Do they go out immediately? Like, you hit the foot switch for the high-beams and all the headlights go out?
That would more correctly be called a bimetallic switch - like turn signal flashers.
That would more correctly be called a bimetallic circuit breaker
A relay is used in applications where a small amount of control current through a electromagnetic coil is used to control a larger amount of current through the contacts. The bimetallic device is really just a switch that is open or closed. Sorry to nerd out, its the electrical engineer in me.
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Sure, but what do we call that little turn signal flasher can? A relay 
Yes, that’s one use for one style of relay, but they aren’t only used for that.
So we at least agree that it can be called a bimetallic switch 
No worries on nerding out. In Electrician A-School we called them relays, so that’s the term that’s stuck with me.
That’s interesting - I guess it’s just different terminology. I’ve never heard the turn signal flasher called a relay before.
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All 4 headlights go out when switched to high beam. My inclination is that this a low voltage issue. I will test the alternator function first. Then begin testing each circuit from fuse to light bulb.
Low voltage won’t cause that… Probably a problem with the Hi/Lo beam switch. But I applaud the fact that you are testing doing real diagnosis. Carry on!
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Bad connection at the high beam switch or a bad high beam switch?
“That’s interesting - I guess it’s just different terminology. I’ve never heard the turn signal flasher called a relay before.”
Most new cars use a relay to control the blinking of the turn signals.
That’s a good point. New cars have a low level electronic signal flashing the turn signal LED’s. So a relay is needed to step up the current to run the LED’s. Our cars are different and rely on the bulb current to heat up and open the bimetal flasher switch.
That’s #2 on my list. #1 is charging system diagnosis. I’m looking for a 3G alternator in Pick-n-Pull today. I will be upgrading to H4 headlights with relays.
What is the preferred donor for 3G that fits our cars?
16 or more yrs ago , the local motor electric shop found a 3g that had the terminals on the rear clocked correctly to clear the motor mounts on our 67 390 . Had to cut the rear flange from the orig pulley for everything to line up . Sorry , cant remember the year but it was a mustang
I have a list of possible donors. I’ve spent a few hours researching. Some of the Mustang forums have been helpful.
Please let us know what you end up with and how it goes
I replaced my old alternator with a Motorcraft NGL7742N. It is a 100 amp unit. I was thinking about adding power windows and wanted a bit more capacity. I stuck with the external regulator. My wiring did not go up in smoke.
I have an alternator from a 92 Mustang 5.0 on my 69 XR 7, 351W. I used an adapter harness from Ron Morris Performance. I had to change the pulley but that was not a big deal. It has been working fine for many years.
Check the grounds. Especially if the car has been repainted. Clean them then install star washers to improve the connection. Dielectric grease can be used to protect the connections from future corrosion. Good luck
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Thanks, good suggestion
Well here is an update on what I have found. 3 headlight bulbs burnt out. 4th headlight only worked on low beam. New 4040 and 4001 bulbs cured the function issue. Now waiting on replacement headlight retainer rings from WCCC as 3 were broken.
Now for the turn signals: They come on but don’t flash. That is until I inadvertently left the key on and the left signal on. I went inside the house and returned more than an hour later. The left turn signal was functioning as it should. I could hear the flasher clicking. When the right turn signal was activated, it too functioned normally. Curious! I have ordered an electronic flasher along with new 1157 bulbs from WCCC.
Back up lights work. Will check brake lights tomorrow.
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