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My 1968 coupe turn signals aren’t working. When I remove the fuse and test at the fuse block terminals, I get 12v. But when I put the fuse back in, it reads zero volts.

The fuse is good and correct amps. I assume there’s a short somewhere but don’t know why I only get 12v with no fuse installed. Does this make sense?

Marc
San Jose, CA
1968 Coupe, 289
 

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The short is grounding out your system hence zero volts. I’d start looking at your turn signal switch then look at the wiring between it and the lights. Since the lights should only be connected when he switch is activated, the problem most likely is in your switch or column wiring.
 

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You're not checking voltage with a probe on each end of the fuse when it's in place are you? With a good fuse you wouldn't have a difference of potential and it would read 0v. I would think if you had a short to ground the fuse would probably blow.
 

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65 Fastback 289 4 spd, 65 convertible 5.0L 5 spd. 3.73 8.8
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My 1968 coupe turn signals aren’t working. When I remove the fuse and test at the fuse block terminals, I get 12v. But when I put the fuse back in, it reads zero volts.

The fuse is good and correct amps. I assume there’s a short somewhere but don’t know why I only get 12v with no fuse installed. Does this make sense?

Marc
San Jose, CA
1968 Coupe, 289
So with a test light proves that there is voltage, however with fuse in and it doesn't light means that it can't carry amperage. When I test circuits I always load test them using a rectangular headlight bulb, just like a test light, if it lights circuit is capable of carrying amps, good circuit. You could have a small draw, like a wire that has 1 or 2 strands shorting which won't pop the fuse, or you have a corroded connection ahead of the fuse box, not capable of carrying a load. I do this on new vehicles testing circuits, I hope this makes sense.
 

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So how exactly are you measuring the voltage? If you removed the fuse and measured across the fuze block you would get 12v since one side is connected to the battery and the other presumably to ground if you have the switch on or a short. With the fuze in if you measured across the fuse you would get 0V because the fuze is effective
y shorting across those terminals. The proper way to measure is from the positive side to a good ground. That proves you have good. Outage to the circuit. You could also measure resistance from the load side to ground. It should have a measurable resistance with w the switch activated, and infinite resistance with no switches activated (assuming no other accessories are running).
 

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Limiting the troubleshooting to the current narrow scope, the fuse, please provide us with six voltage measurements, all using the same ground reference.

1. With fuse removed, measure and document the left or top (depending on orientation) fuse clip
2. With fuse removed, measure and document the right or bottom (depending on orientation) fuse clip
3. With fuse installed, measure and document the left or top (depending on orientation) fuse clip
4. With fuse installed, measure and document the left or top (depending on orientation) fuse body - the metal part of the fuse itself
5. With fuse installed, measure and document the right or bottom (depending on orientation) fuse clip
6. With fuse installed, measure and document the right or bottom (depending on orientation) fuse body - the metal part of the fuse itself

Report back with the results, and we can go from there.

With this information we can assess the quality of the connections to the fuse and the status of the fuse itself.
 

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Assuming you are getting correct voltages e.g. one end grounded to frame, the other on teh fuse, there must be a weak connection in the circuit between the fuse and the battery. Grounded circuit would blow the fuse and / or create more drama than you relate. I would check the connection at the firewall and clean the terminals both male and female. Its not a bad idea to do this maintenance once every 50 years or so...

You could also try powering the circuit south of the fuse and see if the signals work. that would be a clue.

Finally, which I would say check first, make sure the fuse connections are not buggered up with corrosion.
 
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