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Discussion Starter #1
Hello,

I have scoured this forum, as well as others to determine the issues I'm having with my gauge cluster and have come up short. Please save me.

My vehicle is a 1969 Mustang Sportsroof 302. It has a non-tach dash. Currently all exterior lights, driver footwell light, key light, and dash high beam indicator light work. My speedometer, alternator and oil pressure gauges also work.

None of my gauge cluster lights (except the headlight indicator) work, and yes, I have tried turning the headlight switch knob, and my fuel and temp gauges do not work.

Steps I have taken to fix the problem:
  • Replace the headlight switch
  • Verified and cleaned the contacts on the headlight switch plug
  • Replaced the dash circuit board
  • Replaced all of the dash lights with LEDs
  • Replaced all of the dash light sockets
  • Replaced the fuel and temp gauges
  • Covered the metal on the gauge mounting area for shorts/grounds
  • Tested grounding both black wires at dash plug pin 8 separately and together
  • Tested grounding black wire at dash plug pin 11 (youtube video recommendation)
  • Cleaned all of the contacts on dash plug
  • Replaced fuses and cleaned the connectors
  • Replaced the dash Constant Volt Unit
  • Replaced the high-beam floor button

The gauge lights are the biggest issue because I can't see my speedometer at night. With it being winter like 75% of my driving is in the dark, and I've had to mount my cellphone flashlight to see my speedo. The cooling system is brand new, and I can time fuel stops by mileage, so I can live with those being out until I have time to test the sending units. I am under the assumption that the gauges being dead is an electrical issue, as they both have absolutely no life. To my knowledge if it's a bad sending unit the gauges will still move a bit.

I'm at my whits end on this one and at a total loss for what to do, so I would appreciate any and all help you may offer.
 

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I can't say for sure but I think your temp, fuel and oil pressure gauges have to have a constant voltage relay to operate. Other than that, I'd start looking for a loose connection in your dash wiring harness.
 

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Discussion Starter #3
I can't say for sure but I think your temp, fuel and oil pressure gauges have to have a constant voltage relay to operate. Other than that, I'd start looking for a loose connection in your dash wiring harness.
I thought that might be the issue, so I replaced the constant voltage unit. That didn't fix it. I've checked the wiring diagram and tested all of the relevant wires, as well as checked that the grounds are connected to good metal, and made sure the gauges aren't grounding out on the bezel metal. I'm pulling out my hair trying to figure this out. I'm mostly mad about the lights, as a fuel gauge is a luxury I can live without. I have traced the light wires from start to finish and replaced absolutely everything except the actual wires themselves (which look fine). I'd really like to avoid buying a whole aftermarket gauge cluster.
 

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For the fuel and temp gauges, the fact that the oil pressure gauge works rules out the constant voltage regulator.
I would start by disconnecting the temperature wire at the sensor on the intake maniford and temporarily ground the wire. Turn on the ignition key and watch the gauge. If the gauge needle starts its sweep, the gauge and wiring is good and your sensor is suspect.
If the temp gauge fails that test the next step is trace the circuit back through the engine compartment to the firewall plug looking for damaged wire or corroded connections.
The engine gauge feed harness has been known to fail over time due to the harsh environment in which it operates. Using a DVM check for continuity in the sensor wire to the firewall. If you have continuity, move to the dash board. At this point check the harness from the firewall to the instrument cluster plug for continuity. If no continuity there, you are going to either bypass the harness wires or open up the main under dash harness and find/repair the damage/break/corrosion. If you have continuity there continue:
With the gauge cluster out of the car use the DVM to check for continuity between the connector on the circuit board to the gauge itself. If none, your circuit board is suspect and needs closer inspection.
If ok, next use a 9 volt battery and a couple of alligator clip jumpers to power up each suspect gauge briefly. If either gauge needle fails to start its sweep, reverse polarity on the battery. If the gauge needles are still dead,the gauge(s) is/are likely defective.
For instrument lighting, check the fuse. If OK, jump around the main under dash wiring harness from the headlight switch to the circuit board pin that feeds the insrument lighting. If you have lights at that point, your main underdash harness is the problem. Opening up the harness may yield the problem. If you are OK with a permanent jumper on that circuit you are done. Otheriwse refurbishing the underdash harness (Midlife Harness Restoration) or replaceing it with a new one from Alloy Metal Products should remove the suspect circuit from the equation.

