my spedo on my 90 acts like a tacometer. it seams to increase with engine rpm. it will increase speed by just reving the motor in nuetural. I have unplugged the harness from the vss and cleaned it with electronics cleaner but not replaced the vss itself. also if i hold the brake hard on a hill to keep it from rolling with the clutch in, the abs and p brake light will come on and not go off until i shut the pickup off. help please

 

open your glove box then remove it up under the dash there is a box unplug the wires going into it and that will solve the abs and p brake light issue and your brakes will work better too

onto the spedo replace the VSS

 

ok thanks for the help. i cant wait to get it fixed.

 

On your speedo, INSPECT the wiring from the VSS, clear to the plug beside the heater box.
These trucks tend to chafe the wiring, which sometimes bares it to ground and the normal vibrations of the truck will make the speedo do some real funky things.

Your VSS could also be internally shorted and causing the readings, as well, but my money's on chafed wiring.

On the ABS popping on with the brakes applied, check your overall vacuum pressure, if it's low, the vacuum pump may need rebuilt.
The ABS lights popping on like this is actually symptomatic of the problem, LOW VACUUM.

You could also have a brake booster leak, do a vacuum leakdown test on it.

Mark.

 

ok i will check the wiring. I replaced the vss and the speedo seemed to work fine for a couple miles and then it began to bounce slightly, and now its back to bouncing all over the place. As far as checking the vaccum is there a simple way of doing this I dont believe i have ever done that check. Thanks

 

Sounds like the chafe point, or the bad spot in the wiring might be close to the VSS.
There are wire routing tabs close to that end of the harness and many times they chafe the wires, as they are thinly coated tin and can still wear the insulation.
Double check that your plug end isn't also partially broken on the wires.

As far as the vacuum:
#1 and foremost, check that the ABS vacuum valve on the fender, located below the driver's hood hinge will hold vacuum with no leakdown.
A simple hand pumped vacuum pump with a guage will check this (and most other vacuum associated items).

The vacuum at the pump is checked with a simple vacuum guage.
I'm not 100% sure on the minimum vacuum spec. it should need to be.

To check the brake booster for leakdown, hook the booster to a vacuum source, with any other vacuum feeds plugged, save the one you need for the vacuum guage.
Run the engine and note the guage reading.
Shut off the engine and note if there is a reading and if the reading stays stable.
No reading, or a falling reading, indicates a leak.

The Brake booster can also have a major diaphragm leak when applied, but not leak (seriously) when at rest.
To tell if it's an "applied" leak, run the engine, have someone step on the brake for a bit and listen under the dash for an audible leak.
It may be hard to tell with the diesel engine running.

Mark.

 

I thank you for all the help. hopefully I can get it all fixed over the next few days. thanks again,

Clay

 

Mark, thank you, you cured my speedo problem, as well. Bad wiring connection made it jump. I appreciate the help

 

You are quite welcome.

Mark.