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Old 01-02-2017, 12:22 PM
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Join Date: Jan 2017
Posts: 2
Alternator or not

I have a problem with my My 1975 Mercedes 450sl.
I don't drive it that much, but due to an accident, I am forced to drive it. After a couple of months, I started my car and found to be sluggish, but did start. (I attributed that to the fact that it sat for a bit and it was cold) I drove it to a friend's house and when I returned to the car it wouldn't start. I charged it and it started and I drove it home. It sat for 2 weeks. Last week, I attempted to start it and there was nothing, but a short hum, which lasted only a few seconds, then nothing. I replaced the battery, but it still won't start. (only the humming sound) The red ignition light is on and all the electrical works, so I am perplexed.
Any thoughts or ideas?

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Old 01-03-2017, 01:36 PM
Join Date: Dec 2007
Posts: 5,061
I'd suggest a two prong attack on the problem.

One, take the car to an auto parts store with a tester and have them run a free electrical test. Their testers are pretty good at finding out the cause of your problems and best of all, the test is free. Also, check the obvious like a burnt out fuse. Relays are also notorious and need to be checked. The starter itself may also be bad and in need of replacement, check other threads for how to check the solonoid and the internals of the starter. I once replaced the starter on the 1984 300DT with a Beck Arnley brand starter TWICE. I finally ended up buying a genuine Bosch starter which solved my starting problems. Again, test before replacing because throwing parts at a car can get expensive.

Second, if all this doesn't work, the next step is an auto electrical repair shop. The competency of these types of shops varies, so be sure and check out their BBB and online reviews. If they know their stuff, they should be able to find the problem in under two hours.

I've seen Merceces Benz trained mechanics spend hours trying to track down electrical issues. German electrics are different than domestic electrics. Sometimes the problem appears to be obvious, othertimes it takes a real automotive electrical expert to hunt down the problem. You can spend HOURS of your own time trying to track down electrical problems. Paying a competent electrical shop willl save you hours in frustration (ask me how I know).

Just my two cents......
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Old 01-03-2017, 01:49 PM
Diseasel300's Avatar
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Join Date: May 2016
Location: Texas Hill Country
Posts: 5,610
If you've replaced the battery with a known good one and all the electrical systems are working EXCEPT the starter, I'd suspect either the starter has failed, or the solenoid has failed.

If you were cranking with a weak battery, that can be just enough to push a weak starter/relay over the edge.

Once you get the car starting, check to make sure it's charging. With the engine off, your battery should be ~12.8V or so, with the engine running, you should be somewhere close to 14V if the alternator is working properly. Given that the car sat for a long time, it could have just been a failed battery (has happened to my 500SL twice due to sitting).
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Old 01-03-2017, 03:38 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,801
Can you restart the engine immediately upon arrival or are you saying the battery discharges while sitting for a few days? Can you confirm by disconnecting the battery when the car is parked? If so, try to isolate the culprit circuit. One way is to remove all the fuses and see how much current flows across each fuse socket. Start with the circuits that have courtesy lights, the clock, maybe an aftermarket radio or security system...

Are you able to inspect or test the voltage regulator?

Are you able to test the neutral start circuit?

My guess is the hum is the fuel pump.

83 300SD
98 E320 wagon
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Old 01-04-2017, 12:10 PM
optimusprime's Avatar
Trevor Hadlington
Join Date: May 2014
Location: Worcestershire in England
Posts: 1,354
Try to start it with a friend .You need to tap the starter motor with a lump of timber or a mallet or hammer even .As the key is turned .If it starts the starter motor is shot.Use a fully charged battery .
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Old 01-05-2017, 11:41 AM
Posting since Jan 2000
Join Date: Sep 2011
Posts: 6,137
Very simple to diagnose an alternator. Use a DVM in VDC mode. Measure voltage across battery with engine off. It should be about 12 to 12.6 volts. Start engine and measure again. If the voltage is about 13.5 to 14.5 then the alternator is charging as it should. If not, check the brushes. They are very possibly worn to a point that one or both are no longer making contact.

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