Idling RPM drops with headlights on

I have '72 911 S with MFI.
The idling rpm drops 300 or 400 rpm when headlights on.(1000rpm to 700rpm)
Replaced the batteries four years ago, but its voltage is around 13.5V. I think they are OK?

Please tell me the reason you suppose about the idling drop.

Appreciate your help.

I believe it is the same 'effect' as when you switch on A/C ... i.e. there is more load on the alternator, and the extra 'drag' on the fan belt requires more power than the idle setting can sustain, so the idle speed drops. I would suggest cleaning both sets of battery terminals, and the ground strap connections to the body as well. Wire brush them all until bright and shiny, then apply Silicone grease before tightening all connections!

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Warren Hall
1973 911S Targa

Isn't their a "minimum idle circuit" in this car though Warren? If so, shouldn't it compensate?

Warren
I cleaned the terminals and connectors, could not see any improvements. Further I replaced the batteries, with no improvements.
I have to tell you the situation more in detail.
Mine idles at 800 rpm, might be lower.
When I rev at about 4000 rpm or higher or I drive it at high speed, it idles at 1100 rpm. But when I turn on the headlights on or stop at the traffic signal, it drops to 800 rpm.

This is going to be a roundabout answer, because it is practically impossible to know if there is something wrong without going through a conplete tuneup/service. There may be some shortcuts if you already know that valves are adjusted properly, ignition is set and operating properly with new spark plugs, with new fuel and air filter, etc.

What I suggest is that you download and read the factory publication commonly referred to as the 'Blue Book' ... in its entirety before deciding what to do next. It is available here in the Pelican Technical Articles section at the following URL, along with other resources on MFI:

http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

If you go through a complete, total service, then adjust the MFI, I'm sure you can get your engine to idle normally at 900 rpm. If after that, your idle speed still drops with the lights on, then I suggest using the hand throttle to compensate when you drive with the lights on ... that is what I do when operating my air conditioning, and I really don't think the idle should be messed with if properly running at 900 rpm with no load, fully warmed up!

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Warren Hall
1973 911S Targa

Make sure your throttle mechanism is clean and moving freely. Use a voltmeter to see the voltage drop when the lights are turned on. Warren is probably correct and the alternator is having trouble making enough power.

I don't know about idle boost mechanisms on early "S" cars, but one idle would be better than two, and it seems to me that those cars should idle a little fast. I'm not a believer in slow idles. Of course, 1000 or 1100 seems high enough surely.

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'83 SC

Thank you Warren and Superman.
I measures the voltage at the battery teremina.
14.5 to 14.7 V with headlght off and the rpm is 1100rpm.
12.8 to 13.0 V with headlight on and the rpm drops tp 800 rpm.
I am suspecting the regulator with these results. Should I replace the new one?

Turning on your headlamps, while seemingly insignificant, does add a very significant load on the alternator which will normally show up in lower idle rpms.

Today, we are so spoiled with our modern fuel injected cars that automatically compensate for this as soon as they sense the slightest variation in idle rpm with their computer controlled idle air control valves, that we hardly notice a change in rpm, even if we turn on the lights or if the A/C happens to cycle on and off.

I think you might be chasing your tail on this one...

Cheers,

Joe Garcia

[This message has been edited by stlrj (edited 03-20-2001).]

Don't know about MFI motors, but the revs do not drop on my 3.0 SC when *all* the lights, fans, and rear defroster are on together.


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Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm

I don't suspect the voltage regulator. I'm not a technician, but this is just what happens when an alternator is being overworked. Voltage has to drop because the amps are not there.

With the other cars of the world, you can drive into about any tire store and they will test your charging system free. I suppose they could still test 911s, but they would not see any wires coming off the alternator. I guess the load tester would be parked next to the battery.

Anyway, the first ting these guys say is the battery is weak and a fully charged, strong (new) battery is necessary to complete the test. They all say this, so either it is a worldwide hoax or it is true. With a good battery, they can tell if your alternator is capable of running everything. If not, it should be replaced/upgraded.

Be sure your terminals are all clean as a whistle and coated with Dow Corning 111.

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'83 SC