2001 Jimmy 4.3 4WD

So, forever, this thing has flashed ABS and brake lights at me, randomly.

Three weeks ago, they started flashing(no surprise), BUT at the same time, the speedometer started jumping wildly. Then it started shifting from overdrive to 2nd, and even 1st gear, for no reason. I limped it home slowly. Got up the next day, and it wouldn't go beyond 2nd gear(when it would get to attempt 3rd, it was sort of in neutral, until it would finally come back and grab 2nd) and took it to a garage down the road. It was throwing a torque convertor code(I don't know the #). Well, I take it to a tranny shop, and he said it was a sensor coming off the transfer case(?). That the speedometer going crazy was telling the tranny to shift like that. He said he cleaned the sensors, and they seemed okay, and it was shifting normally...

And it WAS, until last night. ABS and brake lights start flashing, then the speedometer starts jumping around. It was still shifting okay, until a couple miles from home, then it starts the crazy downshifting again.

What do you guys think? What do the ABS and brake lights have to do with the speedometer going crazy, and the crazy downshifting? Is the main brain going out? I mean, I'm a shade tree mechanic at best, but that would be my first guess. You guys?
THANK YOU, David

 

Could be the VSS, (Vehicle Speed Sensor) failing. It is in the tail housing of the transfer case, right where the rear driveshaft connects. The data from the sensor is used for transmission shift points, speedometer, and the ABS for the rear wheels. BWD Vehicle Speed Sensor S8310: Get the best Transmission Speed Sensor at Advance Auto Parts 5 minute job to replace it.

 

Thank you. Since I paid the tranny place to fix it, I think they should put it in for free. I'll call him tomorrow, and ask him if I pick up the part, and bring it over, if he'll install it.


THANK YOU. I really appreciate your input!

 

"Could be the VSS" It's possible there's a problem in the VSS circuit from the sensor to the PCM. My point is, it needs to be diagnosed before throwing money at it. $80 in my book, is a lot of money for a guess

EDIT: If equipped with 4WD, the TCCM, (Transfer Case Control Module) also uses the data to prevent a range shift if the vehicle speed is above ~3mph.

 

Should the tranny place be able to test the VSS and the TCCM?

So it's most likely one or the other?

 

If they can read and understand their scan tool, and possibly a DVOM, yes.

The TCCM wouldn't be the problem, all it does is request the VSS data from the PCM.

EDIT: If the PCM isn't receiving the data from the VSS, it can't supply it to the TCCM