Engine runs rough with lights on

Desert Toad

Desert Toad

Member
Hoping for a few ideas here. My '30 sport coupe runs perfectly, starts right up every time. But when I put the headlight switch in any position, the engine cuts out and runs extremely rough. Turn the lights off, and things are back to normal. At idle, the generator charges between 9 and 9.5 volts. With the lights on (any position) it about 7 to 7.5. This sounds about right, so where should I look? I've considered replacing the headlight circuit, all positions on the switch turn on the headlights, I believe there should be low, high, and "running lights".

Thanks for any help!
 
I hope your problem gets solved here as I have another make 1936 car that does the same and I have no idea to solve this.
 
Check the voltage coming to the points with the lights on, then with them off. Now look for the spot that is causing the difference and if you can cure that, you should be good to go. Trace all the wires and connections from battery to the item being powered and its ground. I had a wire that was a bit loose within its attaching point and it gave me grief for years. A fellow club member bumped it with his hand and noticed it moved, we squeazed the clip tighter and all the grief was history.
 
Before going down the road of what is wrong I question 9-9.5 V at idle and 7-7.5 volts with the HL on..
Baby steps, to put it in perspective, in your senario if it was a 12V system your car at idle would put 18-18.5 volts across the battery.
To put 9.5V across a 'good" 6v battery typically would require more current than your generator is capable of..... unless the battery is bad.
Don't forget you are converting a mechanical input into electricity.i.e the generator is not turn fast at an idle.
Take various voltage readings including the coil and from the frame to the engine, preferably under load (headlights). Your reference is battery voltage.
That is to say if you are 6.5V at the battery and say 9.5V somewhere else you can deduce a lot.
Something doesn't make sense.
Dave we were on it together, good advice. I type slowly.
 
Thanks guys. It's a new battery and I recently had the generator rebuilt, had this problem last year but it went away. If you have a list of points to check voltage under load and not, I will gladly report back.

Thanks!
 
Even at the generator, that seems too high a reading for a 6 volt battery. I hope you don't have an 8 volt battery. People that use 8 volt batteries are usually hung at sunrise.

What does your ammeter show at idle and at faster RPM?

As far as the headlights being on in all positions, my first thought is the contacts at the base of the headlamp might be touching each other. Repro contacts were made too large and often fit loose enough to slip around and touch each other.

As far as the engine running rough with the lights on, if the lights draw too much current or have a short, they might take enough current away from the coil to give a weak spark. You could hold the coil wire 1/4" from a head nut while you hit the starter with the key on, and check for a hot blue spark. Now repeat this with the lights on and let us know how the spark looks.
 
I can give better answers tomorrow, but I know driving down the road my ammeter shows charging around 10, as soon as I switch on the lights, it's negative 10.

I sure appreciate all the help!
 
Sounds like a short in the headlight circuit. Possibly a wire has chafed through the insulation or something else has gone a muck like a light bulb socket.
 
If you put on a welding helmet you can look into the light filaments and see which ones are lit up. Only one at a time should be lit.
 
This is a car that has been in my family since 1964. I will confess while checking a headlight last year I noticed wires wrapped in duct dape - yikes! I'll unplug each light individually tomorrow and see what happens.

Thanks again
 
Thats good, find the light overcurrent problem first. Then use an inline ammeter to check output of the generator and set your third brush. Then check voltage drops all the way forward from the battery to the generator. Find the bad connection and repair it. 9 volts is TOO HIGH.
 
Sounds like the generator has a poor connection to battery - or battery is defective.

However, that is at odds with the charge/discharge current shown by the ammeter.

What do the headlights do when the engine is not running - are they similarly bright? Or - do they dim and brighten when the engine is running?
 
GROUND GROUND GROUND.
Poor grounding will increase resistance and voltage drop when lights are on.
Clean your ground cables and attach points and use dielectric grease. Make sure everything is tight.

A frayed wire or partially broken will also cause the same problem.
 
Tom Wesenberg said:
Yep, it sounds like the lights are shorting and running all the filaments at the same time to draw that many amps.
Click to expand...

And the reason your car runs poorly is that there is little current to run the engine. I would also add poor grounds to the things that will cause your symptoms. Both filaments on, and poor grounds.
 
Had time to do some checking last night.

Voltage at generator without headlights 9.5 and 7.8 after regulator
With headlights on its 8.4 and 6.6.

It is now hard to start and runs rough at higher speeds even with lights off. Possibly a points problem from running with lights on, grounding out somewhere?

Now looking at the three light operating positions. Switch to the left I get the small bulb and dim headlights. Center position gives me small bulb and bright headlights, third position gives me just bright headlights. In any position I run about 6.6 volts.

So I'm thinking my generator is running properly, my headlights aren't wired correctly, and somewhere a bad ground is sucking power from the engine.

I recently rebuilt my distributor and things have been great for a couple of months. One more note, when it's hard to start and runs rough at speed. Once I shut it off and restart, it runs just fine. Usually.

So I'm assuming I have two separate problems. I'll be cleaning up some grounds, and will tackle the headlight wiring, appreciate any other ideas.
 
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