MTFCA Forum

Hi Team,

I have a 1926 Model T Coupe that my grandfather left me and I have almost fixed it. However, I am having trouble getting the headlights and horn to turn on. Both work when I power the bulbs directly, but they do not work when I power them through the switch. Can anyone help me with a wiring diagram?

kellanphil wrote: ↑

Mon Mar 24, 2025 11:11 pm

Hi Team,

I have a 1926 Model T Coupe that my grandfather left me and I have almost fixed it. However, I am having trouble getting the headlights and horn to turn on. Both work when I power the bulbs directly, but they do not work when I power them through the switch. Can anyone help me with a wiring diagram?

I don't have a diagram handy, but if you search this forum for "wiring diagram", you'll find dozens of examples.

This should help you.

Roger

If you have an original Ford type cloth covered wire harness it is easy to troubleshoot. If you have a bunch of wire spaghetti it is going to take more effort.

You can use a trouble light/test light to test for power and continuity. Unplug the headlight thimble at the back of the headlight buckets and put the test light with the wire attached to ground and the pointer to the headlight contact on the thimble one contact at a time. If you get the test light to come on you have power to the headlights and probably have a ground problem. A ground problem is usually caused by paint or corrosion preventing a ground. If you do not have power to the thimble, you move back to the junction block on the firewall and test again. If the light comes on there is a problem with the wire running from the junction block to the headlight thimble. If you do not get the test light to come on at the junction block you have an issue with the headlight switch or the wiring from the junction block to the headlight switch.

The horn has its own circuit independent of the lights, so do the test the same way on the horn circuit. Start at the horn terminal with the test light. If it is getting power our problem is probably the internal circuitry of the horn or a bad ground.

Here you go.
Ron P.

Be careful in testing the horn. You should have a battery horn like I do but if you have a magneto horn, then things can burn up. Positively identify the horn first. Also, if you are having electrical problems, now would be a great time to install a fuse if it does not already have one.

I see that this is your first post...welcome aboard!

Here is a photo of our Battery Horn. The back side is larger than a magneto horn.

Here is the switch wiring. You may need to take it apart and bend the tabs a bit for better contact.

Besides a possible wiring issue the headlights, there could be an issue with the switch itself.
But the first thing to do is make sure its not a wiring issue. DO NOT take the switch apart until you have confirmed that the wiring is OKAY

You didn't mention if the tail light worked when the switch handle was on DIM.
There are two different style backing plates, one with pins that secure it in the cup with the switch parts or one without pins that held in place by tabs on the cup as in the diagram.
The switch is actually two switches in one. The key selects the power source, BAT / OFF / MAG, while the handle selects the lighting choice.
Some simple tests

- If you turn the switch handle to select a lighting choice, you should feel some drag & some indexing into the dimples on the underside of the backing plate. If not, possible warped backing plate. - If you turn the switch to DIM or ON, check the tail light to see if it is on even if the headlights aren't on. That is possibility since its powered via the switch independently. enough for now