dm440c
NAXJA Forum User
- Location
- Youngsville NC
ok guys I am brand spanking new to the world of Jeep ownership so I apologize ahead of time for my ignorance of common issues, common acronyms, system names, and anything else that may hinder communication here. I'm short on knowledge but willing to learn.
Disclaimer over- here's my problem:
A couple of weeks ago I bought a 1990 Cherokee Laredo, part time 4x4, auto, 160k miles. Seems to be in decent shape overall. I drove it back and forth to work all last week and had no real issues so I decided to take it to the beach this past weekend. Everything went great, we romped around in the beach sand with it, was able to park right next to the water and set up camp, so far it was doing all of the things that I bought it to do. The trouble started on the way home.
First thing I noticed - and I had kinda noticed this during the week- while holding in-town speeds (for instance ~40 MPH) which requires very little gas pedal input, the torque converter would cycle through lock and unlock after very slight changes to the gas pedal input. I hope I'm explaining that clearly... picture holding a steady speed on a city street, and lifting your foot just slightly as you are gradually getting closer to the car travelling in front of you. When I would do that the TC would unlock, even though the actual commanded change was very slight. Add just a bit of pedal more to accelerate and the TC would lock up again. Once you got up to highway speeds, it would stay locked up and seem to behave normal.
Are you with me so far?
Now again, I have no experience with Jeep systems but I do have some experience with other brand automotive systems so this is where my thought process went: Once I noticed the odd behavior, the first thing I thought of was the throttle position signal not being read correctly by the transmission computer, in other words, if the computer thought you had lifted your foot completely off the gas and was coasting, it would unlock the TC to reduce engine RPM during coasting.
Other than that, everything seemed OK. But wait, it gets better.
So now we're on the interstate on our way home and buzzing along at 75 MPH. I noticed a couple of hiccups spaced several miles apart, barely perceptible. Then suddenly there was a hiccup and the engine RPM went up by about 200 RPM. We didn't lose any speed and kept chugging along but the engine was now running faster to keep the same speed. I decided the TC must have unlocked for some reason, but we're on the open road with wife and kids so I kept going, now with one eye on the gages. After a few more miles I noticed that the temp gage was slowly climbing, which made sense compared to my diagnosis because if the TC is not locked up it will generate more heat in the transmission, which would be transferred to the coolant. I dropped speed to 65 MPH, but it continued to climb very slowly.
When it got to the 210° mark I got off the highway. At the bottom of the exit ramp I stopped, turned, and listened carefully to the transmission shift: 1st, 2nd, 3rd, 4th, aaaaand lockup. So it decided to work again after coming to a stop.
I pulled into a gas station and as soon as I came to a stop the steam starts pouring out. Pop the hood and it's boiling the coolant out from under the cap on the resevoir tank. I punted at that point and had it towed home.
SO- let's be clear on the sequence. Until this episode the temp gage has always been very cool, well below the 210° mark even on these hot days with the AC running. Very clearly there was some sort of transmission issue first, leading to the boil over.
Any ideas on where to start? What to test? common issues with these vehicles?
Disclaimer over- here's my problem:
A couple of weeks ago I bought a 1990 Cherokee Laredo, part time 4x4, auto, 160k miles. Seems to be in decent shape overall. I drove it back and forth to work all last week and had no real issues so I decided to take it to the beach this past weekend. Everything went great, we romped around in the beach sand with it, was able to park right next to the water and set up camp, so far it was doing all of the things that I bought it to do. The trouble started on the way home.
First thing I noticed - and I had kinda noticed this during the week- while holding in-town speeds (for instance ~40 MPH) which requires very little gas pedal input, the torque converter would cycle through lock and unlock after very slight changes to the gas pedal input. I hope I'm explaining that clearly... picture holding a steady speed on a city street, and lifting your foot just slightly as you are gradually getting closer to the car travelling in front of you. When I would do that the TC would unlock, even though the actual commanded change was very slight. Add just a bit of pedal more to accelerate and the TC would lock up again. Once you got up to highway speeds, it would stay locked up and seem to behave normal.
Are you with me so far?
Now again, I have no experience with Jeep systems but I do have some experience with other brand automotive systems so this is where my thought process went: Once I noticed the odd behavior, the first thing I thought of was the throttle position signal not being read correctly by the transmission computer, in other words, if the computer thought you had lifted your foot completely off the gas and was coasting, it would unlock the TC to reduce engine RPM during coasting.
Other than that, everything seemed OK. But wait, it gets better.
So now we're on the interstate on our way home and buzzing along at 75 MPH. I noticed a couple of hiccups spaced several miles apart, barely perceptible. Then suddenly there was a hiccup and the engine RPM went up by about 200 RPM. We didn't lose any speed and kept chugging along but the engine was now running faster to keep the same speed. I decided the TC must have unlocked for some reason, but we're on the open road with wife and kids so I kept going, now with one eye on the gages. After a few more miles I noticed that the temp gage was slowly climbing, which made sense compared to my diagnosis because if the TC is not locked up it will generate more heat in the transmission, which would be transferred to the coolant. I dropped speed to 65 MPH, but it continued to climb very slowly.
When it got to the 210° mark I got off the highway. At the bottom of the exit ramp I stopped, turned, and listened carefully to the transmission shift: 1st, 2nd, 3rd, 4th, aaaaand lockup. So it decided to work again after coming to a stop.
I pulled into a gas station and as soon as I came to a stop the steam starts pouring out. Pop the hood and it's boiling the coolant out from under the cap on the resevoir tank. I punted at that point and had it towed home.
SO- let's be clear on the sequence. Until this episode the temp gage has always been very cool, well below the 210° mark even on these hot days with the AC running. Very clearly there was some sort of transmission issue first, leading to the boil over.
Any ideas on where to start? What to test? common issues with these vehicles?