Lights Dim at Idle

[1969VAGTX]

When I turn on my headlights, the dash lights and exterior lights of the car dim considerably. When I just have the running lights on, it does not seem to have that problem in the dash or markers. As I rev the engine up, the lights in the dash and exterior get brighter. And then as I come to a stop and depress the brake pedal bringing the car back to idle, the lights dim again. Step on the gas, the lights brighten and the process repeats itself. I assume this is related to a ground issue, but I am not certain. Would appreciate any thoughts on what would cause this and how I might remedy this problem. Thanks.

 

[Tony 69]

Put a smaller pulley on the alternator

 

[RR Fan Dan]

Your alternator is not spinning fast enough at idle to provide the voltage you need. There are numerous threads on how to fix this issue. @Crackedback sells a harness that is plug and play that will fix this. Others have upgraded their lights to led. I did both. I have LEDs everywhere except headlights. I have the @Crackedback plug and play for the wiring harness to the headlights. Make sure your charging system is charging correctly before upgrading. Do a search on here for LEDs, you’ll find numerous threads on this.

 

[hunt2elk]

X2 on the crackedback headlight relay kit.

 

[davek]

Its a Mopar

 

[Dennis H]

Characteristic.

 

[QuickBpBp]

Turn the idle up....

 

[Runner 68]

Turn the idle up....

 

[Slipping727]

Had the same problem.
Brought my alternator and regulator to a small rebuilding shop and he tested it. Was less then 30 amps. He took it apart and put a larger rotor in and tested it again. Now has closer to 60 amps and it works perfect. No more dim headlights at idle. Charged me less then a rebuilt alternator at O'Reilly.

 

[Geoff 2]

A smaller alt pulley might help with THIS problem, but bring other problems. If the engine has been modified to rev higher, a smaller pulley means the alt spins higher, might exceed it's design limits.
If it does not cause a problem....leave it.

 

[RJRENTON]

Had the same problem.
Brought my alternator and regulator to a small rebuilding shop and he tested it. Was less then 30 amps. He took it apart and put a larger rotor in and tested it again. Now has closer to 60 amps and it works perfect. No more dim headlights at idle. Charged me less then a rebuilt alternator at O'Reilly.

Your rebuilder must have installed a new STATOR windings. The rotor supplies the power to the stator windings which are AC, which then are RECTIFIED by the diodes to DC ....perhaps you must have misunderstood the repairs...possibly diodes were replaced as well.....fyi......
BOB RENTON

 

[Slipping727]

Your rebuilder must have insisted a new STATOR windings. The rotor supplies the power to the stator windings which are AC, which then are RECTIFIED by the diodes to DC ....perhaps you must have misunderstood the repairs...possibly diodes were replaced as well.....fyi......
BOB RENTON

Yes, very easy for me to misunderstand as he explained everything at once and it was all new to me.
It looked like he replaced half the alternator with another half. Can't remember which half.
Didn't take him very long.

 

[RJRENTON]

Put a smaller pulley on the alternator

How much smaller....10% to 20% ....please state a dimension of the pitch diameter of the proposed sheave compared to the original sheave......or we have to guess ???? Need numbers not just an idea.......
BOB RENTON

 

[Sinitro]

When I turn on my headlights, the dash lights and exterior lights of the car dim considerably. When I just have the running lights on, it does not seem to have that problem in the dash or markers. As I rev the engine up, the lights in the dash and exterior get brighter. And then as I come to a stop and depress the brake pedal bringing the car back to idle, the lights dim again. Step on the gas, the lights brighten and the process repeats itself. I assume this is related to a ground issue, but I am not certain. Would appreciate any thoughts on what would cause this and how I might remedy this problem. Thanks.

Most likely ur Mopar is running the OE stock alternator...
The stock OE alternators are only rated for like 40-50 amps, and
@ idle it will have limited output. A couple of possible fixes..
1. Upgrade the alternator to a higher amperage output rating
2. Revise the pulley size.
3. If ur primary concern is headlight brightness, upgrade
the OE incadescents to LEDs as they draw significantly less current like 70%

Just my $0.02...

 

[Nacho-RT74]

Asperger in action.

 

[Nacho-RT74]

On a side note… aside upgrade the alt for a better output unit (aside the smaller pulley option as MaMopar DID on the late 70s) which not even back in the days was enough to hold loads specially into the cities, I would check the path conditions. Bad conditions paths will become into resistances reducing the power capacity flow along the circuits.

Check for that too.

(I guess your ammeter is constantly on Discharge side)

 

[RJRENTON]

On a side note… aside upgrade the alt for a better output unit (aside the smaller pulley option as MaMopar DID on the late 70s) which not even back in the days was enough to hold loads specially into the cities, I would check the path conditions. Bad conditions paths will become into resistances reducing the power capacity flow along the circuits.

Check for that too.

(I guess your ammeter is constantly on Discharge side)

Congratulations....you are the 5834 th person to reiterate the same point.......loose wires, corroded connections, bad grounds, etc.......show us your voltage drop calculations that support your assumed assumptions ..... that is, if you understand the fundamentals of Ohm's Law ......what you state makes no sense....."hold loads into the cities"....what ever this statement means..and.."become into resistances reducing the power capacity flow along the circuits"......what are you trying to say?? You're better off saying nothing or is it OCD?......just my opinion........
BOB RENTON

 

[Sixpactogo]

As was mentioned, these cars were notorious for lights dimming at idle and the amp gauge going to discharge even when they were new. It never caused a problem that I remember, just a nuisance. Not saying this is the problem in this thread but I do want to point out another problem that people (me included) create for themselves when they switch to a serpentine belt system. Most of them are underdrive systems causing more problems than they are worth.
They look great but they underdrive everything. Alternator, water pump, AC, and Power steering with the idea of saving horse power.
The first problem that shows up is the water pump issue at idle keeping the water in the engine too long causing overheating.
Everyone seems to blame the fan or radiator or thermostat or water pump. Everything but the real problem. I fell into that trap myself years ago.
When I finally figured it out and went back to the factory belt system, all my cooling problems went away and I noticed my ammeter closer to zero at idle too. That said, my point is if you upgrade to a serpentine system, do not fall for the under drive system for a street machine.

 

[RJRENTON]

As was mentioned, these cars were notorious for lights dimming at idle and the amp gauge going to discharge even when they were new. It never caused a problem that I remember, just a nuisance. Not saying this is the problem in this thread but I do want to point out another problem that people (me included) create for themselves when they switch to a serpentine belt system. Most of them are underdrive systems causing more problems than they are worth.
They look great but they underdrive everything. Alternator, water pump, AC, and Power steering with the idea of saving horse power.
The first problem that shows up is the water pump issue at idle keeping the water in the engine too long causing overheating.
Everyone seems to blame the fan or radiator or thermostat or water pump. Everything but the real problem. I fell into that trap myself years ago.
When I finally figured it out and went back to the factory belt system, all my cooling problems went away and I noticed my ammeter closer to zero at idle too. That said, my point is if you upgrade to a serpentine system, do not fall for the under drive system for a street machine.

AMEN.........
BOB RENTON

 

[toolmanmike]

The old car alternators never did put out much at idle. I can stop my turn signals from blinking at idle with the A/C on. Brake lights, blower motor, A/C clutch energized, and lower idle speed because of the drag, makes it happen. There's no kick up solenoid because it is Dealer installed air even though it is Chrysler Airtemp. If I just rest my foot on the gas pedal or pop it into neutral they start blinking again.