Hi,

One of my two 79 300SD's has a short... I think...

I was attempting to remove the becker radio and put it in the other car but did not want to cut any wires so I replaced it without removing it. I did not put it all the way back in it is just sitting in the slot. Shortly after the battery went dead....

My mechanic said it might be an alternator issue so we replaced it and the voltage regulator with some used parts that I was sure worked... now I still have the same issue

Anyone know if it could be a short and what to look for? Is there an short in the area of the radio?

Thanks for any info!

This is not difficult to find, now that the alternator is ruled out.

Get an ammeter that can read milliamps.

Close all the doors and make sure the ignition is off.

Remove the negative battery cable.

Connect the ammeter in series between the cable and the battery negative post.

Read the meter. It will probably be somewhere above 100 milliamps.........maybe more.

Remove the fuses, one at a time, until the meter drops down below 60 milliamps. One of the fuses will be the clock.........so........be somewhat aware that the draw from the clock is normal........about 50 milliamps or so.

You are looking for a fuse that draws 100 milliamps...........it could be as hight as 300 milliamps with a serious leak.

Once you find the circuit that has the excessive draw, repost and we'll see which one it is.

Brian,

How would I do this:

Connect the ammeter in series between the cable and the battery negative post.

Thanks for your time!

Note the picture

Attached Thumbnails

 

Brandon's picture is fine.

However, I don't like to screw around with the postive battery terminal, preferring to use the negative terminal.

So, remove the clamp from the negative battery terminal.

Put one probe from the meter on the negative battery terminal.

Put the second probe from the meter on the negative battery cable (the one you just removed from the battery).

Now read the meter and perform the tests of removing one fuses, on at a time.

The meter must be able to read milliamps............down below 50 milliamps...............or you're wasting your time with this test.

And Start out with the highest setting on the meter (10amps) first before going with the milliamps settings...otherwise if you have a 1-2amp load on the battery your meter is fried. (or you will be looking for a fuse).

Have fun...

Thanks everyone for your help, We used the meter and found the battery drain was coming from the glow plug relay... The plugs were staying on and draining it.

Replaced it with a used one that fixed that issue but now the GP dash light won't come on... Checked all the GP and they are fine

I guess I have another bad relay but will go with it as long as I can or until I find another at a junk yard or...

Thanks again!

Are you saying the relay remained closed with the keyswitch off? Or was the battery drain caused during operation of the vehicle with the glow plugs constantly energized?