Never ending TCC mystery problem (4l80e)

[Turbokat]

For the last 3 years i have been having a problem with my TCC that i can't seem to figure out. If im going over 60mph for more than 15 seconds the TCC will quickly disengage. But if i drive under 60mph the TCC works just as it should without a single problem. Theres NO fault codes at all according to my tech2 and everything else works as it should. I can command TCC on with the tech2 perfectly while going 60mph+ without any problems. I've tried with 2 different PCMs and i have changed all the speed sensors. The only lead i have is that according to the misfire graph it misfires like crazy 60mph+ but the engine is clearly not misfiring at all. Also no misfire codes so i don't know how much of a accurate lead this is. Does anyone know what can make TCC just disengage like this without even giving a hint of whats wrong? This is the most infuriating problem i have ever had with any gmt400 argh.
and yes i have already changed spark plugs, plug wires, dist cap, rotor, the whole distributor 3 times, crank position sensor, knock sensor, maf sensor, fuel injector spider, map sensor, both o2 sensors, speed sensors and 2 PCMs without any luck.

 

[Schurkey]

WHAT VEHICLE??? Apparently, a '96+ V6 or small-block "Vortec" engine.

The computer will not engage the TCC if it senses misfire.

Your computer is sensing misfire, therefore it disables the TCC.

The solution is to figure out why the computer is sensing misfire. Not having a misfire "code" means NOTHING. Look at the cylinder-by-cylinder misfire counter. Figure out which cylinder(s) are acting up; or if the misfire is truly random.

Misfire is determined, in part, by the crank sensor. How old is the sensor, how good is the wire harness?

Doesn't your truck have four O2 sensors? Why replace only two? Which two?

What is the cranking compression pressures of all cylinders?

What is the fuel pressure when the TCC won't engage at 60mph +? Fuel pump or fuel pump wire harness weak, can't supply enough fuel at highway speed?



The '97 K2500 7.4L I bought had massive misfire counts on #8. Turned out to be a cracked spark plug porcelain. Replacing the plug fixed the misfire, and also fixed the lack of TCC. Cheapest and easiest "major repairs to engine and transmission" I ever did in my career.

 

[Turbokat]

WHAT VEHICLE??? Apparently, a '96+ V6 or small-block "Vortec" engine.

The computer will not engage the TCC if it senses misfire.

Your computer is sensing misfire, therefore it disables the TCC.

The solution is to figure out why the computer is sensing misfire. Not having a misfire "code" means NOTHING. Look at the cylinder-by-cylinder misfire counter. Figure out which cylinder(s) are acting up; or if the misfire is truly random.

Misfire is determined, in part, by the crank sensor. How old is the sensor, how good is the wire harness?

Doesn't your truck have four O2 sensors? Why replace only two? Which two?

What is the cranking compression pressures of all cylinders?

What is the fuel pressure when the TCC won't engage at 60mph +? Fuel pump or fuel pump wire harness weak, can't supply enough fuel at highway speed?



The '97 K2500 7.4L I bought had massive misfire counts on #8. Turned out to be a cracked spark plug porcelain. Replacing the plug fixed the misfire, and also fixed the lack of TCC. Cheapest and easiest "major repairs to engine and transmission" I ever did in my career.

Oh yeah sorry its 1999 suburban. I got the last two o2s tuned out by black bear, I tried messaging them but got no answer for some reason.
it "senses" some misfire under 60 too but not the same amount. The misfires is just random, its never just 1 cylinder.

Compression is good too as this is an engine i swapped in myself, it was the same issue before the swap.

I could look into the crank sensor wiring but i kinda doubt its there too, but i guess i will never know if i don't check it out. I don't notice any misfire myself at all which makes me wonder if the readings are just bogus

 

[Supercharged111]

There is a misfire threshold that must be exceeded to prevent TCC lockup. It's a certain misfire count within a predefined quantity of crankshaft revolutions. If it's not misfiring often enough below 60 it won't unlock.

 

[stutaeng]

Isn't there something about the PWM solenoid having to do with the TCC?

What's the history with the transmission? And the truck? Did the problem develop after you started ownership? Or it's been like that since you you bought it?

I don't know enough on how these units work. Maybe a valve is sticking in it's bore? There's also a little tiny screen somewhere on the valve body that I think can get clogged with debris.

 

[Turbokat]

Isn't there something about the PWM solenoid having to do with the TCC?

What's the history with the transmission? And the truck? Did the problem develop after you started ownership? Or it's been like that since you you bought it?

I don't know enough on how these units work. Maybe a valve is sticking in it's bore? There's also a little tiny screen somewhere on the valve body that I think can get clogged with debris.

The problem has always been there. I really doubt theres anything wrong with the solenoid since it i can command it on and off without any problems. I just wish the PCM told me at least a hint

 

[Supercharged111]

The problem has always been there. I really doubt theres anything wrong with the solenoid since it i can command it on and off without any problems. I just wish the PCM told me at least a hint

The PCM has given you much more than a hint.

 

[618 Syndicate]

You're trying to solve a symptom not the problem. The converter is doing what it's told. You already know that if the PCM sees a frequent enough misfire it won't lock the converter. The fact that you don't feel the misfire is irrelevant, the PCM knows what the sensors tell it and acts accordingly.
Seems to me that solving your (non-existent) misfire is the first step.

 

[Schurkey]

it "senses" some misfire under 60 too but not the same amount. The misfires is just random, its never just 1 cylinder.

Is you scan tool capable of displaying cylinder-by-cylinder counts?

Cheap scan tools misrepresent misfire codes. A genuine P0306, for example--misfire on #6--gets displayed as PO300 "random misfire" because the tool isn't sophisticated enough to display the true code.

That's why I'm telling you to look for misfire counts, not misfire codes.

 

[Turbokat]

Is you scan tool capable of displaying cylinder-by-cylinder counts?

Cheap scan tools misrepresent misfire codes. A genuine P0306, for example--misfire on #6--gets displayed as PO300 "random misfire" because the tool isn't sophisticated enough to display the true code.

That's why I'm telling you to look for misfire counts, not misfire codes.

i've been looking into it and the current cylinders that are misfiring is 1 and 3. at highway speeds with higher load the misfires count increases. I can't seem to understand why. I tried swapping the spark plugs and wires around and i looked into injectors and the wiring harness for the injector to the PCM.

I don't really get what cylinder 1 and 3 have in common other than that they are besides each other

At this point I'm so out of leads that i may just try to compression test these 2 cylinders again.
I don't have a way to measure fuel pressure atm but i am running the newer MPFI spider.

Also at WOT it doesn't count any new misfires at all if thats to any help.