GM-GMC: transmission, throttle position sensor, labscope
Question Recently I have been having problems with the transmission on my 1995 chev G 20 van.
The engine maintainence light came on while I was driving and immediately after that the transmission only had 2 nd and 3 rd gears, no 1 st or overdrive.
When I pull away from a dead stop I have to shift it manually or it pulls away in drive. The transmission shop cannot get a reading from the computer to find out my problem.
If anybody has had this problem I would appreciate some imformation on it.
thank you
Barry
AnswerBarry,
It would be a good idea to start with you TPS (Throttle position Sensor) on your Van. What has happened is that the computer has seen a fault in a sensor related to the transmition and has place the transmition in "Default Mode" basicly this mode is the get you to a repair facility but limit any damage to the transmition. The computer used you TPS signal to vary your transmitions shift points and a loss of this signal will definetly give you the simptoms you decribed. The TPS is pretty simple 3 wires and can be tested with a labscope, graphing multi-meter, or a high quality Voltmeter with a quick refresh rate. One wire is reference Voltage from the PCM and one is a constant ground. The third(usually the middle wire) will be the signal to the PCM with the engine off and the key in the run position, your red lead to the wire and the black to a know good ground, open the throttle slowly and watch the meter for a slow gradual rise in voltage to near 5V at full throttle. There should be drop-outs or "dead spots" in the signal. Finally you could try removing your (-) batt terminal for 10 min or so and then reconecting this should remove the code from the PCM memory and then your Trans. should come out of Default mode till the sensor or problem occurs again. Hope this helps.....If problem persists it may be a good idea to find a shop that can access the codes stored in your Vans computer, otherwise diagnostics will be slow and tedious!
