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After some excitement last weekend during a 'parade lap' at COTA (+ cable from battery in trunk shorted on top of transmission!!!  Burned through the ground cable <smaller gauge wire>), I plan to add a fuse (100-150 Amp) at the battery, and a battery cut-off switch too.  I'm wondering if there's a preferred 'order' to these pieces?  

I'm thinking that the cut-off switch should be before the fuse, allowing me to kill power to the fuse block in the event that I need to replace the fuse, but if I reverse that order (fuse between battery and cut-off switch), then the switch is also protected if it shorts out.

Maybe I'm overthinking this, but I wanted to ask the community if there's a 'right' way to do this...

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some suggestions- I never run the battery cable under the car where it can be damaged.  I run it thru the passenger compartment but not near the fuel line.  Slip a rubber hose over it anywhere it can chafe (some for fuel lines).  For track cars we always install a cutoff.  I put it at the A-pillar by the driver.  You need to wire the ignition to the switch as well if you want the engine to stop when you turn it off.  I think a fuse is a bad idea.  The starter draws a lot of amps and the fuse will probably blow leaving you dead on the grid.

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33 minutes ago, tech71 said:

+1, the right way is to properly route, secure and chafe protect. Done properly, a big ass fuse is pointless and redundant.

This is true as long as you can guarantee you'll never be in a accident and have covered all the other possibilities, after all your house is wired to a code that's over 100 years old, inspected,  with wires secured and shielded, yet fuses are still required because shit happens and always will. 100-150 amp fuse is probably to small try a 250 amp fuse after all your trying to protect a dead short on the main cable yet provide the amps for a cold start.

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Meh, the force required to distort the cars structure enough to short a properly installed cable is probably not survivable anyway but your car your choice.

no harm in a false sense of security I suppose😉

Edited by tech71
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22 minutes ago, Son of Marty said:

This is true as long as you can guarantee you'll never be in a accident and have covered all the other possibilities, after all your house is wired to a code that's over 100 years old, inspected,  with wires secured and shielded, yet fuses are still required because shit happens and always will. 100-150 amp fuse is probably to small try a 250 amp fuse after all your trying to protect a dead short on the main cable yet provide the amps for a cold start.

I plan to start with a 100 Amp fuse, and I also have 125 and 150...the fuse holder is good up to 400 Amp, so I'll work myself up to what seems to work in all my normal situations...BTW, I don't race on a track, we were just doing a parade lap after a classic car show.

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I've chosen to leave the battery up front, and went with a panel of 4 main fuses under the hood, as part of a complete rewire.  I didn't put in a cutoff switch. 

 

The fuses are from the junkyard- as used in the E39 battery fuse panel (just above the battery in the trunk).   The panel is made from two layers of very thick G10 epoxy circuit board material, with the studs firmly bolted fast to the front layer and the back layer milled out to give clearance for the bolt heads. 

 

The picture shows it during my "bench test" phase- the battery lead is replaced by a much smaller wire that goes to a power supply with internal fusing that I'm using to sort out the various systems.

 

The big fuse at the right goes to the starter (not attached yet).  The other three are for trunk electrics, cabin electrics, and under-hood electrics as shown from left to right. 

 

I've found  fuses in the E39 with 200A and 250A ratings for the larger fuse, and 50A and 80A for the smaller size.  IIRC the smaller studs are M5 and the larger are M8.  I have not run this but expect that the 200A fuse will be adequate for the starter.  If not, we'll try the 250A.  IF that doesn't work, I'll have to buy a bigger one!

main fuse block 1120.JPG

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I bought a race car with the battery switch on the driver's side.

 

After looking at it a bit, I did the next one with it on the passenger's side.

(that is also really easy to do with an E30.  Because I think THEY were thinking that way when

the car was designed.)

 

Because, really, do you want that much heat next to your face?

Most shorts are about the heat and the arc, and secondarily, about the battery explosion.

Most shorts are caused by a wrench, with a screwdriver a not- too- distant second.

 

If you look at a more modern BMW, the 'disconnect' is done by a one- time explosive 

(it doesn't need to be used much)

and there are fuses on everything except the starter

(and the big ones look like Cal's, above, and are in the trunk)

 

On the street 2002's, I put a 50a marine circuit breaker between the battery

and the hot red circuit, and hope it'll pop before the headlight wiring

sets the whole harness on fire.

 

Hope springs eternal.

Ford springs sag.

 

t

 

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My e30 battery cable only reaches from the trunk to about the servos, which doesn’t really work on my Tii as the air canister gets in the way, might look at lengthening it as I would like a junction point where the battery currently sits,  has anyone added length? Or perhaps I can run it down the drivers side, and then run the starter etc from there, mine is RHD.

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Hi Sydney, rhd here too.

Slightly different install for me.  Battery in the boot, positive feed down the drivers side following the old fuel line route.  Then up and across the dash via an isolating switch and then into the engine bay near the other cables.

 

Dont be frightened to cut the cable and crimp on new lugs.  Use a cheap eBay hydraulic crimper or call into your local auto electrician.

 

I’m using a bulkhead fitting to pass through the firewall.  Maybe something like that will let you add additional length from a terminal?

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2 hours ago, SydneyTii said:

My e30 battery cable only reaches from the trunk to about the servos, which doesn’t really work on my Tii as the air canister gets in the way, might look at lengthening it as I would like a junction point where the battery currently sits,  has anyone added length? Or perhaps I can run it down the drivers side, and then run the starter etc from there, mine is RHD.

Suggest you route the heavy gauge wire from the battery direct to the starter motor...thats where the Amps are needed. Then you can run a much lighter gauge wire from the starter to the wiring loom/alternator. Thats better than trying to mimic the original battery position with another high-current connection that needs protection and mounting. (This is how Rolls Royce did the Silver Shadow in 1965 - with trunk mounted battery)

Edited by dlacey
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