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78/82 300D
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5,180 Posts
Discussion Starter #1
I am getting annoyed. I went out today and when I went to come back home the car wouldn't start..Called AAA...I assumed it was the battery because it was 8 years old, he stated no its not being charged. The Attenuator is a year old and the starter is a few months old. So Took the attenuator and battery down to be tested...attenuator tested good..bought a new battery. The positive connector was bad so I installed a new one...then I noticed corrosion in the battery cable. The car now starts fine but the attenuator isn't charging the battery. When a multi meter is applied, when the car running it only registers at 12.5v. What is my problem? I also noticed the battery light doesn't illuminate when the car is turned on. I am at a loss and before I start replacing more random things I need advice...so please help :crybaby2:
 

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78/82 300D
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5,180 Posts
Discussion Starter #3
Replace the bulb with the proper 3w bulb and everything will work. That bulb compleats the charging circuit.
Yep, after I've done some reading, that is the problem. When I removed the bulb the filaments didn't appear burnt. A few months ago, I did some work on the gauge cluster. The speedo didn't work and the pin in the wiring harness for the high beam indicator was broken along with the center plastic guide pin on the harness plug. I noticed, after I reinstalled everything that the temp gauge would pop up to 40...even when the car has been sitting over night. So there is a possibility that I mixed up the pin arrangement or its just the bulb. Its dark out but I will go out, with the wiring diagram and insure everything is correct and check the bulb...Also, what engineer thought about making the bulb complete the circuit to the attenuator? Most mechanics wouldn't even think of that being the problem...and would start buying the more expensive parts...then after the 4th visit...find out the 25cent bulb is the problem.
 

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1980 300CD
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24 Posts
I had this happen to me. Luckily my car came with a spare alternator so I didn't lose any money. I still felt like an idiot after figuring out it was just the bulb having already swapping alternators. Before cars warning lights were controlled by computers I would assume doing it this way made sense.

Good luck!
 

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78/82 300D
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5,180 Posts
Discussion Starter #5
I had this happen to me. Luckily my car came with a spare alternator so I didn't lose any money. I still felt like an idiot after figuring out it was just the bulb having already swapping alternators. Before cars warning lights were controlled by computers I would assume doing it this way made sense.

Good luck!
Yep that is what the problem was, I opened up the wiring harness that plugs into the back of the cluster. When I switch over connectors because the original was busted(previous owner was an expert repair person :mad:) I seen that pin 4 wasn't in the holder, and pin 4 controls the light. Placed it back in and reconnected the plug to the cluster...and there we have it a battery light again. So this goof only cost me 6 hours of frustration and 105$ for a battery. What I am total shocked by, is the car last for over a month running on only the battery :eek: Like to see a new car do that :p
 

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W108 250s '67, w123 280ce '77, w123 300d-t '85, w126 300sd '85', w140 s600 '94,w210 e230 '97, bug69
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262 Posts
Im pretty sure your alternator is charging but not enough, meaning instead of 14volts it does only 11 to 12, once you open the headlights, the ac, and the stereo then it will already drain the battery as it is not being charged enough, your car ran on entirely the new battery but being partially charged by a weak alternator,
 
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