Hope this helps

P.S. Welcome
 

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Discussion Starter #5
For the fuel and temp gauges, the fact that the oil pressure gauge works rules out the constant voltage regulator.
I would start by disconnecting the temperature wire at the sensor on the intake maniford and temporarily ground the wire. Turn on the ignition key and watch the gauge. If the gauge needle starts its sweep, the gauge and wiring is good and your sensor is suspect.
If the temp gauge fails that test the next step is trace the circuit back through the engine compartment to the firewall plug looking for damaged wire or corroded connections.
The engine gauge feed harness has been known to fail over time due to the harsh environment in which it operates. Using a DVM check for continuity in the sensor wire to the firewall. If you have continuity, move to the dash board. At this point check the harness from the firewall to the instrument cluster plug for continuity. If no continuity there, you are going to either bypass the harness wires or open up the main under dash harness and find/repair the damage/break/corrosion. If you have continuity there continue:
With the gauge cluster out of the car use the DVM to check for continuity between the connector on the circuit board to the gauge itself. If none, your circuit board is suspect and needs closer inspection.
If ok, next use a 9 volt battery and a couple of alligator clip jumpers to power up each suspect gauge briefly. If either gauge needle fails to start its sweep, reverse polarity on the battery. If the gauge needles are still dead,the gauge(s) is/are likely defective.
For instrument lighting, check the fuse. If OK, jump around the main under dash wiring harness from the headlight switch to the circuit board pin that feeds the insrument lighting. If you have lights at that point, your main underdash harness is the problem. Opening up the harness may yield the problem. If you are OK with a permanent jumper on that circuit you are done. Otheriwse refurbishing the underdash harness (Midlife Harness Restoration) or replaceing it with a new one from Alloy Metal Products should remove the suspect circuit from the equation.

Hope this helps

P.S. Welcome
That 9v trick sounds like a great way to test the gauges without digging too deep. I'll give that a shot. Thank you for the recommendation.

For the lighting I'm certain the fuse is good as I just replaced it, and everything else powered by fuse #5 works fine. I'm sure the circuit board is good because I replaced the old one with a 3 year old one, and then again with a brand new one, and it's unlikely all three are bad as they were all in visibly good shape. I even meticulously made sure every bulb plug was making contact with the circuit board. I cut the harness open and tested and traced the wires (pins 2, 8, 12, 13) from the cluster through the firewall and found no visible issues, and good connections. I even cut the the light grounds (pin 8) and tested attaching them to bare metal on the dash bracket, but that didn't solve the issue either, so I connected them back to their original path. The dash turn signal indicator lights don't work either, and they run using different wires than the gauge lights, so it makes me think it's a different issue as I checked those wires and connections also to no avail.

How would I go about installing a permanent jumper? That might be a good hold over until I can figure this out.
 

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Believe me i know what a pia is it to R&R a 69 70 dash.

Try swapping all the LEDs for the correct regular bulbs.

Can be as simple as that
 
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Wow that's an impressive list of things you've replaced. If you can use a VOM, then I can instruct you how to diagnose your problems without replacing more healthy parts. The site I use for hosting my schematics is down due to excessive traffic (Christmas rush?) but when they are back up I'll post one that should help.
There are two separate unrelated circuits involved in your problems.
The LEDs you put in the dash are usually polarity sensitive- if so reverse them. Some LED packages are actually two that are in parallel with one reversed so that it doesn't matter which way you plug them in. As mentioned earlier, put a good incandescent bulb back in and see if it works.
Also, there are a multitude of other lights on the 4A fuse #5 besides the 8 dash lights. Check to see if any of these lights work: ash tray, heater, radio, or clock lights. If they do then you'll know that fuse #5 is good. I suspect it may be the connection to the printed circuit, but we'll see.
Since the oil pressure works, the constant voltage regulator is working ( also mentioned earlier). A very common problem when the gauges are replaced is that one or both posts to the gauges touch the metal dash housing, shorting them out. It sounds like you checked that, but did you use an ohmmeter to verify there was no short? Connect one ohmmeter lead to the dash metal and the other to each gauge post. There must be infinite ohms.
It could be the sensors. Remove the water sensor wire. Do it at the engine and you'll test the circuit in between. Momentarily connect the wire to chassis. Turn the key on and the water gauge should rise- don't peg the needle, you may burn out the gauge. If it does rise, then the water temp sensor is bad. If not it could be the wire, or the firewall plug is defective.
Do the same with the fuel, removing the wire to the tank sensor at the tank and temporarily grounding it to the chassis. Turn the key on and the gauge needle should rise. Again don't let it peg.
Imgur is still over capacity- I'll try tomorrow. This will get you going. We'll find your problems, just be patient.
 

