Two things,

First, check fuse #18 in the cab. Then verify that there's 12 volts going to both sides of fuse #18 while the truck is running.

Second, the ground for the speedometer/odometer is a black and white wire and it should be grounded at ground position H (Behind Right Cowl Panel).

You'll probably see about 5 or more grounds there as well.

Let us know,

Bob

 

I cant find the ground for the speedo and I didnt un hook any wires from the cowl changing the cab?

 

You need to find that ground and make sure it is grounded to the firewall.
And you need to make sure the firewall is grounded to the fender.
And you need to make sure the fender is grounded to the battery.
That would complete the "grounding circuit".

You did replace the cab which means "at the very least" you had to disconnect the firewall ground from the fender.

Check again,

Bob

 

The ground for the speedometer/PSOM is G200.

G200, Behind bottom of RH cowl panel:


The Body ground:


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Thanks Bill (subford) for the pics.
I was thinking the "cowl" was at the upper firewall passenger side.
My mistake.
None-the-less, all of those grounds have to be there !!!

Bob

 

Thank you for the picks I double checked but that ground is hooked up? Im lost I know it worked before the cab swap

 

The fender to firewall ground HAS to be there. Subford (Bill) didn't supply that picture.
The battery to fender ground HAS to be there.

Did you check those as well ????

Bob

 

There is a ground from the firewall to intake manifold bolt and yes the battery ground is on

 

Nope, you still missed it.

You HAVE to HAVE the ground from the firewall to the fender.

Try again,

Bob

 

I hate to say this but there are no fender to firewall (safety wall) grounds.
The firewall (safety wall) is bolted to the fender and that is the ground path.

 

And you know Bill, I'm sure you're correct.
Somewhere tucked back in my mind I seem to recall that there was a silver ground strap, not a wire, going from the firewall to the fender.

Maybe the 1970's caused more damage to me than I thought !!!!

I'm gonna' check under my hood tonight.

Bob

 

Well subford, you're correct again.
I just popped the hood and low and behold there is no ground strap going to the fender there.
There is however, a silver ground strap that goes to the intake manifold as rangergirl94 described.

My mistake...........

rangergirl94, did you check both sides of fuse #18 in the cab while the truck is running ???
It should have power while the truck is running to power the PSOM.

Let us know,

Bob

 

Yes that is the strap that burns up if someone runs the battery NEG ground cable to the frame and not the engine block.

 

Anything else control speedo/odo??

 

The way it works is:
The Programmable Speedometer/Odometer Module (PSOM) receives a speed signal input from the Differential Speed Sensor (DSS) and uses a programmed conversion constant to convert the signal to the standard 8000 pulses per mile speed signal output. The speed signal output is proportional to the road speed of the vehicle. The Programmable Speedometer/Odometer Module supplies this signal to all components that require vehicle speed information including the Speed Control Amplifier and the Powertrain Control Module (PCM).

The DSS is a AC generator and the sine wave goes up and down with each tooth of the tone ring that is mounted on the ring gear. This AC voltage is sent to the POSM & the RABS module in parallel. The PSOM is powered by two fuses, #8 & #18. The ground for the PSOM is G200 as pointed out above. The output of the PSOM goes to the PCM computer, the Speed Control Amplifier with the speedometer and the odometer both plugging into the PSOM.

The gap between the DSS and the tone ring will govern how much voltage amplitude (volts) the AC signal will have going to the PSOM/RABS module.

You can check the AC voltage under the hood at the RABS data link connector plug. But you have to have the rear wheels in the air on jack stands with the wheels turning 30 MPH. The voltage should be approximately 3.5 volts AC.



Note that the DSS was later called the VSS:


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