Weird 4L80E problem

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Nxslt
Posts: 2
Joined: Thu Oct 07, 2021 11:07 am

Weird 4L80E problem

Post by Nxslt »

I've got a 4L80E in a 95 K2500 that has a tendency to go into limp mode quite regularly. It's so bad that I put a toggle switch on the pcm power wire so I can quickly reset it and clear the code. The code is usually 81 or 82 which is a bad shift valve. It's not bad. When it goes in limp mode it goes to second gear with no shifting and gives full line pressure. Here's the weird part, I was datalogging today with datamaster and saw that when I went to start it, right when I hit the key for a brief moment the "abuse condition" lights up in datamaster and it shows my rpm going to 8100 and the trans slip jumped way up too. I'm assuming this is what's causing my issue, the trans thinks it's getting major slippage so it goes into limp mode right off the bat. Anybody ever heard of this "abuse condition" in these older pcm? I was looking thru a bin file for this truck in tuner pro and it has a "700r4 anti abuse routine" listed but nothing like that for a 4l80E. I don't know what is causing it to see this high rpm reading when I turn the key on. Any clue or thoughts on it?
Soppy
Posts: 2
Joined: Sun Oct 03, 2021 8:34 am

Re: Weird 4L80E problem

Post by Soppy »

I had a similar problem on my 93 motorhome disabling 4th gear and the TCC lockup. For some reason the only way I could reset the codes was to remove power from the PCM for an hr. I replaced both shift solenoids and no luck. I finally replaced the input speed sensor and the tranny works perfect again.
Nxslt
Posts: 2
Joined: Thu Oct 07, 2021 11:07 am

Re: Weird 4L80E problem

Post by Nxslt »

So you never got a code for the speed sensor? What made you decide to replace that?

I noticed something else yesterday. Right before it gives the code the reduce upshft and torque reduction comes on and I can see the force motor current drop to about .2 at the same time which means more line pressure.
Soppy
Posts: 2
Joined: Sun Oct 03, 2021 8:34 am

Re: Weird 4L80E problem

Post by Soppy »

If I drove in drive, it would set code 39. TCC would lock up and work just fine. When in overdrive, it would shift into 4th and the converter would lock up. After a mile or so , the tranny would shift down to 3rd and the converter would not lock up. I would have codes 39, 68, 85, 86,and 87. I just happen to know a couple of old guys at a tranny shop and they said it could possibly be the input speed sensor and gave me a used one to try. At the time my data logging software would only show engine rpm and road speed. When the TCC locked , the road speed and rpm would track together, so I knew the converter was not slipping.
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