Weird Idle

[subparmechanic]

Hey guys, at the moment my Celica's idle drops when I hit the brakes and turn on the lights. At first I only thought it was when I pushed the brake pedal so I started looking for a vacuum leak and tested the check valve on the booster, but I couldn't find any leaks. When I discovered it did it as well whenever I turned my lights on, I figured it might be something electrical but before I could test the alternator the car died and I had to replace the starter. Got it back up and running, replaced the alternator as well since I don't know all of the work that's been done to the car and just wanted to make sure it's been done/ possibly fix the issue since it seemed to do it with additional electrical loads (brake switch and lights), but that didn't fix it. Anyone else have this problem? Due to work I don't have that much time to do a thorough diagnosis on it and was just wondering if anyone had ran into the same or similar problem. Thank you in advance!

Edit: It only does it once it's warmed up

[twotone_ra64]

Can you clarify the exact details on how this occurs? That way we can narrow it down.

• Does the idle rpm stay low when you keep the brake depressed and lights on?
• Are the running lights or headlights on when "lights on"?
• Can you put an ammeter inline with the headlights and brake lights fuses and measure their current draw while all lights are on?
• Are there any other vacuum leaks around?

[subparmechanic]

When the pedal is depressed or the lights on it surges down to like 500 rpm and back to like 900-1100rpm and it'll go back and forth as long as it either of them are on/depressed. It's only when the headlights are on, after they flip up. Testing current draw was going to be my next step, and as far as I can tell no vacuum leaks.

[twotone_ra64]

Okay. Check the current draw on 15A HEAD LH, 15A HEAD RH, 15A TAIL and 15A STOP fuses. 15A HEAD LH/RH are for the headlights, 15A TAIL is for running lamps and interior lamps, and 15A STOP is for the brake lights.

[ozzie]

Unplug the brake pedal switch and short across the wires, does it do the same thing? That will tell you for sure if it's mechanical or electrical.