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Discussion Starter #1
Hello Everyone! I have been following some of your posts on an electrical issue that seemed to be resolved, and I thought it was some awesome teamwork (not to mention, it was a recent post instead of those you stumble on from years back and not know if anyone is still watching it or will answer)!

I wanted to reach out because you all might be able to have a simple answer to one of my problems I'm facing. I am by no means an electrical wizard, and I would call that my least strong suit when it comes to diagnosing a repair. Kind of a newby all together, but have made it pretty far!

I currently do not have any instrument lights working. The only lights that do work in the instrument panel are my turn signals and my "high beams" red light that comes on when I engage the brights from the floor switch.

I have gone to the fuse box first, and did find that the fuse that is supposed to power those lights was missing. So, I replaced it with the correct fuse as indicated by the diagram. This did not solve the issue AND I tested the fuse installed and it was not showing any power to it. There were also a few other fuses that were not showing power, but ALL of my other lights seem to be working, which makes it even more strange (especially because the one fuse I replaced did not change anything at all and shoes no power coming from it). In other words, zero changes after replacing the one fuse, and still have other fuses showing now power, but all other light sources seem to be working....go figure...

On a side note, I have experience the occasional moment where ALL of my gauges (other than AMPS) turn off and do not register gas, oil, or temp. The radio also goes off along with it and so do the headlights. However, this is much more rare.

I am beginning to think that the previous owner(s) may have done some rewiring and made things a bit confusing, but I'm hoping that's not the case as it still seems mostly original.

With all of that said, and since I have already gone to the fuse box without luck after replacing the missing fuse with a brand new one that seems to not make a difference, I have gone to the TWO main disconnects under the hood of the car that come in through the firewall. I have tested those with a light tester to see if they have any kind of current running (whether it be positive or negative). ATTACHED is the chart of what I have found. I have not been able to find anything online to tell me what current should be flowing through which PEG or HOLE (as I have decided to call them) from the connections.

Do you have any advice or information on how these main disconnects should be receiving or transmitting power? Should they all give some kind of power indication, or are some of them more "neutral" and would not give off or light up my electric 12v tester light?

Please let me know if you need more details or if I can make the drawings more clear. I'm a bit lost at this point, and I simply want to be able to drive the vehicle at night without worrying about instrument panel lighting or any other lighting issue occurring while on the road (even if during the day, would hate for break lights to stop working).

Thanks in advance for reading this, I know its a bit longer, but wanted to make sure you have the details to be as helpful as possible!

PS, important detail. On the drawn diagrams, where it says "Good," that meant that the 12V light tested DID light up when i probed the hole or the peg. This would be either with the tester hooked up to the negative side of the battery or to the positive. I believe that it will be a mix of positive and negative as the current flows to and from the power items in the vehicle, so I made sure to test them all with both positive and negative hookup. The ones that are not marked as "Good" did not light up the 12v light tester at all, no matter if I was hooked up to positive or negative.

Thanks again!
 

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The best advice I can give you is to search the internet for a 1966 mustang wiring diagram or buy one. NPD sells it for around $10. The diagrams are very easy to read and understand. I referenced mine many times when rewiring my 66FB.
 

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Thanks, @bsolo66 . I have definitely located those online and found some great diagrams to use. I wasn't able to find any information on the actual main disconnects in detail, unless I overlooked it. The other diagrams are very helpful to see what goes where (and this is all if the previous owner kept their wiring to the actual diagram and didn't go too far off course. Thanks!
 

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Hello Everyone! I have been following some of your posts on an electrical issue that seemed to be resolved, and I thought it was some awesome teamwork (not to mention, it was a recent post instead of those you stumble on from years back and not know if anyone is still watching it or will answer)!

I wanted to reach out because you all might be able to have a simple answer to one of my problems I'm facing. I am by no means an electrical wizard, and I would call that my least strong suit when it comes to diagnosing a repair. Kind of a newby all together, but have made it pretty far!

I currently do not have any instrument lights working. The only lights that do work in the instrument panel are my turn signals and my "high beams" red light that comes on when I engage the brights from the floor switch.

I have gone to the fuse box first, and did find that the fuse that is supposed to power those lights was missing. So, I replaced it with the correct fuse as indicated by the diagram. This did not solve the issue AND I tested the fuse installed and it was not showing any power to it. There were also a few other fuses that were not showing power, but ALL of my other lights seem to be working, which makes it even more strange (especially because the one fuse I replaced did not change anything at all and shoes no power coming from it). In other words, zero changes after replacing the one fuse, and still have other fuses showing now power, but all other light sources seem to be working....go figure...

