Fusible link near battery


At Annual Tech last month my car failed because the kill switch didn’t kill the car. I don’t mean kill in the Road Atlanta Miata punt-n-run sense.

It came as a surprise that my kill switch…didn’t. It certainly used to… And the only thing I’d messed with the wiring junction on the right firewall when I was putting in a fan relay a couple months ago.

So after my car REALLY died an hour later, I had time to go around and take some pics of how other people had the wiring junction, well, wired.

What I found is that there is a fusible link back near the battery that was blown. As you probably know we have 2 wires coming from the battery. The big wire goes to the frong wiring junction at the front firewall, and then goes to lots of other places. That’s the wire that everyone interrupts with their kill switch. The small is the one with the fusible link. It goes goes to the wiring junction too, but it’s isolated from the big wire at the junction, and goes to the fuse box, and from there to the ECU maybe. I heard a rumor that the ECU likes it’s own power supply.

I think the the PO was forced to wire the firewall wiring junction funny because the dead fusible link was at the time some electrical mystery. Then I R/R the wires on the wiring junction a couple months ago, do it right, but the isolated wire at the junction isn’t hot so I had to put in a jumper. It seemed pretty strange that the jumper was required at the firewall, but it worked. A little too well.

The failure of the kill switch forced me to look harder at why the small wire from the battery was cold. And that’s when I found the fusible link.

In the pic below look closely at the battery wire just to the right of the fire extinguisher mount. The fusible link creates a thick spot in the plastic insulation.


Here is a close up of the blown fusible link. This I didn’t have in my spares. I’m not sure what the current capacity of the fusible link is, but after looking at some 30A fuses, I’d estimate it at 90A. To fix it I bridged the link with about 1" of wire of thinner gauge then the wire it was protecting. I had to guesstimate the cross sectional area of my wire vs. the blown fusible link.

Damned thing cost me all afternoon.


Scott - have heard of that blowing, not (yet) happened to me. By the way, I believe that you should intereupt the big and small wires with the kill switch. If you search the forum, there are several posts on this issue. The car may continue to run if you don’t interupt the small wire.


I’ll have to look up those old threads I guess. BMW clearly wanted the two wires electrically isolated, after the battery anyways. The reason my kill switch failed is that the two wires were not isolated. That tells me that the generator can run the car if you connect these after the kill switch. So that means a 2 pole kill switch and I don’t think that’s what folks are doing.

But I’ll have to go find those threads. I’m probably just missing something.


Scott - here’s the link.

You will have to read through it as the correct diagram is not the first one shown in the thread.



Thanks Ed. -S


Ranger, fuse part # 12 41 1 706 111 (50amp). If you follow the red wire protected by this fuse, it takes you to the main relay. Main relay feeds the control unit and fuel pump relay. If you kill the battery but not the main relay, there is always a possibility of power looping through this wire to the control unit, which means the engine will continue running. That in addition to the power supply of the alternator… :slight_smile:


Scott, It’s a 50A fusible link (RealOEM).

I replaced ours with an inline fuse holder from Radio Shack (intended for car stereo amps) back when we popped ours due to the original owners direct (ie. no fuse) fan wiring for manual fan control. No fuse=bad, thank god for the fusable link or everything electrical would have melted down.


Ranger wrote:

Those of use that understand the car’s electrical system use a 3-pole kill switch. One pole interrupts the main battery lead, another interrupts the DME power lead and the third pole (which has normally closed contacts) is used to shunt a resistor across the alternator output. The resistor will absorb any spikes from the alternator as the engine winds down and avoid damage to the car’s electronics.

Since the alternator output is connected to the starter main lug it will suffice to jumper from the load side main lug of the kill switch to one pole of the normally closed switch. The resistor then connects to the other side of that switch and then to ground.

As an FYI, the separate DME power lead is intended to reduce noise on the DME’s power supply and to avoid the voltage drop that occurs on the main lead during starting.


Just to follow up on this, because I am in the middle of designing my kill system. The reason there would even be a need for a resistor to shunt voltage spikes is because in stock trim, the battery’s internal resistance is used for that purpose, but since we are unceremoniously removing the battery from the circuit with the relay, we probably need to add something back that can “replace” the battery, but not provide power to the circuit. A resistor is just the ticket for that.


Very few of use bother with the resister. I would go for the simplest possible solution because complexity adds failure points.


Replace the fuse with one that is easy to view. Leave the wiring alone. Race the car. Repeat.



I can put in a durable fuse holder and replaceable fuse for the fusible link. I have some from my old car audio days. No on the resistor? Okay.