1991 caravan, 3.3 with 204,000KM. Both the engine, transmission and both ECU's were replaced 50,000KM ago. The van will only stay in first gear unless I get the RPM upto around 5000RPM, then it will shift up, there are no dash indicators showing while all this goes on. As I slow down, say for a corner it will slam back into 1st gear when the road speed drops below 20KM/hr. I had this problem last winter when the van was cold, removed the main harness at the TCM and found water, so I cleaned it out and packed it with silicone grease and life was good again. This time I removed the connector and all looked OK, sprayed it with a water dispersant just incase. Still not working properly. The van worked flawlessly, then it was parked for 3 or 4 weeks, upon driving it again this problem was there. Any ideas???

 

Have the trans controller scanned for codes.
Does the speedometer read properly?

 

Well here goe. Threw my old OTC scan tool on, shows a code 46 and a code 37. However I cleared those codes and went around the block .. in first gear. No codes were stored from that road test, When I view the sensor readings I see engine RPM and Turbine RPM as well as Output RPM while moving. However the reading for TPS just stays at 3.8 no matter what the throttle position is. When I tried to put it into quick learn mode the message to "reduce throttle opening" appears. Any thoughts?? I would assume if the TPS was faulty the engine CEL would be on all the time - it isn't.

 

The older OBD I vans aren't as good as the newer ones at detecting codes.
I would try a new TPS. It sounds like yours is stuck or shorted.
It should start at a low voltage and smoothly increase as you press the gas pedal down

 

Turned out to be an easy fix. I scanned the engine for codes and a historic code for TPS came up, again I cleared the code and went for a drive, no code was logged. The scan of the engine showed the TPS to be 0.7 volts when I started the engine, as soon as I revved the engine the reading went to 3.8V and stayed there. I shut it down and found the van would not restart. I wasn't able to bring the voltage down in any way. So I unplugged the TPS sensor, this brought on the CEL and put the engine in "Limp Mode", this allowed the engine to start, it also caused the transmission to shift normally. Picked up and installed a new TPS and everything went back to normal. By the way the dealer part was $113, a Blue Streak aftermarket unit was $46. The reason the failed TPS didn't bring on the CEL is because even though it failed and had a constsnt reading of 3.8V, that reading falls within it's normal operating range of say 0.5V to 5.0V. If it had failed and was reading 0.1 V it would see that as out of range and bring on the light. Thanks for the advice.

 

Good job diagnosing it!
You are absolutely right about why you didn't get a code.
The newer OBD systems are "smarter" and will compare TPS to MAP sensor reading to check operation better.

 

where in blazes do you connect into to scan codes on these vans? thanks alot
Wayne

 

what year??

 

Those were codes before the P codes. 1989 up to 1994.