Long post warning, but you may find something helpful here. I just mentioned on another thread, these PRE-9/72 circuits were definitely designed by an engineer, not a maintainer. It’s extremely helpful to understand how the system is designed prior to jumping into a fix not doing so leads to so many “bad” fixes. It can be simplified to four pieces:
1. Hazard Switch is brains of the operation
2. Ignition OFF/hazard switch ON
— battery power
fuse block
hazard switch ➡ flasher
back to hazard switch (load)
— The four signal lights provide the load that triggers the flasher to flash
— Hazards work, turn signals DON’T work
3. Ignition ON/hazard switch ON or OFF
— Power
fuse block
ignition switch
hazard switch to
flasher
back to hazard switch (load)
— This time hazard switch also sends power to turn signal switch
— When turn signal switch is engaged, load from the left or right turn signals cause flasher to flash those lights
— Turning on the hazard switch overrides the turn signal switch and all four signal lights flash
4. Brake pedal DEPRESSED (hazard and turn signal switch positions don’t matter)
— Power
brake light switch
tail lights via turn signal switch
— independent of flasher and hazards so you always get steady brake lights
TROUBLESHOOTING STEPS. If your lights don’t function as above, check the following:
1. Has a PO made any obvious changes to the system (look for a lot of plain red, black, or yellow wires, usually larger than the other wires around them.
2. Check the fuses
3. Ensure good ground contact for each component. There’s not a ground wire in the mix. *Everything* is grounded to the chassis. Making sure clean connection to ground can save you a LOT of time. Even tiny amounts of paint or corrosion can cause funky issues, from not working at all to under volt issues that can make your lights do very strange things.
4. Check for corroded or worn terminals/contacts inside the components. WARNING! There are several small parts that will try to make their getaway. Open or dismantle the switches/flasher carefully and take a picture right away so you know how things go. Getting shorting blocks out of position can cause you more heartache. Clean contacts thoroughly with contact cleaner. Worn contacts can often be renewed with a bit of solder and a light touch. Heat the contact, not the solder or it won’t stick. Too much heat may cause solder that’s already in place to run out. Practice on some scrap metal first!
5. Get out your wiring diagram, multi-meter and some alligator clip test leads, it’s time to start chasing wires. Start from the fuse block back To the switches. Make sure power is going in and coming out. Then check power at the light. If no power, you know your problem is between the switch and the light. Check the ohms between the switch and the lamp. It should be 0 or damned close to it if the connection is good. A high number indicates corrosion or otherwise weak connection. OL (or similar on your device indicates a break in the line. Work your way from the light that’s out and look for breaks in the wire, cracks in the insulation (that will encourage corrosion), brittle/burned wires or shoddy repairs done by a PO.