On a side note, I have experience the occasional moment where ALL of my gauges (other than AMPS) turn off and do not register gas, oil, or temp. The radio also goes off along with it and so do the headlights. However, this is much more rare.

I am beginning to think that the previous owner(s) may have done some rewiring and made things a bit confusing, but I'm hoping that's not the case as it still seems mostly original.

With all of that said, and since I have already gone to the fuse box without luck after replacing the missing fuse with a brand new one that seems to not make a difference, I have gone to the TWO main disconnects under the hood of the car that come in through the firewall. I have tested those with a light tester to see if they have any kind of current running (whether it be positive or negative). ATTACHED is the chart of what I have found. I have not been able to find anything online to tell me what current should be flowing through which PEG or HOLE (as I have decided to call them) from the connections.

Do you have any advice or information on how these main disconnects should be receiving or transmitting power? Should they all give some kind of power indication, or are some of them more "neutral" and would not give off or light up my electric 12v tester light?

Please let me know if you need more details or if I can make the drawings more clear. I'm a bit lost at this point, and I simply want to be able to drive the vehicle at night without worrying about instrument panel lighting or any other lighting issue occurring while on the road (even if during the day, would hate for break lights to stop working).

Thanks in advance for reading this, I know its a bit longer, but wanted to make sure you have the details to be as helpful as possible!

PS, important detail. On the drawn diagrams, where it says "Good," that meant that the 12V light tested DID light up when i probed the hole or the peg. This would be either with the tester hooked up to the negative side of the battery or to the positive. I believe that it will be a mix of positive and negative as the current flows to and from the power items in the vehicle, so I made sure to test them all with both positive and negative hookup. The ones that are not marked as "Good" did not light up the 12v light tester at all, no matter if I was hooked up to positive or negative.

Thanks again!
I am sure that someone on the forum can help you with your questions. I learn something new each time and will be following your post. You mention at one point (your side note) that all of the gauges and headlights went out for a short time, but that it was a rare occurrence. Did you ever replace your original headlights with halogen? If so, then this may be the cause of that happening. When I installed halogen headlamps, I had to install a relay which help prevent this from happening. Of course, if you have not replaced the headlamps, then none of what I am saying applies. I found this information on Daniel Stern Lighting Consultancy and Supply. I do believe that someone else could explain this better. I look forward to hearing how you are able to solve it. Take care
 

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I would probably guess a bad ground somewhere if your gauges all go out intermittently. One of the first steps in diagnosing electrical problems is to very good, clean, and tight grounds.
 

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From what I read, do you have power to the instrument light fuse? If so, check to make sure the contact in your fuse block are getting power on both sides of the fuse and contacts (sometimes the contacts don't connect with the fuse because of corrosion etc.). If you don't have power to either side of the fuse, look for an issue with your headlight switch (it supplies power via the resistance coil on the switch). You can test your fuse/block with a test light to ground. If you have power through both sides of the block contacts, then the lead from the block to the dash is damaged/broken/disconnected. Also, this appears to be a double-entered question (see below).
 

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Discussion Starter #7
I am sure that someone on the forum can help you with your questions. I learn something new each time and will be following your post. You mention at one point (your side note) that all of the gauges and headlights went out for a short time, but that it was a rare occurrence. Did you ever replace your original headlights with halogen? If so, then this may be the cause of that happening. When I installed halogen headlamps, I had to install a relay which help prevent this from happening. Of course, if you have not replaced the headlamps, then none of what I am saying applies. I found this information on Daniel Stern Lighting Consultancy and Supply. I do believe that someone else could explain this better. I look forward to hearing how you are able to solve it. Take care
Thanks for the comment! I have not replaced them with halogen, so I don't think that would be the case here. Did all of your gauges also go out when that happened before you got the relay?
 

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Discussion Starter #8
I would probably guess a bad ground somewhere if your gauges all go out intermittently. One of the first steps in diagnosing electrical problems is to very good, clean, and tight grounds.
I've heard this could be the issue as well. I will try to locate all of the grounds and make sure they are tight. This where I tend to get a little foggy though, if I'm being honest, about what a proper ground is supposed to be and how to test it correctly. I know they are located all around the vehicle and in some tight to reach/see places.
 

