This really has me confused, but as I've stated before, I'm not super strong on electrical...yet. So maybe the answer is simple and just escapes me:

I bought a little light bar for my truck and last night just want to test it out after pulling it from the package. Currently I have my B(-) cable disconnected from the battery due to some other unrelated work I'm doing on the truck (B(+) is still connected however). So I connect the light bar to the B(-) side of the battery and as I'm going to touch the B(+) of the battery with the positive side wire of the light bar, I drop the wire and it momentarily hits the top of the metal casing of my ECM...causing the light bar to light up???!!!

This is has me confused for a couple of reasons:

With the B(-) to engine block cable disconnected, and the fact that I had attached the light at this point only to the B(-) side...where did the light get the positive connection from by touching the top of the ECM? Also, the ECM is mounted on a plastic mount...how could any circuit be completed even if I had not disconnected the B(-) cable from the battery?

Figuring if I didn't fry the ECM with the light bar I wasn't going to fry it with a DVOM (set to DC), I touched my B(-) with one lead and quickly tapped the top of the ECM case with the other lead. Sure enough, the reading showed battery voltage. I went ahead and reconnected the B(-) cable to the battery and ran the same test with the DVOM. This time it didn't read any voltage.

So I'm confused and could really use a good explanation why the top of the casing of my ECM has voltage running through it...and why it is only when the B(-) cable is disconnected from the battery? Even if there was a short somewhere it really seems backwards to me. Please feel free to lay it all out and explain to me like I'm a 12 year old...won't be offended at all.

 

Since the explanation above might be hard to picture, please see the attached image of what I'm trying to explain? Can anyone shed any light? Is there a short in my VCM that is shorting to the outer casing?

 

I want to try asking this another way since it is possible that others are as baffled about this as I am. So, should current be able to flow through the outside of the PCM housing? The best I can figure is it shouldn't since it is mounted on a plastic mount. But on the other hand, I don't know if internally the casing is meant to be grounded with a pin or wire secured to the casing on the inside where I can't see it? Richphotos, rustynuts or anyone else...do you have any opinions or thoughts on this?

 

DC voltage follows the path of least resistance to ground. The PCM housing is wired to ground through the connectors. When the negative battery cable is disconnected, all circuits are open (no ground). When a jumper wire is added (light bar or meter) to B- the circuit is completed. You're creating a circuit that's wired in series. The negative wire coming out is the positive for another component. When the B- cable is connected to the battery there is a direct connection to ground which has less resistance than the component in the jumper wire. Your light bar won't light up when the B- is connected because there's less resistance to ground in the factory wiring.

 

Perfect. Thanks Rustynuts!