I was driving on the highway the other day and all of a sudden my speedometer starts going bonkers, my ABS light comes on along with the actual ABS when I started to slow down and then I noticed my truck downshift to 1st gear. After that my truck won't shift out of first and speedometer doesn't work and my ABS light sometimes comes on.

Is it coincidence that all these things happened at once or not? What controls all these things? Main computer maybe, I tried unplugging all 4 connections and replugging. Worked for 2 days and then everything happened all over again.

Does anybody have any idea. Is my computer shot?

1999 GMC Jimmy SL 4.3L

 

Got any codes?

Seems like it all started with the speedo, Id start with the VSS.

The puter uses info from the VSS to help control the ABS and the tranny.

 

I don't have any codes yet.

I've been looking around and the VSS sounds like it could be the problem. I'll try to get it scanned and hopefully get codes to post soon.
Does the VSS affect the tranny in such a way that it won't even shift. I had no previous problems with anything on it. I recently replaced the fuel pump and haven't changed the tranny oil in quite a while.

Any other suggestions would be great, and if I figure it out I'll let everybody know.

 

YES!
The Vehicle Speed Sensor (VSS) tells the puter how fast your output shaft is turning. It takes that information and compares it to your engine RPMs, and to your wheel speed sensor. The VSS also controls your speedometer. The puter uses the VSS, RPM, TPS, and other sensor information to control engine and tranny performance. So yes, if your VSS is screwed up your puter wont know what to do and it goes into safe/limp mode. Hence your stuck in first gear. There may be other possibilities, however I would start with the VSS.

 

Well you were right. Replacing the Speed sensor tomorrow and hopefully everything works.

Thanks for putting me on the right track.

 

Glad to hear it and your welcfome.

 

Just so everyone knows, the part cost me 170.00

 

I hope you mean installed, the 4wd delphi is only 130 at rock auto and airtex is 31.

 

cool. did that fix your problem?