M42 electrical problem

[e30jr88]

Hey guys, I've been having an electrical issue with my 91' 318i(M42). I've had this car for a while(3yrs) but recently its been having electrical problems and I'm starting to give up on the car. The car keeps draining the battery on the car and what happens is that it completely just shuts off while running.

I have replaced, the alternator on the car twice and also have a new battery on there. I have also replaced the alternator ground and most of the engine grounds to make sure that's not the problem. One thing I have noticed with a multimeter is that when I turn on the AC the voltage drops about 1-1.5V, could there be something wrong with the AUX fan wiring?

About 2 days ago I disconnected the (-) cable on battery because I thought that would help not drain the battery(it was reading 12V before I disconnected) then today I connected it back and measured the voltage and read 11V, it stayed at 11V even after I started the car(alternator is less then a week old).

Any one have any idea on what could be wrong or what I should check next?

[roguetoaster]

Well, since no one else chimed in I would suggest unplugging devices one at a time or as groups and testing until you have found the electrical drain. If I were you I would start with the aux fan and move on to the A/C compressor wiring next. Instead of unplugging the actual cables you can just pull the fuses of course. While you are there you might as well look for blown, but not disconnected fuses, and burned relays.

As for what the actual cause is, I really don't know, but I have the feeling you will know shortly.

[bmwm42]

Make sure you ignition switch is actualy working properly

[Threehz]

I know one of our M42 experts had a mysterious problem that gave him ample troubles and it turned out to be the fuse on the positive cable running from the battery towards the cabin (it's in the trunk behind the trunk trim I believe). His was rusted pretty bad if I recall. Worth checking out.

[series88]

Mine was kinda having the same issue after pulling and re installing the motor. Turned out to be bad alternator ground.


***EDIT: just read the whole post again and you mentioned going through the ground NVM***

[e30jr88]

Well, since no one else chimed in I would suggest unplugging devices one at a time or as groups and testing until you have found the electrical drain. If I were you I would start with the aux fan and move on to the A/C compressor wiring next. Instead of unplugging the actual cables you can just pull the fuses of course. While you are there you might as well look for blown, but not disconnected fuses, and burned relays.

As for what the actual cause is, I really don't know, but I have the feeling you will know shortly.

What is the best way about doing this process with a multimeter? Connecting it to the battery and taking out fuses till the voltage changes or would I need different reading device that reads amps?

I had read some people adding an extra ground wire to the alternator so I did that 2 days ago and the voltage went up .5V. I will work on it this weekend and hopefully find out what is wrong.

[e30jr88]

I know one of our M42 experts had a mysterious problem that gave him ample troubles and it turned out to be the fuse on the positive cable running from the battery towards the cabin (it's in the trunk behind the trunk trim I believe). His was rusted pretty bad if I recall. Worth checking out.

I was curious about this too since I had noticed this fuse on realoem.com, just didn't know where it was in the trunk. I will look into this as well to make sure that it's working properly, thanks.

[bmwm42]

I highly doubt its the fusible link

[roguetoaster]

What is the best way about doing this process with a multimeter? Connecting it to the battery and taking out fuses till the voltage changes or would I need different reading device that reads amps?

I had read some people adding an extra ground wire to the alternator so I did that 2 days ago and the voltage went up .5V. I will work on it this weekend and hopefully find out what is wrong.

That's essentially what I would do, but I am not an expert.

It also occurred to me that the two wires on the alternator might be touching ever so slightly, had that problem once and it made me think the alt wasn't working properly.

[e30jr88]

Make sure you ignition switch is actualy working properly

whats the best way to troubleshoot the switch to make sure its working properly?

[e30jr88]

That's essentially what I would do, but I am not an expert.

It also occurred to me that the two wires on the alternator might be touching ever so slightly, had that problem once and it made me think the alt wasn't working properly.

I will try and do it that way hopefully I can find something this weekend. I will take a look at that as well since I need to make sure my alt belt is tight.

[Gregs///M]

You'll need to run a current load test. DO some research on how this is done but I will summarize quickly.

-You need to disconnect neg side of battery.
-install a amp meter inline. (must read amps, not voltage) Must be wired in "series" or inline with battery cable.
-Normal current draw with trunk closed, doors closed, everything off is around 30mA, no higher.
-Anything higher would be a unwanted current drain.
-unplug fuses one at a time until the current drain significantly drops. (reinstall fuse if no drop)
-Once you have narrowed which fuse has the current drain, reinstall
-unplug one device on that circuit at a time, until drain drops.

You have found the device that causes your drain, replace or repair. A test light can also be used in the multi meters place. normal drain is dim, unwanted drain is bright indicator.

Good luck.

[e30jr88]

You'll need to run a current load test. DO some research on how this is done but I will summarize quickly.

-You need to disconnect neg side of battery.
-install a amp meter inline. (must read amps, not voltage) Must be wired in "series" or inline with battery cable.
-Normal current draw with trunk closed, doors closed, everything off is around 30mA, no higher.
-Anything higher would be a unwanted current drain.
-unplug fuses one at a time until the current drain significantly drops. (reinstall fuse if no drop)
-Once you have narrowed which fuse has the current drain, reinstall
-unplug one device on that circuit at a time, until drain drops.

You have found the device that causes your drain, replace or repair. A test light can also be used in the multi meters place. normal drain is dim, unwanted drain is bright indicator.

Good luck.

Thanks alot for the information, I was able to find that the aux fan was draining some electricity when car was on or off since the resistor on the aux fan was broken.

[e30jr88]

Since I posted this on here what I have done is the following:

- I changed the resistor on the aux since when I would turn on AC voltage would drop by 1-2 volt range, after the resistor was changed it only drops about .5 V.

- added a ground to the alternator since I've read that this is good, and it did help stabilize the voltage while car was on stand still.

- later I was having an issue with the alternator not charging at all, after some testing and talking to one of my mechanic friends I found out that the excitation wasn't working properly.(basically the bulb in the dash that lights up a battery, had a bad connection.)
Without that light working properly the alternator will not charge. Since that bulb didn't work and we couldn't get it to work properly, we connected a dummy bulb in the engine bay to close the circuit.

After all this the battery has not drained in about 3 days and the alternator is charging properly with reading of 14.1V without AC and radio and 13.9 with them on.