I have a 2001 chevy blazer LS. Ok ever since last winter every now and then my speedometer will drop off, pick back up, drop off or flutter when driving. It causes the transmission to down shift (real annoying). This Happens once every couple days on my way to work, could be when i first leave or when i'am almost there. Searched on the forums here for hours and read up.

First thing I thought must be a bad VSS and by the way this is the 2 sensor on the tail shaft housing type. AWD safari type. I replace both, tried it for a couple days and the malfunction still happened, did not cure the problem.

Secondly I thought it has to be a bad ground, so I went searching for ground point under the veichel, cound not find one underneath but managed to find a loose ground on the firewall by the wiper motor. Tightened that up thinking that was definatly the problem. A couple days later, No dice.

Thirdly, this kept happening, so I pulled over on the highway went underneath and grasped and wiggled the part of the harness that both VSS plugged in to, got back in the blazer and it was fine the rest of the drive, and until it happened again i would pull over, wiggle the harness and it would be OK sometimes. Then noticed if I just pulled over and turned the key and everything off and started up again, it would be fine for a while.
Next step was I ordered in the pig tail plugs for both VSS's and rewired them on ($140 from GM later WATA RIP!). A while later, speedometer malfunction STILL happened again and still down shifts again...

What is left? To replace the transmission harness? Is there a ground I overlooked or missed? WTF is going on?
Thanks guys I really appreciate your help..

 

any ideas??

 

Replace the wiring up to the PCM for the VSS sensor. Only one of those sensors is the VSS. The other is the rear prop speed sensor used by the TCCM.

The VSS wiring is purple w/ white tracer for the VSS HIGH & green w/ black tracer for the VSS LOW back to the PCM, connector 1 (blue) cavities 30 & 29 respectively (HIGH & LOW). These wires go through connector C116 which is located above the transfer case. The wiring on either side of C116 is shielded. The shield grounds through G117 (on firewall on the left side of the brake booster - as seen from the front of the truck) between the VSS & C116 and to G104 (engine harness to rear of left side cylinder head - as seen from the driver seat) between C116 & the PCM.

Sounds like you either have a problem with C116 or the wiring somewhere between the VSS & PCM. I would get some shielded 2-conductor cable and re-run the wiring. Just ground the shield to a suitable chassis ground.

 

Thanks swartlkk. Do you have any parts of that circuit of the tech manual that you could send me? I'am not sure which connector C116 is above the transfer case...

Now if i was going to rewire the VSS circuit, which sensor would i do if you say only one sensor is for the VSS? If the VSS wiring is purple w/ white tracer for the VSS HIGH & green w/ black tracer for the VSS LOW back to the PCM, which colors of wires are for the speedometer? Also what is the shield? Is that hard peice of solid wire that looks like mechanical wire running in the loom for each speed sensor 2 wires, is it the shield?

If you have any diagrams or anything I would really appreciate if you could send them to me on here or my email
Thanks again for your help!!

 

Look at the wires. The VSS is the sensor with the wire colors I mentioned. The shield I am talking about is a jacket around those two wires. It would be inside the harness.

C116 just has these two wires in it and the wire colors should be the same on either side of the connector.

I wouldn't go posting your email address out on the open forum unless you want spam mail.

 

You said that it got better for a short bit just by turning the ignition off as you stopped to wiggle the wires. Could it be in the ignition switch???