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From the wiring diagram, power flow starting from the battery goes to:
1. Starter relay, left post when viewing the front of the relay.
2. Wire 37 to the main disconnect at the firewall
3. Wire 37 to a splice
4. Wire 25 to the headlight switch. When the headlight switch is turned on, power is applied to wire 19.
5. Wire 19 to the fuse panel. Note that the feed is not fused up to this point, the design is fault tolerant - don't want headlights to trip off. The headlight has a built in circuit breaker for the headlights, but that does not affect wire 19.
6. Wire 19 from the fuse panel. This is after a fuse in the fuse panel, and protects against a short downstream.
7. Wire 19 goes to the instrument panel lights and transmission indicator light.
8. Each instrument panel light is grounded to the instrument panel. The instrument panel needs to be grounded. I'm not sure of the method for 1966.
9. The transmission indicator light has a separate ground.

Now let's use a methodical approach to resolve the problem, rather than shooting from the hip.

The best practice is to consider splitting the problem at the middle (guess 50% in the lower/higher game) as well as considering how easy or difficult an area may be to reach. For this situation, I recommend starting at the fuse panel.

1. Find the fuse associated with the instrument panel lights. With the headlight switch off, there should be no power on either side of the fuse.
1a. If there is no power, move on to step 2.
1b. If there is power, you are either on the wrong fuse or there is something terribly wrong. Report back here.

2. Turn the headlight switch on. There should be power on both sides of the fuse.
2a. If there is no power on either side, move on to step 3.
2b. If there is power on only one side, you have a bad fuse, or perhaps a weak fuse clip. Replace the fuse and/or tighten up the clip. If you can't resolve, report back here.
2c. If there is power on both sides of the fuse, move on to step 4.

3. Check for any power on any fuse in the fuse panel.
3a. If there is no power, report back here.
3b. If there is power, go to step 5.

4. Remove the transmission gear selector cover plate to access the bulb. Turn the headlamp switch on.
4a. If the light is on report back here.
4b. If the light is not on, check/replace the bulb. Follow the black wire until you find the ground screw. Remove screw and polish everything up, then replace screw. Pull the connector apart and check for voltage on wire 19 (blue/red)

5. Pull the main connector and check for voltage on wire 37 (black-yellow)
5a. report back here.

The reason for the "report back here" is because I would probably get to step 50 if I were to fill out every possible scenario, and I'm getting lazy. Once we know which direction this is heading, I can add more focused pinpoint testing recommendations.
 

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Not to detract from this fantastic troubleshooting plan here ^^^ but I just have to make sure that you know that the instrument lights dim, right? If you turn the headlight knob to the right they get dimmer and to the left they get brighter (or is that backwards?)

When you're in Step 2 up there with the headlight switch on try turning the knob back and forth and see if it changes anything. Unlike modern cars our instrument lights are only on when the headlights or parking lights are on so you won't see them until you get to that point.
 

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Discussion Starter #11
From the wiring diagram, power flow starting from the battery goes to:
1. Starter relay, left post when viewing the front of the relay.
2. Wire 37 to the main disconnect at the firewall
3. Wire 37 to a splice
4. Wire 25 to the headlight switch. When the headlight switch is turned on, power is applied to wire 19.
5. Wire 19 to the fuse panel. Note that the feed is not fused up to this point, the design is fault tolerant - don't want headlights to trip off. The headlight has a built in circuit breaker for the headlights, but that does not affect wire 19.
6. Wire 19 from the fuse panel. This is after a fuse in the fuse panel, and protects against a short downstream.
7. Wire 19 goes to the instrument panel lights and transmission indicator light.
8. Each instrument panel light is grounded to the instrument panel. The instrument panel needs to be grounded. I'm not sure of the method for 1966.
9. The transmission indicator light has a separate ground.

Now let's use a methodical approach to resolve the problem, rather than shooting from the hip.

The best practice is to consider splitting the problem at the middle (guess 50% in the lower/higher game) as well as considering how easy or difficult an area may be to reach. For this situation, I recommend starting at the fuse panel.

1. Find the fuse associated with the instrument panel lights. With the headlight switch off, there should be no power on either side of the fuse.
1a. If there is no power, move on to step 2.
1b. If there is power, you are either on the wrong fuse or there is something terribly wrong. Report back here.

2. Turn the headlight switch on. There should be power on both sides of the fuse.
2a. If there is no power on either side, move on to step 3.
2b. If there is power on only one side, you have a bad fuse, or perhaps a weak fuse clip. Replace the fuse and/or tighten up the clip. If you can't resolve, report back here.
2c. If there is power on both sides of the fuse, move on to step 4.

3. Check for any power on any fuse in the fuse panel.
3a. If there is no power, report back here.
3b. If there is power, go to step 5.

