Hello guys,

I have 2008 trailblazer with 5.3 vortec engine. Its shaking at downshift while criusing down. If I keep it at 3 instead of drive then there is no shaking. So for now I drive at 3rd untill 60km/h and then move to drive if I want to go faster. This way I don't get any vibration. Had the mounts checked and they seem ok. Also once I am in drive, I can cruise to whatever speed up and no shaking. But if I release gas then it starts shaking while slowing down. I would really appreciate any help in this.

 

just a wild guess but could be your tcc. You can try pulling out the pin for the lock up and see if the problem stays

 

What pin are you referring to, the wiring pin in the connector?

 

Hello Wael. Can you please identify which pin are you referring to. Is it inside the transmission. Also I don't have any issues in top speeds. The rpm stay low on highway, so I guess the TC is locking up as it should. Do you thing it can be related to TC failing.Some people pointed in this direction.

 

Hey yeah I’m talking about the pin in the connector it’s the easiest way to rule out the tcc imo. I’m assuming there’s no mechanical issues in the trans since you can hold gear 3 perfectly fine and only happens in drive kind of leaves it to mostly be an electrical Issue. I’ve seen the lockup solenoids have an intermittent shirt to ground cause the tcc to sporadically lockup causing vibrations so that’s why it’s usually the first thing I would look at also the tfp/tft could be giving wrong response back to the tcm but that would usually also cause problems going into 4th while in drive. Testing the tfp/tft could be a bit of a hassle though.

 

if I’m not mistaken it would be the pin T on the connector on the side of the trans.
im just giving it a wild guess it would be hard to find the root cause without fully testing everything but the tc is usually the first place to look at. Also I’m not completely sure of how the tcm controls the tc in drive but placing into manual gear 3 will have it mostly locked up a lot of the time so it could also be that the torque converter has a couple of fins sheared off and that could also cause vibrations. All I can honestly do is point you in the direction of the torque converter and the clutch in it

 

Wouldn't unplugging the plug on the brake light switch do the same thing to disable the TCC? I agree disabling the TCC would help with finding the vibration. A good scanner would also help.

 

Guys. Thanks for the help so far. Can you guide me how to disable tcc in the easiest way. As in
not opening the transmission pan.

 

Unplug the tcc wires on the brake light switch.

 

Hello. So I tried disconnecting the whole connector to the brake switch because it was one piece and no separate wires for tcc. Unfortunately it didn't made any difference. So what do you guys suggest. Does that mean the TC and the solenoid are ok. What should be the next step.
Thanks

 

Did the converter lock-up with the switch unplugged?

 

Good question. Cause if the converter locked with switch unplugged then the switch is likely the issues .

 

yeah it should but I just prefer to unplug the tcc wire from the socket on the side of the trans that way it’s 100% guaranteed that tcc isn’t getting locked up because there could be a short somewhere along the wire between the tcc and the pcm and this way just eliminates all the guess work. So I guess just personal preference

 

im not sure of the brake switch in your car but if it’s a normally closed switch then the problem won’t go away with unplugging the switch for some reason I have a feeling it could be a stuck solenoid you could test it by giving the trans 12v and grounding the tcc wire see if you can hear it engaging

heres a diagram for the socket on the side of your trans to help

 

Hello guys. I got stuck with work and forgot the issue for some time. But the issue is getting worst now. I have tried removing the brake switch and the issue is the same. The problem is there are no good mechanics where I live and I don't want to blindly throw money at it. I just need your help to know the exact problem and then I can sort for repairs. Would really appreciate any help. I also looked under the vehicles on the sides of trans and couldn't find the tcc solonoid connection

 

Have you checked the u-joints?

 

Hello,

Yes I did. Apparently there is no play. Also once I have the shudder and I move to neutral then the shudder goes away. So it leaves me thinking it's not the drive shaft.

 

Or guide me plz if I should do more to verify the u joints

 

Also the trans starts hammering after an hour of slow driving. I.e continuous gear changes. It does not happen on highways. It means the trans is over heating right? So something bad must be happening inside the trans. Sorry if I sound stupid but these are just my guesses