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Help - 4L80e swap - all gears go forward, no reverse or neutral!

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Help - 4L80e swap - all gears go forward, no reverse or neutral!


Old 05-17-2014, 07:29 AM
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Question Help - 4L80e swap - all gears go forward, no reverse or neutral!

Hi all,

I've mostly swapped a freshly built 4L80e (with 2500 stall) into my '08 Silverado 1500 4WD. It now drives forward in all gears: Park, Reverse, Neutral, D! What could it be?

(I've also posted this on LS1Tech, sorry for the duplicate post to anyone who reads both, but I'm kinda desperate as this is my DD and I really need it running again!)

More info:

If I rev to around 2000 rpm in park, neutral or reverse it rolls forward, but it goes forward more easily in D and D2. I haven't been brave enough to go very far on the road to see what it does at higher speeds.

I've tried unplugging the transmission connector to eliminate wiring because I've read that the tranny should still drive forward and backward without the connection (in limp mode I think?), but the tranny still behaves the same and goes forwards. This makes me think it is something mechanical in the tranny.

When I shift into reverse, my reverse lights come on, the truck realizes it is in reverse, just not the transmission.

I'm throwing codes P0700, P0973, P0976 and P2764.

Anyone have suggestions as to what my could be or what I should check?

Lots of searching turned up a possibly similar post (, I've seen Jake suggest "Could be the manual valve not connected to the linkage." I'm guessing I would drop the pan and hopefully I can check if the linkage stud on the outside of the tranny is connected to something?

Any help would be much appreciated!

PS: my programming and wiring
I think my programming/wiring is good, though given that I still have a problem without the transmission connector plugged in makes me think the problem isn't electronic. For programming, I bit the bullet and licensed an '08 Express w/4.8L and 4L80e and did a write entire, then wrote my old ECM tune back over it, but killed the Hp Tuners program before it could overwrite my TCM. I believe this left me with my old ECM tune and a 4L80e tune on the TCM. I rewired the harness including wiring up the VSS and ISS (including the pin 45 to 26 change at the TCM connector for the ISS low signal).
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Old 05-17-2014, 11:06 AM
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Take my advice and definitely do not drive truck at all until you have triple checked everything or you will be rebuilding your trans. The wiring can easily be slightly pressed back on one of the connectors at the tcm or trans, so definitely re-check every pin for being completely inside connector. As far as the tune, I am no help because I had my son's done by nelson's. But I had two pins not completely seated when I did his swap and cost me a rebuild on a freshly built trans. It would barely move in d or r.
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Old 05-17-2014, 01:42 PM
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If it rolls in park then something is very not right. In park the parking pawl locks the output shaft, there is nothing hydraulic or electrical about it, its a purely mechanical mechanism. I would start there.
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