Trouble Installing 2008 Auto Transmission in 2005 Auto (4.0L V6)
 

Hoping someone can shed some light on why I'm running into so much trouble installing a 2008 auto transmission in my wife's 2005 auto Mustang 4.0L V6.

The back story is that the transmission in my wife's 2005 Mustang began going out recently (transmission shop and Ford dealership stated that the clutch packs were likely the cause). Because we don't plan on keeping the car for many more years, I decided to just pull a transmission from a wrecked 2008 Mustang 4.0L V6. Based on what I had read online, the 2008 transmission was the same model (5R55S) and would work as a replacement. Upon trying to install, I discovered that the torque converter that came with the 2008 transmission was unable to mate to the 2005 flex plate (the torque converter studs we're about 1/2" further apart than the original). So I ordered a new torque converter from the dealership and it matched the original. Tried installing the 2008 transmission with the new torque converter today. This time, the engine would not spin once the bell housing was tightened down, despite me ensuring that the torque converter was seated all the way prior to install. Tool the transmission back out and found that the new torque converter is now stuck on the transmission.

So, was I wrong in assuming that the 2008 transmission would be compatible? Or am I missing something else? I appreciate any light you guys can shed!

8-Lug torque converter: http://www.moveras.com/TechnicalBull..._5r55_001.html
Needs a tool.

When you look up the 5R55S to buy one, they say 2005 to 2010 Mustang.

I can't find anything on why a 2008 would not work in a 2005, unless the 2008 has
the 8-lug that needs the tool...

Hmm, that's interesting about the 8 lug. The one that had come with the 2008 trans looked pretty standard so I don't think that's causing the issue.

The replacement torque converter that was sold to me by the Ford dealership for a 2005 trans has part number 4R3Z-7902-AARM. When I look that up at the following site, it claims that it only fits 2005-2006 models .
https://www.fordpartsgiant.com/parts/ford-converter-asy_4r3z-7902-aarm.html

However, if you look up a torque converter on a parts site like Rock Auto or Advanced Auto, the exact same part number is listed for a 2005 and 2008 model. The more I research about this the more confused I become :(

There is a difference but I don't know what the changes are. From what you are saying I guess you are running into them. I replaced my 07 and it came down to date of manufacture. And my 07 fell into two different 4.0 transmissions possibilities for the model year. I know this when I looked it up in www.Car-Part.com. In the search it you had to choose between manufacture date.
In your case I would have went with the 08 flex plate.There maybe an offset to the plate with different converters.
Did a quick search and 05-06 with a cut off date in 06

artsvettes73, thank you so much for that info and the link to car-parts.com. I've had such a hard time finding a source for interchangeable parts information, especially for these transmissions. Everytime I'd search for "Mustang transmission swap" or similar, the large majority of the results were related to auto-to-manual conversions rather than auto-to-auto swaps .

Now that I know for sure that the 2008 transmission is not the same, my plan is to return the 2008 transmission to the junk yard I pulled it at and pull a 2005 or 2006 transmission (thankfully I bought a 90 day warranty that allows returns). I thought about trying a 2008 flex plate as you suggested but I'm afraid that even if that allowed me to get the 2008 transmission successfully installed, in the end I'd end up with some persistent diagnostic code.

Thanks so much to both of you for your replies! I really appreciate the help!

I am surprised the salvage yard didn't tell you this, they should already have this information. Interchange has gotten so bad it comes down to a vin number. Hope it all works out. Let us know.
Back in the day the Hollander books were the bible of interchange. It would tell you what fit what and tell you what you would have to change if it didn't. The recyclers would have these books, but today it all on computer programs

Did you ever get an answer to your question ? I'm wanting to put an 05' transmission in my 08' Mustang but not sure that they wre compatible?

If you read the post the Op. returned the transmission.