I washed the engine in my 88 cherokee yesterday and soon after a few miles down the road it quit shifting. I've got 1st 2nd and reverse. Had a similar prob with my sons dodge truck and it was corroded grounds. I cleaned the grounds and went through unpluging all the connections blowing them out and plugging em back, still wont shift. Anybody know anything to try?
Trans wont shift
- Thread starter flatie46
- Start date
MudderChuck
New member
May be a throttle position sensor. The square plug from the TPS is the one for the transmission TCU. The grey wire from the TPS is the reference voltage, it is often around 4.6-4.8 V (supposed to be 5 volts but it never is), the blue wire is the out voltage and typically right around 4 V (3.8-4.0 V) and the voltage falls at the blue wire as the throttle is opened. The black is ground, the grounds are prone to corrode in numerous locations and eventually end up at the dipstick holder. There is a slight built in resistance in the TCU ground circuit in every Renix ( 87-90) XJ I've ever tested.
There is a formula for adjusting the TPS, but my numbers will give you a good quick check (good enough).
Testing is done with the connector connected and ignition in the run position. I usually slit (sharp knife, exacto etc.) the wire insulation, a little, lengthwise for my probes. I've never had it cause me any grief to leave a little wire exposed, you can reseal it with a dab of silicon if it bothers you. If you run the TPS wires over the rubber TB intake, life will be much easier for you.
If you can manually shift (2-3) but it will not shift itself in drive, it is most often the inline fuse to the TCU in the harness under/behind the glove box. Hard to see, a single, spade type, fuse holder wired to the outside of the harness, you don't have to remove the glove box. You may have to remove the bottom knee panel from the dash, the TCU is on the back of the dash knee panel passengers side.
The other half of the TPS is for engine controls and usually the one that screws up after an engine wash. 3000 RPM idles aren't unusual after the engine half of the TPS gets full of water.
Thanks, I'll give it a look in the morn.May be a throttle position sensor. The square plug from the TPS is the one for the transmission TCU. The grey wire from the TPS is the reference voltage, it is often around 4.6-4.8 V (supposed to be 5 volts but it never is), the blue wire is the out voltage and typically right around 4 V (3.8-4.0 V) and the voltage falls at the blue wire as the throttle is opened. The black is ground, the grounds are prone to corrode in numerous locations and eventually end up at the dipstick holder. There is a slight built in resistance in the TCU ground circuit in every Renix ( 87-90) XJ I've ever tested.
There is a formula for adjusting the TPS, but my numbers will give you a good quick check (good enough).
Testing is done with the connector connected and ignition in the run position. I usually slit (sharp knife, exacto etc.) the wire insulation, a little, lengthwise for my probes. I've never had it cause me any grief to leave a little wire exposed, you can reseal it with a dab of silicon if it bothers you. If you run the TPS wires over the rubber TB intake, life will be much easier for you.
If you can manually shift (2-3) but it will not shift itself in drive, it is most often the inline fuse to the TCU in the harness under/behind the glove box. Hard to see, a single, spade type, fuse holder wired to the outside of the harness, you don't have to remove the glove box. You may have to remove the bottom knee panel from the dash, the TCU is on the back of the dash knee panel passengers side.
The other half of the TPS is for engine controls and usually the one that screws up after an engine wash. 3000 RPM idles aren't unusual after the engine half of the TPS gets full of water.
Bounty__Hunter
Super Moderator
You can unplug the TCU (transmission control unit) beneath the glove box and shift the transmission manually. This will rule out a mechanical problem with the transmission. Then it's likely a sensor or the TCU itself.
Ok , I removed the knee panel under the glove box and didn't see a fuse. I saw a holder where one might go, I put one in it didn't make a difference. Anyway, I unplugged the little box attached to the back of the knee panel and it would shift manually. Is that the TCM? Is it bad? What now? [ note to self, washing is bad!]
Bounty__Hunter
Super Moderator
Sensors don't like water, there's a good chance it's the TPS on the throttle body. Have you checked the TPS output with a voltmeter?
No not yet, it's raining pretty good now and my basement is too full of useless stuff to get it in the dry. This is the first vehicle I've ever owned with fuel injection. I've worked on alot of older stuff, this is all new to me. I trust what your telling me but what does the TPS have to do with the trans shifting?
MudderChuck
New member
The TCU gets inputs from the TPS and the speed sensor and along with the shifter position, decides when to shift.
Been awhile since I saw the TCU inline fuse, if I remember correctly it's about a foot down the harness for the TCU towards the firewall. If you have a holder and no fuse, something is screwy, maybe some creative wiring. Should be a yellow wire going to the fuse holder and it is hot with the ignition on. There is another power wire going into the TCU, it is red battery power, always hot.
I've never seen a TCU go bad, I've seen many TPS go bad from water or even WD-40.
Been awhile since I saw the TCU inline fuse, if I remember correctly it's about a foot down the harness for the TCU towards the firewall. If you have a holder and no fuse, something is screwy, maybe some creative wiring. Should be a yellow wire going to the fuse holder and it is hot with the ignition on. There is another power wire going into the TCU, it is red battery power, always hot.
I've never seen a TCU go bad, I've seen many TPS go bad from water or even WD-40.
Thanks for the help guys, it was the TPS. I fixed it yesterday and have been driving it around quite abit. Drives great and shifts great, it used to have a kind of a surge when I got it out on the highway. That's gone now, I also cleaned that idle air control. Everythings good, I learned a bit in the process too. Once again thanks, maybe I can help one of you guys out one day.
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