Topic: 1940 Ford Voltage at coil?


bwhitworth    -- 11-27-2011 @ 12:54 PM
  While trying to find the cause of a surging-bucking problem during hard acceleration, I standing beside the car listening to it idile when it just quit. I tried to restart it there was no spark at the plugs. I checked at the coil with the key turned on, no voltage. I put a jumper wire from the battery to the coil and it started right up. I checked for loose connections and broken wires and did not find any problems. After taking the jumper off I checked the voltage at the coil again, 2.5 volts. Question; What should the voltage be at the coil after the ignition resistor? It ran better with the jumper in place. Could I have damaged the coil by hitting it with 6 volts?


supereal    -- 11-27-2011 @ 1:47 PM
  Six volts will not damage a coil unless the ignition switch is left on accidently, or it is run on the higher voltage for a long period. There are several reasons for low voltage at the coil, which should measure about 3.5 volts with the points closed. When the points are open, you will read battery voltage, usually just over 6 volts. The places to check include the ignition switch. Old switches usually have a path worn in the Bakelite by the moveable (brush) part. Work the switch as you watch the voltage at the coil. If it changes, replace the switch. The parts are readily available from suppliers. If not, check the connections at the ends of the resistor. They are crimped onto the end terminals, and they often become loosened by the heat/cool cycles. You can recrimp them, or apply solder, The wire is nichrome, and difficult to solder, though. You can diagnose a bad connection by starting at the coil and working back toward the switch using a voltmeter. Place the leads across the connections, with the points closed, and look for a reading. Any found will indicate a loss in the connection that must be repaired. It is also possible the distributor points are worn, and need replacement, as they provide the ground for the coil, and be sure that there is a good ground strap between the battery and the engine block. Your problem is not rare, and should be easy to locate.

This message was edited by supereal on 11-27-11 @ 1:49 PM


bwhitworth    -- 11-27-2011 @ 4:27 PM
  Thanks for the advice, I'll give it a try.


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