Switch for accessory/violet wires

#1

(yes, it’s a photo of a document that I downloaded and printed. sue me.) :slight_smile:

So I’ve got all my wiring stripped and labeled. I’m ready to start wiring up my dash switches and I’ve run into a little bit of a question/stumbling block right out of the gate. I’m wiring an “accessory” switch – effectively mimicking the first position of the key. This would power part of the instrument cluster, washer pump, brake switch, and, maybe in the future, a hazard switch,

What’s not super clear in that photo is that the violet wire running from the ignition to the S231 junction is a 16ga (1 mm^2) wire. But then there’s a 16ga and 12ga wire running from that junction to the fuse box. The 12ga wire then splits out to 3 fuses (15A, 30A, and 7.5A) and the 16ga runs to a 7.5A fuse.

Seems like a lot of current all possibly pulling from a single 16ga wire.

Thing is – I’m not really seeing how this wasn’t the case before I stripped everything out. Seems like – if all of those systems were running hot – that it’s possible for this wire to run too hot without any protection. Is this the case? More likely, I would think I’m missing something obvious.

Thanks,

Som

#2

Without studying that diagram, most cars have two spare fused leads out of the fuse box (one switched hot, the other unswitched hot), and in my case were left basically loose on the passenger side of the firewall near steering column.

I used those two to mount a pair of auxiliary fuse panels on the passenger A pillar and ran all my auxiliary stuff from them. Don’t recall the fuse ratings, etc. but has been rock-solid reliable.

#3

Interesting… there’s a “dotted line square” in the ETM for the “Auxiliary Fuse”. The dotted line says there’s more to it than just the 2 fuses – but there’s no information about the fuse rating, and I can’t find anywhere in the manual that provides more definition.

That said, I’m not sure I can really use that solution here since I’m trying to get power to systems that are already wired up through the fuse box. I guess I could, conceivably, reroute the fused power – through a switch – back through to the fuse box – but it seems like I could just as easily change the 16ga wire to something thicker.

Even so, my switches are only 30A switches – so having that full circuit get powered through the switch is still a possible problem. And even if there’s a fuse for protection, if the collection draw is high, it would keep tripping the fuse – which is still a problem, albeit not as dangerous of one.

I feel like I’m missing something here.

Som

#4

Here’s how I did it. I have one “Ignition” switch that turns on two relays. One relay takes power from the red wire (4 RD on diagram) to the violet wire and the other from the red wire to the green. Those orange relays found all over BMWs work well for this. Using relays is much better than routing all power directly though a switch.

While I can’t solely switch on accessory power with this arrangement, I don’t see a drawback in a racecar. I either want it running or I don’t. I’m not going to listen to the radio while parked or anything like that.

Also, my “Start” push-button switch receives power from the same red wire. No relays here, but the push switch is very heavy duty and rated to be used in such a manner.

I cut all these wires at the switch found on the end of the ignition key tumbler (located underneath the plastic covers on the steering column). Then spliced my switches and relays to those. I could easily revert back to a key setup simply by installing a new ignition switch harness (plugs into connection C200 in the diagram).

#5

Thanks for the feedback!

Totally makes sense to run the 2 wires through relays, given their significant usage. I’m leaning towards doing the same (though, I’ll probably go the 2 switch route).

What I can’t get my head around is – how is this supposed to work in it’s unmodified form? There are no relays for this, and it looks like there’s just the lone 16ga VI wire that runs from C200 to S231.

How does that tiny 16ga wire power so many subsystems that it could potentially draw 60A before a fuse going out? I see 4 options – listed in order of likelihood:

a) It doesn’t. There’s something I’m missing from the schematic.
b) It doesn’t. But the only way the subsystems could draw enough current would be for there to be a short, and that would trip the fuse prior to damaging the 16ga wire.
c) It does.
d) It doesn’t. It’s a design flaw, but not commonly encountered.

I’m still going to add the relays, mainly because I don’t have enough information to suggest it would be safe to do it otherwise.

Som

#6

Just remember that every fuse and relay are potential failure points, you should eliminate as many as you can. Why not just power up everything with a main ignition switch, like a kill switch? They make smaller kill switches rated for 100 amps. Just put a 350 amp fuse inline with the main battery power.

If you ever inspect any high end road race car, you wont find any fuses. If a component shorts out it usually burns up the component or switch. Without carpets or insulation in a race car there is not much to ignite. The easier it is to wire the easier it would be to diag future problems. Power what you want and eliminate what you don’t want.

Peter Thibault

#7

Interesting point. Not sure I have time to ditch the fuse box at this point.

I guess as long as the accessories can run off the 30A switch, I can forego the relays. Just need to beef up the wire, though. Figure I’ll run it the way I understand, rather than trusting a 16ga wire for all that current.

Som

#8

Gosh, don’t ditch all your fuses! I hope Peter wasn’t suggesting you do that. Why would you want to needlessly burn up components? Or god forbid, fry a wire deep within your wiring harness. That would be such a pain in the ass to diagnose. I’ll take the rare popping of a fuse which takes 5 seconds to replace any day over that.

As to that 16ga wire you’re concerned about, the ETM diagram for my 1988 325is shows that violet wire to be 2.5mm[sup]2[/sup] or about a 12ga equivalent. Perhaps it’s a typo in your ETM?

You can power the violet and green wires straight through switches. However, for reliability they absolutely need to be good quality switches rated for the job. Just a warning though. A friend of mine with an E36 race car did it this way and he burnt up 2 switches. The really annoying part was they didn’t fail outright, but instead caused inconsistent starting issues. I don’t know what brand of switch they were. They certainly weren’t cheap junk from Radio Shack.

Yes, there is a chance my added relays could fail, but personally in all my years of owning multiple BMWs I’ve never had a relay fail. Maybe I’ve been lucky. (I’ve definitely jinxed myself now.)

#9

Haha, yeah, I guess I meant “I don’t really have time to do it so I haven’t really evaluated whether it’s even something I want to do”. :slight_smile:

Regarding the ETM – interesting! I just checked out the '88 version and it does, indeed, say 2.5 VI for the exact same wire that says 1 VI on mine. I’ll have to go back and look to see if I have a loose 2.5 wire coming off S231. If not, it would mean BMW fixed this between years.

More than likely, though, I probably cut it and bundled it away because of the ETM typo.

I’ll check and report back. Would be kind of neat to find a case of a “bug fix” like this between years, but my money is on it just being a typo and there being a fat 2.5 VI wire that I have wrapped in electrical tape coming off S231. :slight_smile:

Som

#10

Just checked S231 – there’s only the one 12ga VI wire running to the firewall. That’s exactly what the 1987 ETM shows. The 1988 ETM shows two 12ga VI wires coming out of S231.

That seems to suggest to me that my “option (d)”, below, might actually be right – that this was a design flaw. Unless something else changed that resulted in more current draw between 87 and 88, I guess – but it doesn’t look like it.

Guess I’m beefing up that wire. :slight_smile:

Thanks for the help! :slight_smile:

Som

#11

You know… looking at the wiring some more – that wire powers Fuse 5 – which powers the wiper motors (I thought it was just the washer pump before).

Is it possible this tiny 16ga wire was powering the wiper motors? Any known issues of extended wiper use causing electrical problems on the 87 models?

Anyways… no biggie. I’m going thicker on the wire. :slight_smile:

Som