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Dimming headlights on my 46

Started by Jim Thompson, December 09, 2010, 10:06:58 AM

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Jim Thompson

While driving down the road all of my lights are bright and clear. When I get off the gas and come to a stop, the headlights dim down to almost nothing. Upon acceleration, they are bright again. I know some generators do not charge while at idle, but the headlights shouldn't practically disappear altogether. Voltage drain? Bad regulator? Generator needs rebuild?
1946 Cadillac model 61 fastback

Fred Pennington 25635

Generators don't typically charge at idle so at idle you are running just off the battery.
I would check the battery condition first.
A battery at a low charge or old and / or low on capacity will have significant voltage drop under load
You also could have a voltage relay or regulator set to high, with the high voltage you might be seeing a very bright bulb running on too much voltage just dropping to normal
Put a volt meter on the battery and check the standing voltage. It should be higher than rated voltage.
Start the engine and check voltage at Idle Check it running at about 2000 rpm and a do the same tests with the lights on
If the voltage drop is significant you more than likely have a battery problem


Good lluck
Fred P.
Fred Pennington, CLC 25635
1940, LaSalle 5019
1940 LaSalle 5019 parts car
1968 Ford Bronco
1973 Mustang Convertible
2012 Shelby GT500

Jim Thompson

1946 Cadillac model 61 fastback

The Tassie Devil(le)

Wonder if this feature was a Factory modification and in-built into the electrical system, seeing as the lights don't need to be as bright when stopped as when actually moving. ;) ;) ;) ;)

But, using relays at the headlights and getting direct current from the Battery will solve the problem.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Jim Thompson

I had put a new battery in it a year ago, but I am thinking it might need one of those new 6 volt gel cell batteries. I heard those deliver better power.
1946 Cadillac model 61 fastback

Fred Pennington 25635

Depending on how you maintain your battery a year old battery can be low on capacity
When stored the battery will self discharge if not on a maintainer like a Battery Tender.
While in a low state of charge the battery will sulfide. That is where the sulfur in the acid begins to stick to the plates ( simply put ).
The coating of the plates keeps acid from reaching good lead. No acid / lead contact , No electricity
When you charge the battery at low rates you can not reverse this or break up the sulfur. All lead acid batteries need what is referred to as a mixing charge at least once a year. The Generator may or may not be able to accomplish this. The problem with relying on a generator is they will overheat and damage will accrue. The best way to reverse a sufided battery is to put in on a high output charger, at least 25 amps. Don't pay attention to the amp meter on the charger. Use a Hydrometer to check battery condition. I have had batteries take a week  or two on and off the charger to come up to capacity. yes the battery will get hot.

So long story short, Check your battery with a hydrometer first before you spend the big bucks on a new battery and put it on a good charger for awhile

Fred P.
Fred Pennington, CLC 25635
1940, LaSalle 5019
1940 LaSalle 5019 parts car
1968 Ford Bronco
1973 Mustang Convertible
2012 Shelby GT500

Doug Houston

There is something elso going on here, which needs to be checked. All of our cars have used conventional lead-acid storage batteries from new, and have not had the problem described here.

First, if the battery is sulfated, it won't crank the engine very well, if at all. Also, if the charging system isn't working, the battery would run down, even while driving.

The charging system needs to be tested for operation.  The voltage at the battery while running (above idle) should be around 7.25-7.5 volts. If that passes, then the voltage  (above idle) should be tested at the light switch feed; right at the light switch. It should read  7.25+ volts. Headlights on, voltage should be slightly less than 7.25-7.5 volts.  Next, test voltage at the dimmer switch, and voltage should be just about the same as at the dash switch.  Then, headlight voltage measured at the terminal block on the inner fender should be down to about 7 volts. That's where the feed goes to the sealed beams, and there should be a good bright ground at the terminal board, to the fender.

If all is well at the fender terminal board, remove the headlight lamp, and inspect the three blade conector for the Sealed beam lamp for corrosion, coating the contacts. Corrosion can be cured with some of the sink and bowl cleaners, rinsing and drying the connector afterwards.

The headlights will dim a bit normally at idle engine speeds, but won't go almost out, if the charging system is working properly. the lights will pick up immediately as soon as the car is under way,
38-6019S
38-9039
39-9057B
41-6227D
41-6019SF
41-6229D
41-6267D
56-6267
70-DeV Conv
41-Chev 41-1167
41 Olds 41-3929

Jim Thompson

The headlights do brighten up immediately upon acceleration and the car is easy to start. I did check that fender wiring block you suggested and I have some of the insulation coming off the wires now. The car has the original wiring harness in it. Looks like I will need to get that remedied asap.
1946 Cadillac model 61 fastback

Doug Houston

Something just doesn't go together here. The generator output will go below 6 volts at idle, but the battery should be supporting the lighting load during that time. When the generator voltage drops below the battery voltage, the cutout relay in the generator regulator disconnects the generator from the system. When the engine RPM increases, the generator begins to put out higher voltage until the cutout relay sees enough voltage, and closes, putting the generator on line. The generator will be putting out around 7.25-7.5 volts. This supports the electrical load, and provides charging current for the battery.

Thinking about it now, the generator is doing its job OK, but for some reason, the battery isn't getting enough voltage to the car's system. The fact that the car starts OK, means that the cables from the battery to the starter and to ground (frame) are doing their stuff.  For some reason, battery feed to the system isn't getting there. Have a look at the main feed wire (about a #10 wire, connected to the starter solenoid with the battery cable, and  check voltage at that point, with engine off. Then, try to measure the voltage at the light switch feed  (the end of it), with the lights turned on. You're losing voltage badly somewhere along the way to the light switch, it appears.

Now, on harnesses.  I had designed lots of hrnesses for the Redstone and Jupiter missiles, and  also supervised the manufacture of them. Granted, the hardware for that application was different than the things on our cars, but the layout of the harnesses  was very similar. At that time, I took the harness out of my '41 Cadillac, and a few other collector cars, made  pegboards from them.,  I made new harnesses for thiose cars. Them days is gone forever, and I needed a new dash harness for my 38-90 sedan. I ordered one from Rhode Island  harness company, (or however teir company is named). When I received it, I was almost in shock over the beautiful way the  thing was made. The harness could have come right out of Packard Electric  division in 1938. It was well beyond my expectations. It possibly cost a tad more than other companies charge,  but I'm so happy with the way it was done, any price differential would never bother me.
38-6019S
38-9039
39-9057B
41-6227D
41-6019SF
41-6229D
41-6267D
56-6267
70-DeV Conv
41-Chev 41-1167
41 Olds 41-3929