I’m having trouble with my battery going dead on my 300CD. I recently replaced the voltage regulator. My favorite electrical shop says the alternator is otherwise OK. Main question is what should the system resistance read across the battery leads when they are disconnected. I am looking at about 3k Ω using an analog VOM. The digital meter gives a really varied reading. I removed each fuse with no change in the reading. So, is 3k Ω normal? Any suggestions? - JA

What are you trying to do?

If you suspect an electrical "leak" you want to measure milliamps with everything in the vehicle shutdown.

Put an ammeter in series with one of the battery leads and see what you get.

OK So I see about a one second pulse from 1.5 to 4.5 mA. Don't know where zero is.

4.5ma is negligible. In fact, I'd be surprised if it was that low. The clock draws more......

If the reading is correct, then replace the battery, It's not holding a charge.

Battery is also new. Should I reconnect it and see what happens?

If the battery is new.....and it not defective.....you must have more than a 4.5ma leak.

What type of meter are you using?

I haven't connected the new battery except to drive onto blocks to facilitate access to the voltage regulator. I'm using a small analog VOM instead of my fancy digital meter. I guess I could drive the car again and keep the jumper cables handy.

Usually a small analog VOM won't measure milliamps. You typically need a big analog VOM or a digital meter.

The amount of current draw you describe when the car is off doesn't sound like much. If we assume your battery is good and your alternator is good let's look for a bad connection, here is what you should try next. Start the car and let it run for a minute or two. At idle, check the battery voltage across the battery posts at tell us what it is. (It is important you do this at the actual posts of the battery and not the cable connector. Also check the voltage from the alternator to a ground on the engine block or the block it self. It is difficult to place a probe on the back of a MB alternator because of the well engineered connector. But you can trace where the two wires come to the fuse on top of the fender well and check it there. Once again, let us know what this voltage is. (These two voltages should be identical.)

Next step is the bring the car up to a high idle (the right foot of a friend may be helpful) and turn the head lights and heater blower motor on high. Take the same measurements. Also take the measurements from the negative battery post to the engine block. (remember you are measuring voltage.) Also measure from the battery post to the edge of the battery terminal cable connector. Do this for each post. This may seem wierd to have the probes only a couple of inches apart on the same part of the electrical system that you know to be the negative or the positive side. What you are looking for is voltage drop. If you are seeing any difference in voltage across points of the system that you know should be the same voltage, then there is resistance at that point. This could be in the form of a bad connection, corroded connection and such. These bad connections can prevent your battery from charging properly.

Let us know what you find.

TimK

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83' 300D Turbo - 300K + Miles
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85' 6.5L Turbo, GV-OD, GMC Suburban,
67' 2.5L Diesel NA - Land Rover Series IIA

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Brian and Tim,
Thank you for guidance.
First, I apologize for my hasty reply that the ‘leak’ was 4.5 mA, I now see ~2.3 A with my good digital meter.

At idle, check the battery voltage across the battery posts at tell us what it is.

12.2 V at idle

Also check the voltage from the alternator to a ground on the engine block or the block it self.

Next step is to bring the car up to a high idle and turn the head lights and heater blower motor on high. Take the same measurements.

13.5 V (13.7 V with no lights or blower)

Also take the measurements from the negative battery post to the engine block.

Both ~0 V

I think the voltage regulator and battery connections are good. I don’t understand what I am now seeing for leakage current – It was a constant 2.3 A, but now it is 2.2 A when initially connecting the meter, then dropping to near zero after 6-8 sec. This can be repeated by reconnecting the meter after a few seconds.

Perhaps the problem has gone away after all the unplugging and reconnecting (I hate when that happens). Never did find IT.

Unless you have more ideas, I guess I’ll drive the car and keep the jumper cables handy.

Thanks,
Jacob