Hey all
I;ve got a 89 TP magna, lately the battery charge light has being coming on if the revs drop below about 1500rpm for an extended period of time. Testing with a multimeter gives 14.25V across the battery terminals at idle but this drops to 12.25V with Highbeams, stereo, hazard lights and demister all on. Is it most likely worn alternator brushes causing the light to come on?

With all those accessories on, anything above 12v sounds right, maybe it's the voltage regulator that needs replacing. This is about a $60-70 fix. Either that, or you could recondition the alternator

you need a larger regulator and alternator -like a ford one better than 85 output
a marine size battery

or stop trying to run too many devices with only 12 volts

do the math and go from there

Yeah it's the charge light that I'm having problems with. According to the workshop manual i have the alternator should be under load when I'm testing it, thus having all the accessories on.

Your voltages sound okay...

Can you confirm whether the Charge lamp is lit or not when you get those readings?

Belt tight?

Cheers,
Mattt.

The belt is the correct tension and the voltage readings are with the charge light on.

Hmmm... Okay...

Quite possibly brushes then, and possibly not much to worry about - yet ;-)

Cheers,
Mattt.

Those voltages sound perfect. I'd say the problem isn't in the charging circuit, it's may be the indicator circuit that controls the warning light. However, the regulator may still be at fault, as the indicator light will start flashing if the sense voltage from the alternator varies by more than 0.3v.

You can get a new alternator from bursons for about $70 I think, and if that doesn't solve the problem, you may have to look at the indication circuit. The regulators fail very commonly, so even if it isn't the cause, they are a good thing to replace before they actually fail while you are driving! ;)