Electrical and Ignition Diagnosis

[Mitch]

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Original Thread








Lets use the above wiring diagram to make some basic electrical checks. Follow along on the diagram as you read through this.

The solid yellow (Y) wire is the main feed for the entire Model A circuit. This connects to the battery wire at the starter or feeds through an after market starter mounted fuse. If the fuse is blown you will lose horn, lights and have no spark. This yellow (Y) main feed wire goes to the junction box then feeds the ammeter. Black and yellow (B/Y) comes off the ammeter back to the junction box and out to the generator cutout. (If the ammeter is blown or has a bad connection you'll lose power to the drivers side junction box lug). From the cutout are the feeds to the horn and lights. Once the generator produces and the cut out closes, that will supply the entire electrical system with a constant current and charge the battery as well.

With that being said the basic initial electrical check points with the key off should be the starter mounted fuse, both wing nuts at the junction box, both wires at the coil and the black and yellow (B/Y) at the cutout. All should have full battery voltage. This takes about 10 seconds to check. If you are losing voltage at one of these points it should be fairly easy to determine why.

Poor junction box connections is a popular spot for issues.


Let's look at another part of the electrical system, the primary ignition circuit. Notice the black (B) wire feeds battery voltage to the negative side of the coil via the junction box. There is a red (R) wire from the positive side of the coil that feeds the ignition switch. From there the ignition switch feeds the power to the points when the key is on. With the points contacts open there should be battery voltage at the moveable point arm. (the arm that has the rubbing block). When the points close the power provided to the moveable arm goes to ground. This opening and closing of the points collapses the field in the coil to produce a spark from the coil wire.

If you're not getting that battery voltage to the moveable points arm key on points open, it could be a bad ignition switch, shorted wire in the armored cable or maybe the armored cable is screwed to far into the distributor housing. Another common problem on an aftermarket type switch is the terminals can ground on the fuel tank when the instrument panel is mounted.

A common no spark condition is dirty or mis adjusted points. This will stop the power from reaching ground when the points are closed.

Okay I was bored but now i'm off of my soapbox.

See our other related technical threads located in the tune-up - performance forum.
such as this one. https://www.vintagefordforum.com/for...-and-misc-tips

[Jeff/Illinois]

Thanks very good!

The late great Alex Janke once said it was a good idea to trace your known good circuits with a Fluke meter while things were all working and write down your findings, then if something goes afoul go back and check again until you find a fault and that will help you diagnose your problem. Good advice!

[old31]

Mitch, I love it when you're bored.

Please get bored again soon.

[WMWS]

Great job Mitch. Very clear and easy to follow. Thanks.

[Bjunkans]

Thanks for the clear explanation. I will have it handy this weekend when I try to figure out my ignition problems. This may be off topic, but is the wiring diagram in the les Andrews book the 1929 and earlier wiring set up? it looked different than the one in the tech section here. Or it could just be that wiring diagrams are not my friend.

[Mitch]

Bjunkans The early cars were wired differently. The ignition system did not go through the ammeter. Therefore the ammeter would not register or show when the points would open and close. There was a Service bulletin on this which i'll find and post here. I would rewire all cars the revised way per the diagram above.

[Mitch]

Bjunkans Thanks for asking, that was a very important question and change in the wiring. Here is the service bulletin which clearly explains it.

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[1930 Closed Cab PU]

I tried to write something up in laymans terms. It's good to see it received a positive response, and hopefully it helps others. Over the years I have helped folks on the phone by basically walking them through the same basic checks. Now we can direct people in need right to this thread and eliminate 5000 duplicate repetitive answers like which oil

Thanks all

Mitch. Was recently looking for how the Pop Out switch works, could not find info on how it works as a theft deterrent. Does the pop out style affect trouble shooting? If yes perhaps add to your troubleshooting doc.

[Mitch]

Mitch. Was recently looking for how the Pop Out switch works, could not find info on how it works as a theft deterrent. Does the pop out style affect trouble shooting? If yes perhaps add to your troubleshooting doc.

The popout info is in the electrical tech forum at this post
https://www.vintagefordforum.com/for...0552#post70552

What I posted above is a basic guide to perform checks which will help someone get on the right path to troubleshoot an issue. The popout would still be the same as it lets the power flow through to the points. There are differences internally which can be checked further

[Mitch]

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