4L80E doing some weird stuff

[Lt.Del]

On my 1991 3/4 ton 4x4 5.7 sub  with a 4L80E, electronically controlled trans, she starts out fine, no problem.  I drive 10 minutes to pick up  my son from day care, park the sub for about 10 minutes, get back in and drive home, and just like clock work, for at least the past week, she doesn't want to come out of 1st gear.  Needless to say, it's a slow ride home.
I have check all the obvious, fluid level fine, I changed that about a year ago.

What is a mystery is that it runs fine when cold.   I can drive it for hours in fact.  But on short trips, i turn the motor off for 10 minutes, get back in, and the the thing doesn't wanna get out of first.  I tried unhooking the battery and resetting the ECM, no change. 

Is my ECM going?

[VileZambonie]

A complaint of no upshift may be caused by a bad Transmission Input Speed Sensor. Reading data with a scan tool may show no vehicle speed signal. Replacing the output speed sensor, DRAC, and even the computer may not restore normal vehicle speed signal, if the computer does not recognize a valid transmission input speed signal.

High line pressure (no Force Motor current) may also occur depending on the software strategy. This condition is most common on two-wheel drive diesel applications, but may also apply to others. The software strategy used on the different models, years, and calibrations are too widely varied to be absolutely certain which vehicles may be affected in this manner.

Verify input speed sensor operation with a scanner before replacing other components needlessly.

[Lt.Del]

Thanks Vile.

The VSS is towards the front of the trans, just back from the bellhousing, correct?  Doesnt that also give signals for cruise control and ABS?

My cruise and ABS (no idiot light) work fine. 

I have read that the early models of the 4l80e (like mine, since it is a 91) have speed sensors (both input and output--the output i believe is on the transfer case for 4x4's) that become faulty due to design flaws that were later fixed.   This will cause no upshift or erratic upshift.

What gets me is that everything works perfectly.  Only after shutting off the engine for 10 minutes and restarting (like picking up my kid or going into a store for a minute) then going, it will not upshift.  No idiot lights at all when this happens.

I am glad the sensors are relatively cheap.  I believe I'm gonna start with the output sensor on the transfer case and input sensor, if nothing else, to rule them out and we'll see.  It'll cost upwards of $100 to put this baby on a scan tool because I'll have to go to a shop to do that.   

[VileZambonie]

Input speed is turbine speed. This is near the bellhousing. Output speed is the vss signal.

[Lt.Del]

thanks for clarifying.  I know nothing 'bout this stuff.