 

Do you have a tachometer??? If not then I am suspecting the Crank sensor wiring on the engine. If that sensor is kaput you loose your cruise, but not your speedo. However if the wires going to that CSS are shorting or crossed, maybe the PCM doesn't know whats going on..

Just a thought..

J-eh

 

If it was the crank sensor it wouldn't be charging, would it? And he'd have no A/C.

 

Good point WD...

This is a weird one..

 

Thanks for the help guys......I got the speedo working, and the cruise came back with it. But, it is running about 20 MPH faster than actual speed (as clocked with a GPS unit). Also, it seems to jump around and could read anything when first started up and driven, but settles down and gets more consistent as the truck warms up (consistently 20 MPH faster than actual speed). It sounds crazy.......any ideas. Oh, the dead speedo problem was the new connector that came with the new VSS.....the ground in the molded plug was loose and was being pushed back by the pin the fits into it, so it was seldom if at all making contact. It was barely contacting when first installed, so I was getting no resistence, and getting readings when spinning the VSS....after a short drive, it vibrated out of contact, thus the dead speedo. Exchanged VSS and got new plug. Speedo working, but with above mentioned problems still present. At least I have cruise control.

 

Hi,
I had a similar problem with my 93 and it turned out the tailshaft nut on the transmission was loose. What would this have to do with the speedo? I think it was proving a ground to the rear end and the other sensor.

 

I don't understand the inconsistent readings when cold, but the reading wrong sounds as though you may have the wrong VSS?

 

Alec, it is NAPA/Echlin part # VSS100, and it is the correct one as far as I can tell. Maybe not.......I'll double check.

j_stickers, was your transmission leaking out of the tailshaft? Mine has developed a slight leak, not even enough to notice on the garage floor, but enough to spin it on the underside of the truck.....Maybe the loose nut allows the pinion gear for the speedo to slide (or wobble?) slightly out of place.......I don't know. Worth checking out, though. I just got the truck back last night. A buddy had borrowed it to go pick up a tractor in Texas. I'll check it out this morning. Thanks guys..............

 

uriah, come fix my speedo problem!

I call it "THE PLAGUE"

I've done everything you've done...but I have the L-shaped gear housing adapter between the VSS and the trans (5spd Getrag 2wd). I've replace the shaft & key stock piece, and pressed the gear on. I've also heated up both gears with a torch while they were on the shaft, which kept them from spinning on the shaft. Still nothing.

I could care less about my speedo...I just want cruise.

For now I've installed the VSS directly to the trans, since the speedo works great but is 20-30mph off depending on speed.

At 70mph, my speedo reads 100
Still no cruise control.

 

Well, the speedo has settled down on its own. It isn't jumping around when it is cold anymore and it now consistently runs 10 MPH over. I don't know what the fix was, so who is to say it won't come back......but for now, it is working again. Thanks for the help guys.....TexasHardcore, I don't know why your cruise isn't working if your speedo works at all. On mine, I didn't lose cruise until my speedo died completely. Maybe your cruise problem is somewhere else??? Have you checked the wiring harness? I went over mine and found two bad sections in the VSS wire. In one spot it was completely broken and the previous owner had simply twisted it back together and taped it. I'm surprised it was working at all before I "fixed" it.