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OK, Imgur is running again. This is a schematic, not a wire diagram. You will need a wire diagram also. Schematics show how the circuit works by simplifying. They don't generally show wire numbers, colors or plugs. I'll run you through the circuits for the lights, starting at the top left of the page, at the battery positive terminal:
1. It goes though the 15A circuit breaker inside the light switch.
2. Through the "PARK, ON" switch, so the light switch must be in switch position PARK, or ON.
3. Through the knob adjusted rheostat. If you turn the knob counter-clockwise all the way it will turn on the courtesy lights. Go clockwise just past the detent and the dash lights should be at their brightest. Leave the knob in this position.
4. Power then goes to fuse #5
5. Power then goes to these lights: ash tray, heater, radio, two clock lights, and the 8 dash lights. Each of these lights goes to chassis ground and thus back to the battery negative terminal completing the circuit.

The gauges are shown in the middle on the right side of page 2:
1. Power comes from the battery+ terminal to the ignition switch B terminal and out A terminal when the switch is in Accessory or ON positions.
2. Power goes through 10 ohm resistor wire #30 (which is violet) to the IVR (instrument voltage regulator)
3. Power is applied through each gauge to the sensors. The gauges are all the same internally, the only difference is the scale. The sensors are just variable resistors that range from 10 ohms on high to 78 ohms on low. Even though one measures the water temperature, one oil pressure, and one fuel level, they are all variable resistors that range from 10-78 ohms.

You are going to need a volt/ohm meter, know how to use it, and a wire diagram. Do you have these and can you use them? If not- get them and I'll explain. Without them you will just continue to replace parts, and NOTHING pisses me off more than BLINDLY replacing parts. This isn't rocket science, and anyone can be taught how to do it.
 

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This is what I bought in 1969. It is from the Ford Shop Manual and is a 37 page wire diagram: 1969 Ford Mustang and Mercury Cougar Wiring Diagram Original. each page is 11 x 17.
This is what I "backward engineered" into a 5 page schematic on 8-1/2 x 11. It is the only classic Mustang schematic in existence*. Schematics are what the engineers use to make the wire diagram. The wire diagram shows you how to wire the car, while the schematic shows you how it works. You need both. Wire diagrams are like a topological map. They are very detailed and filled with so much information that it is difficult to see how things actually work.
* Ford may have schematics buried deep in their vaults somewhere, but none have ever been published- all you ever see is wire diagrams. In case you are wondering, I'm a retired electrical engineer.
 

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Discussion Starter #11
Wow that's an impressive list of things you've replaced. If you can use a VOM, then I can instruct you how to diagnose your problems without replacing more healthy parts. The site I use for hosting my schematics is down due to excessive traffic (Christmas rush?) but when they are back up I'll post one that should help.
There are two separate unrelated circuits involved in your problems.
The LEDs you put in the dash are usually polarity sensitive- if so reverse them. Some LED packages are actually two that are in parallel with one reversed so that it doesn't matter which way you plug them in. As mentioned earlier, put a good incandescent bulb back in and see if it works.
Also, there are a multitude of other lights on the 4A fuse #5 besides the 8 dash lights. Check to see if any of these lights work: ash tray, heater, radio, or clock lights. If they do then you'll know that fuse #5 is good. I suspect it may be the connection to the printed circuit, but we'll see.
Since the oil pressure works, the constant voltage regulator is working ( also mentioned earlier). A very common problem when the gauges are replaced is that one or both posts to the gauges touch the metal dash housing, shorting them out. It sounds like you checked that, but did you use an ohmmeter to verify there was no short? Connect one ohmmeter lead to the dash metal and the other to each gauge post. There must be infinite ohms.
It could be the sensors. Remove the water sensor wire. Do it at the engine and you'll test the circuit in between. Momentarily connect the wire to chassis. Turn the key on and the water gauge should rise- don't peg the needle, you may burn out the gauge. If it does rise, then the water temp sensor is bad. If not it could be the wire, or the firewall plug is defective.
Do the same with the fuel, removing the wire to the tank sensor at the tank and temporarily grounding it to the chassis. Turn the key on and the gauge needle should rise. Again don't let it peg.
Imgur is still over capacity- I'll try tomorrow. This will get you going. We'll find your problems, just be patient.
Thanks for the recommendations.

