Brake Lights and Turn Signals not working

[E9Pilot]

I have been hunting electrical gremlins on my 1972 3.0CS for two straight days now and am hoping the experts here can help me out with some diagnostic prowess.

Symptom: Brake lights and blinkers are not working at all. The hazard flash, but no turn signals.

Cause: When trying to find a switchable power source for a small Air Fuel Ratio gauge, the #4 fuse blew when I grounded something I should not have near the steering column. It was the resistor under the cover labeled 122 on the electrical diagram and 30/31a on the resistor itself. It's right next to the ignition switch. The #7 fuse never blew. This has made the brake lights and turn signals not work. ??

I replaced #7 fuse anyway and still no lights.

Diagnostic efforts: Chased the Green/black wires from the brake switch to a relay #53. No power when the car is off, which is normal, but the power is there when the key is turned to the on position, also normal. The kicker is that I get no power on this circuit when the ignition is on and the brake switch is connected. The fuse remains intact. Tested wire continuity, good there and all the way to the lights via Green/Red wire. Light bulbs are good. The switch has continuity when open, which is normal.

what would cause the no-power situation after the brake switch being connects with the ignition on?

By the way this circuit worked flawlessly before. Is there something I am missing? Thanks in advance for any help!

 

[aearch]

ae

check continuity of brake switch

 

[E9Pilot]

check continuity of brake switch

The switch has continuity when it is open (pedal pressed). I even tried a spare brake switch known to be good with no luck. The problem is that when the switch is introduced to the circuit, the power no longer is on the Green/Black wires on one side of the switch.

 

[Arde]

No power when the car is off, which is normal, but the power is there when the key is turned to the on position, also normal. The kicker is that I get no power on this circuit when the ignition is on and the brake switch is connected. The fuse remains intact. Tested wire continuity, good there and all the way to the lights via Green/Red wire. Light bulbs are good. The switch has continuity when open, which is normal.

what would cause the no-power situation after the brake switch being connects with the ignition on?

By the way this circuit worked flawlessly before. Is there something I am missing? Thanks in advance for any help!

This is a long shot but the only explanation I see for your symptom is that the ignition contact that feeds fuse #7 (or a connection to it) has developed some high resistance. The voltage is there when there is no load (no current) and drops when you load it with the brake light circuit. The resistance has to be large enough not to blow the fuse.

Check if the horn works because it is on the same circuit but has a relay so the load is much less. If the horn does not work then you really have a problem with the #7 circuit before the fuse. If the horn is kind of faint or intermittent it confirms the theory. If the horn sounds loud and continuous not only you are still in the dark but your neighbors are also pissed at you and your old cars.

 

[E9Pilot]

This is a long shot but the only explanation I see for your symptom is that the ignition contact that feeds fuse #7 (or a connection to it) has developed some high resistance. The voltage is there when there is no load (no current) and drops when you load it with the brake light circuit. The resistance has to be large enough not to blow the fuse.

Check if the horn works because it is on the same circuit but has a relay so the load is much less. If the horn does not work then you really have a problem with the #7 circuit before the fuse. If the horn is kind of faint or intermittent it confirms the theory. If the horn sounds loud and continuous not only you are still in the dark but your neighbors are also pissed at you and your old cars.

Arde, good call. If I get a chance to get to my shop tonight, I'll see if that is the ticket. Thanks!

 

[E9Pilot]

It turns out that I had two issues. The first was a few very dirty spade connectors on the rear of the fuse block and the second was a bad fuse. The fuse had continuity and almost no resistance, so I thought it had to be good. Once I replaced it with another, bingo!

I feel wicked smart after getting bested by a fuse.....