Electrical gremlin - fog light

So the patient is a 1979 BWM with many previous owners. The headlight/fog light wiring is a Frankenstein of botched old mods.

For example, when I first got the car, if you flashed the high-beams on the headlight with the fogs on, it blew the fuse for the cabin lights.

Current problem: The left fog light uses a 55W H3 12V bulb and it fried. When I test the ground lead to the bulb, it's solid. When I connect the fog light hot lead to the bulb, it's not bad -- about one volt below B+. The fog light relay functions fine, and voltages at the relay end of the circuit test good.

The right fog light is working fine. At this point I don't know whether the left side is wired in series with, or parallel to, the right side.

With a new bulb installed in the socket, I measure the voltage at the fog light B+ lead (connected to the hot side lead of the bulb) as 20-something millivolts. If I disconnect the bulb's hot lead from the B+ connection, it measures regular (i.e., 11 volts running B+ off a battery that's down to 12 volts). The bulb doesn't light. But it will light if I jumper 12v to the hot side of the bulb off another B+ connection.

So, testing the ground and B+ leads to the fog light bulb socket is hunky dory 12-volt-ish, but when I put the bulb in the circuit, it drops to more or less nothing?

Hmm, maybe the fog light housing is grounding the bulb socket to the chassis somewhere. I should check that.

It's always a ground problem. That's what I tell friends when trying to diagnose electrical problems and it ends up being right pretty often. From the description of the harness it sounds like it wouldn't be a bad idea to just start over from the relays and rewire.

You have excessive resistance on the power side of your left fog light bulb. The lights are in parallel, if they were in series they would both be on at half voltage (dim). I'd trace back look for a bad connection between the left bulb and the fog relay.

What model BWM/BMW is it? Wiring diagrams for many models can be found online.

Hi Stuart, it's a 323i euro. But all semblance of stock wiring scheme is a distant memory (I have the stock diagram for the E21).

If it's a euro car, chances are the wiring has been screwed up since it was imported... I know how it is, my euro M535i has all kinds of funny stuff going on in the wiring harness that was done to make it EPA/DOT compliant.

I assume the e21 used relays to operate the fog lights? In my car there's a separate fuse and separate relay contact for each fog light.

My old '70 Mustang had a problem in the back up light circuit that would sometimes blow a fuse. One of the bulb socket housings turned out to be defective and shorting to ground intermittently. It took a while to find that.

But in this case I think the suggestion to just start fresh with the wires is the way to go.

HAhahahah this E21 does that too; the back-up lights are currently out yet again. Last time it was a short to the chassis in the wiring to the reverse switch, where the insulation wore off. For some reason, that short blew the fuse to the instrument panel. But this time, the instrument panel is fine, so my short must be somewhere else.

Oh the joys of the car with many previous owners.

Euro M535i? Damn, Daniel, do tell us about that, please!

It could be a bad ground, but the fact that it blows fuses makes me lean more towards a short to ground on the positive side. The drop in voltage while under load could be bad connections on either side of a circuit.

If you had a battery cable that was severed through 99% of the wire you would still get 12V on the other side, but as soon as you load the circuit the voltage will drop considerably because of the restriction in current flow.

I would yank all the fog light wiring from switch to light and replace with an easily purchased universal kit on fleabay. Save yourself a -lot- of trouble trying to track down any and all silliness in the butchered harness

JBasham wrote: Euro M535i? Damn, Daniel, do tell us about that, please!

Not much to tell...the e28 M535i isn't much different than a regular euro spec 535i, it's more of a trim option than anything else. It has the Mtech body kit, rear spoiler, sport seats and other various bits and pieces. It does have specific Mtech springs and came with specifically valved Bilstein shocks from the factory. Being a euro car it has a higher compression engine with more hp than a US car (factory rated at 218hp vs. 178hp).

They are pretty uncommon in the US since they were all grey market imports, a few more have probably been imported now that they are old enough to bring in without having to Federalize them.

Fixed it.

The grounds were fine.

Each lamp has it's own B+ lead from the fusebox; same relay but separate fuses.

The wire/connection from the fusebox to the left lamp was messed up and had a high resistance. With a super-low current draw through the circuit (the volt meter draw), it conducted pretty well - only 1 volt below B+.

But at the high current draw occurring with the bulb in the circuit, the high resistance blocked the voltage (Ohm's Law, voltage = current x resistance). D'oh!

Run_Away got it in one. Thanks.

Jamey