Possible short in starting circuit? Red charging light stays on

[franjo_66]

Yesterday, I took my 230SL out for a drive, and after 15mins the red charging light came on and I lost all power. Car stalled and I called roadside assistance.

I suspected the alternator/regulator (even though both are almost new, and battery is near new).
We couldn't get the car started initiallly. Battery had a full charge, and I could tutn on the lights, radio, etc
But the fuel pump, starter etc wouldn't engage when attempting to start.

Started fiddling with the fuses, and when we jiggled fuses 8 & 9, voila, managed to start the car.
Checked the alternator/regulator and battery was charging at 14.4V even though red charging light remained dimly on (irrespective of revs).
Switched on headlights, etc and alternator still charging correctly.

So I managed to at least drive home.
Took out all the fuses, got some wet & dry sandapaper and cleaned up all the connectors and then sprayed them with electrical connector spray.

Car starts fine, but the charging light still stays on (much dimmer than the bright redwhen you first start the car).

So if my battery is charging fine, am I looking at wiring/connection issues ?  Has anyone else had similar symptoms?
Given the fact that I had to jiggle the fuses to get the car able to start, leads me to suspect that there is a short of some other issue in the circuit
Any suggestions appreciated !

[Jonny B]

I had the same issue, the dimly illuminated charging light, on my 250 SL. I put it in two new alternators and a new voltage regulator (solid state replacement). Also a new battery, which may have been unrelated as the battery it replaced was an older one.

In any case, what we found were two things. One was an intermittent or poor connection in the wiring harness at the alternator, by jiggling some of the wires discovered an issue, and replaced and cleaned the connections.

The other item on the early cars is a grounding strap from the alternator to the engine block. My mechanic was not convinced this was an issue as he thought the mounting post was good enough. He acceded and installed the small strap (part A 615 150 02 32). Photo attached.

I highly recommend an annual ritual of rotating each of the fuses in the fuse box to ensure a good contact.

[ctaylor738]

One function of the light is to "excite" the alternator so that it will charge when turning,  My understanding of this is that the warning light gets a positive feed from the ignition circuit, and the alternator provides ground when it isn't operating.  Once the engine starts and the "excited" alternator starts charging, it ceases to provide ground, and the light goes out.  So, from intuition and experience, if you are still getting a dim light, the alternator is providing a marginal ground and is probably on its way out.

[franjo_66]

Thanks for that insight Chuck

I have a remanufactured alternator that is only 12 months old.
So even if all the checks reveal that the alternator is charging properly, it's possible that it may be on the way out?
Also could it imply that the ground for the alternator need to be looked at?

Regards
Franjo

[ctaylor738]

I have had rebuilt alternators and generators fail literally out of the box. 

Possibly what is happening is that one of the three (?) diodes is bad and allowing a weak ground but the others are OK, so you still get a charge.  When an alternator is rebuilt, the "diode set" is usually checked but not necessarily replaced.

I think if you have a bright light, then the alternator has a good ground.