Side and main lights not working

Bass1st · Posted: Tue Jul 30, 2024 1:16 pm Post subject: Side and main lights not working

I have a 1936 4 1/4 and all the side/main lights have suddenly stopped working. Brake lights are fine, as are the indicators (which are shared with the front side lights). I also have a central spot light between the horns and that works fine also.

All the fuses seem to be good, and I checked the electrical connections on the back of the master switch which also seem good.

Any ideas what might cause a total failure of all the lights?

Nicholas Simons · Posted: Wed Jul 31, 2024 10:36 am Post subject:

Have you checked you are getting 12v to the lighting fuses? If not, you may have a failed light switch in the main switch box

Bass1st · Posted: Thu Aug 01, 2024 9:46 am Post subject:

I took the fuse that feeds the side lights out and I get 12v when the switch is turned onto S&T. With the fuse in I get zero volts, so its clearly finding a way to get to ground, just not through the lights themselves.

When I do the 'fuse out' test with the switch at H,S & T setting I get no volts at all at either the side or main light fuse, which is a little odd. Not sure if the switch in this setting disconnects the S&T fuse or not? Either way, its not what I would have expected.

I double checked all the bulbs and they are fine. The side and tail lights share a common ground wire with the brake and indicators, so I don't think its a grounding issue.

Of course, this all assumes the original wiring diagram I have is correct. The car does have a more modern indicator setup installed so no guarantee that the wires have not been adjusted from the original state.

Nicholas Simons · Posted: Fri Aug 02, 2024 8:20 am Post subject:

Sounds like a fault in the switch box. With the light fuses out, you should get 12v at the top of the S&T fuse and H fuse when the switch is in the appropriate positions. In H position the S&T switch will also be on.
You could check the downstream continuity from the bottom of each fuse, ie, to the lights themselves.

Bass1st · Posted: Fri Aug 02, 2024 1:34 pm Post subject:

I carried out a few more tests today. I bypassed the master switch at the fuse box and all the lights worked, so I don't think I have a downstream problem.

I did a resistance check on the switch itself with the battery disconnected. I do get a closed circuit between the fuse that feeds power to the switch and light fuses as I would expect as I rotate the lever, however the resistance is high. I would expect the resistance to be close to zero, but I was seeing >350 ohms. I am not sure if this is normal or not?

I checked the resistance from the main power fuse and the fuse that feeds the master switch (the one that goes via the amp meter) and I get a closed circuit with no resistance at all.

I think I will take the master switch out and check all the connections, plus clean up the contacts if I can get to them.

Nicholas Simons · Posted: Sat Aug 03, 2024 1:16 pm Post subject:

The switch, when ON, should give zero Ohms. Remove the switchbox and overhaul the switch contacts. There is a moving blade that runs across a series of fixed contacts, made up as a stack. When you remove the stack, make sure you keep them in the correct order as they may look identical but are actually all slightly different lengths. Clean everything and add a bit of extra bend to the fixed contacts to make sure they make a good contact against the moving blade.

Bass1st · Posted: Sat Aug 03, 2024 6:37 pm Post subject:

I removed the switch box today (not the easiest of tasks) and found that the washer under the bolt that feeds the +12v into the light switches was completely corroded and was generating the resistance. I replaced that with a good washer.

I cleaned up the contacts as you suggested, although they do look warn so I might try and source some new ones. .

End result is I now have my lights back....!