03 transmission in 01

should bolt right in as long as trans cooler lines are the same.im putting an 04 in my 02 next week.
 
Yeah...that was pretty vague.

Taking a 2003 Allison and putting it into a 2001 also with the Allison.

And that was the first hurtle we've found. Cooler fitting's in the 2001 are smaller, not sure about the lines yet as our 2003 has no fittings. Not looking like they are pipe either.
 
For some reason, something sticks in my mind that says there is a difference that causes and issue, but I damn sure don't remember what it is.
 
I copied this from another forum. Not my words, but sums up the differences.

01-02 trucks have a unique bellhousing and cooler fittings.

03-05 have the same bellhousing.

03-04 have unique cooler fittings that will not work with 01-02 Allisons, or 04.5+ allisons

You CAN use an 03-04 bellhousing in an LLY truck IF you swap the cooler fittings on the bellhousing.

01-02 have a different thread size on the belhousing for the cooler fittings. You can not use 03-04 or 04.5-05 fittings in an 01-02 bellhousing

LLY cooler fitings WILL NOT FIT into an 01-02 bellhousing. 03-04 LB7 allisons have the same bellhousing as the LLY Allisons, but different cooler fittings.

01-03 allisons dont have the G-solenoid in the valve body
04+ allisons have the G-solenoid. There is an exception...All LLY's have the G-solenoid, SOME 2004 LB7's had the G-solenoid (but it didnt do anything). Probably just an overlap in production; Allison started making the LLY valvebodies with the G-solenoid a bit early and decided to throw some extras into the last of the LB7's.

You can use an LLY valvebody (with the G-solenoid) in any 01-04 LB7 allison without problems. You can use an 01-04 LB7 valvebody in an LLY, but you might get a check engine light/set a code unless you reflash the TCM with an 01-03 Operating system (that does not ahve the G-solenoid enabled)
 
01-early 03 used a unique cooler fitting, 03 had the newer style cooler fittings used up to at least 2010, 04-05 went to a single plug NSBU and added in a G solenoid to reduce line pressure at idle and some other situations(although this solenoid was not enabled until 04.5 with the LLY). 01-early 03 trannies work ONLY with the 01-early 03 cooler lines. 03-05 trannies can be run in the earlier trucks with the use of special adapter fittings from GM as GM no longer offers the earlier unique bellhousing. If it is an LLY you must either use an 04-05 with a G solenoid, or swap in a 04-05 internal harness and plug in a G solenoid to it and leave it I nthe pan to keep it from coding. You can also swap in an 06-early 07 6 speed trans by swapping shift shafts, removing the IMS, swapping valve bodies, swapping internal wiring harness's, and installing an external NSBU. The 07.5-10 can also be used, but you will have to swap bellhousings and all the rest listed for the 6 speeds.
 
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