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Transmission disengages after braking


craigerc1985
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Hello,

My son recently purchased a 1991 SE, automatic transmission, with 130k miles. We think it is a Canadian model. A perplexing problem arose yesterday. Every time he hits the brakes while the rig is in gear the transmission goes dead. No forward or reverse gears. If he turns the engine off and restarts the rig, the transmission functions as normal until he hits the brakes after putting it in gear. We have no idea what kind of maintenance has been performed over the life of the tranny. Could this be clogged filter(s) etc, or some type of sensor that is or has failed? Thank you for any suggestions.

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It happens when he's driving around. He can press the brakes to put the it in gear after restarting and drive off as normal, but the next time he hits the brakes the tranny acts like it's in neutral (no forward or reverse gears) until he turns the rig completely off and restarts it.

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My best guess is that the circuit for the brake lights is shorted to the transmission harness, and an unexpected +12v connection on some wire that's not supposed to see +12v is making the trans control unit shut down. Ignition off/on resets it. My first test would be to unplug the brake light switch and see if it still does that. Put the trans in neutral first, I think the shift lock works off the brake lights, so it wouldn't let you out of park otherwise. Then test it again. If I'm right and the brake light wire's shorted to something it shouldn't be, the trans should work like normal.

 

The TCU is located in the passenger's rear quarter panel, should be the big square module, and I think the wiring for it runs forward along the bottom of the door openings on that side under the carpet. I think it branches off and goes to the trans through the floor somewhere under the passenger's front seat, but it was a while ago that I had my carpet out and I wasn't in there for the wiring so I don't remember for sure. The service manual probably shows the routing in the EL section. Anyway, I'm pretty sure the brake light wire for the passenger's side runs through that same bundle along the edge of the floor, so I'd be removing the passenger's side seats, step plates (careful, the screws are plastic and can be an SOB to remove), the trim over the wheel well on the inside and the lower B-pillar trim, pulling the carpet back, and checking the harness for damage between the TCU and wherever the trans harness branches off. It should have plastic covers over it but who knows what didn't get put back the last time someone had it apart, or what someone pinched under a fastener.

 

If the trans still acts up with the brake lights unhooked, then, well, I'm as confused as you are.

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Start with checking the condition and level of the fluid. It seems to be slipping, which can be caused by low fluid.

 

If you already know this then forgive me, but check it with the fluid warm, tranny in park and engine on.

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My best guess is that the circuit for the brake lights is shorted to the transmission harness, and an unexpected +12v connection on some wire that's not supposed to see +12v is making the trans control unit shut down. Ignition off/on resets it. My first test would be to unplug the brake light switch and see if it still does that. Put the trans in neutral first, I think the shift lock works off the brake lights, so it wouldn't let you out of park otherwise. Then test it again. If I'm right and the brake light wire's shorted to something it shouldn't be, the trans should work like normal.

I had thought about this too, but I don't know. If so, then when he pressed the brake to shift out of park, it would do the same thing, shut the TCU down. Also, I would think you would not have brake lights, or they may be really dim. It would probably blow a fuse too. It is worth checking though, would be easy to do and a quick thing to rule out.

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Hadn't considered that. Yes, it's odd that hitting the brakes would only cause issues when the truck is moving, and not cause any other obvious issues. Depending on what the brake light circuit shorted to, though, it might not be adding enough load to the circuit to pop fuses or noticeably dim the bulbs--not a short to ground, certainly, but maybe a short to a sensor. Since it only happens when the truck is moving, I'm starting to wonder it it might've shorted to the wiring for the revolution sensor. I had a look at the diagnostics for that sensor in the manual and it looks like it makes an AC signal, with the voltage rising as speed rises. Perhaps the TCU is using the frequency of that signal to determine speed, so when the truck is stopped, putting +12v to it and then taking it off again just counts as one revolution and it doesn't care. Or maybe it only listens to that sensor when the truck is in drive. When the truck is moving, though, the TCU sees that signal go from oscillating to stuck on, and thinks the trans has suddenly locked up. The trouble with this theory is that I would expect the TCU to put the trans in limp-home mode if it thought something had locked up, and neutral in all positions is not limp-home mode.

 

One other thing to consider: the TCU is supposed to tell you when it thinks something is wrong. When you turn the ignition off, then on again, the light in the E-AT switch should flash if it thinks there's something wrong, and with the help of the AT section of the manual, you can decode the flashes into what the TCU is upset about. I'd be interested to see if your TCU has anything to say for itself, especially if it turns out I'm all wet on this brake light wire idea.

Edited by Slartibartfast
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Put it in D and allow the trans to creep to 1/2mph. Does pressing the brake at very low load cause the issue?

 

Put it in D and take the trans thru 4th @55mph? Does pressing the brake at moderate load cause the issue?

 

Put it in D and take the trans to a speed slow enough not to be dangerous, 35mph?, and @3500rpm or so and WHILE STILL ACCELERATING hit the brake. Does the brake disengage the trans at high load?

 

Check the trans oil health. Fluid couplers are designed to operate with oil that is in a certain spec and very poor oil can cause your torque converter to malfunction.

 

 

Sent from my iPhone using Tapatalk

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Forgot the most important one:

Put it in D and without releasing the brake press the accelerator. Does the trans disengage in this condition? At what RPM does the engine stop increasing at, with accelerator on the floor/at max and the brake fully engaged? Make sure you have your parking brake on and be careful not to let go of the brake with the trans under load, unless of course it is safe.

 

 

Sent from my iPhone using Tapatalk

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  • 3 weeks later...

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