Brake light Issue

[browndog9]

I have searched and cannot find answer to my problem.
When I check voltage to the brake switch, I am getting 12 volts on each wire in the plug. I figured you would get it from the fused side, but why am I also getting the bl/rd wire to show 12 volts. The plug is disconnected from the brake light switch.

[bluptgm3]

Three pole plug? I believe it is validation by OBC, if Zs have OBC.


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[rf900rkw]

Assuming an M, it will be backfeed from the cruise control unit. This will only happen if you have removed all brake light bulbs. The bulbs normally sink the pull-up current to ground. No bulbs means no ground path means a DVOM will pick up the floating +12.

[browndog9]

Thanks for the help. That is exactly what I have going with my 99 m. Will put back bulbs and check again.

[rf900rkw]

A note in case this is what you're dealing with. LED replacement bulbs can cause the same effect. It is a common technique to dribble a very limited current into the light bulb circuit, using the bulb as the ground source. It's often done to test bulb function. In our case, it's used by the cruise to indirectly test the brake switch position. This is what you measured. LED bulbs without compensation will cause the cruise to not work. Don't laugh. Many a 90's F150 burned up the transmission, in some cases the whole truck, because of unintended backfeed from aftermarket LED tail lamps interacting with a too-powerful bulb failure system.