So, this one has me scratching my head. The original alternator in my 1998 XK8 Convertible (AJ26) is not charging the battery, so I took it out. The plug connector in the back was very corroded, so I thought I would bench test it the alternator, along with another alternator from a donor AJ27 motor.

I have a battery connected with the negative on the alternator housing, and the positive on the battery post. With the battery connected, the (L) Lamp pin in the connector socket is at ground, giving 12v between it and the battery post. That voltage should light a lamp when one is connected, as you'd expect. The G (Ground) pin is also at ground, as you'd expect.

Here's where it gets weird. On both alternators, when I connect a lamp across the L pin and the battery post, the L pin lifts to 12v, meaning that there is no voltage to light the lamp. The G pin is at around 6v, compared to the battery post.

What am I doing wrong here? Does the G pin need to be grounded to the battery negative? I tried connecting the lamp across the L and G pins, but they both stay at ground when I do that. It's almost like the 6v between the raised L and G should be what's driving the lamp?

I guess it's possible that both alternators are faulty in exactly the same way, but I rather doubt it...

 

I do not remember which pin is what, but in the car, one is connected to a 5A fuse and has +12V with the ignition on, and the other is connected to the instrumentpanel for the alternator lamp

 

Th live or B goes to battery positive Battery negative to the casing. A 12v 5w bulb goes from the L for lamp to the battery positive .
the light should go out when it starts charging

 

it doesn’t work like an old eurotrash alternator. voltage regulator has permanent sense line, when there’s a field the lamp line gets powered and the cluster receives it. cluster shuts off the light

 

Yep - I figured out that what I thought was a scratch on the pin out label next to the 'G' was an 'I' - so 'IG' for ignition. Doh!

Thanks for confirming!

So, for those following along at home, this is the answer:

- Battery negative to alternator casing
- Battery positive to the output post
- Lamp between the output post and the 'L' pin in the plug
- Battery positive to the 'IG' pin in the plug

I'll update with what I find out - zero, one, or two working alternators.

 

And the answer is.......

One working alternator! The oil-encrusted one from the project car is dead (probably regulator failure - no lamp, and no volts). The salt-encrusted one from the donor motor puts out a healthy and steady 14.5v and extinguishes the lamp - huzzah!

That's $200+ dollars I can spend on something else...