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A different headlight question

77-4speed

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Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Hey all,

I got my headlights hooked back up tonight, yeah! After I confirmed that everything work I decided to unhook all the vacuum lines from the manifold, except for the brake booster. I had nothing connected to any manifold vacuum source and had the headlights in the open position.

Now, when I turned the headlights on the rpms dropped about 100. Any ideas why? Once again, I reiterate that I had the vacuum lines plugged and the manual override switch was open (i.e. headlights up).

I can't figure out why the rpms would drop when I turn on the lights.

Russ


Oh, by the way, when I turned on only the parking lights, no drop in rpm. Is there some other vacuum connection that I'm missing?
 

69MyWay

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Jul 3, 2001
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4,364
Location
Auburndale, Florida
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1969 Killer Shark
Russ,

It is possible the extra amps of the head lights are drawing down the charge on the alternator and causing an RPM drop.
 
V

vmrod

Guest
Did you unplug any vacuum lines at the override switch or headlight switch?

The vacuum lines must be connected to the proper place on each switch, otherwise there will be a vacuum leak when the switch is put into the 'headlight up' position. If you had unplugged the tubes at one of the switches, then switch them around and try it.

HEADLIGHTS DOWN: The vacuum that comes from your manifold ties into a 'normally open' port on the headlight switch. The other port is the 'common', which continues onward to the override switch (which works the exact same way as the headlight switch). So, vacuum passes through the switch(es) then to the headlight relay, which keeps the headlights down.

HEADLIGHTS UP: When you either turn on the headlights, or the overide switch, the 'normally open' port becomes 'closed' (not allowing vacuum to bleed). The 'common' port is allowed to bleed off (through the switch body), which allows the headlight relay diaphram to relax, then the headlights will raise. That's why I think that you may have 2 tubes swapped.

To determine what port is which, install one small hose to a port, then suck on it. Move the switch to the other position and suck. If you can suck air through the port at both switch positions, then that is the 'common' port. The other port will allow you to suck air in one position, but NOT allow you to suck air when you change the switch position (so that is the 'normally open' port).

It just sounds like a vacuum leak like this could cause the problem you are having.

let us know what you find out.
 

77-4speed

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Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Chris,

I guess it's possible that the rpm drop is due to an increased load on the alternator due to the lights. But is that usual? I've never heard of a cars rpms dropping because of an increased electrical load. Any ideas how I could check?

Vmrod,

I had all vacuum disconnected from the car when this happened! The only vacuum connections that were attached were the PCV valve to the Carb and the brake booster hose. All of the A/C and headlight controls were disconnected.

I've attached a picture to show you what I disconnected.

The red X's are the lines I disconnected and the blue circle shows the tee that I plugged.




Russ
 
V

vmrod

Guest
Well, that should rule out the vacuum problem. Maybe Chris is onto something with the alternator.
 
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Norcross, Georgia, United Stat
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2017 Arctic White Grand Sport
The rpm drop due to alternator load is common when you engage heavy draw components. On computer controlled cars it comes back up immediately. I'm not sure if that would be the case on a carb car as it relies on a static adjustment where the computer is more dynamic.
 
V

vmrod

Guest
I know that they compensate idle during the A/C use. Perhaps they do it for the lights as well.

If I remember correctly, my car doesn't change idling rpm at all when I turn on the headlights.

You are positive that there are no other vacuum lines hooked up to the manifold?
 

77-4speed

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Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
The only vacuum lines that I see connected are the PCV to the carb and the brake booster to the manifold.

I might try removing the headlamps and then turning the headlights "on" to see what happens. I figure that if the rpms drop with the lamps removed, then there is some sort of vacuum leak somewhere.

Otherwise it's the alternator load.

By the way, it's only a 100 or so rpm drop so you wouldn't even notice it if you weren't looking. I just happen to be paying a lot of attention because I was listening to see if there was a vacuum leak in the headlights.

Russ
 
V

vmrod

Guest
...or pull the fuse.

If you are still puzzled, then put the headlight switch in the 'off' position and the overide switch in the 'headlight down' position. Then, pull a tube off the headlight overide switch (any tube), and see if there's vacuum there. (I know you said that everything is plugged up, it just doesn't make sense.)

...As Mr. Spock says, 'When the logical is ruled out, you must contemplate the illogical'.
 

77-4speed

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Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Good idea with the fuse. Probably a bit easier than removing all the lamps :)

If that doesn't solve the rpm drop I'll look into the override switch to see if there's any vacuum getting there.

I'll let you guys know what happens this evening.

Russ
 

vigman

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Valencia, CA,USA
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88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
As the load increases

Electrically speaking.. the mechanical resistance internal of the alternator does change... when you energize the feild you in reality make a BIGGER MAGNET so it is harder to turn.

The lights increase the load......

So as far as the A/C goes.. there is a soloniod to push the carb open a tad more to offset the resistance of the compressor turning/pumping.

Easier trick...if you remove the 2 pin connector on the top of the ALT ( power to feild winding)

That will accomplish the same effect.


Vig...
 

77-4speed

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Joined
Apr 30, 2002
Messages
249
Location
Houston, TX
Corvette
1977 Black L-82 4-speed
Well,

I've confirmed it. Chris (69myway) among others, is in fact a geniuse :BOW

I went to pull the fuse, but to my surprise, the headlights only have a circuit breaker and I wasn't about to search for that.:nono So I removed the lights, fired up the car, pulled the headlight switch and no dip in rpms. I guess the headlighs pull more current than I thought.

That's it for tonight. This weekend she should be aligned and ready to tear up the streets again!

Russ
 

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