Hi So i started running into a weird issue with the battery light on my 81 w126 300sd. I would start the car - let glow plugs warm up and the battery light would come on while I waited on the glow plugs (normal). I would then crank the engine up and the battery light would stay on. As I always place the parking break and like to give the car a little gas before getting it moving I would press the gas pedal and the battery light would then click off at that point. This was a bit weird but nothing that I was too worried about until...

Now when I give the car a bit of gas the battery light will not turn off. Now the battery light will not turn off until I drive the car a little bit (few miles) and then it flicks off?

I thought it might be a grounding cable? But I haven't had any trouble starting the car? Could it be faulty wiring in / around the instrument cluster? There are other buggy signal issues with the cluster as well that need to be addressed as well...

Thoughts?

I had a very similar problem on my 85 300d, not sure the exact cause, but I think it was a loose bulb in the instrument cluster.

What you describe isn't unusual, depending on how long it stays on.

On these old cars, the voltage regulator needs to "wake up" to start the charging cycle and generally doesn't do so until the RPMs pass 1200.

On really cold mornings I have to goose my 300d well beyond that in order for the light to go off.

(An electrical purist will shortly eviscerate me for the underlined part of the explanation, but the general thrust of my answer is sound.)

If you are on the road with normal transmission shifts and the light is still on, then you need to check the charging system.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Definitely not wrong.

But, there is supposed to be a provision that allows the 'wake up' to occur at low rpm.

Its usually a thin wire leading to the alternator and attaching to the back.

When the wire gets destroyed, or has a bad contact, it will not 'wake up' on low rpm and you will get a battery light until you rev it over 1200rpm-ish

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Would it be wise to replace the voltage regulator then? This was my first instinct as it is probably the original when the car was purchased in 1981.

Thanks!!

And, put a volt meter across the battery posts to confirm that the glow plugs are pulling measured voltage down. I was having charging issues and Autozoo wanted to condemn the alternator when in reality the voltage regulator going and battery was bad. The VR was a $30 problem vs $100+ for an alternator. I found a Blem battery for $20. Too bad the battery guy has retired.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Without placing a voltmeter across the battery terminals and measuring both DC and AC volts, you are pretty much shooting in the dark.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Take the regulator out and inspect the brushes. Sometimes if lights stay on you can have a diode in the alternator going bad .

...until I tightened up the alternator belt. It would go out after I turned up the idle knob. Something else to check.


snapped_bolt

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'81 240D For now, a good place to borrow new parts
'80 300TD Probably will be put back into service!
'79 240D BACK IN SERVICE SINCE 09/16; limited use, oil leak. Guide pin r/sealed/replaced. Still a leak. Front crank seal....

I had the same "problem" on my 85 300D. Checked the following which was found to be in good working order:

idle rpm,
alternator belt tension
electrical contacts at battery terminals, chassis, ground strap, alternator, starter motor
voltage regular brush length, tried different voltage regulator
charge lamp in cluster

This car had a battery from the PO. It looks clean and not very old (4 years) and starts the car fine but I would always have to rev the throttle after a cold start to get the charge light to go out.

I wanted to get an AGM battery and found a "take off" from an Interstate distributor for $40. Installed it and the "problem" went away immediately. Upon starting the engine, the charge light now extinguishes immediately w/o revving the throttle.

I was surprised to say the least.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks I will review all points noted above... plus the battery came with the car when i purchased it.

Question - Not being a battery expert - what is the advantage of AGM battery?

The last time I looked, prices were twice the standard lead battery cost and warranties were less. I concluded that regardless of technical improvements my money was preferred over AGM. Batteries rarely give a problem and the fix is simple and quick - replace with the highest CCA available weighted for price and warranty. No painful R&R or even much troubleshooting required.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

My 84 SD does the same thing though I just have to tap the accelerator to put the lights out.

I'm probably going to use the 80amp alternator off the 380 engine so if there's a bad ground or regulator it should be remedied.

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84 300SD
85 380SE
83 528e
95 318ic

Thanks!!!

Resolved... The new alternator I installed a couple months back kicked the bucket ... Replaced and working like a charm. Luckily still under warranty