Electrical issue: Battery or Alternator again? - PeachParts Mercedes-Benz Forum

steamboatsykes · #1 01-23-2015, 06:37 AM

Yesterday morning my 1981 300D refused to start with what seemed to be a battery without enough charge to spin the starter. After an hour of charging the battery I was relieved to hear the starter turn and start the car. The battery has been in the car as long as I have owned it, +4 years, and has given no issues of failing until now.

But I am wondering if it may be the alternator again. The alternator was just replaced with a rebuilt unit about two months ago. On colder days I have been noticing that the group of three indicator lights on the lower left of the console stay lit until the engine would rev above idle speed or warm up. I have been assuming they were an indication of low alternator output due to the lower engine speed. Perhaps it was an indicator of a failing battery?

I guess I’ll pick up a new battery today regardless. I am hoping that I don’t need to replace the alternator as well again so soon.

enoch · #2 01-23-2015, 06:46 AM

I had a somewhat similar problem.. Mine turned out to be the belts just a little loose. Not loose enough to squeal but just loose enough to slip. Check those out.. But the new battery definitely is a good idea. Good luck.

steamboatsykes · #3 01-23-2015, 06:53 AM

Thanks. I forgot to say that I did check the alternator belt and it was very tight.

strelnik · #4 01-23-2015, 07:56 AM

Quote:

Originally Posted by steamboatsykes

Thanks. I forgot to say that I did check the alternator belt and it was very tight.

Consider your ground cable.

IIRC, the original ground cable set-up was a short cable from the battery to the car and then a woven-type ground underneath between the chassis and the tranny.

I trashed this arrangement on my 123 in favor of a short ground cable from the (-) post to the body and a second one from the (-) side of the battery directly to the rear of the engine.I used a marine battery clamp, then used two pieces of cable that ended in an eyelet and attached both to the (-)battery post.

Poor grounds are a big cause of problems in gassers and less so in diesels, but they can reduce the charge sent to your battery in any system.

That's why your battery works so well after charging, because it got the full charge.

Often cables can be corroded internally. You never see it.

i bought a Citroen DS21 from a museum that waxed all the wires monthly. The car was sold because it wouldn't start. I replaced the perfect-looking wire between the coil and the distributor and the car ran perfectly and still does!

greazzer · #5 01-23-2015, 10:09 AM

news ...

Maybe upgrading all the cables, et cet., to ensuring alternator, belts, pulley are in good order. Overall, the electrical charging system has room for improvement IMO and I am guessing after 30+ years, issues will present themselves ...

mespe · #6 01-23-2015, 10:35 AM

Most likely corrosion in the wiring, like prior posts have suggested. And green copper wire near battery can cause same issue.

Green copper wire (corroded wire) adds resistance to the wiring, which causes the alternator to increase it's voltage to charge the battery. This extra voltage is "dropped" across the corroded wire (electrons flow on the outside of copper wire)

So check ground (from engine to chassis) and cables at battery, should solve your cars electrical issues.

Since you replaced the alternator, I'm wondering if the bearing was bad, or if it overheated when it produced the excess voltage.

Zacharias · #7 01-23-2015, 10:45 AM

If you just replaced an alternator, it wouldn't surprise me if the battery was also done. A dying alternator will gradually kill an older battery, especially if there are a lot of short trips in the driving habits.

I agree with the comments on cleaning and checking the ground cables.

It's easy enough with a meter to check the alternator output, just in case you got a bad rebuilt alternator. Wouldn't be the first.

I highly recommend getting one of the charging system monitors that plugs into the lighter socket.

vstech · #8 01-23-2015, 10:53 AM

for some odd reason, MB chose to ground the battery to the frame of the vehicle, not to the engine... so they rely on the chassis to get ground to the motor, through a battery strap under the car... it's a good idea to pull the bolt that holds the strap to the car, and motor, and clean them, and the surrounding metal, then paint over the tight bolt. it's amazing how many problems can be fixed by starting here.

also, on the 240, the main power goes directly from the battery to the starter bolt. cleaning that makes a big difference as well.

steamboatsykes · #9 01-23-2015, 12:09 PM

Just replaced the battery so its really looking to me like it could be a grounding issue. I still see the battery indicator lights come on at starting and then fade with RPM increase.

