1998 C2500 4L80e Transmission Has 3rd Gear and Reverse Only

PrchJrkr

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So out of nowhere my truck decided it would switch to limp mode (my guess). No warning lights and no codes stored in the OBDII system. After searching I found the likely cause was the 1-2 shift solenoid, so I dropped the pan and swapped 1-2 and 3-4 solenoids and filter, buttoned up, and refilled with fresh fluid. Test drive revealed no change. I'll try to reset the computer by removing the ground cable for 30 minutes. If that doesn't do it, I'm going to unplug the harness from the trans and try to diagnose and clean the connections. Any suggestions?
 

Grumpy

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Ugh...cleaning all the grounds is all I have. Strange that no codes show up. FWIW, I am not in any way,shape or form a mechanic, but I had an issue similar to what you are describing and cleaning the ground points on the car cleared up the issue.
 

PrchJrkr

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Ugh...cleaning all the grounds is all I have. Strange that no codes show up. FWIW, I am not in any way,shape or form a mechanic, but I had an issue similar to what you are describing and cleaning the ground points on the car cleared up the issue.
Thanks! I plan to clean up any grounds I can locate, as this is most likely caused by an electrical issue. Supposedly, 3rd gear is the only one not effected by the electrical system so that's what it defaults to in limp mode. I just find it strange that I get no codes. I'll hook up the scanner again, but I'm sure it was clear last week when I checked.
 

PrchJrkr

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Additionally, it is an old municipal truck so it's seen all kinds of nasty weather and road salt. The selling point on it was the regular maintenance it received by tax payer funded mechanics.
 

PrchJrkr

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Could your PCM have failed?
Anything is possible at this point. I hope not, but the thought did cross my mind. The thing that gets to me most is the fact that it was running great, I parked it, and when I jumped in at lunchtime, it didn't shift. I know electronically controlled devices sometimes do that, but it's not like I've been having any other issues with it. The ignition switch has been known to cause issues on these trucks, but that was replaced a few years ago and has been working fine ever since.
 

Grumpy

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Googling your issue looks like you are on the right track, solenoids/harness/connectivity. I would think that a PCM issue would throw a code.
 

PrchJrkr

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I just hooked up the code reader and went to "enhanced codes" and cleared out P0758 code for the 1-2 shift solenoid. That's one I changed out yesterday. Fingers crossed.
 

PrchJrkr

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Unless you have weird shifting issues prior, it may not the issue, but is cheaper than a PCM.
It keeps setting a P0758 Shift solenoid B code. It looks like I'll be replacing the speed sensor next. Thanks for the advice. Auto electrical is usually a gamble. Pick the cheapest fix, then work your way up.
 

Chris0nllyn

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It keeps setting a P0758 Shift solenoid B code. It looks like I'll be replacing the speed sensor next. Thanks for the advice. Auto electrical is usually a gamble. Pick the cheapest fix, then work your way up.
And of course more and more cars today are all-electronic. You almost need a degree in electronics in order to properly diagnose something.
 

PrchJrkr

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I haven't tagged anything historic since my 69 Gutless, but I'm tempted to.

Okay, so I unplugged the harness from the connector on the side of the trans and fluid started dripping from the connector. I'm fairly sure that fluid isn't supposed to be inside the connector. I don't know if this would cause my no shift issues, but it can't help matters. It looks like I'll be replacing that. I'm glad I broke the drain plug loose when I had the pan dropped.
 

Sneakers

Just sneakin' around....
I haven't tagged anything historic since my 69 Gutless, but I'm tempted to.

Okay, so I unplugged the harness from the connector on the side of the trans and fluid started dripping from the connector. I'm fairly sure that fluid isn't supposed to be inside the connector.
:sshrug: Keeps the contacts lubricated?
 

PrchJrkr

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They're all nice and shiny. Before I do anything major, I'm going to run a new ground from the trans to the negative side of the battery just to rule that out.:yay:
And, that didn't change a thing. The connector on the Echlin solenoid was too basic and universal for my liking. The pins were actual pins like a circuit board connection for low voltage. I know it's just a switch, but the original was much beefier. It's like comparing a 20awg to a 28awg wire. Not a huge difference unless you're dealing with resistance in already undersized wiring. Anyway, re-ordered another aftermarket solenoid with bigger contacts...
 
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