4. Remove the transmission gear selector cover plate to access the bulb. Turn the headlamp switch on.
4a. If the light is on report back here.
4b. If the light is not on, check/replace the bulb. Follow the black wire until you find the ground screw. Remove screw and polish everything up, then replace screw. Pull the connector apart and check for voltage on wire 19 (blue/red)

5. Pull the main connector and check for voltage on wire 37 (black-yellow)
5a. report back here.

The reason for the "report back here" is because I would probably get to step 50 if I were to fill out every possible scenario, and I'm getting lazy. Once we know which direction this is heading, I can add more focused pinpoint testing recommendations.
Jo
From the wiring diagram, power flow starting from the battery goes to:
1. Starter relay, left post when viewing the front of the relay.
2. Wire 37 to the main disconnect at the firewall
3. Wire 37 to a splice
4. Wire 25 to the headlight switch. When the headlight switch is turned on, power is applied to wire 19.
5. Wire 19 to the fuse panel. Note that the feed is not fused up to this point, the design is fault tolerant - don't want headlights to trip off. The headlight has a built in circuit breaker for the headlights, but that does not affect wire 19.
6. Wire 19 from the fuse panel. This is after a fuse in the fuse panel, and protects against a short downstream.
7. Wire 19 goes to the instrument panel lights and transmission indicator light.
8. Each instrument panel light is grounded to the instrument panel. The instrument panel needs to be grounded. I'm not sure of the method for 1966.
9. The transmission indicator light has a separate ground.

Now let's use a methodical approach to resolve the problem, rather than shooting from the hip.

The best practice is to consider splitting the problem at the middle (guess 50% in the lower/higher game) as well as considering how easy or difficult an area may be to reach. For this situation, I recommend starting at the fuse panel.

1. Find the fuse associated with the instrument panel lights. With the headlight switch off, there should be no power on either side of the fuse.
1a. If there is no power, move on to step 2.
1b. If there is power, you are either on the wrong fuse or there is something terribly wrong. Report back here.

2. Turn the headlight switch on. There should be power on both sides of the fuse.
2a. If there is no power on either side, move on to step 3.
2b. If there is power on only one side, you have a bad fuse, or perhaps a weak fuse clip. Replace the fuse and/or tighten up the clip. If you can't resolve, report back here.
2c. If there is power on both sides of the fuse, move on to step 4.

3. Check for any power on any fuse in the fuse panel.
3a. If there is no power, report back here.
3b. If there is power, go to step 5.

4. Remove the transmission gear selector cover plate to access the bulb. Turn the headlamp switch on.
4a. If the light is on report back here.
4b. If the light is not on, check/replace the bulb. Follow the black wire until you find the ground screw. Remove screw and polish everything up, then replace screw. Pull the connector apart and check for voltage on wire 19 (blue/red)

5. Pull the main connector and check for voltage on wire 37 (black-yellow)
5a. report back here.

The reason for the "report back here" is because I would probably get to step 50 if I were to fill out every possible scenario, and I'm getting lazy. Once we know which direction this is heading, I can add more focused pinpoint testing recommendations.
John, thank you so much, this is incredible! Thanks for taking the time, I appreciate it more than you know. Okay, so I made it all the way to step 5 as I have power to all fuses OTHER THAN the fuse associated with the instrument panel lights...so, I will go back and try to find the black-yellow wire you mentioned as #37 which leads into the main connector and see if I am getting voltage through that wire to the main connector. I will report back here as soon as I get up and do that tomorrow, and say a little prayer that I do have power to that point!

Thanks again for your help.
 

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Discussion Starter #12
Not to detract from this fantastic troubleshooting plan here ^^^ but I just have to make sure that you know that the instrument lights dim, right? If you turn the headlight knob to the right they get dimmer and to the left they get brighter (or is that backwards?)

When you're in Step 2 up there with the headlight switch on try turning the knob back and forth and see if it changes anything. Unlike modern cars our instrument lights are only on when the headlights or parking lights are on so you won't see them until you get to that point.
@Magnus yes sir! I did actually try that and I had the headlight switch engaged and moved through many different variations of the dimming and brightening of the dash lights by twisting left and right, but had no luck at all with having any lights working or the fuse receiving power. I will go back and do what @67gta289 said at step 5 and then report back. Stay tuned....
 

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Keep in mind that wire 37, from the starter relay, through the main disconnect, to the fuse panel MUST be good because that provides power to the other fuses, and you said you had voltage there.

If you look at the diagram, there is a splice of wires 21 and 25 into wire 37, just before the fuse panel. Wire 25 is going to source power for your instrument panel lights.

Wire 25 connects to the headlight switch, and the headlight switch, when turned on, provides power to wire 19, which is the final wire feeding the affected lights.

Based on no power on either side of the fuse for the affected lights, I think you need to focus on the headlight switch. There are two options.

