ALT light troubleshooting. FIXED, Specifically, what makes it come on? Options

[malcolm2]

I noticed the last time i parked that the red "G" light had come on slightly. Got in yesterday and it was on bright and the volt meter was at 10.5. Revving the engine did not help either.

I checked the belt tension and decided to tighten it.

Took a 20 minute drive to Nashville and all was well. At the last stop i saw the dim G light again. Then bright for a while. Then out.

What is the prognosis? I have about 20k miles on the alt wire harness and gel battery. Maybe 10k on the rebuilt alt.

[jim_hoyland]

Another good source is Ratwell's:

http://www.ratwell.com/technical/ChargingSystem.html

[malcolm2]

Another good source is Ratwell's:

http://www.ratwell.com/technical/ChargingSystem.html

WOW, that is great. I had forgotten that Ratwell had so much good info. (IMG:style_emoticons/default/piratenanner.gif)

I'll get to the VR tests this afternoon.

Clark

[malcolm2]

Another good source is Ratwell's:

http://www.ratwell.com/technical/ChargingSystem.html

WOW, that is great. I had forgotten that Ratwell had so much good info. (IMG:style_emoticons/default/piratenanner.gif)

I'll get to the VR tests this afternoon.

Clark

Has anyone done the RATWELL test? I got some crazy numbers. Plus I seem to have an intermittent problem. Once the engine is warm the ALT light goes out at idle. But if I shut it down then restart the warm engine, I get the light on at idle....



RATWELL says to check the voltage at the ALT harness

I took the plastic cover off of the board behind the driver and was able to get my VOM probe into the back of the ALT harness plug as follows.... the black probe was alligator clipped to the battery neg.


Red wire (D+) with the ALT light on and engine idling. I need 14v and I get 1.8v
Grn wire (DF) with ALT light on and engine idling. I need 5.0v and I get 1.8

Red wire again, with the ALT light on and engine reving, I need 14v and I get 1.7v
Grn wire with ALT light on and engine reving, I need 5v and it goes to MAX. If I slowly rev the engine, it creeps past 18volts to 35 volts then the meter goes to --1-----.

Something ain't right, but what is it?

BTW i took emory cloth to the spades for the VR and the ALT harness and cleaned them up.

[Spoke]

I took the plastic cover off of the board behind the driver and was able to get my VOM probe into the back of the ALT harness plug as follows.... the black probe was alligator clipped to the battery neg.


Red wire (D+) with the ALT light on and engine idling. I need 14v and I get 1.8v
Grn wire (DF) with ALT light on and engine idling. I need 5.0v and I get 1.8

Red wire again, with the ALT light on and engine reving, I need 14v and I get 1.7v
Grn wire with ALT light on and engine reving, I need 5v and it goes to MAX. If I slowly rev the engine, it creeps past 18volts to 35 volts then the meter goes to --1-----.

Something ain't right, but what is it?

BTW i took emory cloth to the spades for the VR and the ALT harness and cleaned them up.

D+ voltage is generated by the alternator and should equal the battery voltage.
DF voltage drives the magnet in the rotor which in turn generates voltage to D+ and charges the car battery.

If the VR is bad, it will not drive DF properly and the alternator may produce too much or too little voltage.

If the alternator is bad, it may not drive V+ (which drives the VR) and the alternator will not charge the battery.

You can test the alternator by removing the VR and shorting D+ to DF. This is equivalent to the VR providing maximum drive current to the rotor magnet. With the car running, the alternator should generate more than 16V. If the alternator doesn't provide >16V and the GEN light stays on, likely the alternator needs replaced.