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Discussion Starter #1
Took the car out for a test run after replacing breaks and had something odd happen. After about 10 miles, the tach started bouncing around and a mile or so later the car died. After I got to the side of the road and looked under the hood, it started up again. Got it back to the shop, about 1/2 mile away. Now the car starts and runs as before, in the shop, but something seems to be drawing down the battery. Any suggestions where to start looking? I can see nothing obvious.

88 560SL with 135k on the clock.
Thanks
 

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1987 560SL
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59 Posts
I'm no spark but maybe you forgot to tighten the battery terminals after you did the brake work? is the alternator charging, what volts do you get across the battery terminals with the engine running? A battery going absolute dead whilst you are driving could mean bad alternator, bad battery or a short somewhere and be the cause of the car dying but it seems your car started up again so battery could not have been dead. A bouncing rev counter could mean bad alternator, not sure about this car on that point but on my Jag it did on my boat it didn't go figure.

Check battery voltage after car has been at rest for say 6 hours a good battery should be full and read 12.75 volts and and then check again next day after 12-18 hours should be nearly the same volts or just a tad less like 12.74 or 12.73. If it's a lot less like 12.5 volts then you have a drain or battery is not holding charge. I think this is how you test a drain, someone chime in if i'm wrong. With ignition key out, doors closed trunk bulb removed or disabled, start to check a drain by removing the positive terminal from the battery put your tester on to amps and rig the tester between + battery post and removed battery cable. You should be reading zero amps, if you are getting a reading then pull your fuses one at a time and see when the amps stop flowing, then that circuit is the drain circuit, test them all.

I hope this helps you get started and trust others with more knowledge will chime in and add and correct.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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This will be hard to find when the car is running normally. Having just fried a main wire harness, I would suggest the following test to be performed first.

1) Disconnect battery and remove all fuses.
2) Using an Ohm meter, check for continuity to ground at the the center terminals of fuse 11, 12, and 14, These are the circuit 30 terminals (live at all times terminals). There should be no continuity
3) Turn the key to the run position and check for continuity to ground at the center terminals of fuses 5, 7, 8 and 10. These are the circuit 15 terminals (live with key on). There should be no continuity
More than likely if the car is running fine, These should measure > 1000 Ohms. But make sure your connections are good and get a helper to wiggle wires to look for intermittent problems.

Next reconnect battery an perform a voltage drop test at the same terminals.
1) Using a long lead like 10' on the positive terminal of the volt meter, connect the positive lead to the battery Make sure this is a good connection, like take off the nut on the battery terminal and use a ring terminal attached to the lead
2) Measure voltage at the center terminals of circuit 30 fuses 11, 12 and 14. There should be no more than 0.5 volts
3) Turn the key to the run position and measure voltage at the center terminals circuit 15 fuses 5, 7, 8 and 10. There should be no more than 0.5 volts
Again wiggle wires and look for changes.

Having done all that you may not be out of the woods unless you find the problem but at least you will have checked your un fused circuits (The ones that can fry a main wire harness).

If any of those test fail you will need to work your way up the circuits performing the same test at the terminal blocks under the hood, ignition switch and headlight switch. But post your results first.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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10,743 Posts
So I did a little more research and found that I presented the voltage drop test incorrectly. It is required to have a load on when doing the voltage drop test. Therefor at least 1 fuse needs to be installed and its component powered. I would recommend for circuit 30 to install both aux fan fuses 7, 14, and bypass the aux fan temperature switch on the engine and key needs to be on. For circuit 15 I would recommend the rear window defogger, Fuse 10, with key on and rear defrost on. Make sure your rear defroster works, Should about 150 Ohms across the two pins going to the rear defroster plug connector in the right side back seat.


I think this might be even better.

 

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Discussion Starter #5
I'm no spark but maybe you forgot to tighten the battery terminals after you did the brake work? is the alternator charging, what volts do you get across the battery terminals with the engine running? A battery going absolute dead whilst you are driving could mean bad alternator, bad battery or a short somewhere and be the cause of the car dying but it seems your car started up again so battery could not have been dead. A bouncing rev counter could mean bad alternator, not sure about this car on that point but on my Jag it did on my boat it didn't go figure.

Check battery voltage after car has been at rest for say 6 hours a good battery should be full and read 12.75 volts and and then check again next day after 12-18 hours should be nearly the same volts or just a tad less like 12.74 or 12.73. If it's a lot less like 12.5 volts then you have a drain or battery is not holding charge. I think this is how you test a drain, someone chime in if i'm wrong. With ignition key out, doors closed trunk bulb removed or disabled, start to check a drain by removing the positive terminal from the battery put your tester on to amps and rig the tester between + battery post and removed battery cable. You should be reading zero amps, if you are getting a reading then pull your fuses one at a time and see when the amps stop flowing, then that circuit is the drain circuit, test them all.

I hope this helps you get started and trust others with more knowledge will chime in and add and correct.
Thanks for the information on how to check for a battery drain. The alternator in new as of a couple of weeks ago when I did the timing chain.
 
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