1. If you can safely access the terminals, you can use a jumper wire to replicate the contact in the headlight switch. You'll need to jump between 25 (black-orange) and 19 (blue-red). Take great care because you have no fuse protection at this point. If the lights come on, you most likely have a bad switch. I suppose that the connector terminals might have an issue (corroded or pushed out of place, no longer locked in)

2. Replace the switch. If you can, plug it in hanging below the dash so that you can test everything out. Check all functionality not just the dash lights. If there is still a problem you might need to pull it again.


756444
 

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I agree, in fact I would have investigated the headlight switch first. I've seen a few of them with failed rheostats. Easy to check- Just use a 12V probe to see whether the circuit has power in, and power out. You can get one of these for under $10, and you'll get a lot of use from it.

 

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Discussion Starter #15
Keep in mind that wire 37, from the starter relay, through the main disconnect, to the fuse panel MUST be good because that provides power to the other fuses, and you said you had voltage there.

If you look at the diagram, there is a splice of wires 21 and 25 into wire 37, just before the fuse panel. Wire 25 is going to source power for your instrument panel lights.

Wire 25 connects to the headlight switch, and the headlight switch, when turned on, provides power to wire 19, which is the final wire feeding the affected lights.

Based on no power on either side of the fuse for the affected lights, I think you need to focus on the headlight switch. There are two options.

1. If you can safely access the terminals, you can use a jumper wire to replicate the contact in the headlight switch. You'll need to jump between 25 (black-orange) and 19 (blue-red). Take great care because you have no fuse protection at this point. If the lights come on, you most likely have a bad switch. I suppose that the connector terminals might have an issue (corroded or pushed out of place, no longer locked in)

2. Replace the switch. If you can, plug it in hanging below the dash so that you can test everything out. Check all functionality not just the dash lights. If there is still a problem you might need to pull it again.


View attachment 756444
Thanks, John. This makes a lot of sense and I feel like you have likely pinpointed the issue, especially with the further explanation on #37 likely working due to all of the other fuses getting power. One question (because I am no electrician)...when you say "take great care" when using the jumper wire to connect between #25 and #19, do you have any tips as to how to properly do that and not have an issue? I am just not familiar enough to know what "taking great care" in doing this would mean. In my mind, it's simply taking the jumper wire and clipping one end on #25 and the other on #19 to see if the instrument lights work. Definitely let me know if you have any tips on this maneuver so I don't harm myself or the vehicle!

And thanks again, you have been so awesomely thorough and clear!
 

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I agree, in fact I would have investigated the headlight switch first. I've seen a few of them with failed rheostats. Easy to check- Just use a 12V probe to see whether the circuit has power in, and power out. You can get one of these for under $10, and you'll get a lot of use from it.

Yes sir, that is the exact instrument I have been using the whole time to test the fuses, etc. I have not used it to probe one of those really small wires yet...never done that. Or do you mean simply using the probe to touch against the terminals of the lights switch after it has been disconnected from plug that fits into the light switch itself?
 

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Thanks, John. This makes a lot of sense and I feel like you have likely pinpointed the issue, especially with the further explanation on #37 likely working due to all of the other fuses getting power. One question (because I am no electrician)...when you say "take great care" when using the jumper wire to connect between #25 and #19, do you have any tips as to how to properly do that and not have an issue? I am just not familiar enough to know what "taking great care" in doing this would mean. In my mind, it's simply taking the jumper wire and clipping one end on #25 and the other on #19 to see if the instrument lights work. Definitely let me know if you have any tips on this maneuver so I don't harm myself or the vehicle!

And thanks again, you have been so awesomely thorough and clear!
John, additionally, are my turn signal lights on a different circuit than my instrument panel lights? They have always worked just fine, and I can't tell from the diagram I they are coming from a separate power source. They must be, but just making sure that is normal.
 

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By taking care I mean make triple-dog sure that you don't short power to ground, because there is no fuse protection on wire 37. You just need something to connect the two terminals.

As far as turn signals go, yes they are on a different circuit. You are having a problem with wire 19 specifically.
 

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You can buy an inline fuse holder and make up a jumper wire with an alligator clip on each end and the inline fuse holder with a low amperage (5 amps) fuse for protection in case you accidentally short something. O'Reilly and the other parts stores keep them in stock. You can get one that uses ATC style fuses or the old AGC glass tube fuses.
 

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I have this same problem. I seem to recall reading a post here a couple of years ago that a certain combination of screws holding the instrument cluster to the dash actually ground the instrument lights. If I were to guess, I think it's the two bottom screws (with their corresponding J-nuts) that are needed to complete the ground.
 
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