Ignition switch and cable

Tom F OHIO

Tom F OHIO

Well-known member
Can you tell me how to check the ignition switch and cable to see if it is working properly when the key is turned on and off and the cable is wired to the switch.
I have a volt meter or test light. Thank You.....
 
Is it an original pop out switch or aftermarket on/off? If a popout, when the key is on, you should have continuity from the terminal on the switch to the button at the end of the cable that goes into the distributor. There should be no continuity from the terminal to the body of the switch key on. With the key off, you should have continuity from the terminal to the body of the switch. The popout grounds out the ignition current when off. If it's an aftermarket on/off, continuity from the terminal to the button when on, no continuity when off. There never should be continuity from the terminal to the body of the switch.
 
Will, its an aftermarket setup. So are you saying put one probe on the screw with the cable attached and the other probe at the end of the cable that goes into the dist and see if it has continuity turned on and none if turned off.
 
An easy way to check is to unscrew the ignition cable from the distributor and with the key off, scratch the cable's spring-loaded tip against a cylinder nut. There should be NO spark. Now turn the ignition key on and repeat the test. The tip should spark this time. This simple test will tell you if your switch is functioning or not. Of course, if you want emperical data provided by test equipment, fine. That will only re-inforce the scratch test, however. The switch either works or it doesn't.
Checking for spark at the ignition points will also tell you if the ignition switch is working. But you might get spark at the cable's tip, but not at the points due to an electrical problem inside the distributor.
By the way, when you replace the cable into the distributor, screw it in all the way by hand and then back it out one or two turns to prevent the tip from shorting out the lower distributor's tang on the bottom side against the distributor's metal body, which will result in no spark at the points. That's confounded many a Model A owner!
Marshall
 
Mornin' all, Tom F, I'm up early today, lookin' fer some Model A knowledge. X2 on all above. I'd like to add to the above stack of info:

Leave it to me, a few years back when our car was new to us, I took the above warning regarding over tightening the heavy cable ferrule a bit too seriously, and somehow left mine just a thread or so too loose.

After a morning with two closely spaced stumbles of the engine on a tight turn and dropping over a hilltop, several days later, I removed the distributor. I discovered that the ferule could be jiggled and was loose in the distributor. After tightening just a thread or two, the intermittent stalls have never reoccurred. I suggest trying to jiggle the ferule to prevent the too loose side of the equation before reinstalling the distributor as well as not over tightening it. I also push on the spring loaded tip to make sure that it is flexing as it should. The cable that came in our car was old, and I was not able to flex it with my thumb at all. I purchased a restored original pop out switch from Dick Crabtree.

Electrical failures on machinery are often intermittent. I've watched a few hundred Youtube videos where professional mechanics advise one to test the circuitry immediately upon failure and without touching or moving/removing components, at first.

If the car starts and runs, the switch and all the connections within the primary ignition circuit are being made. To stress test the switch, with the engine running, jiggle the key up, down, left, right, in and out, in the off and on positions and all points in between. Tap on the switch lightly with a plastic screwdriver handle. If your stumble or stall does not occur while doing these tests, then move outside. Tap lightly on the heavy cable, the ferule and the distributor casting with the plastic screwdriver handle.

If you cannot recreate the failure, then it may be time to disassemble some things for further tests. If you suspect the switch or its cable to the distributor, you can bypass the switch and its cable using a bypass cable. These cables can be purchased at the vendors. Be prepared to pull the safety fuse to stop the engine, else remove the positive cable from the battery.
 
Last edited:
Tom F OHIO said:
Will, its an aftermarket setup. So are you saying put one probe on the screw with the cable attached and the other probe at the end of the cable that goes into the dist and see if it has continuity turned on and none if turned off.
Click to expand...
Yes
 
Testing the switch (and its heavy cable with a multi-stranded wire connected to the spring loaded terminal on the distributor end) with an end to end continuity test will provide reliable information if the test indicates an Open / OL condition. If the test indicates that continuity is there, it does not prove that the circuit can continuously conduct enough current to properly saturate the ig coil windings.

I.E. if the wire has broken strands or loose / corroded end terminals, or the green crusties of corrosion internally, it likely will not support the amperage required for proper ignition.

I recently tested the primary ignition circuit on our A, which was and still is running great. I placed the leads of a DVOM across the blocked open ig points. The DVOM was set up on the ammeter circuit to reveal the current flow. All accessories were off. The only circuit live was the primary ignition circuit. (Ig switch on) The reading, as I recall, was ~3.6-3.7 amps initially. I didn't watch for more than maybe 30 seconds. The amperage seemed to be slowly dropping a bit, which I surmised was due to heating of the curcuit. (due to current flow)
 
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