Original and new alternator put out 14.5 volts on bench but only 11 volts in running car and then the battery dies. I cleaned and tightened all connections. I also tried a new battery. Battery does not discharge with the ignition off. Any ideas are welcome!

Bill
1960 XK150 FHC
95 XJ6
98 XK8 cv

Submitted by wilmar6@juno.com on Tue, 06/27/2006 - 00:06

Thanks,
The battery condition and charge indicator light works properly. The battery voltage reads 12 volts with key off but actually drops when running with lights and AC on. It reads erratic in that it briefly hits 13 volts, then for no reason it drops to 11.5 volts and stays there.

Bill
1960 XK150 FHC
95 XJ6
98 XK8 cv

Submitted by mfrank@westnet.com on Mon, 06/26/2006 - 00:20

First of all, I doubt your alternator is putting out 11 volts: what you're seeing is battery voltage, and it's near exhaustion.

The alternators on these cars usually have a trio, which is an internal circuit that powers the field coil. Trios require a bootstrap voltage to induce the alternator to start charging. This voltage is usually provided by using the trio as a ground for the dashboard indicator light. The lead may be labeled "D+" or perhaps "I" on the alternator end. The usual problem is that the bulb in your dashboard charge indicator is bad. It sounds improbable, if not silly, that a bad charge indicator lamp could cause the alternator not to charge, but it's a very common problem. If this proves to be the case, it's a good idea to wire a 330 ohm resistor in parallel with the new light bulb. That way, if the bulb fails again, there is an alternate path to provide bootstrap voltage to the alternator.