Idle increase when headlights on
 

I recently purchased a 1977 911S. Tonight when I turned the headlights on for the first time, the idle jumped from 1000 rpm to 3000 rpm. I'm stumped. Any ideas?

Did the idle remain high or just jump up and settle back down?

Headlights are one of the biggest loads on your charging system. Id start with swaping out the battery (test), then test the Alt.

The idle remained at 3000 rpm. When I revved the engine, It came back to 3000 and stayed there.

What happen when you........
 

Baker,

What happened when you turn the headlight switch off? Does the RPM drop back to 1000 rpm? Could you replicate the incident by turning ON/OFF the HL switch? I'm curious and baffled too. Keep us posted. Thanks.

Tony

Yes, when I turn off the lights the rpm's drop back to 1000. Turn the lights back on and they go back up to 3000. Also, I have had batterys' go bad before. When you turn on the lights the rpms' drop and the car dies, not rev up.

Very strange as I see no connection between an electrical issue and engine increase of rpm on a 77. Just a wild guess--could the headlight circuit somehow be activating the cold start valve, causing an injection of fuel into the intake? Is that even possible? What other electrical device would cause an increase of rpm?

Baker - does the engine actually increase in RPMs or does the tach just show an increase? The only reason I ask, is because I have a similar occurance, but w/ my gas gauge. It shows about an 1/8 of a tank more w/ the headlights on than w/o. In my case, I guessing I have a bad ground somewhere.

cool!

i would look at the charging circuit first as said above.
the only other thing i can think of is someone has been splicing some wires and/or put something on the engine that is not suppose to be there. either way, PO may have done a bad thing perhaps trying to cover up another problem

Yes, the engine actually revs to 3000 rpm and stays there, not just showing on the tach. I had the battery tested when I bought the car and it is good. The PO said that when he bought the car there was a high end stereo wired into it (now gone). He also started to add cruise control, but didn't finish. Those are the only things that I know of that were added to the electrical system.

Bingo--cruise control. I have absolutely no knowledge of the workings on a Porsche, but it would make sense that a poorly installed (or partially installed) system could activate the idle through bad wiring with the headlight switch. Most cars with cruise automatically deactivate the system through the brake light circuit. It's possible that bad wiring did just the opposite and the activation starts with the powering of a circuit connected to the headlight switch.

Also, does your car have AC? Is it possible a miswiring of the light switch is activating the AC compressor which, on some cars, also activates an increased idle?(Though a 2000+ increase is way over typical.)
Just a couple of guesses to help, if possible.

Random thoughts: Does the idle increase only when the headlight switch is pulled all the way out, or does it increase from the first pull--parking lights?

Do you notice any unusual wires coming from the headlight fuses at the fuse block or tapped into wires coming from the fuses meant for the headlights?

Have you tried to remove fuses to determine which circuit is causing the idle change? It could be from the headlights or the tail lights which are on different fused circuits. See if you can isolate when the idle is affected.

report back anything you can think of, no matter how insignificant, ie. the cruise control.

Thanks, the cruise control is going to be taken off this weekend.
No ac.
Yes, the rpm's go up at the first stop (parking lights) of the light switch.
If taking the cruise control off doesn't fix it, I will start checking the other things that you mentioned. I will let you all know.

sounds like something with the cruise.