Brake Lights And Ac


New Member
Nov 19, 2013
i have got a 96 mustang 4.6. the ac and brake lights stopped working.I am pretty sure it has to be the pcm. i am able to get the brake lights to work if run a wire from the brake switch to the back lights. also the compressor works when given 12v. i dont want to bastardize the wiring harness. is this a common problem with 96 stangs? are these circuits close to each other on the pcm? and yes i checked the fuses
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The brake lights don't go through the PCM. IMO it's silly to replace the PCM to fix any problem unless everything else has been ruled out. But replacing the PCM to fix a problem with the brake lights is an especially long reach IMO.

Do the high mounted 3rd brake lights work? This is an important trouble shooting aid as the 3rd brake light does NOT go through the multi-function switch where as the lower brake light do.

Sooooo if the 3rd brake light works and the lowers do not, think bad multi-function switch.

If neither works, think bad brake pedal on/off switch or fuse. But don't overlook the possibility of a bad G300 ground in the trunk.

As for the AC problem, are the any outstanding DTC codes? Why is that important? Because some DTC's disable the AC by design.

If no DTC codes and the AC clutch engages when directly powered, think blown fuse or bad CCRM.