I replaced 3 year old regular bulbs with the LEDs, I'll try adding them back to see if that helps, as they still appear to be in good shape. The other lights on fuse 5 work fine, so I assume the fuse box isn't the issue. I've changed the printed circuit twice (car came with a spare), and made sure every light is contacting. They are made weirdly so that the bulb's full turned position barely makes contact. It's annoying. I took out the gauges and covered all possible contact points with electrical tape to ensure they weren't grounding out, but I didn't use an ohmmeter to verify. I've been using a voltage tester pen. Sounds like I need to bite the bullet and just buy a real VOM.
 

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Discussion Starter #12
OK, Imgur is running again. This is a schematic, not a wire diagram. You will need a wire diagram also. Schematics show how the circuit works by simplifying. They don't generally show wire numbers, colors or plugs. I'll run you through the circuits for the lights, starting at the top left of the page, at the battery positive terminal:
1. It goes though the 15A circuit breaker inside the light switch.
2. Through the "PARK, ON" switch, so the light switch must be in switch position PARK, or ON.
3. Through the knob adjusted rheostat. If you turn the knob counter-clockwise all the way it will turn on the courtesy lights. Go clockwise just past the detent and the dash lights should be at their brightest. Leave the knob in this position.
4. Power then goes to fuse #5
5. Power then goes to these lights: ash tray, heater, radio, two clock lights, and the 8 dash lights. Each of these lights goes to chassis ground and thus back to the battery negative terminal completing the circuit.

The gauges are shown in the middle on the right side of page 2:
1. Power comes from the battery+ terminal to the ignition switch B terminal and out A terminal when the switch is in Accessory or ON positions.
2. Power goes through 10 ohm resistor wire #30 (which is violet) to the IVR (instrument voltage regulator)
3. Power is applied through each gauge to the sensors. The gauges are all the same internally, the only difference is the scale. The sensors are just variable resistors that range from 10 ohms on high to 78 ohms on low. Even though one measures the water temperature, one oil pressure, and one fuel level, they are all variable resistors that range from 10-78 ohms.

You are going to need a volt/ohm meter, know how to use it, and a wire diagram. Do you have these and can you use them? If not- get them and I'll explain. Without them you will just continue to replace parts, and NOTHING pisses me off more than BLINDLY replacing parts. This isn't rocket science, and anyone can be taught how to do it.
This information is perfect. Thank you very much! I'll pick up that wiring diagram book and a VOM. It certainly can't hurt. If you're an active imgur user my username on there is Calicious. It's usually over capacity on Selfie Day.
 

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If you're an active imgur user my username on there is Calicious. It's usually over capacity on Selfie Day.
No, I'm not active, I only store my schematics there so they can be posted on sites like VMF that don't allow large uploads. You may want to visit 69Stang.com which does allow large uploads and has lots of technical information under the "How Tos" section.
 

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Discussion Starter #15
I managed to get my gauges working! My fuel gauge is working perfectly, but I'm not certain if if my temp gauge is totally functioning. It definitely has life, but it doesn't raise very far. It probably only moves about 1/5th of the way up the level. I assumed it should settle around half way, but I don't have enough experience to know for certain. I'll stress about it later. Right now it isn't a priority. The lights are still not working, so that's my biggest headache.
 

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It would be so much easier if this site allowed larger and more varied uploads. This is the wire diagram page showing the power for the dash lights- just above the word "Interior" at the bottom it shows wire 19C go into the plug. It doesn't directly identify the wire color, but it may be blue with a red stripe. It doesn't identify the plug number either, but shows an image of the shape of the plug so you should be able to figure it out. Connect one lead of a voltmeter to 19C and the other lead to a good chassis ground. With the light switch on and dialed up, you should get 12v here. Also check the path on the printed circuit to see if you have a good connection through the plug. Wire 57 is a black wire and is the ground lead- make sure that has continuity too.
 
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