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Roscommon and the PS Alternator


Well-known member
When you do the 12 volt Roscommon Conversion ( I am not advocating doing so, just passing along info) the instructions tell you to use the ground from the DS alternator to ground the PS alternator.

7. Add a ground wire from the ground terminal
of the passenger’s side alternator to the
engine. The wire removed in Step 12 can
be used.
12. Remove the ground wire that connects the
driver’s side alternator to the engine.
The Wiki tells you to use the DL7157 on the DS only as it does not need the isolated ground. This is true in the 24 volt configuration, but you may use the DL7157 on the PS after the Roscommon Conversion when running only one alternator. This alt is grounded to the case and will excite itself just fine. I did leave the ground wire in case I ever want to use a CUCV alt again.

Again, the PS alt can use the DL7157 and not be grounded only when running one alt after the 12 volt Roscommon Conversion.

Charging System
Dual 100 amp alternators. Batteries are wired in series to make the 24V. Very strong charging system that will handle any load you can toss at it. Changing over to 12V is NOT recommended.

The passenger side alternator MUST HAVE an isolated ground. The driver side does not. An Autozone part number for the DRIVER side alternator is DL7157 (also found in a 1975 Cady with a V8 engine). This alternator is 78 amps and works just fine. You MUST have an Isolated Ground Alternator for the passenger side. Ebay, www.saturnsurplus.com, and many other places stock this alternator.

NAPA has a remanufactured Isolated Ground Alternator, part number WIL 90014277. Wilson Alternator rebuilds them. Cost ~$220. Just make sure you test the alternator before you install it. They have had some issues with the rebuilds of NOT being isolated grounds.

27si Type 100 Series
View attachment 442428

Links to part numbers from retailers (these are 100 amp alternators):



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