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1985 300TD
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77 Posts
Discussion Starter #1
I'm having problems starting my 80 300d. It's been on on-going thing that I think I fix, then something comes up down the road. Can't really tell how related they are. Any troubleshooting help GREATLY appreciated.

Here's what's happened chronologically (long):

-last August: Car wasn't starting. Bought new battery and alternator. Seemed to fix the problem until...

-1.5 months ago: Car wasn't starting. Seemed to be loosing charge. Battery and alternator both tested fine. Changed the voltage regular (suggested by my mechanic) and seemed to do the trick.

-1 week ago: Ripped out dash to repair HVAC & replace cracked dash pad. In process, ripped open the headlight switch pod. Picked up a used one from mbsource and just replaced the entire switch wire for wire. At this point, the lights are not coming on at the right points on the switch (for instance, now to turn headlights, I must turn switch all the way to the right vs just one click).

-2 days ago: Appears that car looses charge. I take in the battery and have it checked and charged. Seemed to get the thing to start again until...

-yesterday: Would barely start...I finally got it to crank over and then drove it around for 50 mins to recharge battery. It ran fine throughout the entire 50 mins. When I got back, I disconnected the battery (so nothing would be draining it).

-now: Still won't start. Glow plug light and other instrument lights coming on strong but still is cranking very slowly.

What could be the issue(s). I think I have a short somewhere in my system, but even if I did, you think it would start this am if the battery was disconnected since the last time I drove.

Any help appreciated. Thanks.
 

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'84 300D/ dakota benz trk. Sold... miss it. "88 260E '91 190E 2.6 '89 Volvo 240DL "85 300D
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1,252 Posts
check voltage reg at back of alternator. diesels have alot of comp. so you still may have a strong 10+ volts left in the batt. but no enough ass to crank the car over. once running, check batt with load tester/volt meter, it should be running at least 13.5 or so volts.
 

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1981 300CD, 1973 280C
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448 Posts
The headlight switch must be turned fully right to the second click to engage headlights, thats normal. When off, the switch is vertical, one click to right, park lights/taillights, 2 clicks, headlights. I would verify the alternator belt is tight. If you can rotate the alt. pulley by hand against normal rotation with engine off, its too loose. That will cause the battery to never reach full charge. A short will not kill a battery, it will blow a fuse, a current draw will continue to draw power from the battery, car off. The man that verified alternator and battery are good can do a simple draw test on the battery. It either has one or it doesn't.
 

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1989w126
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708 Posts
It sounds to me like your alternator is dead. It has not charged anything during the 50 minutes drive. Or your battery doesn't take charge.
But since you said a manual charge lasted 2 two days and a drive charge lasted nothing : i would stand charges :) against the alternator.
 

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1985 300TD
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77 Posts
Discussion Starter #5
I'll try that stuff

Thanks to everyone for the advice. I'll take the alternator in again to be tested and have the battery charged and tested. Like I said in the text, I just recently changed the voltage regulator, so other than if it somehow got installed wrong that doesn't seem like it should be the problem. I'll double check that installation when I have the alt out.

I checked the belt tension on the alt. belt and it seems to be tight, so unfortunately its not that simple.

If the alternator tests ok, where do I go from there? I heard one post say it couldn't be a short (that would just blow a fuse). Is there another way something could be drawing power from the battery when the car sits??

Does the draw test on the battery have to be done in the car.

Electrical sys. is definitely not my strongest area vehicle-wise.

Also very weird about my lights, my old switched turned the headlights on at the first click to the right. I guess it was weird wrong originally... Go figure.

thanks so much.
 

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1981 300CD, 1973 280C
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448 Posts
YES, the draw test is to check the cars electrical system for any draw on the battery in the car!
 
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