Completely baffled by parking brake sensor

[Dbacks2k4]

I thought I knew electrical systems pretty well but this one has me completely baffled.

I'm trying to tap into the parking brake sensor on an air-brake bus to activate a relay for an accessory. Basically if the parking brake is on, activate the relay; when parking brake is off, shutdown the relay. Seems pretty simple, right?

The bus is a 2006 IC CE300. Behind the dash the air lines feed into the standard yellow diamond air parking brake knob that activates the air brakes. In one of the air lines is a sensor with two wires on it. You can tell when the sensor is activated or not because the parking brake light on the instrument cluster illuminates. Unhook this sensor and that light no longer works.



Now the fun part...

Wire #1 always reads +12V
Wire #2 when parking brake IS NOT applied reads 0V
Wire #2 when parking brake IS applied AND the wire is DISCONNECTED from where ever it runs to that eventually feeds the dash reads +12V.

All of this makes perfect sense to me so far. Here's the baffling part:
Wire #2 when the brakes are ARE applied AND the wire IS CONNECTED to where ever it runs to that eventually feeds the dash reads +0.07V.

I mean it's obviously designed to work that way (the bus has no warning lights or DTCs in ServiceMax, and the parking brake indicator light on the dash works as it should) but it makes no sense to me electrically.

[joeblack5]

Wire 2 is neutral... Wire 1 goes to lamp or relay and the other side of the relay or lamp goes to 12 volt.

Good luck
Johan

[cadillackid]

Maybe this is a pull down circuit floated by the ECM? Does wire 1 still read 12? Or has it dropped to 0.07 also?

[TheHubbardBus]

Originally Posted by joeblack5

Wire 2 is neutral... Wire 1 goes to lamp or relay and the other side of the relay or lamp goes to 12 volt.

Good luck
Johan

Thanks, Johan!

It's amazing how hard it is to think outside the box sometimes. I was baffled too, assuming it a foregone conclusion that wire #1 was the 12V source, with current flowing towards the guage cluster rather than away from it.
So the .07V is just the voltage drop / potential difference between probes (the wire/conductive path to ground).

[Dbacks2k4]

Thanks everyone. That makes more sense now. I hadn't even considered that the ecm might be regulating the voltages and was thinking more in terms of a straight 12v system. Silly me, I must've worked too hard yesterday

It dawned on me this morning the easy solution to my relay problem is to just order a second sensor to put into that air line for my accessory relay...

[TheHubbardBus]

nevermind, misthought.