HI,
I searched for my situation, but couldn't find the anything.

I have a 1989 (summer 88) Grand Caravan 3.0l w/Ulradrive tranny (the first year of the A604). I bought the van new in August, 88. It's on it's 3rd tranny guts (rebuilt 1st in 1996, right at the end of 7/70. Second rebuild was in 2001 (cost me $900).

The van has been very good to me. I use it now for hauling musical instruments to practice.

I recently replaced the main cranshaft seal (very bad oil leak) and timing belt. It smokes (bad valve seals), but runs ok and still gets 23 mpg loaded.

Here's the problem which just appeared about a week ago. After a fairly long drive, either at 65mph or up long hill road back to the town where I live, when I come off the throttle and the van slows to about 45, the engine either goes into LIMP mode, or the torque converter is locking up. I think it's going into LIMP mode. IF I pull over, turn off engine, restart, it runs OK again until I coast off-throttle and go below 45. Then SLAM, back into either lockup / limp mode. I have the service manual. It mentioned something about if the tranny gets too hot, it will "lock the torque converter" to prevent further overheating the fluid. I just can't quite tell if the TC is just locking, or if its really going into LIMP mode. I would have expected a check engine light if it was LIMP mode.

The engine temp appears to be OK. I run a slightly cooler thermostat (last 6 years) which has never been a problem. After the long uphill drive, temp barely gets to halfway.

I just changed the tranny filter and fluid (yeah, I know that that changes only a portion of fluid) thinking it may be clogged / old (about 18 months). No change.

The check engine light is not coming on. Should I go ahead and "pull" the codes anyway? Does the electronic Ultradrive give it's own codes versus the engine when you do the key switching thing to get the codes?

I put the effort to getting the oil leak stopped. I need to get another 9 months out of this vehicle. I hope I can find the problem.

Any ideas? or is it finally time to play taps for my 17 year old Caravan?

Thanks,

Stu

 

UPDATE:

I went to Autozone and bought a output speed sensor ($25). The sensor looked like it had been used (a little dirty, and the plastic had scrape marks). In a rush to see it work, I just up and replaced the sensor. I forgot to pull the original codes, so lost those.

Disaster. The tranny immediatly went into Limp Home mode as soon as I reached 5 mph. I'm on the right track. Either I got a bad sensor, or their's something wrong with the sensor connector. Before, it would take 10 minutes / 10 miles for the limp mode to occur, and then only on decelleration. There is a Technical Service Bulletin on these connectors (21-06-99). The TSB says to cut off the old connectors and splice in a new improved sensor connector that makes better contact with the sensor pins. This problem also causes problems with the Cruise Control cutting out, a problem I've had for 10 years.

I pulled the new sensor and compared response with my DMM. The resistance of the old one was about 500 ohms. The new one, 200k ohms. Looks like an open circuit on the new one. Next, I went to millvolts and checked for "response". I connected the DMM leads to the pins in the sensor. Since the sensor is really just a magnet sensor, it responds with a millivolt pulse as steel is brought near the sensor head and removed from it. The pulse only lasts as long as metal passes by the sensor head. The sensor reads gear teeth passing by inside the transmission and gives a series of pulses. More pulses per second means faster shaft speed. The computer senses the input and out shaft speeds and decides how the torque converter is working. If it senses a problem, either real or false, it can force the tranny into Limp Home mode. I was able to get about 100 millivolts + and - pulses as I brought a piece of steel to the sensor and pulled it away. Conclusion: The old sensor was at least giving a proper response. I got NO response from the new sensor.

I cleaned up the old sensor and put it back in. Back to Autozone for a refund (luckily, it has a 3 month guarantee).

Next, I inspected ALL the speed related sensor connectors to the tranny and engine. I also inspected all the SMEC 60 pin computer connectors (near battery, at the tranny, and on the firewall). Remember, the A604 Ultradrive has it's own computer. I cleaned all the male and female connectors with carb cleaner and then some CRC contact cleaner/lubricant. I blew out the residue with compressed air. I made sure to check the solvent with the plastic so that I didn't end up with a sticky goo for a connector (been there, done that). The light lubricant helps keep moisture and corrosion out in the future. I pulled every connector I could find. Some were OK, some had visible signs of water infilatration and were lightly corroded. Some were nasty with caked up grease / oil / dirt (I had a 2 year oil leak that pretty much caked the engine compartment). I was pretty obsessive with the power washer at times over the 17 year history of this vehicle (bought new in August 88).

Well, I put it all together. Since the battery was pulled to get access to the 2 speed sensors, the computer memory was cleared. I fired the van up and took 30 minutes to do a proper "shift pattern learning". 15 dead stop to 60 mph with constant throttle position shifts all the way into overdrive and torque converter lockup. I did the wide open kick downs, etc. Things smoothed out. I made 2 trips up a long hill (4 miles) to really raise the temp at 60 mph. Before, this would have caused limp mode at the top of the hill. Things look good.

What did I really do? I'll never know. Was it cleaning / improving the wiring connectors? Did I get lucky last night and still have a bad sensor? I really think it's either a failing sensor or a bad connection. Hopefully, I fixed the bad connections and I'm ok now. Thursday, I drive it 30 miles to Santa Fe.

We'll see.

Stu
89 Grand Caravan 3.0l V-6 / A604 Ultradrive