'92 602.962 losing 2.5 AMPS when not running - PeachParts Mercedes-Benz Forum

srbays · #1 05-06-2010, 09:03 PM

Dear Brethren,

New MBz battery showing 11.16 volts when off or at idle (yep, it's dead all right), up to 12.5 volts at >2000 rpm, so 2 year old Bosch alternator may be ok. Became suspicious when 2nd new battery discharged while sitting at the office all day.

Inductive DC meter reads 2.5 to 2.78 AMPS discharge across both positive and negative cables (measured individually of course) with everything turned off and the car not running.

Any thoughts?

Thanks,

Sam Bays

aaa · #2 05-06-2010, 09:17 PM

First stop: fusebox.

Of course, after trying all the fuses one by one I found out it was flaky alternator wiring.

babymog · #3 05-06-2010, 10:58 PM

I looked at a friend's '92, same issue, it was a shorted OVP relay (water in it).

layback40 · #4 05-07-2010, 07:31 AM

Sam,
Make sure your trunk light is going out when you shut it.

pimpernell · #5 05-07-2010, 05:52 PM

Quote:

Originally Posted by srbays

Dear Brethren,

New MBz battery showing 11.16 volts when off or at idle (yep, it's dead all right), up to 12.5 volts at >2000 rpm, so 2 year old Bosch alternator may be ok. Became suspicious when 2nd new battery discharged while sitting at the office all day.

Inductive DC meter reads 2.5 to 2.78 AMPS discharge across both positive and negative cables (measured individually of course) with everything turned off and the car not running.

Any thoughts?

Thanks,

Sam Bays

Had almost the same thing happen with my 96E300d this past winter. I have a scanguage attached all the time, and the voltage was indicating 12-12.5 at almost the same rpm that yours did. Parked the car overnight, next morning, the battery was almost dead. Removed battery, charged it up, put it back in, and in 6 hrs it was almost dead again. I again removed the battery, recharged it again. This time I decided to check the alternator itself. With the engine off, I disconnected the battery cable, and the rest of the wires from the alternator. I took my multi tester, in continuity mode, touched the alternator case, and the alternator battery cable post, and got a positive continuity reading. Apparently, one of the diodes had shorted, and was allowing current to flow from the battery to the engine block via the alternator case. Replaced the alternator, and all has been well ever since. Hope yours turns out to be and easy fix.

Jeremy5848 · #6 05-07-2010, 10:13 PM

Pump for door locks stuck "ON"?

srbays · #7 05-08-2010, 10:04 AM

My Friends,

Thanks to all for your valuable insights.
Update:
-New battery out of the car and charging up at 6 AMP/hr on two-stage charger, after four hours it is up to 50% (about 12.2 volts).
-With the alternator disconnected there is no continuity between any of the three alternator terminals and the alternator frame (ground).
-With the alternator disconnected there IS continuity between two of the alternator harness terminals and ground. Is this correct? I only expected one ground in the harness.
-Pulled OVP relay; all dry and "normal", fuses okay. De-energized, the relay only has continuity between poles 30 (power) and 30a. Other four poles are not active so the relay is open (normally open I suppose).

Pump for door locks stuck "ON"? Sorry Jeremy, I don't understand. From my vacuum tracing I believe the locks are purely mechanical/vacuum actuated.

Thanks again,

Sam Bays

srbays · #8 05-08-2010, 10:09 AM

Oh, and the trunk light is verified to be "out" while the power is draining.

Thanks,

Sam

babymog · #9 05-08-2010, 10:17 AM

Locks have a pump in the 124s, 201s, 126s (gen II anyway), etc.

2.5Turbo · #10 05-08-2010, 02:45 PM

It's hard to hear it running because it's covered in sound insulation under the passenger side of the rear seat. All the door lock vacuum lines run to that location.

Junkman · #11 05-08-2010, 02:58 PM

Try isolating the circuit by: 1) connecting ammeter & measuring drain. 2) pull a fuse until you find a fuse that reduces the drain. 3.) Disconnect load items on that particular circuit.

Confirm accuracy of gauge before you travel far along this path.

layback40 · #12 05-08-2010, 08:08 PM

X2 with Junkman on diagnostic method. Fuse pulling will soon find the problem if it is fused. You are much better off looking for the actual leak path than testing conductivity.

srbays · #13 05-08-2010, 09:48 PM

Thanks to all.

The alternator doesn't show a dead short between terminals and frame, but the leak only occurs when the alternator harness is connected so I'll take the 18-month-old (just off warranty) Bosch to an alternator shop on Monday to see what's wrong.

Best regards,

Sam

bigblockchev · #14 05-08-2010, 10:55 PM

Use your DC inductive ampmeter to check any and all bundles of wiring. you don't have to separate the wires individually to do this until you find one with current flowing. This is the beauty of the the inductive meter. I have one and it has made troubleshooting this kind of thing much easier Try every bundle under the dash and elsewhere. Cheers Dan

pwogaman · #15 05-09-2010, 03:39 PM

Quote:

Originally Posted by srbays

Oh, and the trunk light is verified to be "out" while the power is draining.

Thanks,

Sam

Don't forget to let your "assistant" out now.