Lights "Pulse" at low RPM. 1974 Challenger

[Scooter]

Have a slight annoyance with the interior lights "pulsing" at lower RPM's. This car was upgraded to a Painless wiring kit under the hood and interior before I took possession but seems to be wired fairly normally. System is running alternator with external regulator and ballast resistor setup for ignition. Things I have already done to troubleshoot:

1) Bypassed the Ammeter, put all connections on same post.
2) Added two 14ga. wires from alternator 12v to the starter relay. Tried running with just the 2 new charging wires to the starter relay and it made no difference.
3) Replaced Alternator with 100amp Powermaster square back externally regulated. Grounded the Alternator from ground post on the alternator to chassis.
4) Replaced voltage regulator and ensured the box was grounded properly to chassis.
5) Battery is a Optima Red Top, unknown age. Reads 12.6 static and seems to turn the starter fine. When engine is running shows it's getting charge at 14.6v.

I'm at a loss here. The only hi amp draw item are the electric fans. When they kick in the headlights do dim. The engine compartment and passenger compartment harnesses seem to have all been properly ground. The battery is charging at 14v and change so I think the regulator is doing it's job. Would I be getting 14v if it was not? Only thing I can think of is, could I still be dealing with a weak battery?

Thanks in advance.

-Scoot

[dodj]

If your headlights are still fed through the B/H connector, it could be the source of your interior lights pulsing. When the headlights are on and the B/H connections are not pristine, a larger voltage drop is created across the B/H. This leaves less power for your lights and harder for your alt to maintain a steady power flow to those lights - especially at low rpms where the chrysler charging design is particularly challenged.

[Burdar]

The factory style alternators aren't known for putting out a lot of power at idle speeds.  Even the higher amperage units don't put out a lot at an idle.  Not sure if that is adding to your issues or not though.

Also, the stock wiring powers the headlights through the headlight switch.  With an alternator that doesn't put out a lot of power at idle AND the fact that the wiring has to go through the bulkhead connector, leads to dim headlights at idle.  A popular mod is to add relays to the headlight circuit.   I bought a relay kit from member Crackedback.  This relay mod will power the headlights directly from the battery and only use the headlight switch to trigger the relay.

I'm not 100% sure this is your issue but it seems plausible and is a problem that all old Mopars seem to have.  Make sure all of your wiring connections are tight and dry.  Dielectric grease is good to use on those connections to keep out moisture.

[Katfish]

You could switch to LED lights, use less current and less susceptible to voltage variations.

[Scooter]

Thanks for the replies!

Happens when the headlights are not on. Harness, alternator and voltage regulator are new. No booming stereo or other high amperage draw on the system. With a 100A alternator and light power draw I would not expect to be having power issues at any rpm. I might just have to bite the bullet and try a new battery.

[Scooter]

You could switch to LED lights, use less current and less susceptible to voltage variations.

I'm currently tracking the thread on LED headlights.. definitely on my my to do list regardless of this interior light pulsing issue. 

[Chryco Psycho]

This is normal , all of the older cars do this , the wipers would slow at idle too , you can raise the idle speed which is wasteful or us a smaller pulley on the Alt if anyone makes one so it will turn faster , LED lights will have more stable lighting .

[redgum78]

This is normal , all of the older cars do this , the wipers would slow at idle too  .

Yep and the turn signal pulse gets longer at idle.

[Scooter]

This is normal , all of the older cars do this , the wipers would slow at idle too , you can raise the idle speed which is wasteful or us a smaller pulley on the Alt if anyone makes one so it will turn faster , LED lights will have more stable lighting .

Honestly don't remember any of my previous cars doing this at idle. The slower wipers and slightly dimmer headlights yes, interior lights pulsing.. nope. then again my memory ain't what it used to be.

[Burdar]

When it happens, give it a little gas and raise the RPM.  Does the "pulse" stop?  If so, it's the alternator not putting out enough power at idle.  You might look into installing a higher amp Denso alternator.  They put out lots of power at idle speeds.

[Scooter]

When it happens, give it a little gas and raise the RPM.  Does the "pulse" stop?  If so, it's the alternator not putting out enough power at idle.  You might look into installing a higher amp Denso alternator.  They put out lots of power at idle speeds.

It does go away. But with a brand spanking new 100A Powermaster Alternator, new voltage regulator, 2x 14ga charge lines right to the starter relay and nothing taxing the system (headlights and fans off), I would think there would be plenty of juice to go around even at idle to run the dome light solo without pulsing. Could the ignition be pulling down that much juice? Something just does not seem right. 

[dodj]

It's not that it's pulling a lot of juice, it's that it is only feeding you a trickle at idle. It's a design feature.

[Scooter]

Thanks for all the good input guys!

Guess the thing that gets me is the pulsing. I'm accustomed to interior lights being dim at idle on older cars and brightening as the rpm's come up. Does not seem to be effecting anything other than my brain so I'm going to let it ride for now.

[Chryco Psycho]

Another possibility is the Alt , Powermaster has proven to be very unreliable

[Scooter]

^^^^ Could be.. but it runs normally otherwise and I get good charge volts from it. Anyone else try one of these as a fix?

Mopar Performance Voltage Regulators P3690731
https://www.summitracing.com/parts/dcc-p3690731

13.5 constant volts

-scoot