97 Jimmy intermittant ABS combined with shift problems

[Rockyrider]

Hello All. I've done a lot of reading but couldn't find any similar problem/solution, so I thought I'd try my own thread....here goes:
I have a 97 Jimmy, about 225000 km (about 140k miles). I had a problem with the ignition (no dash lights, crank but not start, couldn't remove key) and had that replaced. That solved the start problem so I know it's not the ignition module.
Prior to and since then, I would be driving the truck and if I slowed or came to a stop, the ABS light would come on, the speedometer wouldn't work and the transmission wouldn't shift out of 1st. The tach continues to work, so does everything else.
Sometimes if I pulled over and turned the vehicle off, it would start up and be fine, sometimes I'd cycle it a number of times and the problem wouldn't go away, so I'd continue to drive home (although slowly) and again the problem would just go away next time out, and be ok for a week or so.
I replaced the computer with one from the scrap yard, that didn't change anything.
The last time I had to drive a fair bit to get home after it had this problem and the engine light came on, so it's been sitting out front ever since.
I suspected an electical issue as it's the ABS/Speedometer/Shifter combined, but the only common item I could think of was the computer. Some indications here also pointed to the ignition switch, but that's also not it.
I'm out of ideas, has anyone seen this or anything similar?
Odds are it will clear up on the way to Autozone, will there still be a code in the computer?

Thanks everyone, your forum is an excellent resource!
Rockyrider

[blazee]

The first component that comes to mind is the VSS (vehicle speed sensor).

You need to pull the DTC, to help with diagnosis. Even if the light goes out on the way, the code will still be stored.

[Rockyrider]

Thanks Blazee, I'll look into getting the code read. That would certainly make sense, I take it that the VSS is directly related to the ABS, Speedometer, and transmission shifting?
Is the VSS something I can access/inspect/clean?

Cheers,
Rockyrider

[blazee]

Yes the VSS is tied to all those systems.

1) The ABS uses the VSS to determine the speed of the rear tires, and wheel speed sensors to determine the speed of the front tires. It compares these speeds to determine if any of the wheels are locked up. When it sees a problem, such as one sensor not reporting, it sets a code and turns on the light.

2) The VSS signal is used to determine the vehicle speed and displayed through the Speedometer.

3) The transmission won't shift properly if it doesn't know how fast the vehicle is moving.


The VSS is located at the tailend of the transmission on 2wd vehicles and on the tail of the Transfer case on 4wd vehicles. You can do basic checks of the VSS, connectors and wiring, but proper diagnosis would be difficult to do yourself without a scanner considering that you'd have to have a voltage meter connected to the VSS and ran inside the vehicle so that you could see the VSS output went the problem occured, again.

Keep in mind that the VSS is just a guess at this point. Your best bet is to get the code scanned (ABS codes require a special scanner, but because your check engine light came on, you should be able to pull a powertrain DTC with any OBD II scanner) that way you have a solid starting point. If possible use a scanner that will show you the freeze frame data stored when the code was set so that you can see what the sensors were reporting.