Big Voltage Drop - Headlights at Idle

[EddieE]

74 Cuda with Rallye gauges. Volts at idle (lights off) is 12.5 to 14. New Mopar voltage regulator. New alternator. New Optima Red Top battery.

When I flip the parking lights on it goes down to 11.5 at idle (1000 RPM). When I flip the headlights on it's down to 10 volts!  The voltage goes so low that it make the car want to stall with the stock electronic ignition.  The ammeter shows discharge.

If I rev the engine above 1000 RPM the volts go back up and the ammeter is centered. The car runs fine without the lights on, and it runs fine with the lights on as long as I don't go back down to idle. The headlights seem to work fine on low and high beam.

I noticed that the previous owner (lunkhead) has messed with the wiring. It has a few of those yellow clip-on jumpers to parallel some wires off the headlight switch.  The gauge light bar is disconnected from the headlight circuit and connected directly to the fuse panel with a spade terminal. The light bar is on whenever the ignition is on.

Could it be as simple as the ignition switch or some corrosion at the connector that goes through the firewall?

Please tell me the next steps in testing and share your experience on what would cause this in order of cheapest fix to rewiring.

[burdar]

Headlights dimming very low at idle is a problem most of these old cars suffer from.  Add in corroded connections and it gets very bad.  The first thing to do is make sure the bulkhead connector is clean and dry.  Make sure all the contacts fit tight.  Use dialectic grease so moisture doesn't corrode the connections.  Then check all of your grounds.  Adding additional ground can't hurt.  Your 74 should have 3 from the factory.  The main negative cable goes to the drivers side head.  A smaller wire comes out of the negative post and hooks to the rad support infront of the battery.  The third ground comes off the back of the passengers side head and connect to the firewall.

[Strawdawg]

If you are reading voltage directly at the alternator, I would tend to believe it is the altenator and the speed it is being driven.  Seems to me that many of these alternators don't put out much a low shaft rpm....I think it is typical

If you are reading the voltage down the line, it may be wiring related, but start at the alternator first

[JayBee]

Headlights dimming very low at idle is a problem most of these old cars suffer from. 

 
I've fixed every bad/corroded connection and used the dielectric grease in my bulkhead connector. Replaced every wiring harness in the engine bay. I also have an electronic ignition. Well, after all that I still get a voltage drop and the ammeter swings to "D" with lights on at idle. Not as bad as to feel the car wants to stall though.
I bet buddy hard wired the light bar in because the dimmer on the dash went faulty.

[msbaugh]

What size is your crankshaft/alternator pulley?  No one has mentioned this, but an underdriven pulley can cause your lights to dim like that... I've got one on mine, it causes issues running with all the radio, lights on.  If the dimming is very excesive at idle I would look into it

[AARuFAST]

This article might help.
http://www.moparmusclemagazine.com/techarticles/interior/mopp_0808_fixing_dim_or_blinking_headlights/index.html

[cuda346pk]

This article might help.
http://www.moparmusclemagazine.com/techarticles/interior/mopp_0808_fixing_dim_or_blinking_headlights/index.html

I did this upgrade as well as headlight relays so the headlight switch and bulkhead does not take the full draw of the lights, no dimming at idle at all now, even with high power headlights. Mine pulled down quite bad before too. I replaced alternator and voltage reg at the same time as both other upgrades so I cannot say what made the most difference but well worth the time to do them. I used an amplifier install kit to get my 8 ga wiring and inline fuse.

[1of1]

Headlights dimming very low at idle is a problem most of these old cars suffer from.  Add in corroded connections and it gets very bad.  The first thing to do is make sure the bulkhead connector is clean and dry.  Make sure all the contacts fit tight.  Use dialectic grease so moisture doesn't corrode the connections.  Then check all of your grounds.  Adding additional ground can't hurt.  Your 74 should have 3 from the factory.  The main negative cable goes to the drivers side head.  A smaller wire comes out of the negative post and hooks to the rad support infront of the battery.  The third ground comes off the back of the passengers side head and connect to the firewall.

I had the exact problem, The fix was very hard to find....I simply stumbled on it....1st the ground to the rad support was fine but inside the wire there was only about 1/2 of the copper left in the wire sheild that had not broken away from the crimp..2nd the isolator rings on the amp gage were so old and crispy when i took it apart they simply fell apart. I reworked the gound wire, then use new rubber o rings to isolate the amp gage ....and wala no more problem....

[EddieE]

Thanks for the laundry list of things to check.  I'll check 'em all.

Let me throw something else into the mix.  When I first start the car, I'm getting a voltage spike up to about 17 volts.  It only happens in the first minute of operation, so I keep it at idle which is about 14 volts.  Also, when the voltage spikes the ammeter spikes as well.

This spiking doesn't happen after it's warmed up for about a minute, even if I drive it for an hour.

So the car runs great, but these little gremlins must be crushed!

[EddieE]

Forgot to add that my voltage limiter from Auto Instruments is "in the mail" so that will be the perfect time to isolate those leads to the ammeter.

[MEK-Dangerfield]

Let me throw something else into the mix.  When I first start the car, I'm getting a voltage spike up to about 17 volts.  It only happens in the first minute of operation, so I keep it at idle which is about 14 degrees.  Also, when the voltage spikes the ammeter spikes as well.

This spiking doesn't happen after it's warmed up for about a minute, even if I drive it for an hour.

So the car runs great, but these little gremlins must be crushed!

I think that is normal. I get a spike on my ampmeter after I wake it up after a few weeks. It settles down after a minute or so, and acts fine after that. I just think the voltage regulator needs time to adjust.