2000 TH400 problem

Hey all, my question is about a th400 automatic transmission. it is out of a GMC Utilimaster Boxvan. i realize this may be a little out of place but noone reads the commercial GMC threads.

Anyways this transmission is exactly like any other chev/GMC th400 computer controlled blah blah.

Heres the deal, about 2 weeks ago we began having problems with this shitbox, it would shift hard occationaly, which then progressed to where it is now shift from 1st to second no problem... your gonna love this... if i dont let off the gas from a normal accelerating situation it can easily get up to 4 grand and will stay there unless you flutter the gas.
so far we have replaced the filter and fluid, and threw in some transmission fix (which doesent work by the way)

Im led to believe either the tourque converter is on its way out or the valve body is shot..... any suggestions??????

This is a non overdrive old style non pcm controlled 400?

im sorry i thought i forgot something.....
yes it does have overdrive, and as far as i know it is computer controlled. we've so-far checked the computer settings and the shift points are perfectly factory... its just the transmission has an attitude problem and doesent wanna listen

This sounds like a 4l80e, have it scanned on trans function scanner for slippage errors, solenoid functions and trans codes, a regular scanner will not do this.

Anyways this transmission is exactly like any other chev/GMC th400 computer controlled blah blah.

Never heard of one!! 700R4 used computer signals to lock up the TC. Your header says 2000 TH400. Only 2 tranneys I know of in 2000 are the 4L60E & 4L80E (and maybe the Allison behind the DuraMax) all of which are computer controlled. You said what it came out of (no year) but not what it's in now??

So, you say it's computer controlled. If so it needs to be scanned like "MaxWedge" says. That's the only way!!! Either way I highly doubt the torque converter.

Third vote to have it scanned (or do it yourself) before it gets expensive. Right now, it could be as little as a PWM solenoid, spring for a pressure regulator, or something else which is well below a $25 part to repair. Is the "Check Engine" lamp illuminated while you are driving?

This is all presuming it is in fact a 4L80E (TH400 with Overdrive, electrically shifted).

i appoligize im not that informed of newer transmissions model names :dupe:
the original vehicle is a 2000 gmc utilimaster 5.7L 350 engine, its a lot like the Frito lay vans
where can i pick up a scanner? or where can i get it scanned? and no the service engine soon light is not on.

one more littel peice of info...what is it in now?

You can get a basic code scanner which will display numerical error codes. They cost as little as $45. However, they only display and clear stored (and some pending) error codes. Since the "Check Engine" lamps is not on, there will likely be no stored codes. Your case seems to indicate that the transmission parameters need to be monitored while the transmission is in operation. That would require a bit more sophisticated scanner. The minimal fee you would pay for a diagnostic scan could potentially save you a lot in unnecessdary repairs.

one more littel peice of info...what is it in now?

still in the original vehicle.


i do have a snap-on OBD 2 scanner available and have hooked it up but as you said it didnt throw codes since the engine light wasnt on.
im not sure if this scanner has transmission code cards available but i can definately look into it....

heres a bit of a predictive question for you guys.

Just judging from whats goin on right now, the fact it doesent like to go from 2nd to third without a fight. how long do you think it has to live, and/or what in your experience do you think is the most likely cause.
weve got a pool going at work on how long this thing has to live.

If the transmission is holding second without going to third, it could "live" indefinitely. That would indicate that the valving is holding second and not applying third. If the transmission is going from second to "no-man's land" and then finally engaging solidly third, that could indicate a valving or actuator problem not engaging a clutch, or not releasing the band, in which case the transmission may have a short time left. About all you could do externally (short of a scan) is measure resistance of all the solenoid coils (from the outside) but this will only tell you that they are operational. Apparently, they are, since it evidently finally does shift. You might also check line pressure with an external gauge to verify that the pump is operational and line pessure is correct in the various ranges.

You really need to monitor the transmission pressure sensor/switch data while the trans is in operation.

Another thing you can do as a general test is to operate the transmission manually through the ranges to see if it shifts when the selector is moved, or delays.

Also, does it engage both REVERSE and DRIVE normally, without delay?

A Snap-On Solus MT-2500 can read all powertrain parameters with the correct personality module and keys.

pops into reverse and drive just fine, manual shifting works about the same, still need to flutter the gas.