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Old 02-22-2009, 10:55 PM   #1
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Fuel cutoff, spark cutoff; saving the ignition.

I noticed that some people have devised cutoff switches for optimal EOC and it's suppose to save the ignition switch from getting prematurely worn out. I can't remember where this thread was or if it was even on this forum but it was quite a lengthy discussion and one even posted pictures of his kill switch which was a red button on his steering wheel.

Anyway some mentioned the issues with a fuel cutoff and spark cut off, that cutting off the spark but not the fuel could waste fuel and cutting off the fuel but not the spark could lead to a lean condition which could be bad; that the solution is to cutoff both! The problem lies in creating relays for both which would be a pain in the *** and that's pretty much where the discussion ended.

Well I have an idea; why not have a relay that cuts off electricity to the ECU? By cutting power to the ECU, you'd be cutting out BOTH and it'd be simpler. What do you guys think of this idea?
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Old 02-23-2009, 07:06 AM   #2
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All the gauges (speedo odometer etc) and other monitoring that the ecu does would stop working if you cut power to it. Maybe even the abs, seatbelts, airbags and stability control also.
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Old 02-23-2009, 07:16 AM   #3
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Lean condition? From what? Unless you have a leaky injector something like that you shouldn't have any fuel in the cylinder after fuel cut off. Even a TBI engine will only run 1 or 2 cycles before the cylinders run dry.

Damage to an engine from running lean only really happens when the engine is lean under load. You can actually tell when you are there because you've leaned to a point where the engine feels like it is misfiring. It isn't actually misfiring though; the combustion process has gotten slow enough that all of the fuel isn't burnt by the time the exhaust valve opens causing reduced power and EGT skyrockets. This is what burns valves and what causes engine damage.
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Old 02-23-2009, 05:31 PM   #4
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Quote:
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All the gauges (speedo odometer etc) and other monitoring that the ecu does would stop working if you cut power to it. Maybe even the abs, seatbelts, airbags and stability control also.
Airbags are always working, even with a disconnected battery, that's why when working in the area around the airbags, one has to be VERY careful because they still can deploy at ANY TIME. I don't see why cutting power to the ECU momentarily is any different from turning the key from ON to ACC and then back to ON. As for seatbelts, wth, what are they going to unlatch themselves?
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Old 02-24-2009, 07:43 AM   #5
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Air bags are fired by an electrical signal sent from a computer when the crash sensors mounted in the front end are triggered. I don't think they work when the key is off or else parked cars would be deploying air bags when hit by moving cars.

Seat belts have explosive charges in them to tension the belts during impact . . . in my xB anyway . . . the tensioning system in mounted in the column between the front and rear doors.
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Old 02-24-2009, 09:03 PM   #6
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Air bags are fired by an electrical signal sent from a computer when the crash sensors mounted in the front end are triggered. I don't think they work when the key is off or else parked cars would be deploying air bags when hit by moving cars.

Seat belts have explosive charges in them to tension the belts during impact . . . in my xB anyway . . . the tensioning system in mounted in the column between the front and rear doors.
If you read the service manual for a car or know anything about the transportation of hazardous materials, one of the things you'll learn is that airbags and seatbelt pretensioners are dangerous not only from the material they hold, but because they can accidentally be deployed even when there is no battery. Now you're going to make me have to re-lookup this stuff again just to confirm what I've been saying. sigh
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Old 02-25-2009, 03:36 AM   #7
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I suspect that airbags can accidentally deploy by malfunction with the battery disconnected, but I can't imagine how they can be properly triggered with the battery disconnected. There would need to be linkages or hydraulic/pneumatic tubes from the front bumper that go to the airbags' charges. There are not; there are just wires.
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Old 02-25-2009, 07:01 AM   #8
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I know there is a battery back-up in the control units but I don't know how long it is good for after the battery has been disconnected. It's meant to be the backup in case a crash interrupts the power to the unit.

The FSM for my ex's Cavalier and my Tracker both say to disconnect the two front airbags when doing front-end work and under-dash work around the computer.
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Old 02-25-2009, 08:50 PM   #9
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Quote:
Originally Posted by theholycow View Post
I suspect that airbags can accidentally deploy by malfunction with the battery disconnected, but I can't imagine how they can be properly triggered with the battery disconnected. There would need to be linkages or hydraulic/pneumatic tubes from the front bumper that go to the airbags' charges. There are not; there are just wires.
airbags, seatbelts, and even some battery cables that have explosive disconnects all have one thing in common: they need to function if any amount of the rest of the car has been removed in the course of the accident. that means the system has backup power... capaciters, batteries, whatever the mfr feels like. This is so that if the battery is damaged or disconnected in an accident (like the accident disconnect) the other safety systems work. (all 3 of them are considered hazardous materials for DOT purposes)

NEVERTHELESS they won't be affected by cutting power to just the ecm/pcm/ecu or whatever you want to call it. the problem i forsee is that the ECU has it's own reserve power and it may start throwing codes for intermittent power or something.

I wouldn't worry about rich/lean conditions for the reasons mentioned: engine's not under load and it's not making very many rotations in that condition. same goes for lubrication: oil pressure doesn't lube the engine, presence of oil does. I know people who've run engines without an oil pan 20 minutes before it siezed. tho it was a toyota 22RE
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Old 02-26-2009, 08:12 AM   #10
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I say a moderator makes a poll for this page.

How do you kill the engine when you EOC?
  1. Turn off w/key
  2. Ignition kill Switch
  3. Injector Kill Switch
  4. Fuel pump kill Switch
  5. Other
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