4l60e in limp mode has me confused

[tryingtolearn]

So I have an 02 suburban with 200K+ miles on it.
Looking for some troubleshooting advice so Ill describe everything Ive done up until now.

The other day it was driving fne and I stopeped at a stop sign and when I went to go it was really sluggish (Didnt know about limp mode at that time)

Got it home parked it
Went out a few hours later and started it
Service engine soon light came on

Hooked up the scanner (Innova 3130c)
Got
P0785
1. 3-2 Shift Solenoid Circuit - electrical
2. 3-2 Shift Solenoid Control Circuit
3. Shift Timing Solenoid

Replaced the 3-2 control solenoid
Cleared the code
Took it out on the road
Started off great shifted into 2nd gear fine got to the stop sign
started again just fine
Drove about a mile
Stopped at a red light
Light turned green and it was sluggish again
Turned around
Service engine light came back on

Got home
Scanned again
Same code.

Took it to a transmission shop, they drove it scanned it said they would replace the solenoid ($695!!!) but wouldnt guarantee
Suggested I replace the trans since it was over 200,000
$2695 for that job.

I get that its high miles but the truck has never given me a single problem and it still runs strong as the day I got it (Minus this little glitch) so

Read some forum posts and decided to replace the 1-2 and 2-3 solenoids as well as the TCC PWM Solenoid (Only cause that is what Amazon sent the first time instead of the 3-2 Solenoid.

Cleared the codes
Took it out again
Ran great down the street
Got out on the road for about a mile and then stopped
Same problem
Sluggish from the start
engine light on again
Same code.

Got home
Read some more
Ignition switch was the next hot topic on the forums for a fix for limp mode.

Replaced the ignition switch yesterday
Cleared the codes
Got out on the road and it ran fine.
Made it past the mile point
Took it on the interstate and all was shifting great.

Drove about a total of 30 miles stop and go city and highway and then hit a stop light.
Took off and it was sluggish again
No light this time
Hooked up the scanner anyway when I got home
Same code again...

This morning the light is on.

So I know its a lot of info but trying to get a good diagnosis of what to look at next. Hoping the trans shop knee jerk replacement idea isnt the only way to go. THanks for any help.

 

[zora]

You have read the description for the code?
http://engine-codes.com/p0785_gmc.html
Assume you replaced the fluid and filter?
Have you looked for a break in the wire harness, especially at any connector they may be hard to see visually and may be intermitant.

 

[tryingtolearn]

I have read it,
Yes new filter and fluid

I did look at the harness while I had it open but I suppose Ill drop the pan again and maybe take the harness out and inspect it closer.

Seems like alot could point to the problem, PCM, Throtle position sensor, any of the wiring on the circuit etc...
So with it starting out cold driving fine until I make a stop, does it sound more electrical rather than mechanical?

 

[Spriteman]

I can't remember which system does this but the trans goes into a shift-down mode when braking for a stop light. The computer remembers downshift points and kicks the solenoids at the right points. The faster the braking the faster the shift points.

Here's a test - when you're out driving the next time - find a place where you can get up to 50 mph or so and then just pull your foot off the gas and let it coast to a stop - then try accelerating. See if that does anything different.

Another test. Do your usual stop with brakes at a light - but before you go again turn off and restart the engine and see if it takes off correctly.

If it behaves any differently on either test that might narrow down where the problem is.

 

[tryingtolearn]

Thanks for the ideas, Im getting ready to test the electrical connections now but Ill try those tests as well.


Just one more question, If I get 12V Key on engine off at the harness going into the trans (Looks like pin e), would it be safe to assume that the problem is in the internal harness?

 

[tryingtolearn]

Im getting ready to test the electrical connections

Easier said than done, Doesnt look like I can get to the connector with the trans installed......