TCC locks & unlocks frequently

[offroadrider12]

I have a 2008 Trailblazer LT with a 4.2L & 4x4.

I assumed this was the typical TCC bore wear, so I installed the following:

Transgo SK 4l60E to eliminate PWM
Sonnax Pinless accumulator pistons
Torlon check ba***
New OEM separator plate
Corvette Servo
One bottle Lubegard Red additive.


I still have an intermittent problem where at constant speed & no load, say 55 or 70 mph, the converter will lock, then unlock, then lock again.

I took it to a transmission shop and they put it on their scan tool. They said the transmission itself is working properly and the PCM is commanding the converter to lock and unlock. They checked the TPS and said it was fine, however they are unsure on what is causing it. They said it is some sort of electrical issue on the chassis and recommended another shop.

Has anyone had an issue like this before? I'm stumped on what to do next.
Anybody have some suggestions or areas to look at?


Thanks

 

[gmcman]

There is a thread on this currently and here's the link I included.

Look at the pieces that went back in, does yours look the same?

There's also a chance you wore the clutch in the converter but that's hard to do unless it slips for a long time, those are quite beefy.

Sorry, pics are backwards in sequence.

http://gmtnation.com/forums/topic/7238-removed-pwm-control-from-tcc/?hl=%2Bpwm+%2Beliminator

 

[gmcman]

The brake pedal switch is also very sensitive and this helps with trailer brakes as the brake light comes on as soon as the brake pedal is moved.

You could check that your brake pedal switch could be not adjusted properly and the trans thinks you're using the brakes.

The TCC will unlock on deceleration amd when the brake light is activated.

 

[offroadrider12]

Thank you. I actually saw that thread before.

I never noticed my TCC unlocking when I would two foot it to lightly depress the brake pedal. I'll take a look at the switch.

The trans shop did say the PCM is commanding it to unlock. It being an intermittent problem makes it hard to diagnose.

 

[gmcman]

I never noticed my TCC unlocking when I would two foot it to lightly depress the brake pedal. I'll take a look at the switch.

The trans shop did say the PCM is commanding it to unlock. It being an intermittent problem makes it hard to diagnose.

When you are at no load, are you coasting? If you lift your foot,the TCC will disengage.

The TCC stays locked only when under a slight load and when the trans has warmed up. If you are cruising steady and notice it going in and out of TCC lock, try pulling back on the brake pedal with the other foot.

Have you checked the fluid level, trans hot, level ground, at idle?

I think there may have been a TSB or just an issue with the brake or tail light circuit boards on the older models. I don't think this could cause a problem since I would think it's powered from the brake switch but I'm not sure.

The fact the PCM is commanding the unlock my guess is it's outside of the trans...hopefully anyway.

 

[Sparky]

Mine always drops out of TCC lock when coasting. That is an improvement on mpg as it "free wheels" vs engine braking.

 

[offroadrider12]

No load was bad terminology on my part. I did not mean while coasting.

I meant light, consistent throttle pressure, at constant speed, without a trailer or anything like that.

It can happen whether I have the cruise control on or if I'm using my foot to maintain speed.

 

[gmcman]

How old are your plugs and if you changed them what did you put in there? If the PCM is commanding the unlock, seems it could also be caused by misfires that are not setting codes...(yet).

How many miles? Maintenance history?

 

[offroadrider12]

Bought the truck with about 105,000 miles. Replaced the plugs when I bought it with new AC Delco Iridium plugs. It currently has 117,000 miles. I've done basic maintenance on it since I bought it, cleaned MAF, throttle body, changed plugs, diff fluid, etc.

The trans shop I took it to had their scan tool on it and got it to act up while driving. They didn't seem to think it was misfire related, they said something electrical on the chassis.

 

[gmcman]

ure

The trans shop I took it to had their scan tool on it and got it to act up while driving. They didn't seem to think it was misfire related, they said something electrical on the chassis.

I think so as well, I don't think it's trans pressure related but it's a remote possibility. Do you have Torque and if so can you see what the misfire count is?

The most likely culprit is the brake cicuit, then the TPS, but misfires won't show up unless your scan tool is equipped to show them obviously. Did this start to happen after you changed the plugs? Any moisture in any of the plug wells, mainly #4? #4 is under the cowl seal and there's a void in the sheet metal and it tends to drip right in the area of #4

Have you changes the trans fluid? What condition is the fluid?

 

[Tiggerr]

I'd check the harness for the trans. Check brake light switch/circuit as said above. Check Speed sensor maybe? Possible you got a bad TCC valve? Seems you did everything right to correct it under normal circumstances. Maybe a glitch in the PCM? Idk if there would be a relearn procedure after doing all the valve body work?

 

[offroadrider12]

Given that the transmission does this whether it is in cruise control or not, I'm starting to doubt that it's the brake switch. I'm thinking that if the vehicle thinks the brakes were applied it would not only unlock the TCC but turn off the cruise control?

I've read the following things affect TCC lockup. Guess this gives me a place to start...

VSS
Trans Fluid Temp
TPS
Engine RPM
Coolant Temp
Brake Switch
A/C request

 

[Mike w]

I had this problem and the pwm eliminator worked. However my problem originally was a ground wire on the harness to the trans. The pwm elimination makes lock up mechanical instead of electrical i believe.

 

[Virgiliolvp]

Hola, mi problema es que el convertidor se bloquea y se desbloquea a velocidad constante y 2000 rpm, o si acelerar para bajar de 4ta a 3ra se queda en un bucle de rpm que salta como si bajara y subiera de velocidad. ....

No DTC.

 

[azswiss]

Hola, mi problema es que el convertidor se bloquea y se desbloquea a velocidad constante y 2000 rpm, o si acelerar para bajar de 4ta a 3ra se queda en un bucle de rpm que salta como si bajara y subiera de velocidad

Translated: Hi, my problem is that the converter locks and unlocks at constant speed and 2000 rpm, or if accelerating down from 4th to 3rd it stays in a rpm loop that jumps as if it goes down and up in speed.

 

[Mooseman]

Hola, mi problema es que el convertidor se bloquea y se desbloquea a velocidad constante y 2000 rpm, o si acelerar para bajar de 4ta a 3ra se queda en un bucle de rpm que salta como si bajara y subiera de velocidad. ....

No DTC.

As per the rules, please post in English only. Google translations are fine .