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flickertheory

Reverse issue SOS

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My '87 auto (three speed) SE has just lost its ability to reverse. The gear engages, but as I let my foot off the brake it just lurches at the point the truck would (normally) start moving back and stalls. If I keep 50% pressure on the brake, I can get it to creep back slowly, riding that threshold before it stalls. All three forward gears work fine.

 

Interestingly, switching the transfer case to 4L and the auto gearbox to R, it works somewhat. There’s a rough judder as I let my foot off the brake at the point that it stalls in 2H, but then I’m able to use the accelerator freely to reverse in 4L. Switching the transfer case to N and the gearbox to R, the engine revs without issue.

 

Has anyone come across an issue like this before? I’m in the wilds of northern BC 1500km from home, and my main concern at this point is making it back to Vancouver without triggering anything further. There aren't any mechanics up here who can take a look at it, but chatting with an auto wizard friend back home it's unlike most of the transmission issues he’s come across. We're totally baffled.

Edited by flickertheory

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Obvious, but have you checked the fluid level with the engine running in park?

Reverse is normally the first indication of failure on an automatic transmission, being slow or failing to select reverse is usually a warning sign. 

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When the 4 speed in my 93 died, the first gear to go was reverse. In my case, no warning and it was complete. It worked one morning on the way to work, then at lunch, reverse was simply not there anymore. In my case, it acted like it was in neutral. Over the next few weeks, 1st, 2nd, and 4th disappeared leaving me with just 3rd. The transmission shop was impressed that my truck made it in under its own power. 

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Thanks guys, appreciate the ideas! The AT fluid is new (~4000km) but was on the low side, although a top-up hasn't changed anything. Speaking with my friend, we think it might in fact not be a transmission issue at all. Will update this once I've found out more...

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Chiming in a little late with the (most unusual) solution. I coaxed my truck back to Vancouver and to my local mechanic, who spent a couple of days diagnosing the eventual issue...which turned out to be electrical! Typical...

 

The transmission control module under the rear passenger-side quarter panel trim had been scorched in a past mishap (the less said about that the better), and the wiring wasn't conducting properly. So in short, whenever I shifted into reverse and let off the brake, a short somehow caused the brake light to kill the engine as it went off. And pulling out the related brake light fuse from the fuse box killed the engine entirely. Running a relay to keep the brake light on permanently somehow solved the problem. Obviously not a tenable long-term solution, so instead they just ran new wires and I had my reverse back! Mysterious, but I'll take it.

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Sounds like something was telling the computer to lock the torque converter when it was in reverse. It's supposed to unlock when you press the brakes, which would explain why it was fine when the brake lights were on. Good that it wasn't the computer, '87 has a different trans vs other years and that might've been a tricky unit to find a replacement for. Good to hear you got it in the end, though, and thanks for closing the loop!

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On 12/29/2020 at 11:59 AM, Slartibartfast said:

Sounds like something was telling the computer to lock the torque converter when it was in reverse. It's supposed to unlock when you press the brakes, which would explain why it was fine when the brake lights were on. Good that it wasn't the computer, '87 has a different trans vs other years and that might've been a tricky unit to find a replacement for. Good to hear you got it in the end, though, and thanks for closing the loop!

 

That's it exactly, it was the lock-up solenoid that wasn't operating properly. New wiring solved the issue!

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My hat is off to your mechanic!  I pride myself in being a decent mechanic, that one would have taken me a while...OK, a long while to find!  LOL  I wonder how many people would have replaced the tranny before finding this issue?

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