So I was driving in to town this evening (after dark) and about 10 minutes into the drive my radio started cutting in and out, then the instrument cluster lights dimmed and failed. I hadn't had any trouble starting, except for needing a jump the other day. I was worried when I had to jump it, but it has started up fine since then. Well, I got to where I was going, and happened to run into another 300D entusiast. I let the car run while I was in the store for fear of it not starting again. I talked to him, and we shut it off since I had cables, and voile! no start. Couldn't jump it either, actually didn't have the time to charge the battery. The car is a greymarket 84 300D wagon, normally aspirated. The alternator looks brand new (I just bought the car and the guy did make mention of replacing it). I wound up leaving the car and am going to go jump it with my Dad's cummin's tomorrow, but was wondering where I should start looking for problems? Is there any way to test the alternator (lick my fingers and grab it? Where should I start looking for wiring troubles, or what should I look for? Everything appears to be connected and "normal." Any advice? How much does a new alternator cost? I would put it in myself, seems easy. Thanks,
Will lyons

This is usually a sign of the alternator beginning to go out. I had similar symptoms on the SD.

Note, however, that the symptoms may show up one day and the vehicle will behave fine the next day. It would be my guess that the brushes are just about shot.

When the vehicle starts "acting up", take a voltmeter and place it across the battery terminals. With the engine at 2000 rpm, the voltage should be above 13.2 volts. If it is below this value, or anywhere near 12V, the alternator needs servicing or replacement.

Cool, thanks. One more question...I don't have a tach, only a huge clock, so is 2000 about normal cruising revs?

No, you don't need to go that high.

2000 would be normal cruise at about 45 mph.

2000 is not critical. The alternator may put out full voltage at 1500 rpm. I'm just not sure of the exact number. Just rev it up a bit and watch the voltmeter. At idle, it may be around 12V. As soon as you rev it, the voltage must go up to 13.2 or higher, or the alternator is NG.

Cool, thanks!

Just follow up and let us all know how you make out.

If it is the alternator, you can probably fix it with just a new voltage regulator (held in by 2 screws). I got an adjustable voltage one for my mb, cost was around $10 shipped. The voltage regulator is simply the set of brushes and a diode.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

This was something that bit me not too long ago. Intermittent problems ended up as a loose ground. Check batt.-chassis-engine grounds. Mine was the bracket the engine ground strap attached to.

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Sam

84 300SD 350K+ miles ( Blue Belle )

This is the order that I'd try based on ease and probable cause:

1. Voltage regulator (as previously mentioned). Piece of cake to change. 2 screws in the back of the alternator

2. Short in the wiring. Look at the heavy wired coming out of the back of the alt and eye ball it. Anything shredding?

3. Alt

dp

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed

well, went to the mechanic and he hooked a voltmeter up to it and the alt was fine. he said it was the battery, I didn't know what else it could be, so I just put a new one in (old one looked old). I guess time will tell, hope I didn't make a $100 mistake!
Will

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84 300TD 4 speed, green

kind of funny as your description did not sound particularily like a bad battery. Quote: you were driving around and the radio went out etc. As long as you were driving the alternaor should have been putting out enough in combination even with a marginal battery to keep the radio functional. Although a need to get a battery jump to start a day earlier could easily have indicated a bad battery. Always best to get a battery charged and checked prior to changing it if not sure as that service is offered free at a lot of places.

I agree that it doesn't sound like a battery but if it was old anyway then you really aren't out anything.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Could still have been a bad battery of course. People that know everything must have a hard life. Your post does bring up the importance of having a book or system to check your car over once a year. Our cars are almost antique and any effort taken to avoid a flat bed plus paying a mechanic to do it all is effort well expended in my opinion. The exception is of course if you are just trying to beat the car into the ground. Use many systems in a haphazard way myself to try to accomplish this. Generally speaking my old cars I drive are in better condition every year. Even simple things like cleaning and treating wire terminal connections are important. They will cause trouble if left for long periods of time. Nature of the beast.The other thing is to catch things as they fail or becoming noticeable. If one just lets them accumulate you will think you are looking at a mountain. On the otherhand taking care of them gives you a feeling that your car is reliable plus the incentive to improve your maintenance program becomes automatic. If you buy an older car that has been in my possesion for a time it will be better by far than when I aquired it. Now one of our new cars when sold is really another story. When you develop pride in your car everything goes better with yourself and it.

Don't forget as Mercedes Benz owners we are entitled to free towing to the closest MB dealer and free roadside assistance as long as the MB lasts.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Sadly MB Australia does not have such a service