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Classic/Antique Car Repair/Low voltage to lights 39 Mercury


1939 Mercury 4 dr still has 6 volt positive ground as original. Only change I see is sealed beam headlights.  Have 6.2 volts at battery but only 3.5 volts at headlights and taillights. I have shop manual with wiring diagrams but I am a novice.  Could you suggest a place to start to repair the problem?

The first question that I have  is the wiring harness original  or has it been altered or replaced? Let me know. To check the system check the voltage at the input side of the headlamp  switch with the lights on. The voltage there should be within .2 volts of battery. If ok check the voltage at the output side of the switch. Again this should be within .2 volts of the battery. If it is lower the switch is bad. Net check the dimmer switch the same way and the at the head lamps. This will point out where the voltage drop is. The final check is the ground connection for the headlights themselves.

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Brad Sears


All automotive including antique and collectible. However if the car has been modified I can only answer in general terms and maybe get you pointed in the right direction.


Automotive tech instructor. Syndicated auto columnist 1970's though the early 1990's. Syndicated auto radio talk show, Ask Brad About cars, CBS Radio 70's through 90's TV Show "Last Chance Garage" 1980's PBS-TV syndicated. Auto instructor for the following companies: Fram Autolyte Holly Carter AMF Ford Motor University Of Conn Blue Hills Technical School Sugar River Technical Center Grew up in a family garage in Needham Mass and turned wrenches from the age of 14.

Manchester Union Leader, Nashua Telegraph, Motor Service Magazine, Yankee Magazine, Popular Mechanics (Saturday Mechanic early 80's), Los Angeles Times, New York Times, and lots more.

More than I care to remember. Basically Franklin Technical Institute in Boston, Northeastern University, Fitchburg State Teachers College, Tufts University, and a lot of factory schools along the way.

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Moto Award winner. And much more.

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