tuffmeat

Forseveral miles/minutes my 1999 K1500 will drive normally from a cold start. After some miles the speedometer will bounce about from 0mph to whatever the driving speed is thendrops and stays at 0mph.
At exactly the same time loss of drive in 3rd & 4th. Drives only in 1st or 2nd but no shifts from 1-2, 2-3 or 3-4. It will recoverafter few minutes sometimes if I stop for a while with or without the engineidling. NO "service engine soon" light and NO diagnostic codes appear. Changed ECM and still the same behavior.

Please help. I ama stumped do-it-yourselfer.

Fmlywgn

Sounds a lot like a problem my dad recently had with his Ford F250. The speed control module (I think that is what it was called) had to be re-placed. I'm not certain if GM's are similar, but the conditions sound the same.

McGuireV10

The 4L60 has two sensors -- the Vehicle Speed Sensor (VSS) and a tailshaft speed sensor. The VSS controls shift points, the tailshaft speed sensor is what drives your speedometer.

I've never replaced a VSS but as I understand it, it basically screws into the transmission, and the wires connect to the back of it. Should be pretty easy to replace. You can see a VSS <here> and how the body of the sensor is threaded. This will be up near the bellhousing since it's concerned with RPMs, not output speed.

The tailshaft sensor is very easy to replace -- you unplug the wires, take out the retainer screw, and slide it out of the hole. Replace the O-ring, which you can find at virtually any auto parts store in their O-ring bin. And you may have to top off the tranny fluid. The sensor is located in the circular hole below the red thing at the back of the tranny in <this> photo.

No idea what it'll cost you, and of course I can't guarantee this is it -- could be the gear inside the tailshaft that spins the speed sensor, for example -- but those are the places to start looking, I bet.

tuffmeat

Thanks foryour suggestions.

I found three VSSs under there. Looks like two in the transfer case and one in the transmission. They all looked exactly alike. I boughtone VSS new and didseveral test drives swapping it with each original VSSwith the same erratic behavior. As a shade tree mechanic- that was no fun. Iguess I'll look at the tailshaft gear next.
I can always wait a few minutes and it'll behave just fine for another few miles. I guess I need to start looking for bad connections and grounds. (groan)

I'll take a look this weekend. Let y'all know what happened.

Snowman1975

I had the exact same problem on my 99 Suburban when it had about 92K miles on it. Turned out to be the ignition switch went bad. After paying 300 tohave a dealer replaceit I've had no problems for the last 15K miles. Hope this helps.

typhoon

I don't know if you ever resolved this issue but I will tell you exactly what is wrong. Your K1500 uses a combination speed/ABS sensor for the rear. When the sensor acts up the abs computer thinks the rear wheels are locked up and at the same time the speedometer will not read. At the same time the tranny will try to downshift to 1st gear. Replace the rear ABS/speed sensor near the output of the transfer case and it will solve all your problems. Available at most parts stores. Ask for the REAR ABS sensor for your K1500.

vf171

I believe these sensors were the subject of a massive recall IIRC. I had some (front maybe) replaced on my 99 Silverado and I think the wife's Suburban was inspected and fronts replaced. Maybe the rear isn't covered under the recall. Good luck.

Richpblaze

Had same problem have replaced all 4 sensors 1 on tranny and three on t-case still having issues????? its tripping a 502 code VSS nad the speedo quits working and has erratic shifts????

gator101

I am having the same issue, can anyone help... Thanks

gator101

Hey guys just wanted to let you all know that I found my problem, after I bought a new vehicle speed sensor and it wasn't the problem. I went back to the drawing board and rechecked the plugs and wiring to the vss. The main harness plug was in a bind and wasn't getting a connection, think it happened when I dropped the transfer case a few months back. What I noticed all the time my SEL was never on, but my brother suggested I hook it up to my ob2 program and thats when I found the po502 code,
P0502 - Vehicle Speed Sensor 'A' Low Input