Vaguely described electrical issue...

[Eecen]

Guy is selling an E30 ~2 hours away from me. I called him today, and will be going to see the car this weekend.

The car has an electrical gremlin...yay. Other than this it looks good, with a fair price.


These are the details he gave me. ('84 318i w/ m10.)

-Car died.
-Taken to a shop, new alternator.
-Car dies again
-Taken to the shop again. New alternator x2, new battery. The car was also left idling for ~7hours, no issues.
-Drives the car around, makes a few stops.
-Gets on the highway and the car dies. Lights fade at first.
-Gives up on car, it sits for the winter, he posts it for sale. (has 2 other cars, doesn't need this one).

Doesn't remember if the bat. light came on or not. Doesn't sound very mechanically inclined.



This sounds dumb to ask, but could there be any causes other than a bad charging system?


I'm assuming there is a bad/intermittently bad ground somewhere...or idk I'm not too good with electronics yet.

I'll be bringing;
DMM
Tools
Spare charged battery (wonder how many batteries I'd have to bring to make a 2 hour drive @ 100kph w/o an alternator)
At least one spare ground wire.


Anyone wana give some advice on my vaguely described problem:D?



Off to read the etm.

[Sagaris]

Some sort of current draw issue, not worth the hassle in my opinion. If it was a cheap M3 or 325is then I MIGHT give it a shot but that is just me. I have fought electrical gremlins in the past and rarely won.

I would say learn how to measure current draw with an ammeter and begin unplugging fuses and relays to determine what circuit the large current draw is on, then focus on that circuit.

[m3recon]

And bring a spare harness just in case!

[LJ851]

Take a look in the wiring diagram to see if the batt light is in series with the alternator (energizes the alt.) I just found out from a buddy that my 85 911 does this as do a lot of cars from that era. Bad bulb or connection = no charge. Could give the intermittent charging you describe. If you do go get it , take a voltmeter and wire it in so you can catch problems before you are S O L

[DREWHALL]

Was just going to say what LJ said. The battery light is typically in the circuit with the exciter wire to the alternator. When the alt starts pumping out voltage it no longer acts as the ground for the battery light, and voltage cannot pass through the bulb either direction causing it to stay out. If you remove the wires to the alt you should see b+ obviously at the large battery cable but with the key in the on pos b+ should be at the smaller wire as well. If not there is a problem with that circuit. I have seen that wire grounded out and kill alternators over and over. They didn't last 7 hours, more like 5-15 minutes. As stated try hooking the DMM in series with the battery set to amps, make sure the key is off or it may blow the fuse in the DMM. If it has current then it obviously has a short to ground somewhere and it's draining the battery. Check the engine to chassis ground for high resistance, and the alternator to engine as well. There isn't much in that charging system so it shouldn't be too hard. A draw would have to be massive IMO to kill an alternator.

A quick trick to see if the battery light circuit is in working order is to disconnect that wire from the alt and ground it with the key on engine off. Do not ground out the b+ cable obviously. Grounded the battery light should come on. If you disconnect that wire and it stays on, it's shorted to ground before the alt.

[Eecen]

..

Thank you!


And yes, from the bentley;

On models built up to late 1986, a burned out
alternator warning light will prevent the alternator
from charging. On late 1986 and later models,
a resistor is wired i n parallel with the alternator
warning l i g ht. This will allow current to
reach the alternator d u ring starting when the
warning light is burned out

[Ryann]

Sounds like you may have it figured out, but I thought I'd mention that an alternator without a good ground connection won't charge the battery. This has happened to me a couple of different times with bushing mounted Bosch alternators.

[Jaxx_]

rev it to ~3k and and the alternator will turn on without the exciter circuit even connected.

There are two wires on the alternator, yes, but one is the large one that goes to B+ and the other is smaller (blue) that is D+.

D+ is the exciter circuit. This basically starts the alternator.


Electrical issues are time consuming but relatively easy to troubleshoot I think. I prefer doing electrical work to getting my hands dirty, so maybe it's just me.

[Jaxx_]

Sounds like you may have it figured out, but I thought I'd mention that an alternator without a good ground connection won't charge the battery. This has happened to me a couple of different times with bushing mounted Bosch alternators.

yes, some 318s need a separate ground wire.