So my speedo quit working some time ago. The mechanical side, VSS, adaptor, etc, all seems to be working fine. I took it apart and tested it, and it all checks out.

The needle on the speedo will go crazy under heavy acceleration, and bounces all over the place (higher than my actual speed) I'm not sure why this is, but it leads me to believe there is an electrical/PCM problem.

So the next thing to check is the PCM right? I don't have any codes, or any other PCM related problems. Is there anything, other than checking power/ground to the PCM, that I should check out before buying a new one?

 

Just did the PCM check, but it only actually tests power/ground in. Can I be 100% certain the PCM is bad?

 

I have 5 volts at one wire coming in to the VSS, and good continuity to ground at the other. Is this right?

 

Look at the wiring from the VSS to the plug at the back of the engine.
It's not unheard of to have the wiring get chafed and cut, as well as ground out.

This may explain the erratic operation of the speedo under hard acceleration.

Mark.

 

Ok thanks! The only other wires on that harness are the reverse lamps right? If possible I'd like to pull the whole harness out to inspect it, and then put it in loom before I put it back.

 

It also has the 4x4 switch wiring in it, as well as temp sender, fuel heater, FSS and a couple others I can't recall.

It is the doubled harness, with the black and gray plugs, near the rear of the head.

Mark.

 

Thanks!

 

There is 1 other possibility that myself and some others have found succseful - if you have a G360 manual trans. The speedo drive gear is sandwiched between 2 spacers and can move thus creating the problem you describe. The solution is to drop the driveline at the back of the trans and tighten the jamb nut to 390 ftlb of torque. This was originaly found in 4X4s but worked in my case.

Bob

 

I've got a 4X4, so I'm thinking this is a different scenario, but thanks!

 

The problem was first found in 4X4s!

 

Er, no, that is totally incorrect information.

It's the 2WD Getrag that uses the speedo gear and spacers, not the 4WD.
The 4wd Getrag is backed by an NP205 transfer case and the speedometer assembly is located on the NP205's extension housing, NOT on the tranmission!

Mark.