4L80E tranny headache

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09-04-2010, 09:56 PM

JimmyD55

Junior Member

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Join Date: Sep 2010

Posts: 1

4L80E tranny headache

So... I traded two P.O.S.'s for a slightly polished turd. I knew when I bought it that it had a problem with the tranny but I was hoping it was just dirty. It is not that simple. I've spent the past two month (on and off) trying to get the tranny out of "limp" mode with no success.

Here's what I have:
'95 Chevy 2WD Dually with a 7.4L and a 4L80E tranny.

Here are my symptoms:
Will only shift between 2nd & 3rd but has reverse.

Problems revealed with an engine analyzer:
Bad MAP sensor - swapped.
Bad spark control module - swapped.
Bad Quad driver module
Bad ABS wheel sensor - checked and found section that feels like it was pinched (heading to the junkyard for a replacement).

Other work/testing I've done:
Could not find a downshift module for a 4L80E only a 4L60E. Changed the filter and shift solenoids. Checked for fluid on the tranny harness connector and it is leaking so the internal harness will need to be swapped, in the mean time I cleaned the connector with brake clean, blew it out with air and put dielectric grease in the contacts. Swapped the input speed and output speed sensors. I've checked the TPS and pulled the IAC and cleaned it (the code for the ABS is the same as the code for the IAC). Tightened all electrical connections to the bat. Swappped the ignition switch harness with a junkyard unit. Checked all fuses. Pulled an ECM from a junkyard van to test and this did not change the symptoms. Pulled out the meter and test light and started testing pins at the ECM and the tranny plug, I have bat & ign power to both.

My questions:
Will a faulty ABS sensor cable cause the tranny to go into "limp" mode?
When testing the pins with the ignition on I have 14.5 VDC on the 1-2 Sol. and 245 mVDC on the 2-3 Sol. does the ECM need to see certain perameters before it will allow the 2-3 to work? This is a ground switched unit.

I know I'm not listing some of the symptoms I've found but I can't think of anymore right now. I'm certain this issue is electrical but I'm not familiar enough with the system to know what steps to take next. I've been using the Haynes manual and the ATSG Tranny manual and have checked everything they have recommended.

Something else... this has a OBDI system in it. The only code I have found manually is on the ABS side. I had a local parts house hook a scanner up and it did not flash any codes. I took it to a local shop and they hooked up a Snap-on computer and found the codes for the sensors and modules.

Any help I can get would be greatly appreciated.

Thanks!