4L80e Help!

[journeyman]

I have a OBD1 1992 P30 Chevrolet chassis with the 454/4L80e combo.

Unfortunately, I can't remember exactly how/when it started, but it has a problem that happens when it kicks down from 4th to 3rd (passing gear).



Everything works perfectly on the transmission UNTIL the computer kicks it down under acceleration. If I manually move the shift lever from 4 to 3, all is good. But as soon as the computer kicks it down, OD is lost. It stays in 3rd gear and If I let off the throttle, it freewheels down to near idle.

Pull over, shut it off, wait 10 sec (about) and start it up... All good again.



I am completely stumped by this. Because it is very repeatable, my thinking is that it is electronic. I am thinking about buying an aftermarket TCU, but before I shell out the cash for that, I really want to make sure that it is indeed computer related.

Ideas? Thoughts?

[RobertISaar]

there's a reason it is happening.... most of the time, those reasons are pushed out over the datastream, such as "high slip caused 4th disable".

[journeyman]

I am out of the country right now, but my nephew says he can test drive it with the scan tool. I am not sure what it can tell us other than rudimentary error codes.

Because the transmission works perfectly even if you shift down (manually, with the selector) until a computer requested kickdown, and because this is very repeatable, my assumption has been that it has to be electronic.

Does that make sense?

[RobertISaar]

considering what is happening, it may be electronic(I seem to remember hearing about the pressure control solenoid being problematic on some early 4L80E applications), or it could be the electronics saving something mechanical. you may not have much/any 4th gear(or TCC) clutch slip until high throttle(at/near kickdown thresholds) when suddenly the friction material isn't capable of holding up to engine torque, the PCM sensing it and then disabling 4th/TCC operation.

this is why I mention the datastream as being a source of answers. the condition may or may not set a DTC, if it does that would certainly help narrow things down. this PCM doesn't output nearly as many status bits about TCC/4th operation as some of the later PCMs, but it does give a few hints as to the reason it could be happening.

[journeyman]

So how do I get a data stream from the factory GM TCU?

My brother just test drove this with the scan tool attached and sent me this:
Drove the Alumalite. Trans temp: 170. Locked and unlocked fine. Might have to have a load. No codes.

I replied and asked if it indeed kicked down from 4th to 3rd (typical passing/hill situation). I assume he did that, but am waiting for a reply. The vehicle weighs approximately 17,000 Lbs.

[buddrow]

Might also have a peak at the MAP sensor. If it is "sticking" in a high load signal it may inhibit 4th. Just a thought.

Buddrow