After upgrading to dual electric fans (30 amp load), my 75 amp alternator is struggling to keep up on a warm day at idle. The needle on the charge gauge slowly drops and the V12 idles progressively rougher. I have no need for the air pump in my part of the world, thus, I was contemplating on ditching it for a GM alternator (an AD244 to be exact if I can make it fit).

I know I'll have to swap out the pulley for one that'll work with a 13 mm V-belt. Does anyone know the smallest diameter I can safely go on one of these?

It's been awhile since the XJS Bible was written. So, there have been several new GM alternators since the ones Kirby wrote about. I realize the latest ones are PCM controlled. So, which GM alternator works best at idle in place of the air pump? Or should I consider something else?

 

PCM control will bring you nothing but misery since your
car was not designed with a PCM control circuit.

Instead, consider something a bit older that you have rebuilt
personally, or rebuilt locally at a good shop.

Maybe one from a loaded Cadillac, or taxi/police special Impala?

The Impalas had to run a lot of electrics, including the radio
and AC while idling.

 

Just a suggestion of you cannot find a suitable GM alt. A 115 amp alt from a later XJS will fit in the original space with just a drill out of the bracket arm holes to 10mm and the a 10mm fulcrum bolt to replace the smaller original. An ordinary V pulley will go on the 115 amp spindle just as on the old one and just needs a washer or two to align it with the crank pulley. This will fix the current problem.
Greg

 

IIRC the one on mine is from a 1989 V6 Camaro, 105 amp and the right clock positions to fit where the air pump was. Just had to change out the pulley.

 

Thanks for the suggestions. Greg, do you happen to know how many amps the 115 amp alternator can handle at idle? I have yet to find a performance curve for that alternator. In the right conditions at night, I could easily need 50 amps at idle.

 

Sorry, no idea. Mine can definitely handle anything you throw at it at 1,000 rpm, always showing 13.7 - 14.3 v, but at 5 to 600 the volts on my plug in digital thingy can go down to 12.8 (eg both electric fans on in traffic). All I can say is that compared with the original 75 amp alt the entire electrical system is transformed. To overcome the tickover volts point, when the main fan comes on I have wired the aircon-compressor-on extra air solenoid to come on with the main fan.


Greg

 

I wholeheartedly recommend the later model Bosch alternator. Take it to an alternator shop and swap the pulleys around. Some Lucas alternators have 17mm shafts allowing the pulleys to be swapped, others have 16 meaning you'll need a new pulley for the new alternator. The shop I took mine to put a new pulley on for free and spaced it out for me as well. Some Lucas brackets will require about 5mm of grinding down as well as the boring of the thru bolt hole. Adjust the tensioner and bolt the thing back in. Probably less work than the GM alt with the same result. I didn't even need a load dump module, it charges at low rpms just fine.

 

I believe my Bosch, the the one that came with car in 1991 (OEM) is rated at 140 amp's. Of course it probably cost $1 million to replace it and I don't know if it would work in the air pump spot.
Larry