Hi, New member here! I have a 99 sl2 wagon and the automatic transmission is slowly dying on me … but luckily I have a 93 sl1 parts car with a manual transmission… I was wanting to see if anyone had any information on what the biggest hurdles would be… what parts are different between the manual and auto? What parts will need to be changed over? Will the PCM be mad at me? Any help would be appreciated thanks!

 

I believe 99 was the first year for the S car having a body control module (BCM) which controls most of the non-engine related control duties. This means that the transmission is probably talking via a transmission control module TCM on your 99 sw2 automatic to the BCM in the 99.. this transmission is designed for the dual overhead cam engine in terms of gear ratios, etc.

The 1993 SL1 parts car with the manual has no BCM. The transmission is monitored by the PCM directly. Therefore, the interface for connection to BCM is not present
This transmission, the MP2, is designed for the single overhead cam tension in terms of gear ratios etc.

Your 99 would be continuously throwing errors looking for the automatic and the sensors it gets feedback from.

Conceptually, you could reflash the 99 BCM and the PCM to the programs for a 1999 sw1 with manual transmission, but I suspect the BCM would still be throwing errors since it cannot communicate appropriately with the 93 transmission.

Additionally, the reflash may be costly because most dealers will not flash an existing BCM. They make you buy one from them that is blank and program it. If that's not enough of a pain, some GM dealers will refuse to flash anything onto the modules that was not originally there at delivery. Meaning they won't put the manual firmware on your automatic PCM and BCM. If those are not big enough potential hurdles, many GM dealerships will claim they do not have access to the source files to flash the Saturn PCM and BCM. This is false and true. They will not have them in house but they can be shipped from GM upon request.

Also, You would end up with an eternal service engine soon light which would be a problem if your state has annual inspections.

It is much simpler to put a 99 through 2002 dohc version transmission into the vehicle. No flashing, no SES.

Just be careful what you buy. Most of the transmissions unless they have been rebuilt will have the same general level of subpar behavior as the one you are trying to replace. Just because it was running and shifting on its way into the wrecking yard does not mean it was doing so very well.

Please double check on whether your 99sw 2 does in fact have a BCM. Not all of the S cars got them in the same year. Some in 99, some in 2000. Even if it does not have a BCM, you'd still have to get the PCM flashed to keep it from looking for the automatic transmission and throwing errors.