My name is Pete and I'm new to the forum!!

I have an 1986 XJS with a new alternator, new battery, and car charges fine running at idle wiith the AC off (13.5 or better) as soon as I turn the AC on volts running at battery drop to 12.5 or less and if i add lights even worse. As soon as i increase the idle it jumps back up. Rackin my brain need HELP!!

It doesn't idle up when AC is engaged is that normal on these?

HELP!!!!!!!!

 

Pete, welcome aboard.

NO IDEA where you are, so some quess work here, due to market availabilty of some items.

That NEW alternator is the issue in MY opinion. I have had this many, many times. Some of them DO NOT have the "idle charge" capacity that others do.

Down here we use the 110amp BOSCH unit from the XJ40 6 cyl Jags, and run an extra +ve cable from the alternator +ve terminal direct TO the firewall battery connector on the RH side (near our brake booster mounting), and then an extra EARTH cable from the alternator casing TO the chassis. This will sort heaps of stuff regardless.

Please also bear in mind that ALL aternators have very little idle charge OOMPH, and then add the lousy wiring into the equation, you are at where you are now.

The V12 idles very slowly by any standards, which helps even less.

The a/c clutch is usually a 7amp draw.
The internal fans (cabin) are about 5 amps +/- on LO.
The LH radiator fan is about 8 amps.
There you have 20 amps you just dumped on the system.

Mine runs that alternator I mentioned (10 years + now), and twin thermo fans, and idle charge is 13.9v unloaded, and 13.1v with a/c, 1 thermo fan, headlamps ON. Rises to 13.8 at 1200 RPM.

 

Thanks for your quick Grant
What year xj40 works? and do they bolt right up?

 

Here ya go.

V12 Alternator Upgrade.doc