4L60E Limp mode problems

[Charlie C.]

New user here, and I hope I have this in the right section.

I have a 1993 C1500 pickup with a 4L60E. It was not giving me 1st or 4th gears, but I had reverse. At first, I thought I had just lost the “A” solenoid (1-2 solenoid), as if I recall correctly, it would shift between 2 and 3 with no problems. I ohmed out the “A” solenoid and it showed bad (taking measurements at the PCM).

I bought 2 solenoids and a filter kit while I was at it. I changed out both solenoids and installed the new filter and fluid. Went to test it, and I had no change in how it acted. Doing a test, I was able to force a ground to the A solenoid and got it to take it off in first that way. The PCM was not calling for a ground to the A solenoid at all. For the hell of it, and because it looked like the original switch anyway, I changed out the ignition switch. No change. Obviously not the problem.

Did some research and troubleshooting, and just ordered a new PCM. Transferred the PROM to the new PCM, installed it, and now it just seems to have gone into and stays in limp mode. If I let it sit for a while, or if I disconnect the battery for a few minutes, it’ll take off in 1st, but within a few seconds, it goes back into limp mode. I shorted the ODB-1 terminals to get the codes via the Service Engine Soon light, and it just pulls a 12 code. There are no errors being presented that would explain the limp mode.

I rigged up a couple of switches to be able to force ground to the A and B solenoids, and took her on a test drive on the highway. I can cycle through all 4 gears with no problems at all by manipulating the A and B solenoids to call for the gear I want. Disconnect the switches and let it go back to PCM control, and she stays in limp mode.

I am at my wits end, and feel like I have searched all over to no avail. The thing that’s really messing with my head here is that I was able to get all 4 gears by using the manual switches, so it’s not a +12V supply problem to the transmission. The PCM is providing grounds tothe solenoids due to this limp mode I can’t get rid of. No codes via OBD-1 adds to the frustration.

Has ANYONE dealt with this before? I feel like I have tried everything. I’m willing to try any level of suggestion (mechanically, electrically, and controls experienced).

Thank you in advance!

Topic

[none]

I found a wiring diagram that I don’t know if I can trust. It does point out a transmission range sensor in it though. I’m wondering if the range sensor isn’t sending out a voltage when it should. If the PCM thinks it’s in park, it makes sense why shift solenoids aren’t grounding.

[Craig Furbush]

Did u ever find fault?

[Author]

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