Allison 5sp: P0748

[THEFERMANATOR]

Can somebody post up the diagnostic flow chart for p0748? My burb has been setting for a few months, get in it to drive and all is well, get it up to temp, flog it a few times, bam limp mode hits then the light. Cleared it and it was fine for a few minutes, then it would do it after it
was in 2nd for 5+ seconds, and now it pops as soon as I start it up. I haven't checked power at the tcm, but I have 13.8 at the fuse, and p0748 is the only code. I know it's an electrical code for the a trim solenoid, just was hoping to get a flow chart to go by.

 

[gmac32]

Pm me your email and i will send it to you

 

[THEFERMANATOR]

No signs of any chafing anywhere. I've got power to the 3 inputs(connected a test light so there was some load), both grounds, 6.7 ohms to solenoid a, but it fluctuates alot, solenoid b(for comparison) is a rock solid 6.4 ohms. There is some atf in the plug, but I cleaned it out good, made no difference. The code pops everytime I start it up. Thinking of putting my spare tcm in just to see as I had been getting some nasty downshifts the last time I drove it(it would change which one would be bad every time I cycled the key).

I just powered them and both trim solenoids click nice and loud. A draws 1.9 amps and B draws 1.8 amps.

 

[Mike L.]

You have a wiring problem with shift solenoid C. You need a wiring diagram.

 

[Mike L.]

You have a wiring problem with shift solenoid C.

 

[THEFERMANATOR]

Pm me your email and i will send it to you

Thanx for that file, it helped me track it down. Had to replace the internal harness because of oil leakage, and the A trim solenoid was going open circuit when it warmed up. Replaced the trim solenoid's, harness, and all is well again.

 

[gmac32]

No problem, glad you got it figured out