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Gmc envoy no overdrive

I have a 2004 GMC envoy. The code was trans shift solenoid b po756. Replaced. Wouldn’t shift into OD. Replaced the trans and the new one still wont shift into OD. Any ideas?

You may not in fact have an overdrive problem. A faulty torque converter lockup is more likely. I’m pretty sure your transmission has four speeds, of which 4th is overdrive. If it’s not going into 4th gear that’s a different problem.

17 yr old 4l60e. There’s ur problem.
I’ve been inside them. But not yours

Torque converter lockup? Does this mean the Torque converter itself doesn’t lock up or would this be a solenoid? I cant understand why both transmissions are doing the same thing. A torque converter came with the replacement transmission. I didn’t use the old one.

The replacement transmission is a 2007.

If the torque converter is at fault, my money would be on the lockup solenoid since the same thing happened with two different torque converters.

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Wondering if something is up with the shifter that it thinks it is in D3 or whatever where overdrive does not kick in.

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I am wondering if the TCM for the 2007 transmission can communicate properly with the ECM for the 2004 engine. Or, perhaps the TCM for the 2007 trans wasn’t installed along with the “new” transmission. In any event, this could be a case of bad “communication” between the TCM and the ECM.

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Your wire harness is damaged.

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P0756 is a performance fault, it indicates a mechanical failure.

Below is a service bulletin that explains fault P0756;

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Thanks for all the input. I’m off today and I’m going to use all this info to try and figure out what is going on. I had a local shop take a look at it and the tech told me the same thing you guys have. It’s just going to be a process of elimination. I’ll post an update as soon as I can.

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UPDATE: I inspected everything that has been recommended short of taking it to a shop. Finally shifts into overdrive but kicks down into 3rd when going uphill. When the vehicle levels out and I give it gas it kicks down into 3rd. Real sluggish as well when in overdrive. The vehicle doesn’t want to get faster than 60-65. I’m going to try and upload the video I took. It probably won’t help much but I thought it may give better insight.

Tried to upload a video and it declined.

A brain reset and relearning might be in order, disconnecting the battery for 30 seconds, though this was probably done in your recent repairs. Make sure all ac and fans are off, battery disconnects seem to lead to actuator failures.

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UPDATE: So after many days of testing all the theories turns out it was a Catalytic converter problem. Someone in another shop talk site suggested checking the exhaust pressure. It was low. Pulled the cat out and sure enough it was stopped up. Allowing just enough exhaust to pass through. I al

                                                          sways believed that if the cat is stopped up as bad as it was you:d get codes on

Wow. That sure came outa left field. Thanks for the update.

MAybe it is a software problem? because you changed it

That has been several people’s reaction. The guy that suggested the exhaust pressure said that after pulling new o2 sensors in the computer was still reacting to the cat converter outlet o2 sensor. It basically didn’t know what to do because all the other sensors where telling it the car was fine but it wasn’t getting the proper outlet pressure. When it was getting the proper pressure ok now shift. Then the pressure dropped so it would down shift to gain speed and pressure. Hope that makes sense.

Well that’s a new one. I wonder how often that happens? Plugged cats are pretty rare, no?

Just for the sake of discussion though, I have had two failures of the 4th gear. One was a bad wire harness that the rebuild shop swore they replaced (which they didn’t). Cost me $700 for another shop to replace it and a week’s wait for GM to ship it.

The other time was a tech putting 12 volts direct to the solenoid and blowing it out looking for the cause of a hesitation on hard throttle. I had told them to leave the solenoid alone and that wasn’t the problem since I had just had the trans overhauled but he did it anyway before giving up on the problem. The next day the cracked crank sensor came apart on the way to work costing me a tow and a day off. So the kid blew my solenoid (the trans shop agreed to just replace it without charging me again) and missed the large crack in the crank sensor.

So yeah, if I had a problem with going into 4th gear, I suspect the harness or solenoid. Also engine temp had to be up to allow the shift. After that I’d worry about hard parts but would be in a shop anyway. I feel like I’ve paid the tuition for some kids though learning on my cars.