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4l80e stuck in first

Thread starter #1
Hello all! New member here. Was hoping someone could help! I have a 93 gmc 2500. I just dropped a vortec tbi 5.7l in. The truck had been sitting for close to 2 years waiting. Got it running, took it round the block, won't shift out of first. Speedometer works. Checked the iss and the oss, both are sending signals. Had a custom chip burned, so I replaced with the old one, still nothing. I hooked test lights to the solenoids, ecm is not sending signal to ground the a solenoid. Then I discovered this truck has a oss buffer. I have 12v coming in, a good ground. Have not checked signal going to ecm. Could the buffer have gone out and still send a signal to the speedometer? I did get several codes at first... 59, 85, and 86. The 85 and 86 haven't been set in a couple weeks, but I haven't driven far at all. The 59 is for Trans temp sensor voltage too high. Could this cause no shift? What could I be overlooking?


Staff member
Lead Moderator
Staff #2
91-93 is a unique setup. 1st off, is this the stock 93 trans? 91-93 4X4 4l80E's are UNIQUE in that they are the ONLY 4X4 4L80E to use the output speed sensor in the transmission. Your speedo in a 93 truck works off of the speed sensor in the tail shaft of the transfer case, and has NO input on the trans shifting. There are 2 speed sensor in the trans on a 93, 1 up behind the bellhousing for input speed, and another mounted just in front of the extension housing for the output speed. BOTH sensors feed directly to the ECM for a 91-93 rig. You also have to make sure you have the correct chip as some will use later bin files from a 94, and they worked differently. Your best bet is to get a scanner on it, otherwise you're guessing at it. For checking if it's commanding the correct solenoid, the only way you can test this is with a bidirectional scanner connected, otherwise the ECM will sense electrical issues(even back probing the plugs), and go to limp mode. Unfortunately with an all electronic trans, you need to see what the controller is seeing to know what is going on, otherwise you're guessing, and might as well just start lighting money on fire as many times you get the same end result. And any 3 of those codes can send it into limp mode. You NEED to get a scanner on it.