My son's alternator light is on in the '90 240, but when I check the voltage across the battery it reads 12.5 volts with the engine off and 14 volts with the engine on. What do I need to check, it doesn't appear to be the voltage regulator or brushes?

That's the same conclusion that I eventually came to, but wasn't sure it was correct. I told him to have it checked ASAP. Thanks for the help.
Marlin

The light comes on because the small red wire at the back of the alternator is getting grounded somewhere. It could be grounded anywhere between the back of the cluster and the back of the alternator or it could be receiving it's ground from the alternator which would mean that the alternator is bad. A voltage check is not the last word in diagnosing a bad alt, it is possible to have voltage but no amperage. You need to see what the output of the alternator is in amps and find out why that red wire is going to ground somewhere.

Mark

Yeah I had a very similar problem with my 240. My lights would go away if i revved the car but return a short while later. My alternator turned out to be missing almost all of it's amps but it was still spitting out good voltage. I replaced the unit and everything has run fine since. Most auto stores will give you a free amp check on your car. Might also want to check your wiring to make sure its not touching anything. I know on my 93 240 that it's really cramped in that area.

I had a similar problem and turned out to be my ignition switch going bad.With the car running i could wiggle the key back and forth just slightly and alt light would go on and off as I did so.A friend replaced ignition switch and no more problems.Just in case all else checks out ok.good luck.

I had a similar problem and turned out to be my ignition switch going bad.With the car running i could wiggle the key back and forth just slightly and alt light would go on and off as I did so.A friend replaced ignition switch and no more problems.Just in case all else checks out ok.good luck.