Battery light glows brighter with higher RPM

[zeke]

I changed the belt (it was dry rotted and slipped when wet anyway), but the problem remains.....I think it is likely a bad voltage regulator, and I want to see if anyone on the forum can validate this guess...

It glows dimly at idle and gets brighter as I press the throttle.

[vstech]

odd. usually when the regulator fails, the light does not come on. it's the simple thing to try though, so it's always the first thing I try when the alt is not performing.

[zeke]

OK, ordered one from Phil....
The output is low - like 12.7-13.2V.

[layback40]

when I had the dreaded glowing idiot light problem it turned out to be the main wires from the alt to the battery that were bad. I put a wire from one of the big tabs on the alt direct to the battery & it fixed it. A bad connection on those wires can soon destroy a reg.

[Diesel911]

I made the below Alternator Test Cord.

If my Alternator works with this Test Cord Installed I know it is not a problem with anything on the Alternator.

I got the Alternator Connector at the Junk Yard but you can buy the parts at the Dealer and you need a 3 watt Light Bulb.

[jonbobshinigin]

Im having the same problem. I also replaced my belt and the issue persisted.
You can read my post on it:
Voltage Regulator again??? 1987 300TD

I will have my resolution posted as soon as I have a resolution.

[tyl604]

Vstech - my experience on my 81 300SD was different. The red light glowed faintly all the time and brighter with higher RPM. I replaced the voltage regulator(?), the easy removable thing that fits into the alternator. Did not help. Turned out that the alternator had gone.

So I am wondering if his problem is alternator.

PS - dear old Sears checked my system before I made the repair and said both my voltage regulator and alternator were fine.

[Diesel911]

Originally Posted by tyl604

Vstech - my experience on my 81 300SD was different. The red light glowed faintly all the time and brighter with higher RPM. I replaced the voltage regulator(?), the easy removable thing that fits into the alternator. Did not help. Turned out that the alternator had gone.

So I am wondering if his problem is alternator.

PS - dear old Sears checked my system before I made the repair and said both my voltage regulator and alternator were fine.

When Sears Checked was the Alternator on or off of the Car?

[Diesel911]

This is from my notes and from one of our forum members:

ALTERNATOR DIODE check with volt meter
With your meter across the battery, on Volts DC, you should read 12.5 - 14VDC

If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.

Not from my notes;
These Alternators seem to be sensative to grounding issues.
Where one if the Screws goes trough the Voltage Regulator there is a grounnd strip that presses against the Alternator Casing/Frame. If the Frame area is corroded it will not ground well.
Besides the Battery to Chassis and Chassis to Engine Ground Straps needing to have good contact I am beginning to wonder from what I have been reading if the Alternator Bracket to Engine areas are also getting so rusted that are providing the Alternator attached to them with a poor ground.
Layback40 suggested a separate Ground Wire going to the Alternator frame as well as replacing the Wiring to the Alternator if your problem does not go away.

[Biodiesel300TD]

I've had dim flickering charge light before. It ended up being the alternator. I changed the voltage regulator, no dice. Made sure the plug was clean and connected, no dice. New alternator fixed the issue.

I've also had it just be the voltage regulator.

[bustedbenz]

It was the alternator itself when my SDL did that.

[piccolovic]

All three of my cars (190D, 240D, and 300D) did that more than once, and each time is was a dead, or dying alternator. Usually they are rebuilds from Delco, and there was a bad series of diodes, which Pierre replaced on the 240D, without removing the alternator (R & R is a real *itch!!!!). I would start it, drive to a shop, and have them test it while running. It should be between 13 and 14 volts. The lights wil l come on when it drops below 12 and a half . . . somewhere around that. General formula = 2.1V or greater per battery cell (6), for a mimimum of 12.6 Volts. Places that sell batts will usually do that test for free so they can see you a $600 alternator . . . .LOL, yeah, right!!!!

Good luck!

Torie

[zeke]

Diesel 911...can I check for continuity between the alt body and engine block or neg batt terminal to style in or out grounding issues?

[tyl604]

When Sears gave me a clean bill of health, the alternator was tested on the SD. From everything I see in this post I think the alternator on the original poster's SD has gone bad.

[zeke]

Update: I changed the volt. regulator and it had no effect. I changed the alternator and it solved the problem.

Thanks everybody,