91 R2500 4L80E problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

91r2500 · 03-10-2018, 08:51 PM

Hello, Welcome to my first thread

I am having an odd issue with my 4l80E.
When shifting automatically it skips second, it shifts through all other gears 1-3-4, does not slip, nor give any other problems.
When I shift it manually all gears work 1-2-3-4 no problem. It will not automatically downshift to 2nd either, 4th to 3rd is fine and if I go slow enough it will go into 1st as well, it will downshift to second if I manually put the shifter into 2nd though.

I have replaced all of the solenoids and wiring harness in the trans., rewired the external harness (someone did the conversion to the newer harness but the connections looked iffy) While resplicing the connector I checked to make sure that the wires were in their proper places.

Anyone have any ideas?
Thank you in advance for any help.

85raiderburban · 03-28-2018, 06:51 AM

I've got a 4l80 in my 80 suburban. I shift it manually. Going from 1st to 3rd skipping 2nd is an indicator of solenoid B powered on too early. Both solenoids have to be off to get 2nd gear.
As follows:
1st - A ON, B OFF
2nd - BOTH OFF
3RD - A OFF, B ON

the computer/TCM is the obvious culprit here, as you wouldn't be able to shift it in all gears manually if it was something internal.

Ski-me · 03-28-2018, 12:51 PM

^^^ I have no idea but that sounds pretty dang logical!

03-10-2018, 08:51 PM	#1
91r2500 Registered User Join Date: Mar 2018 Location: Melrose, Florida Posts: 1	91 R2500 4L80E problem Hello, Welcome to my first thread I am having an odd issue with my 4l80E. When shifting automatically it skips second, it shifts through all other gears 1-3-4, does not slip, nor give any other problems. When I shift it manually all gears work 1-2-3-4 no problem. It will not automatically downshift to 2nd either, 4th to 3rd is fine and if I go slow enough it will go into 1st as well, it will downshift to second if I manually put the shifter into 2nd though. I have replaced all of the solenoids and wiring harness in the trans., rewired the external harness (someone did the conversion to the newer harness but the connections looked iffy) While resplicing the connector I checked to make sure that the wires were in their proper places. Anyone have any ideas? Thank you in advance for any help. Last edited by 91r2500; 03-10-2018 at 08:53 PM. Reason: missing info.

03-28-2018, 06:51 AM	#2
85raiderburban Registered User Join Date: Nov 2014 Location: between AZ & NV Posts: 67	Re: 91 R2500 4L80E problem I've got a 4l80 in my 80 suburban. I shift it manually. Going from 1st to 3rd skipping 2nd is an indicator of solenoid B powered on too early. Both solenoids have to be off to get 2nd gear. As follows: 1st - A ON, B OFF 2nd - BOTH OFF 3RD - A OFF, B ON the computer/TCM is the obvious culprit here, as you wouldn't be able to shift it in all gears manually if it was something internal. __________________ UNDER CONSTRUCTION '80 Suburban C20 (C6P) (Silverado) - '68 472 (Edelbrock 1813, PH8026 LT's, H pipe) / '91 4L80 (manual shift,vac modulator, 350 converter) / 14B FF 3.21 G80 34' s/ '96 GMT400 interior

03-28-2018, 12:51 PM	#3
Ski-me Registered User Join Date: May 2012 Location: Colo Springs, CO Posts: 870	Re: 91 R2500 4L80E problem ^^^ I have no idea but that sounds pretty dang logical! __________________ - Jeff 89' K5 Blazer, 2.5" lift, 35" tires 04' GMC Yukon XL, 3/4 ton, 8.1L

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