Ive looked not only on this forum, but also all over the net fir a straight answer and can not find one, so here we go.

Lets make a long story very short. My stock tranny blew. My monte is an 02 LS. I then replaced it with a used transmission from a 3800 grand prix. Making that tranny a 4t65e-hd. The car still shifts very rough but everything bolted up.I bought it on craigslist and The guy that I bought the transmission from was kind of shady and lives 2 hours away (he has no reason to lie to me now being that ill never see him again, dont know where he lives because we met at a gas station and did not sell it with any warranty. But still swears up and fown that this trans is good), so I'm not sure if its just that the transmission is bad or if its just not matching up with the pcm.

Can ANYONE tell me if they have done this swap before and tell me what you did? I need to know if I should rebuild the 4t65e-hd, get my pcm reprogramed, or start looking for 4t65e. Thanks for the help!!!

 

First the 4t65e-HD is on the L67 (supercharged 3800). The 3800 non-supercharged has a 4t65e (just like the 3400). So if the trans you bought came from a non-supercharged 3800 it's NOT an HD trans.
Also, if my memory serves, if you have a 4t65e-hd, you would also need to replace one of the axle shafts (as one side requires a shorter shaft as a result of a slight difference in the transmission). To my knowledge all that makes the trans HD is a different differential that is supposedly more durable (but in practice, it's not the differential that fails in the transmissions).

But, in theory, yes, mechanically you can bolt up a 4t65e or 4t65e-hd to your 3400. The problem you are going to run into is what gearing is in the transmission. I don't believe all 4t65e and 4t65e-hd transmissions are geared the same. That being said, the PCM might not be properly adjusted for that transmission (so it's shift times may be way off because of the gears in it). Or it could actually be bad.

My advice, find a reputable transmission shop that you can trust and talk to them. First they can probably look up if that transmission should have worked or not. If it shouldn't work properly, they can probably tell you why. And they can probably diagnose you issue you are having as if the trans is bad or incorrect for your application.

 

• How Much Do You Know About The GM 4T65E Transmission ...
  
  <cite style="color: rgb(0, 153, 51); font-size: 14px !important; font-style: normal;">etereman.com/.../a-history-lesson-on-the-gm-4t65e-transmission-and-dis...</cite>‎
  
  
  
  Jan 1, 2013 – Let's take a quick look at the history of the GM 4T65E transmission and... The transmission was equipped with a 2.92 first gear ratio and a ... The 4T65E was also available in a heavy duty version, referred to as the 4T65E-HD.
  
  
• PATC 4T65E Performance Transmission, 4T65E and 4T45E Parts
  
  <cite style="color: rgb(0, 153, 51); font-size: 14px !important; font-style: normal;">www.transmissioncenter.net/alto_2.htm</cite>‎
  
  
  
  This is our level two 1997-2005 performance 4T65E-HD transmission for L67 motors. ...Tested by GM with an 1100 horse power motor. Fits the direct .... Upon rebuilding the 4T60 transmissions the final drive sun gear is often found damaged.
  
  

 

Agree. I know, from working with Will at Overkill, that the 3400 and 3800 transmissions have different gear rations. Will informed me that a tune would need to be done in order to make the 3800 transmission shift correctly in a 3400 car. He said that the shift points are different which could be causing your rough shifting (I think. I am no transmission mechanic by any means).

 

Gearing is off I almost guarantee it. A HD trans is geared at 2.93 a non HD trans is geared at 3.29. Idk what gears are in a 3400. Maniac you are correct if you install a HD trans into a non HD vehicle the passenger side axle needs replaced because the HD axle is shorter than a non HD axle.

My best guess is your pcm needs tuned for the trans. But if you have put many miles on it this way you could of trashed the trans. Wrong gearing will make major heat issues and cook the trans fast.

 

Well when I put this trans in, I did not have to replace the passenger cv shaft. So does that mean the one I have now is not an hd? And if I'm understanding you correctly, even though if its not an hd, the gearing could still be off because its from a 3800?

 

if you did not replace the cv shaft it is a non HD trans. but yes the gearing is different from the 3400 to the 3800. after some digging around on here i found conformation of this.

https://montecarloforum.com/forum/en...uestion-18670/

^^^click^^^

 

here is some more information on how to get the correct transmission.

https://montecarloforum.com/forum/en...2/4t65e-43421/
^^^click^^^

 

I took the car to AAMCO transmissions. and like you guys said, the gear ratio is defiantly off. he said to take it to the dealership, and I called the dealership and he said that he doesn't see why they wouldn't be able to do it but even if they cant its still gonna be about $150. Does anybody for an absolute fact that they will be able to program it? I really don't want to waste even more money on this car.

 

Do not go to aamco!!!!!!!