Key switch kill switch?

[Diesel_Dave]

Let me start by saying that I'm pretty bad when it comes to electronics.

I've started to do more EOC now, and the issue is that I "lose" some of mileage (on the odometer) during the period between when I turn the ignition off, and when I turn it back on after the engine stops. I estimate this is up to about ~5% of my miles when I do lots of EOC. I believe (but am not entirely certain) that the odometer is controlled by the body computer--not the ECM.

Why can't I just put a kill switch in the ECM key switch wire? Wouldn't this shut the engine down by shutting down the engine down "normally", but leave the body computer (and thus odometer) on?

I've done some research and this seems like a pretty simple and easy solution, however, I can't find anyone who's done it and I'm not sure why.

[ksa8907]

im familiar with the chrysler LH line. i assume your trucks is similar, so yes the mileage is stored in the bcm. i know on gas engines there is an ASD (automatic shut-down) relay that kills injectors and coils, i would assume there must be something similar on a diesel engine and if you just wire a switch into that relay and force the ASD to kill the engine it might be easier. if im not mistaken, the key powers the ECM and BCM at the same time with the same wire, probably to another relay, so that might not help you.

[Diesel_Dave]

Originally Posted by ksa8907

if im not mistaken, the key powers the ECM and BCM at the same time with the same wire.

I think that it may come from the ignition as the same wire, but at some point there's got to be one wire that goes to a pin on the ECM, That's the wire that I'm thinking of switching.

[ksa8907]

For less stress on components you would want to switch the ground, electrical current actually "flows" from negative to positive.

But since the ecm is probably on a relay that switches ground, you should be ok. Be sure to check the underhood pdc for relays.

[Diesel_Dave]

For more information, here are the wiring diagrams:
http://www.dodge.com/bodybuilder/200...owermodule.pdf

I'm not smart enough to figure everything out from them.

[mechman600]

Originally Posted by ksa8907

For less stress on components you would want to switch the ground, electrical current actually "flows" from negative to positive.

But since the ecm is probably on a relay that switches ground, you should be ok. Be sure to check the underhood pdc for relays.

Not true on any ECM made in the last 20 years. ECMs always have constant +12V and ground with a "wake up" +12V signal from the keyswitch.

Dave, your plan should work great. Put an interrupt in the ECM keyswitch signal wire and the BCM will continue to count miles. Than all your problems will be solved!

[baldlobo]

yup they do have a constant 12volt source but that's only for memory storage so your ecm doesn't reset everytime you turn on the car; it's a computer, turning the key-switch on means you want it to run(thank gawd no cars run windows).

i'd go for the relay it's in the pdc

[Diesel_Dave]

Update: I finally got around do doing it and it worked:
http://ecomodder.com/forum/showthrea...tml#post355748