84 Charging system

[jimbo79]

Volt meter does not read correct voltage. Alternator light dimly lit at all RPM's. Battery drains overnight. Stop lamp and brake fluid lights dimly lit. However, Headlights bright while under way. Any suggestions?

[calebz]

Classic symptoms.

 

New alt time.

[Cougar]

The alternator may be causing both problems. There may be some shorted diodes in the alternator causing the battery to drain. When you replace the alternator hopefully that will cure the battery drain also.

[wolfdog]

The alternator may be causing both problems. There may be some shorted diodes in the alternator causing the battery to drain. When you replace the alternator hopefully that will cure the battery drain also.

 

Strangely enough, developed the exact same symptoms within days of installing a new alternator on the convertible, though not having the battery draining issue. Looks like that one will have to go back!

 

--wd

[robm]

If the diodes are open, instead of shorted, I believe you will get this lights on effect, without the battery draining. And a shorted diode can burn itself open with a jumpstart, so your symptoms can change fast.

 

Check for AC voltage at the battery terminals with the motor running at 1500-2000 RPM. There should be almost nothing. When mine blew last winter, it was putting out about 30 VAC, as well as 14.7 VDC, so it actually worked OK for a couple of days. This was AFTER the diode blew, before that, it drained the battery flat overnight.

 

The new alternator put out just a few millivolts of AC, and 14.7 VDC. Full story:

 

Charging System Anomalies!