W123 electrical - - PeachParts Mercedes-Benz Forum

unkl300d · #1 11-22-2006, 09:07 PM

Hello and Happy Thanksgiving!

I have gone through the archives and gleaned alot of good info.

Until my car battery recharges, I am submitting these questions for extra tips.
1979 300D. 163K miles. Garage kept. No water/rain interaction this past week.
When running, a multimeter reading shows 13.8 volts at the battery terminals with engine at higher than idle revs.
Seems like alternator is good. Archives confirm 13.8 volts as adequate. (Alternator is a Bosch 005-154-98-02 88 remanufactured replaced June 1995 at 110,825 miles).

I replaced the battery on Sept. 19 2006 with an appropriate Interstate battery. The old one lasted about 7 yrs. So battery is new.

Yesterday my car's battery was very low. (11. volts.approx)It had enough juice to keep the clock working and interior lights. Car would not turn over at all though. I jumped it and the car started easily but I shut it off after 2 minutes because I had no time to do a freeway run .

I also made sure that I turned off the radio properly thinking that I may have accidentally activated a stanby mode which may have drained the battery since last used 2 days previous.
Tested the trunk switch and light. It works and bulb was not hot to indicate it had stayed on.

~Discovered glove box light did not work. Removed the bulb and the bulb filament looks whole but untested yet. Yet to remove the glove light switch to inspect. Forgot to look at cig lighter.

Both front and rear interior lights work but the switch that activates the rear int. light does not activate the light. Other accessories on that dash panel not tested at that time.

Used contact cleaner on the alternator connections and general area.

Today I tried to jump start. Did not start. Battery read about 2.3 V with multimeter before jump.
After leaving jumper cables on for over 5 minutes with other car at fast idle, still could not start the 300D. Not enough juice to activate starter. Seemd to want to go.
I tested battery with jumpers off. Tested around 11.7 V (as a result of recent boost). After about 20 minutes it went to about 2.3 V

~ I noticed that when I took a multimeter reading of the battery terminals intact with battery cables, the reading was about 2.3V
When I removed the neg. battery cable and placed the meter terminals onto the battery terminals (pos terminal still connected to pos cable) the reading was about 6.8 or 5.8V. It went up.

~~So, is this normal that the negative cable will cause a lower reading of the battery?

I inspected the battery cables as best as possible. No visible evidence of bad.

I could not yet locate the ground wire from engine block to chassis.
~~Where is it?

~~ I read that the grounding of the alternator should be checked. Where is that?

As soon as the battery recharges- it is on a charger... slow... I will do the ammeter on series and individual fuse tests as per archives.

I already remioved the glove box light/radio fuse today. just to remember that this may be one source of a leak.

Also could the alternator brushes still be suspect?? If so how to know if my alt. has the replacable ones?

By the way, the battery cables were clean and tight previous to any dinking around after the fact.

Any insights at this point or tips, opinions???
When last running, all basic functions seemed to work.

Thanks

Denis

running-snail · #2 11-22-2006, 10:02 PM

I'll post just to bump your problem up again.

Definitely agree that an ammeter will help - sure sounds like a voltage drain/draw to me. I believe, although am far from an electical expert, that this is what explains pulling your neg. cable and having an increase in voltage. While the neg. cable is connected, you're completing the circuit and the draw is being made.

I like your approach to solving this - one of the 'experts' will need to chime in to help however....

Try not to let your battery drain down so low while troubleshooting.

barry123400 · #3 11-22-2006, 10:02 PM

Pull your clock fuse. Put your meter in the line of the battery circuit. Between the negative terminal and cable is fine. Keep pulling fuses till you get a zero reading. Then you have no drain at all. If it does not do it by pulling fuses disconnect the alternator. You might have a shorted or leaky diode in there. I have run across a few cases of aftermarket sterios draining the battery when off as well. Kind of sounds like you have a pretty heavy drain.

Lycoming-8 · #4 11-22-2006, 10:30 PM

Could also be the glow plugs draining down the battery thru a faulty glow plug relay. This is probably the highest draw on a sleeping car except for an accidental short thru a frayed wire, and can really dump a battery in short order!

unkl300d · #5 11-23-2006, 04:54 PM

Thanks all.

Update, I had the battery on a charger overnight.

It measured 12.3-4 in the morning.

I did the systematic ammeter in series. Initial reading was 2-3 mA
Pulled all fuses methodically. No drops at all. Same 2-3 mA reading with all fuses out.
Retested radio/glove box fuse and it still did not waver the ammeter.

I checked the voltage drop between the alternator case and the neg battery post. This was zero.
I did not yet check the V drop of alt. pos connection and pos battery post.

Started the car up and drove it 30 min. on freeway. Back home I tested the charging at high idle and it read (edited)13.6-13.7V.
Once the car was shut off I tested battery voltage at posts with cables connected and it read 12.47 V
~This time the voltage difference between testing with neg cable on and off did not produce such a dramatic drop. Very minor drop. Perhaps my first such test yesterday was dramatic because the battery was very low. In the second test the battery was charged up 12.3-4V

So, I shall retest tomorrow and see whether the battery goes low again.
If so, more tests to be made.

barry123400 · #6 11-23-2006, 05:55 PM

If after your highway run you had no load like lights on etc. I am not sure with mercedes alternator at high idle you should have been that low in voltage. You could check the archives for what is standard on them. Or check the voltage across the battery at high idle with the lights on. I still feel you should see 13+ volts. If you do not pull idle up 1 k with lights on and read again. It should be 13+ in my opinion. I might go over the grounds in the engine compartment. Especailly if the archives indicate you should have more. I realise that you did the alternator case to negative terminal voltage test to see if You had a bad or marginal ground. You might have found the basis of your problem. An under charging alternator might be a possibility. If so you will find it now. The possibiliy of a load when car is off off you seem to have eliminated pretty well. Something brought that battery down. Or run with your headlights on and see if battery holds the 12.47 volts with everything off. If not of course the alternator is not keeping up with the load.

unkl300d · #7 11-23-2006, 11:41 PM

Barry I made a typo error and corrected it above. Should have read 13.6-7 V reading. Yeah 12.6 would be way low. Two days ago it read 13.8 so it ought to be OK.
At 12:45 PM today the battery read about 12.47V after the freeway ride. Just now, 8:40PM, the battery read 12.08 both readings with the car off.

I'll see tomorrow if the car starts.

Cheers.