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post #1 of 3 (permalink) Old 07-21-2016, 07:02 PM Thread Starter
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2008 Chevy Cobalt Transmission Swap HELP!

Hello I'm new to the forum and I am in need of help. My fiance and I recently bought a used 2008 Cobalt LS and I am having transmission problems. Long story but I'll try to summarize, I took it to a mechanic and said the tranny will have to be rebuilt or replaced, he said he would have to send it out and gave an estimate of around 2500. Right now we can't afford that as we are both going to school in the fall, I have a friend who was a mechanic and suggested to just swap it with a used transmission as it will be much cheaper. I found one today that belonged to a 2010 Chevy HHR for a good price and bought it since I was told it will work for my Cobalt. I went home and did some research and while I did find some threads and people claiming it will be compatible, I also found a closed post on here with someone doing basically the same swap albeit out of choice, but was told he would have more things to do electronic wise as the gear ratios were different for each transmission. My question is will doing the plain swap work? and if so, will the different ratios have a negative effect on the car or ruin it farther? also will I HAVE to reprogram the TCM or can I just leave it as is? Thank you all for your time!
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post #2 of 3 (permalink) Old 07-21-2016, 07:36 PM
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Well, the gear ratios are the same in the trans, but the HHR has a 3.91:1 final drive and the Cobalt has a 3.63:1 - so it'll just be like going to shorter gears in a muscle car - you're going to get more pep down low at the expense of top speed as well as cruising RPM.

This means you're not going to see as good of fuel economy on the highway as the engine will be spinning faster. But it will be faster off the line - so it all depends on what you are looking for.

The ECM will need to be recalibrated as changing the final drive will change the speedo calibration.

I'd recommend just picking up a used Cobalt trans, as it will be far easier - better yet if it's the same year as yours.

Out of curiosity, what symptoms is it currently exhibiting?

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post #3 of 3 (permalink) Old 07-21-2016, 08:40 PM Thread Starter
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Thanks for the reply! Yea i figured it would be easier to just get the Cobalt one, I only purchased the HHR one as it was newer so I was expecting it to be in better shape, they did have cobalt ones but were older and not as good looking. As far as performance i don't really have anything in mind i just want the car to run well as intended haha. By chance do you know where and how much it should cost to have the ECM re-calibrated?

For symptoms, shortly after we bought it i noticed a SERIOUS lack of power on first gear, we would be pressing on the pedal but all it would do is make the RPMs go up (to about 4-4500) till it finally changed gear. It wouldn't kick, make noise or anything it was just weird. After a while i couldn't ignore it anymore so i took to my dad as he does a lot of home repairs and is more knowledgeable than i am. He sugested i ground a wire with a nut and bolt to the TCM as apparently that is a common problem with both Cobalts and Malibus and it worked! for about a week... after, my fiance called me and said the car was struggling on the highway i asked her what RPMs she was running, she said 6... SO when i got home from work i drove it around, it felt normal, did the first change then when i got to 40 MPH it felt like something was pulling it back, also the rpms were going up and down between 2500 and 3000 and would not shift, i turned around as i didnt want to get to far and risk being stranded and it did the same thing when it hit 40. I finally decided to take it to a mechanic to see what could be done assuming it would just be replacing the TCM, 2 days later he tells me the tranny needs to be rebuilt.

Again thanks for your help!

Last edited by MannyFallout; 07-21-2016 at 09:00 PM.
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