battery cut off switch

[seatown88]

I use a 2-pole kill switch, which interrupts the battery cable and the DME supply separately.

I just finished welding in the kill switch plate and mocking up the wiring for KEVORM's PRO3 build this evening.

[!kid]

I use a 2-pole kill switch, which interrupts the battery cable and the DME supply separately.

I just finished welding in the kill switch plate and mocking up the wiring for KEVORM's PRO3 build this evening.

what are the two smaller wires? if one is the small battery positive and the other the DME, thats how I had my last switch hooked up. and the small poles failed.

[jlevie]

what are the two smaller wires? if one is the small battery positive and the other the DME, thats how I had my last switch hooked up. and the small poles failed.

That is a fairly common failure. As I said earlier the low power switch(s) aren't rated for as much current as the DME draws. The switch will work for awhile, but it will eventually fail.

[!kid]

That is a fairly common failure. As I said earlier the low power switch(s) aren't rated for as much current as the DME draws. The switch will work for awhile, but it will eventually fail.

Understood, I do not plan on doing it that way again. I made another drawing on the first page. Can you let me if it's correct?

[seatown88]

interesting, good to know. What is the rating of the fusible link?

[Jparkr]

I'm still not understanding why this is so complicated.

Splice the two positive cables together and run them into and out of the main switch. this will kill any and all power to the block.
Then look up the wire that supply's ignition power to the DME and run it through the secondary circuit as well.
Cap the remaining two pins you did not use since you have 2 sub circuits.

Im just not following the resistors. It is a kill switch. It should make the entire car Off or On. why over complicate it.

[jlevie]

interesting, good to know. What is the rating of the fusible link?

The fusible link is rated at 80 amp. I don't know what the peak current draw is, but I have one measurement at idle of 45A. I imagine that the current draw is greater at high rpm. I do know that replacing the fusible link with a 60A fuse works.

I'm still not understanding why this is so complicated.

Splice the two positive cables together and run them into and out of the main switch. this will kill any and all power to the block.
Then look up the wire that supply's ignition power to the DME and run it through the secondary circuit as well.
Cap the remaining two pins you did not use since you have 2 sub circuits.

Im just not following the resistors. It is a kill switch. It should make the entire car Off or On. why over complicate it.

BMW powers the DME directly from the battery so that voltage drop on the main cable during cranking doesn't affect the DME. This may not be a problem on a race car as they generally aren't used in cold weather when the starter draw (and voltage drop) can be quite high. But in less severe conditions it may be a problem if the battery is partially discharged.

The small wire contains the fusible link (near the battery) and will protect the car from an electrical fire if something in the engine management system shorts out. If you eliminate that protection you need to add a fuse between the kill switch and the DME.

If you flip the kill switch on a running engine the current sink and feedback voltage of the battery goes away. That will result in the still turning alternator spiking it's output. That spike can go to 16-20v, which can fry electronics in the car. The resistor supplies a load that bleeds that spike to ground.

So this isn't a case of over complicating the issue, but rather a matter of doing it the right way and avoiding problems.

[jlevie]

I've got an M52b28, this set up wont work for me.





okay, think I got it now. let me know if its wrong.

Yep, you got it.

[!kid]

Out of curiosity I looked up the specs for my failed kill switch. the small poles are rated for 20A continuous. this explains why the switch failed, the DME pull much more Amp's than the switch is rated for on the small poles. Friend of mine that is in the Mini STEP program, he mentiond the DME in the current Mini's pull about 70Amps. I am sure thats a bit more than what an M20 or an M50/M52 DME will pull but is not far from. I think Jim mentiond the M20 DME will draw about 40Amps.

Jparkr, if i were you I would look to see what the secondary poles on your switch are rated for so you dont end up with the same problem I had.

[jlevie]

I'm not sure what the max draw is, but I measured 45A at idle. Id expect a higher current flow at high rpm.

[seatown88]

Out of curiosity I looked up the specs for my failed kill switch. the small poles are rated for 20A continuous. this explains why the switch failed, the DME pull much more Amp's than the switch is rated for on the small poles. Friend of mine that is in the Mini STEP program, he mentiond the DME in the current Mini's pull about 70Amps. I am sure thats a bit more than what an M20 or an M50/M52 DME will pull but is not far from. I think Jim mentiond the M20 DME will draw about 40Amps.

Jparkr, if i were you I would look to see what the secondary poles on your switch are rated for so you dont end up with the same problem I had.

I too looked up the specs of my switch, and it is identical in power rating with yours. 20 amp on the small pole. There are switches I have found with 125amp on the alternator("small pole") side and 175 amp on the main lugs. I say "small pole" because they use the larger, 3/8 lugs similar to the large pole.

[Jparkr]

Mine is also 20A. I doubt an M20 coil pulls more than that, but I will have to wait till i install the new engine to see. It's been a year with no issues so i'm not very worried.

How long did it take yours to fail the first time?

[!kid]

Mine is also 20A. I doubt an M20 coil pulls more than that, but I will have to wait till i install the new engine to see. It's been a year with no issues so i'm not very worried.

How long did it take yours to fail the first time?

Maybe a little over a year.

[dsobering47]

stupid question:

If I have a 4-pole kill switch (two large and two small terminals), could I install the kill switch directly in line with the battery cable? I was going to use the small terminals for the fusable link side in the cable, and the larger terminals for the main voltage supply. Should I also incorporate additional ground protection for the DME with this setup?

[jlevie]

stupid question:

If I have a 4-pole kill switch (two large and two small terminals), could I install the kill switch directly in line with the battery cable? I was going to use the small terminals for the fusable link side in the cable, and the larger terminals for the main voltage supply. Should I also incorporate additional ground protection for the DME with this setup?

I think your questions will be answer by a complete reading of this thread. If not I'll try to explain it again.