More 8l90 (maybe electrical) issues

Philip Chandler · October 3, 2020

Long time reader, very active in hptuners forums posted similar questions on other forums with no luck. This is kinda long bear with me as I'm quite lost.

2016 gmc sierra all terrain

Stock 8l90 3200 circle d converter

Whipple 2.9

lt4 fuel system

Snow perfoemance meth inejction

Tsp 236/241 cam(kit)

Truck is on low boost has the capability to make upwards of 900 but we are at 600 wheel right now

Engine was out for the cam install, threw in the new converter, taken to a shop for the dyno tune for the new cam and converter.

Made good power, trans was fine. I pick it up all seems well sitting at a stop light and go to accelerate and nothing. Like its in neutral. Shut the truck off turn it back on and its fine for about 10 seconds and then slips out of gear. Called the shop, waited on tow truck. truck wouldnt drive into the shop they throw it on the lift and the fluid is nasty black. So we flushed trans new mobil 1 fluid. And all seemed fine again. Drove it up the road let it get up to temp seemed normal. Headed on my way to the house.

(San antonio-marble falls tx)

Got about 10 minutes down the road and I'm coasting down a hill and go to accelerate up the hill and it does the same thing. Well in my experience when the fluid is that black it usually is to far gone. So I assumed trans was toast. Called a buddy to grab my car hauler and he came and got me. Ordered new trans, sent converter to get cut and cleaned. Threw it all back together, primed converter, checked tcase fluid.

Started it up let it get up to temp checked fluid per gm procedure.

Went on a test drive seemed great shifts were smooth never slipped converter engaged at exactly 3200, never even floored the truck. Next day I get in the truck to go wash it as its dirty from being in my shop for a couple weeks. Going through an auto car wash( where you put it in neutral and it drags you through it) I go to leave and nothing, like its in neutral. I get out and push it into a parking spot pull out the code reader bc my CEL is on now p0722 output shaft sensor no signal, I restart the truck and its fine. Limped the truck home having to shut it off and turn it back on twice. Only 4 miles. tested the output sensor was fine replaced it with no change. Think its throwing it bc of the input shaft vs output shaft numbers aren't lining up. Check wires to the tcm. Check grounds all over the truck. Replaced the battery and alternator(was time anyway) Checked the fluid level and it was a little over maybe 1/4 quart. So what gives. Any information would be very helpful ive tried almost everything. replacing the entire tranny did not fix the issue as it is doing the same exact thing as before. Fluid is nice and pink filter has zero debri in it. Only codes are p0700 and p0722. I just don't understand how it can run on the dyno for an entire day. I can post the tune as well if someone wants to look it over but I've been through it already and everything seems in order. Tune was done by weaponxmotorsports using local tuners dyno.