Mainly noticeable on low beam. Headlights dim at idle and at speed, and at any RPM. Appreciate any ideas what would cause this ? I’m realy hoping it’s not the alt. Thanks

 

Sean. Im assuming you are not running an old odyssey race battery and RS pulley? If so could be the battery. Otherwise almost certainly the aternator Im afraid...

M

 

Thanks Macca, I'm dreading you're spot on. Had my fill rebuilding these alternators on my SC. Think I'll leave this one for experts.

 

...or voltage regulator.

Measuring the battery voltage during the conditions you mention, would tell you that.

Using a cigarette lighter plug connected to a digital multimeter is the easy way, or wire in a permanently install voltmeter directly to the battery as I did...

 

"Otherwise almost certainly the aternator Im afraid..."

Guessing results in lost time and money!

The Bosch 993 alternators are very reliable and have basically two failure
modes; worn regulator brushes and/or slip rings, and/or failed bearings.
Worn brushes/slip rings usually result in intermittent charging. Obviously
an incorrectly jumped battery will damage the alternator as will a bad battery.

One really needs to test the charging system using a good voltmeter before
concluding that the alternator is the problem source. The possibilities for
a low charging voltage are:

1. a battery drawing too much charging current,
2. voltage drops from the alternator to the battery, e.g. bad connections
3. excessive draw from non-standard accessories, e.g. high intensity lights
4. And not so common, could be a bad voltage regulator (very rare).

 

On a local owner's '83 Carrera that I was working on the other day, a ground cable coming off a casing stud on the alternator looked like it needed replacement.

I took it in to work and measured the resistance on this roughly 12 inch long ground cable. It measured 0.12 Ω using an upper end 4 wire meter. The replacement cable I made measured 0.0007 Ω.

The importance of low resistance connections where higher currents are flowing!

I wonder why they even use a ground cable; maybe as a back-up. The alternator is attached to the fan housing which is attached to the engine block.

 

Thanks Loren.
On further inspection, it’s as though the headlights “blink’ dim for only a half a sec, an amps drop when the a/c comp’ cycles on and off its thermostat setting. Quite distracting.

All good feedback, thanks everybody. I’ll check the voltage .....