I am wiring my BMW 2002 during quarantine and I've encountered two questions I can't find answers to. To set the scene, I have two Bussmann RTMR fuse/relay units under the hood feeding about 100A worth of accessories. I have 1/0 wire running from the starter positive to the battery in the trunk and I'm using a 3-wire Bosch alternator.
1. Can I use the positive starter lug as a junction to draw power for the RTMRs? Will that harm anything?
2. Can I send the alternator output directly to the starter positive lug and through to the battery? My gut tells me no based on what I know about DC current, but my gut is often stubborn and ill-informed. I have attached an illustration of how I would like to wire this. I want to thoroughly understand why this is or isn't possible.
My e36 race car goes battery direct to alternator (through a kill switch) and starter using the same cable run which also feeds the main fuse/relay power so I dont think there would be an issue with that plan. My kill switch has studs I ran the lower gauge wiring for the fuse panel but the electrical system doesnt know the difference.
I believe stock BMW uses those engine bay positive power blocks to pull power from the main engine lead to feed the fuse blocks. Do you have any stock power distribution wiring to use or starting from scratch?
My dune buggy is wired that way. Alternator wire goes directly to the solenoid. Ignition switch is powered from the solenoid. It works.
Olemiss540 said:
I believe stock BMW uses those engine bay positive power blocks to pull power from the main engine lead to feed the fuse blocks. Do you have any stock power distribution wiring to use or starting from scratch?
I'm starting from scratch with the accessory wiring harness, keeping it independent of the factory harness until I move all that stuff into it. My '69 doesn't have a factory distribution block but most of the stock parts are long gone. I will use a kill switch for sure.
Saron81 said:
My dune buggy is wired that way. Alternator wire goes directly to the solenoid. Ignition switch is powered from the solenoid. It works.
Awesome, it sounds like this will save me laying another 1/0 wire. My concern was that power would be flowing against itself by traveling two different directions in the same wire.
No worries using the exisint lug/conductor.
Think of it as weight savings.
That's actually how a lot of domestic cars and trucks were wired for years. I'd put a fuseable link off the starter in the wires going to the RTMRs.
Curtis73 (Forum Supporter) said:
No worries using the exisint lug/conductor.
Think of it as weight savings.
12 feet of 1/0 will definitely offset a couple hamburgers.
APEowner said:
That's actually how a lot of domestic cars and trucks were wired for years. I'd put a fuseable link off the starter in the wires going to the RTMRs.
I had been wondering that. Someone raised the point that if it were possible OEMs would do it to save on materials. I'm wiring a circuit breaker ahead of the RTMRs and ensuring that the total draw on each RTMR is within spec. Thanks everyone for clearing this up.
Dame Judi Dench said:
Curtis73 (Forum Supporter) said:
No worries using the exisint lug/conductor.
Think of it as weight savings.
12 feet of 1/0 will definitely offset a couple hamburgers.
APEowner said:
That's actually how a lot of domestic cars and trucks were wired for years. I'd put a fuseable link off the starter in the wires going to the RTMRs.
I had been wondering that. Someone raised the point that if it were possible OEMs would do it to save on materials. I'm wiring a circuit breaker ahead of the RTMRs and ensuring that the total draw on each RTMR is within spec. Thanks everyone for clearing this up.
The advantage of a fuesble link is that you can put it right on the terminal and it protects the wire running to the RTMRs. On a lot of cars that wire runs very close to the exhaust manifold so there's a danger of it getting shorted if it's not anchored as well as you think or if it comes adrift in a wreck.Depending on the breaker you may be able to accomplish the same thing. The important thing is to think about what you're protecting and why.
