98 GT stalls when brakes are applied at idle

hybridEF8 · Mar 29, 2017

Hey guys I've been searching all week and haven't had any luck finding a solution to my issue. Other people seem to have had this issue but I haven't found anywhere where it was resolved.

The issue I'm having is my car will die whenever I touch the brakes including at idle. I drove the car with no issues about a month ago and came back to the car last week and since then I haven't even gotten out of the driveway. The car idles fine but as soon as any amount of brakes are applied the RPMs drop and the car dies.

I had a second 98 GT which I just sold that I used to narrow down the issue such as swapping the entire upper intake manifold which includes the TPS, IAC, and MAF but the issue remains. I also just put in a new VSS. I know the brakes aren't vacuum assist otherwise I'd disconnect them to diagnose this.

The car is fairly new to me but as far as I know it has a PI head motor out of a crown vic as well as a 5 speed transmission out of a 99-04 due to the shape of the VSS. Otherwise no crazy mods just a 76mm TB, upper intake manifold, and a K&N intake.

Any help is appreciated!

hybridEF8 · Mar 29, 2017

Okay so I found the real cause of my issue. I disconnected the fuse for the brake lights and the issue went away so I realized that the car is actually stalling from load on the alternator as the car would turn off when I turned on the headlights or the reverse lights.

ttocs · Mar 29, 2017

not sure the brake lights could draw enough current to kill the alt unless its on its last leg. The wire just isn't big enough to carry the current to kill it that fast or the fuse would blow assuming its the right size. Try disconnecting bulbs in the light and see what happens.

mcglsr2 · Mar 29, 2017

Normal headlights or have projectors been installed?

I agree with ttocs, I'd be surprised that the draw on the lights would pull your alternator down. More likely, I think, is 1 of two things:

You have a short in your lighting wiring. When you turn the lights on, something somehow shorts, making the draw way more then it would normally be
Still with the lighting wires, you have a nick or cut in a bundle of wires, and the lighting wire(s) are touching something important like a fuel pump wire or an engine sensor wire or something; something that when the lights are on it effs up some other wire that engine/ECU needs to run or idle. I'm not sure what this could be without tracing the lighting wires in a wiring diagram.

Also, does your car not have a battery? I find it weird that your car dies from just the alternator, unless your battery is not present or dead. The car will pull from the battery if the alternator isn't keeping up. If your battery is present and fully charged, and your lights cause the car to die, then that makes me lean more towards item #2 above.

hybridEF8 · Mar 29, 2017

ttocs said:
not sure the brake lights could draw enough current to kill the alt unless its on its last leg. The wire just isn't big enough to carry the current to kill it that fast or the fuse would blow assuming its the right size. Try disconnecting bulbs in the light and see what happens.

I had someone help me measure the voltage when I turned the lights on and the car is staying at a consistent 14.3-14.4V both when the lights are on or off. I'll try disconnecting bulbs next.

mcglsr2 said:
Normal headlights or have projectors been installed?

I agree with ttocs, I'd be surprised that the draw on the lights would pull your alternator down. More likely, I think, is 1 of two things:

You have a short in your lighting wiring. When you turn the lights on, something somehow shorts, making the draw way more then it would normally be

Still with the lighting wires, you have a nick or cut in a bundle of wires, and the lighting wire(s) are touching something important like a fuel pump wire or an engine sensor wire or something; something that when the lights are on it effs up some other wire that engine/ECU needs to run or idle. I'm not sure what this could be without tracing the lighting wires in a wiring diagram.

Also, does your car not have a battery? I find it weird that your car dies from just the alternator, unless your battery is not present or dead. The car will pull from the battery if the alternator isn't keeping up. If your battery is present and fully charged, and your lights cause the car to die, then that makes me lean more towards item #2 above.

Stock headlights with HIDs installed. The car dies with just the parking brakes on though or even the brake lights so I don't think it's the cause.

I have been checking and cleaning grounds all day but the issue persists. I agree there could be a short though I'm not even sure where to start. It must be far up somewhere since any and all lights are causing this issue.

The car does have a battery hooked up that I charged overnight, and given that I see 14.4V at the battery and alternator when the car is running I think the two are healthy? The battery is from 2014. I should probably also note that the turn signals only seem to work when the car is off and they blink really fast. Last night the turn signals just stayed stuck on and didnt blink with the car running but today they're blinking 10x slower than normal.

I guess I'll start tracing wires and try disconnecting bulbs. Thanks for the help so far guys.

mcglsr2 · Mar 29, 2017

14.4v at the battery with the car running (meaning alternator) just means that the alternator is putting out 14.4v and *should* be charging your battery. It is entirely possible to have a bad/dead battery and for it to not accept a charge. When you start the car, how is it? Does it fire right up? Or does the starter seem to drag a bit?

If you suspect the alternator, you could always take it off and go to a Auto Zone/Advanced Auto/Equivalent and they will test it out for you for free on a bench. You just have the inconvenience of having to remove the alternator from the car.

Assuming your battery and alternator are good, it really sounds like a short/electrical issue to me. Especially if things are acting weird, like turn signals sticking on and such. For the lights, I would start at the light switch. For turn signals, maybe around the steering shaft area.

Have you done anything recently around that area - the steering shaft or instrument cluster?

Has the car been sitting outside and/or do you have problems with rodents/mice/squirrels sometimes getting into the car?

Does the car have an aftermarket alarm system on it, installed either by you or a previous owner?

Lastly, it *is* possible that a bad battery and/or bad alternator can cause really weird electrical issues. I think step one on your Search for the Answer is to have both your alternator and battery checked out at a parts store (Auto Zone, etc.). You absolutely must be able to rule them out as the problem before I'd recommend going on a witch hunt for a short somewhere...