Problem with '91 4L80E

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Gary Gable

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Thanks Tayto, and I appreciate your sentiments, but given everything it's not a terrible idea to replace 30 year old parts with new if they're even remotely suspect. The cost is minor, especially when weighed against what the 3 shops I took it to charged me to come up with nothing. I think it's probably a broken wire in the external harness. It could be the DRAC unit (not likely, given the speedo works) or it could be one of the internal issues you've described. The thing is this: Different than earlier periods of my life, nowadays I have slightly more money than time. Because of that it's far easier for me to shotgun blast approach the problem. IE, replace a bunch of parts and see what happens. If the transmission shops around here were better I'd be happy to pay them to figure it out. But that hasn't worked, so I'm back to buying parts and replacing them, working backwards from most likely to the least. Eventually I'll find the cause.

I've ordered a US shift stand-alone controller along with the wiring harness to fit it up. Before installing it I need to drop the pan again and install the Holley style solenoid it evidently requires. While there I'll air test the items you've mentioned and give the visual to the rest. When I had the pan down previously I carefully looked (and smelled) the remnants in the pan. There were no bits and pieces whatsover, and it didn't smell at all burnt. I'm not a novice around tools or metal fabrication, or machine shops. That's actually my 40+ year working background in a nutshell. But even with that, and even with the knowledge gained here or in the numerous articles I've read and the ATSG manual I've read cover to cover, it's still a bit of a challenge because automatics are new to me and this problem isn't neatly fitting into a diagnostic chart.

Again, thanks for the tips and advice, it's definitely appreciated.

Gary
 

tayto

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an 8' hose and a 0-300 psi gauge is a lot cheaper than a US shift controller+new wiring harness and will probably tell more about what is actually wrong with your transmission.
 

GMC Josh

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I do know that a bad Ignition module that intermittently fails to send the tach signal to the pcm will cause a 4l80e to lose 4th gear. When you shut the truck off and recrank it, 4th will work until the ignition module acts up again
 

Supercharged111

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Mindlessly swapping parts IS dumb, because you're potentially swapping a good part with a garbage made in ****** China part guaranteed to fail out of the box for something that was made with love here in the USofA back when people gave a F. 20 years ago, that approach worked. Now? Not so much. I'd sooner trust some 30 year old junkyard OE part over new BlowReilly's garbage.

Seriously?! C O M M I E is censored? Get a f'in life.
 

yevgenievich

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Poor signal from the speed sensors on the trans will also cause issues but speedo can still work.
 

Gary Gable

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Hi folks,

Update:
The US shift stand alone controller solved all the shifting problems. I still have a couple other issues to overcome however. The US Shift talks about connecting their device to a speed sensor coming off "an" electronic speedo. Since hooking it up I have no speedo output from my stock speedo, so I need to know, wire for wire, what to hook up to. Does anyone know? The tech help from US Shift is falling short. Also, I need to find a "when ignition is switched on and remains on during cranking" wire that feeds the PCM. US Shift steered me to an "always on" connection. Don't want that , I need an "only when ignition is on" connection. Hopefully both connections I seek are near the PCM as my new controller is mounted in the glovebox, and the PCM is directly behind it.

Anyone have any thoughts? Reminder, this is for a 1991 GMC 3500 with a 4L80E, completely stock.

Again, as always, thanks for any and all help. No bad answers here.

Thanks, Gary
 

Erik the Awful

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Out of curiosity, which shift controller did you use, the Quick2 or the Quick4? I'm going to need a controller for my C3500 at some point, and I was wondering if I would need to spend another $150 for the Quick4. From what I'm seeing, a Quick2 would work just fine for a tow pig.

I don't know the differences between my earlier trucks and a '91. Did your truck have a speedo cable? Don't '91s have a VSS behind the instrument cluster?

For key-on power, I'd tap into the "convenience center" next to the fuse block under the driver's side dash. Use a test light and see what empty spots power up when you turn the key on. I nabbed a bunch of spare convenience center pigtails from a junkyard truck so I can wire stuff into my truck easier.
 

Gary Gable

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I got the Quick 2. It had all the features I needed and If I want I can still tune the shifts. It wasn't hard to install but took most of a day to get the wiring harness routed. I wound up routing it from the transmission following the same route off the main harness then popped a new hole into the cab just under where the main harness enters and routed it up to the glovebox where the unit is mounted. Yes, the speedo is electronic, no cable.

Thanks for the tip on the ignition-on wire. I was hoping to find that at the existing PCM and there may be one there, but I can certainly run a wire from the spot near the fuse block.

The tech help at US Shift was medium helpful, but they're not experts on every installation. So they weren't all that helpful with my truck. I think they're geared more toward those installing electronically controlled transmissions in early vehicles, not so much in bypassing the transmission controlling portion of the PCM in the 89-91 GMC trucks with 4L80E. The instructions that came with the unit had some errors I had to clarify with tech help, but at least they were responsive.
 
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