Guys,

I need some help. My 1987 300E has a battery drain somewhere and I don't know where. If the car sits for about a day it dies and I have to recharge the battery.

Last night I check the battery by doing a power drain test using a test light and pulled out each fuse one by one. The drain is still there after all the fuse were pulled. No lights are on (headlights, tail lights, interior lights, or trunk light.)

Where else can I look guys.

 

Had a similar issue on my 86 300E.

If you have a factory stereo in it - It is likely that it is the source of the leak. I hear it is common with the old becker stereo's. It has a built-in line switch that tend to short out when the stereo gets old. I spent $200 at an electrical shop that diagnosed the issue and said it was fairly common (then why didn't they first check it without charging me the 2 hours labor?? hmmm)

Anyways a replacement stereo was all that was needed.

Now I didn't pull all the fuses and I don't know if the unswitched power to the stereo is on a fuse (Should be I guess).

Have you verified it's not just your battery not holding a charge?

 

I have a brand new battery so thats not it. there is a constant draw somewhere. I will check the radio and see what happens. Been searching through the forum all night and learned that there is other stuff I should check too. Alternator (voltage regulator) and a fuse behind the battery case.

Anyone with anymore ideas?

 

yogibearal may be right because our 89 300SE did the same thing with the becker radio, so when i disconnected radio, the problem went away, dont disconnect the fuses in the engine bay, just take the radio out, and disconnect all the wires from the back...

 

I think I found the culprit. Ok, here is the update. I checked three places where the drain could be coming from. First, was behind the battery location there is a red 10 amp fuse. It was broken so I replaced it still a battery drain. Next, I checked the aftermarket radio. I pulled it apart and still the drain is present. Last, I checked the alternator. I jump started the car (battery still dead) and when the car was running I pulled the positive cable from the battery and the car just went dead. So the problem is most likely the alternator! Going to fix it later next week and will let you guys know the details.

I still don't understand why there is still a draw from the alternator becase the alternator doesn't need any power when the car is off correct? Maybe the draw is still coming from somwhere else?

 

The red fuse you replaced was on the OVP relay.
It could have blown when you jump started.
Possible the alternator has an internal short.
It could have been damaged by jump starting.

Have you read the voltage and current output from the alternator when the car is running ?
You should read 13+ volts and 50-60+ amps.

 

I havn't read anything on the alternator...but I'm thinking it has something to do with the battery not charging. But tell me this...the alternator cannot be possibly drawing out power from the battery. Is this correct. The reason I ask is because there is still a draw coming from the test light. Do you think the diodes or the brushing has something to do by drawing the power from the battery.

Is there anyother places I can check for the draw. Where are the other fuses and relays I can check. So far all I've checked are the ones on the driver side engine bay fuses and don't know where the other relays are. THanks for your help.

 

Voltage regulator - on the back of the alternator.

 

Brand new alternator replaced and still a drain. I will go out and purchase a voltmeter to check it more accurately (been using a test light)

Update: I took out all the panels underneath the steering wheel and found the alarm and I unhooked it and still a slight draw. Will inform you guys what the miliamps when I get the voltmeter.

 

6HIFT,
It might be time to make an appointment with a professional, and save your self the anguish. Your problem is difficult to diagnose across the internet. You seem to be going in the right direction but missing something.

YOGIBEARAL,
To answer your question about the diagnostic charge, you had a problem, and they had an answer. I'm assuming the folks at the electrical shop that solved your problem were not volunteers and need to cover overhead and make a living like the rest of us. Also in my opinion they were very thorough to be sure that was your only problem. They obviously followed a diagnostic procedure that proved their results. I'm confident that they stand behind their work and would take care of things if they were incorrect. I would have gladly payed for their service knowing that if they were incorrect, the door was open for me to request satisfaction. If my request were brushed off, believe me things would get ugly.

I don't mean to sound harsh, but after 35 years in the service industry I have time to preach the word about why good service is somewhat expensive. Speaking of expensive, has anyone taken a ride in an ambulance lately? What does a fully outfitted ambulance cost? What about insurance? You have to pay drivers and EMTs while they are doing nothing.

Getting back to the cost of automotive service, professional shops pay $200.00/month for internet service and repair info. When you talk about a volt meter from Kameapart for $20.00, a professional pays $300.00 to $400.00 for a Fluke which is about as good as it gets for accuracy and reliability. Autozone will give you codes from a fifty dollar scanner, but to break those codes down and start questioning components for voltage, current, signal, and wave forms calls for a scanner costing between $3000.00 and $20000.00.

Thanks for listening to an old MB factory trained veteran trying to put things into perspective. I hope everyone reading this post understands why shop rates are what they are. I haven't even covered the investment in hand tools. A professional quality wrench which has to stand up to daily use can cost as much as half the price of a whole set of tools made out of hardened butter from China. An investment in hand tools and box can easilly reach $30,000. It takes a heavy investment to be totally armed and ready to correctly diagnose and repair your Mercedes.