91 K2500 4l80 gets stuck in 2nd

[amh]

Thanks in advance, hope some one can give me some ideas.

91 GMC 4wd Sierra HD (8600 GVW) sometimes will only drive in 2nd gear (reverse OK).

History:
Started down shifting a bit jerky 2nd to 1st (for several months in summer and only after it warmed up, downshifted smoothly while still cold)

This winter in north east started lighting up SES light.

When SES light comes on, shifting into drive will result in 2nd gear only if I turn the engine off, wait some a random amount of time sometimes it will return to normal. But when light is on and stuck in 2nd it has not popped into 3rd if I drive over bumps, wot or start stop traffic)

Took it to trans shop when SES light was on, no code could be scanned out with shop scan tool.

Tried the blink method myself (jumper 2 pins on scan connector), same results only a pair of 12s (pause-blink-pause-blink-blink-pause & repeat - diagnostic code indictor, nothing in middle)
Local trans shop mechanic did not want to guess what was wrong & did not want to do random replacements (though when I pressed him he said the solenoids worth a guess, if I wanted to roll the dice on $170)

Standard service performed not too long ago (gear oil & filter), trans oil looks & smells ok. Oh yeah, speedo works fine.

Anyone with early K2500 4l80e experience please share your ideas on this puzzle.

thx

[hquick]

Doesn't second and reverse only indicate 'limp' mode?

http://forum.efilive.com/showthread.php?t=6785

[Lextech]

Put the solenoids and internal harness in it. If it still does it, then it probably needs internal trans repair/rebuild and they will replace the solenoids/internal harness anyway. You can do the solenoids and harness yourself---It's easy.

[amh]

hquick, yes I do realize that 2nd & reverse only is "limp" mode.
I was hoping someone else has seen and solved the problem of a Service Engine Soon (SES) light going on with no codes, scanable or viewable.

lextech, I may do that if my research & help pleas do not help. It is just that like hquick said, limp mode means control unit is sensing something wrong but I can not scan it. I am not aware of older models having temp sensors the only thing I can guess that would cause control unit to be unhappy without running engine & engaging gears (I think speed sensors should not matter unless there is motion) and warning light goes on before I shift in gear and go.

[amh]

Thanks to all for suggestions. Looks like 4l80e is fine, the 2nd gear only behavior is caused by ECM forcing limp home mode and I have to figure out what is causing the check engine light with no codes.

TCS is ~400 ohms (legal range for coolant temp sensor).
TPS seems ok varies from 1K ohm to 4.5K ohms on a 5.6K full range (should be equivalent to a 0.88V to 3.98V voltage divider pot)
IAC ohms out at 30 or so ohms on both windings
Vacuum lines look fine, don't feel or hear leaks

Did notice that when the Check engine will go on, Idle will be higher than normal, which pints to leak - intermittent one? (in addition to the 4l80e going into limp home mode).