Good afternoon. New here so hopefully posting this right. I have a 1988 XJ with automatic 4 speed. It inadvertently downshifts when ever it feels like it winding out the motor. It has 180K on the Transmission. It shifts and drives fine until it does that weird crap. Does anyone know what could cause it? Is it probably replacement time???
I would guess a bad TPS sensor, especially if it's the original. They do wear out eventually. It can be tested with a voltmeter if you have one.
It's a waste of time and money to throw parts at this.
It could be as simple as a bad ground for the TPS circuit and TCU.
It could also be a C101 connector with too much resistance.
It could also be corroded connectors between the TCU and trans solenoids.
I provided a link in post #3 to address these known issues. Takes less than an hour and needs to be done anyway.
Yup, good advice on going through the grounding points (and Cruiser has some great how-tos on his site). Get a cheap voltmeter and test the TPS sensor before throwing money at it.
An 88 had the dual-output TPS sensor, so I think it's just the +5-volts coming through the C101 connector and the trans side of the sensor signal goes through the firewall on the passenger side near the TCU.
Good afternoon. New here so hopefully posting this right. I have a 1988 XJ with automatic 4 speed. It inadvertently downshifts when ever it feels like it winding out the motor. It has 180K on the Transmission. It shifts and drives fine until it does that weird crap. Does anyone know what could cause it? Is it probably replacement time???
Everything you ever wanted to know about the AW4 transmission:
https://www.naxja.org/forum/showthread.php?t=1053970
Not sure it was clear earlier, but the shifting is electronically controlled by the transmission computer (aka the TCU) based on inputs from the throttle position sensor (TPS), output speed of the transmission (rarely goes bad, usual symptom would be faling to upshift out of 1st), and to some extent the sensor that tells it where the shifter is (the NSS, also does the backup lights). Poor connections, blown trans computer fuse, failing sensors, bad grounds can all confuse the computer and make it shift inappropriately.
The TCU controls the shifting by means of two solenoids, and the torque converter lockup with a third solenoid. Bad solenoids usually manifest as shifting into or starting in the wrong gear and can be intermittent based on temperature. There is also a throttle position cable that often needs adjusted (often incorrectly called a kickdown cable) which controls the pressure in the transmission and hence the shift firmness.
So boiling all my rambing down, the good news for you is that downshifting when it shouldn't is usually not a mechanical problem. The electronic problems are usually easily diagnosed and remedied. Personally, I'm betting a bad TPS sensor if it's the original with 180k miles- you pressure wash the engine recently?
