Electrical short - front power distribution?

#1

I was installing something last night and wanted to make sure there were no shorts between pos and neg in the installation. Sure enough there was resistance. I started tracing the wiring from the battery forward. I traced forward to the junction and determined one of the two legs of smaller guage wiring that bolts to the larger guage wiring (with the battery cable-style lug) has a short.

The leg I am referring to is the one that has the smaller of the two lugs that attaches to the battery cable-style lug at the junction (there are two bolts that connect to this connector, one 6MM and one 8MM I believe — the cable in question is the one that connects to the 6MM bolt). Looking at the ETM I cannot determine exactly where this branches off to.

Assuming you followed what I am referring to, can anyone provide some direction on what this particular leg feeds to? I wasn’t able to test it further last night, but I hope to get out there tonight or tomorrow. Some direction may be helpful versus chasing everything.

I haven’t added any electronics, so I’m afraid something is abraided. This was not a problem at the end of the season that I am aware of, as well. First tornadoes, then 5" of snow and now this.

Thanks!

#2

I think that one of those wires goes to the master power relay to become “switched” 12V, and other one goes to the fuse box to become “always on” 12V. I don’t recall which is which tho. If you pull the master relay you can probably figure out which is which.

#3

Check out this link - http://www.armchair.mb.ca/~dave/BMW/e30/e30_89.pdf

Page 0670-2. One of the points of distribution is starter/alternator circuit.

Ed

#4

If I had to guess I’d say that “switched 12V” is the bigger wire so the one with the 6mm end would be the car’s “always on 12V”.

#5

Thanks guys. That helps. I will proceed in that direction.

I’ve copied a shot of the ETM below. It looks like the smallest wire goes to the fuel pump relay.

In order of size: biggest to starter, next to fuse box, then to diagnostic connector, and the smallest to fuel pump relay (if I’m reading it correctly. I guess I know what I’ll be doing this weekend…

[attachment=1790]Powerdistribution.jpg[/attachment]

Thanks!
John

#6

[quote=“hans.ackerman” post=63918]Thanks guys. That helps. I will proceed in that direction.

I’ve copied a shot of the ETM below. It looks like the smallest wire goes to the fuel pump relay.

In order of size: biggest to starter, next to fuse box, then to diagnostic connector, and the smallest to fuel pump relay (if I’m reading it correctly. I guess I know what I’ll be doing this weekend…

[attachment=1790]Powerdistribution.jpg[/attachment]

Thanks!
John[/quote]
That’s how I’d interpret the diagram also. That’s kind of at odds tho with what I found when I’ve pulled things apart a time or two. Strange.

#7

Would it be possible that your fuel pump is getting long in the tooth and therefore you are reading some resistance? Is that not how some motors die?

#8

I guess I should have posted earlier…

The small wire from the battery positive post supplies power to the engine management system. The DME has both switched and un-switched power suppled to it by that wire. A resistance check will always "fail"because of that. What really matters is the parasitic draw when the ignition is off. As long as the current draw from the battery is no more than 60ma everything is okay. If present, the cluster and OBC also draw a small amount of current with the ignition off.

#9

Thanks, that is very helpful. The resistance is actually under that 60mA value (somewhere around 55mA).

Michael’s point is interesting about the fuel pump.

FWIW, I took a better meter to the car yesterday. Using the “beep-for-continuity” function of the meter it indicates continuity. Unfortunately, I wasn’t able to dig any deeper as my wife and child are sick and needed me inside the house.

I assume even if some resistance is normal there shouldn’t be continuity between positive and negative, right? I apologize in advance for my lack of knowledge here.

I hope to get back to the car before the weekend, but…

Thanks!

#10

Continuity or resistance checks on an E30 are meaningless if the OBC, cluster, or DME are present and connected. Each of those receives un-switched power and will result in some resistance between the positive and negative battery cables. The radio also gets un-switched power, but I presume that has been removed.

What matters is the current draw from the battery when the ignition is off (the parasitic draw). That is best measured from the battery ground cable to the battery negative post. Normal parasitic draw on BMW is 30-60ma.

#11

Okay, great, thanks! I’ll disconnect those and check just to be sure, but it sounds like it’s a non-issue.

I’ll check and let everyone know.

Thanks!
J

#12

The continuity business isn’t going to fly. That’s really a resistance check with some unknown threshold of resistance triggering “continuity”.

Checking resistance on these devices isn’t a great approach either because there’s active components all over the place and measuring for resistance will give misleading results. Imagine, for example, measuring the resistance across the primary of a relay, which is just an electromagnet. You’d get little or no resistance and maybe interpret that as a short.

Jim’s solution is best. Get a decent multimeter that can measure at least 10A and than put that in series with the suspect power sucking wire. Ideally, that will lead you to the culprit…or you might find that the problem is intermittant and will therefore be a bastard to find.

10A @ 12V isn’t much so be careful you don’t blow the multimeter.

#13

Thanks, Ranger.

I have a good Fluke digital, which I think will suffice. With the meter in series am I looking for voltage or amperage draws (or does it matter)? I’ve only ever done it as Jim suggests at the battery ground.

Thanks for all the help guys!

#14

When you put the meter in series with the battery negative post and negative lead you have have the meter set up to measure current. It makes no sense to do a voltage measurement that way. Start on the highest current measurement range that the meter supports to (hopefully) avoid damage to the meter. A Fluke DMM has a 10a capability and is where you start.

The reason to do the test using the negative cable is that noting “exciting” will happen if something gets shorted to the car body.