When I picked up the battery this morning I ordered new positive and negative cables but I didn't realize about the 2nd ground cable at the time.

That's really a strange grounding system, one would think that the shortest path to ground would be the best.

Just moving the braided cable a bit I can see where it may be where the issue is. Especially after 35 years.

mach4 · #10 01-23-2015, 12:29 PM

When I had my alternator issue last sumer, I resolved not to drive without a volt meter as a warning and monitoring system for the alternator and battery. While I ended up incorporating a voltage monitor in my Arduino/Android bluetooth engine monitoring gauge setup, my original thought was to just get one of these
off dBay

....actually, I still may for the other cars.

JHZR2 · #11 01-24-2015, 09:03 PM

Quote:

Originally Posted by mach4

When I had my alternator issue last sumer, I resolved not to drive without a volt meter as a warning and monitoring system for the alternator and battery. While I ended up incorporating a voltage monitor in my Arduino/Android bluetooth engine monitoring gauge setup, my original thought was to just get one of these
off dBay

....actually, I still may for the other cars.

+1 to this. There are a few brands of these and I do recommend them. For convenience, there are some that have a voltmeter and USB charger in one.

Very useful tool.

OP, the voltage regulators on these cars don't take the battery voltage as high as is desirable, and so that doesn't help. Neither does GA summer heat. 4 years on a battery in a hot climate, if perpetually undercharged, can be condemnation time.

Diesel911 · #12 01-26-2015, 02:38 AM

Quote:

Originally Posted by JHZR2

+1 to this. There are a few brands of these and I do recommend them. For convenience, there are some that have a voltmeter and USB charger in one.

Very useful tool.

OP, the voltage regulators on these cars don't take the battery voltage as high as is desirable, and so that doesn't help. Neither does GA summer heat. 4 years on a battery in a hot climate, if perpetually undercharged, can be condemnation time.

That has the same construction as one of the ones I have.

Bosch Alternator Regulator Modification with diode to increase resistance
http://www.detomaso.nu/~thomast/alternator/
I am not sure the above link still works but there is also a thread some where were one of our Members did the Mod to the Voltage Regulator.

Also the wattage of the Charging Light makes a difference.

When I changed the Chassis to Battery Ground cable my Charging Voltae went up slightly.

Diesel911 · #13 01-23-2015, 12:30 PM

All of the above suggestions are good but where is the voltage information that a cheap $6 or less Harbor Freight Voltmeter tell you?
Less useful would be one of those Volt Meters that plugs into the Cigar Lighter.

steamboatsykes · #14 01-23-2015, 12:37 PM

I suppose a good check would be to look at the voltage across the alternator terminals and also the battery and compare for any differences, and if so then look where the drop is.

mach4 · #15 01-23-2015, 01:22 PM

Quote:

Originally Posted by Diesel911

Less useful would be one of those Volt Meters that plugs into the Cigar Lighter.

You are absolutely correct!

All I was suggesting was that most of the electrical issue posts start with something along the lines of "all of a sudden my car would't start", and then a string of helpful suggestions ensue. Since the first sign of trouble is only the result of a period of time where the problem existed previously, it would be nice to conveniently at a glance reference the battery voltage before starting and monitor the voltage while driving as early warning of an electrical problem headed toward a non-start situation, often in less than optimal conditions. I realize that volt meters that plug into the cigar lighter do not provide that information (unless they are plugged in at the time) and are therefore less than useful. A cheap HF multimeter is preferred for troubleshooting, but it's kind of inconvenient to try to get a voltage reading while driving as manipulating those little probes are tricky one handed and popping the hood to get a battery reading all the time is kind of a pain.

Recognizing the lack of utility I didn't get one of those cheap cigar volt meters, choosing to add voltage monitoring to my other system status monitors - fuel pressure, EGT, trans temp, oil temp, intake temp, engine bay temp, boost and vacuum.