EFI and Cutoff Switches

[CTX-SLPR]

A long while ago I was talking to the guy at MAD Electrical and he told me I'd fry my ECM with a traditional cut off switch for a trunk mounted battery.

Since I'm still running an OEM ECM for the time being but may switch to a MS3x in the future, how do you wire a remote battery cut off that won't hurt the ECM?

Thanks,

[NewEnglandRaceFan]

A long while ago I was talking to the guy at MAD Electrical and he told me I'd fry my ECM with a traditional cut off switch for a trunk mounted battery.

Since I'm still running an OEM ECM for the time being but may switch to a MS3x in the future, how do you wire a remote battery cut off that won't hurt the ECM?

Thanks,

do, stock factory cars fry the ecu's when you change out a battery? as long as it's not on run, when you connect power it should be fine.. if not, just add a switch to kill the power to the ecu for when you turn the power back on to the whole car it can't hurt it..
never seen a factory car fry a ecu from a disconnected battery and then hooking it back up.. but the key is never on when you'd do that, in the racing kill switch, you could turn it back on, with the key still in run by mistake.. can always add a fusable link to power the ecu to add some extra safty. of a spike.. but the battery only gives what the ecu draws. so?

[CTX-SLPR]

Cut off switch in terms of the emergency switches required by sanctioning bodies, not something like pulling the battery.

[Bamfster]

Honestly, I don't see the difference, but I'm far from an expert in the world of electrical and or EFI. In either case you're just disconnecting the battery. But what about one of the self resetting circuit breaker with an appropriate rating on the EFI main feed?

[NewEnglandRaceFan]

Cut off switch in terms of the emergency switches required by sanctioning bodies, not something like pulling the battery.

how is it different, switch kills all power just like pulling the battery cable..
no difference.. electricly

[BKBridges]

Most "modern" aftermarket EFI ECUs will be fine with a battery disconnect switch. In the many times Ive pulled the batteries, had melt downs, welded on the car, etc. Ive never had a problem with the ECU "frying". If you are communicating with the ECU with your PC/laptop when the power is disconnected bad things may occur depending on the mfg. of the ECU. I don't think the sanctioning bodies will approve of a separate ECU cut out switch.
BKB

[CTX-SLPR]

All I remember was that it had to do with the Alternator kicking some sort of transient onto the line that would smoke the ECM. Right now I have a GM OEM ECM in there but an MS3x+GPIO is calling my name I fear.

If I have nothing to fear, well then I'll get one and wire it up.

[NewEnglandRaceFan]

All I remember was that it had to do with the Alternator kicking some sort of transient onto the line that would smoke the ECM. Right now I have a GM OEM ECM in there but an MS3x+GPIO is calling my name I fear.

If I have nothing to fear, well then I'll get one and wire it up.

why would the alt act any different than you turning the off? once you cut power if the kill switch is wired right it kills the field and the alt is off.

[Bob Holmes]

Mine is only anecdotal evidence, as I can't give you a engineering reason as to why the gentleman alarmed you, but we have an aftermarket EFI (Haltech) in our racecar. To pass tech the emergency cut off has to be operated in the presence of the inspector. We have never had an ECU failure. And, I'm not aware of any of the other racers having a problem.

Just my take, but he might have been blowing smoke up your skirt.

[Bamfster]

A 3 wire ALT will feed back power thru the charge circuit .... This when the engine is running and you disconnect the + battery cable, the engine will stay running. When I put the kill switch in, I had to add a relay to shut down that back feed so it would kill everything ...

[CTX-SLPR]

A 3 wire ALT will feed back power thru the charge circuit .... This when the engine is running and you disconnect the + battery cable, the engine will stay running. When I put the kill switch in, I had to add a relay to shut down that back feed so it would kill everything ...

There we go... I knew I was remembering something like this and I don't like 1-wire alternators so I knew it was related to that as well.

Solution: add a big relay on the charge wire so the whole thing goes dead and the still spinning Alt doesn't mess something up.

[Bamfster]

Painless sells a specific one just for that ... And I have one on the shelf if u want it. It was destined for a customer's car, he paid for it and disappeared a few years ago so it's yours for the taking.

[CTX-SLPR]

Can't pass up a deal like that!!

I'll take it, gives me more of an excuse to try and meet up with you anyway!

Got to get the rewire done on the Riviera so I can make the 3 Aug cruise here in town!

[Bamfster]

Can't pass up a deal like that!!

I'll take it, gives me more of an excuse to try and meet up with you anyway!

Got to get the rewire done on the Riviera so I can make the 3 Aug cruise here in town!

In the springs? Hmmm

[CDMBill]

I have an MS3X setup and a trunk mount battery with the typical cut off switch. The Alternator charge wire (00 welding lead) goes to the (+) terminal side of the switch, the hot back to the power buss, and everything else is on the switched side. Alternator is Powermaster 200 amp Ford police car type unit that uses an exciter wire typical on early Ford's. the shut off has passed the NHRA 2500 rpm kill test multiple times with no issues, no run on, no dead MS3.