Blinkers, break lights stopped working

[dwmcompton]

1973 Series III here. Hoping for some troubleshooting suggestions. for no apparent reason my blinkers and break lights both stopped working mid-drive yesterday. When I move the blinker switch, the interior green arrows do not light up and the blinkers outside do not work. And no break lights now either. When I put the Hazards on, the interior green blinker lights both work and the blinker lights in the back both flash like they should. Where do I begin?? Thanks for your help!

[siiirhd88]

Check the fuses and fuse holders as the power for the turn signals and brake lights are both supplied by the "Green'" wire circuit, which is fused "ignition ON" power. A white wire from the ignition circuit supplies the incoming power to two of the fuses, and green wires exit to power various loads. The fuse box terminals use riveted connections that can can corrode or loosen causing loss of power.

Assuming the wiring on your truck is not modified, the hazard circuit on an SIII usually uses a different flasher than the turn signals and has a different 'always hot' power supply. The turn signals are powered from the fused 'ignition ON' green wire circuit off of the fuse box. Power from the fuse box goes to the two prong flasher and then to the turn signal switch. The turn signal switch routes the power from the flasher to either side when selected.

Depending on the hazard switch, the switch can have the green wire power for the turn signal flasher routed thru the hazard switch. With the hazard switch OFF, the green circuit contacts in the hazard switch are closed routing the power to the turn signal flasher. With the hazard switch ON the green contacts open isolating the power to the turn signal flasher.

[jimrr]

I sure appreciated that explanation, I replaced all my broken lights with aftermarket stuff and they never work. now i'm having instrument issues..... gas, oil press etc. Jokes abound about
Lucas being the ''prince of darkness'' but considering what my S3 has been thru i'm lucky one headlamp still comes on.

[siiirhd88]

jimrr, PM sent

Bob

[vlad_d]

I agree with siiirhd88 , check the fuse farthest from the dash. It has all those things running through it. With only 4 fuses, many systems share the same fuse. That green wire one is all the lights and turn signals and some other dash things. So, you get hilarious sounding mystery wiring issues like “my lights and gas gauge stopped working” . It’s just because they’re on the same fuse(and actually the telltale sign it’s a fuse because multiple systems are on one fuse, but after the fuse they split into different wires. So what are the chances of multiple shorts on different wires 3-10 feet apart?) Replace the fuse. And with the right one. Those are 35amp fuses, not the 10, 20amp ones like in a VW Bug or old muscle car. Sometimes it could blow 10amp fuses a lot and make you feel like you got wiring issues when it’s just under the actual specs.

Doesn’t hurt to check your grounds, too. Or even add some ground wires. Lots of wiring gremlins are just bad/weak grounds. Bad grounds cause higher resistance and heat, so more likely to pop a fuse. Just find steel for the grounds. Don’t use the fenders(“wings” in Brit) or roof, as those are aluminum and not usually connected back to the battery negative.

Put a multimeter (on the Ohms setting) across the fuse terminal(bypassing the fuse) and check if the terminals make a good connection. Then do the same with the fuse in your hand to make sure it’s good(don’t just eyeball it). Finally, put the key in and turn it on, because those are ACC ON activated(which you can check with the multimeter set to 20v setting. I spent a good hour testing those ones only to realize the circuit was fine, I just needed to turn on the car.

ProTip: If it’s working, the little power plugs in the dash are a convenient place to test for voltage (red) or grounding(black) when testing the dash wiring. Can’t tell you how many times I just shove the negative terminal of a voltimeter in there and the positive on some switch or terminal I wanted to test. Much easier than trying to find a good ground somewhere.

Lastly, you can get a new fuse box for less than $20. I bought one and had it in a box in the garage. When I was diagnosing my own issues earlier this year, I just replaced it while I was down there. It took 15 minutes and I can cross that worry off the list about possible bad terminals and whatnot. Just replace it. You won’t regret it and it’s dirt cheap. It’s a satisfying little upgrade you can do fast and feel like you played with the Series over a weekend.

PS. I also have a ‘73 Series 3!!!

[siiirhd88]

Note that the headlamps and parking lamps on a Series are not fused originally. However the trucks are old enough now that anything can be found in the wiring..... A solid Brown wire (unfused, always hot, directly from solenoid or starter button) powers the lighting switch. There are other unfused loads depending on the year and type. Best to check the wiring diagrams.