Wiring diagram

78 Mach1 · Sep 24, 2024

I’m having electrical gremlins with my MII after sitting for 30 plus years. Specifically the turn signals and windshield wipers. Does anyone have a wiring diagram they can share to help me troubleshoot these issues?

Passenger side turn signal works fine. Hazard lights work fine. But the front drivers side front signal light never works. I have already replaced the bulb and the socket. Voltage is there with bulb out of socket, but as soon as I insert the bulb, voltage goes to zero. Very odd. I believe it must be a bad or corroded connection somewhere that once it has current flowing, loses connection a schematic or wiring diagram would be super helpful for troubleshooting this issue.

Windshield wipers just don’t work at all. No clue on them yet.

Any help is great appreciated.

78 Mach1 · Sep 24, 2024

It’s a 1978 hatchback originally a 302 engine automatic if that helps with the correct wiring diagram.

LILCBRA · Sep 25, 2024

Straight from my Haynes manual.

Hope these help! Beyond that, it sounds like a ground issue. How's the connection at the radiator support? I'm fairly certain that's the ground for the front lights. If that's good you're probably going to be stuck chasing the wire from the connector.

Bullitt347 · Sep 25, 2024

78 Mach1 said:
Voltage is there with bulb out of socket, but as soon as I insert the bulb, voltage goes to zero. Very odd.

A voltage reading does not really mean anything.
If the wiring is corroded, the circuit can show 12 volts, but it would be unable to flow enough current to light the bulb.
I would guess that somewhere on that circuit there is high resistance preventing current from flowing through the wire.
It may be easier to overlay the harness (run a new wire) than it would be to strip apart the harness to find the corrosion on the circuit.

Blackhawkxx · Sep 25, 2024

78 Mach1 said:
Voltage is there with bulb out of socket, but as soon as I insert the bulb, voltage goes to zero.

Did you try more than one bulb?

78 Mach1 · Sep 26, 2024

Thanks for all your help. Blackhawkxx, yes I did try more than one bulb. lol. Not a bad question though. I’ve learned that the hard way.

LILCBRA, thank you so much for the diagrams. I’ll print them and go searching tonight.

My belief is it is a grounding issue. As you may know, this car was a ground up rebuild. I paid close attention to my ground from battery to frame and from frame to engine. But I believe I may have to add a couple more grounds from engine to body and frame to body.

Bullit347, you are exactly right. I am an avionics technician by trade and am very familiar with ohm’s law. It’s funny how the simplest circuits can be the most difficult to diagnose.

But with the diagrams in hand, I should be able to figure this out. I’ll let you know what I find.

78 Mach1 · Sep 26, 2024

Well the diagram came through. After chasing the wire from the connector on the firewall to the socket I figured it out. And let me just say, removing that big connector at the fire wall with a tunnel ram Cleveland in the way is no easy task.

The issue ended up being, someone wired up the socket wrong many years ago. I’m guessing it never worked. Apparently the lower element on an 1157 lamp pulls more current than the upper element. The upper element wasn’t allowing the blinker to work. As soon as I rewired the socket correctly to match the provided diagram. That put the turn signal wire to the lower element, everything started working correctly

Now to figure out those darn windshield wipers.

78 Mach1 · Sep 26, 2024

In the process of fixing these lights, I noticed my reverse lights aren’t working. In the diagram, it shows the reverse lights switch c-317 as part of the neutral safety switch. But it also shows another option of a reverse switch as c-303. Does anyone know if the 78 has the reverse switch built into the neutral safety switch? Or is there another switch located somewhere else to power those lights? Like in the shifter itself.

LILCBRA · Sep 26, 2024

It's been a LONG time since I've had a C4 car, but I think I remember there only being one wire harness hooked up to a switch looking thing around the shift lever on the transmission - like this:

Now, just in that search, there are listings saying it's also the reverse switch, as I highlighted here:

Hope this helps!

78 Mach1 · Sep 28, 2024

Thanks lil cobra with your help I was able to quickly troubleshoot it down to the neutral safety/reverse switch at the shift lever on the transmission. In true MII ownership experience, all of my searches came up with switches for 65 through 73 Mustang and 79 and up Mustang. They all skip our years. So now my question is, does anyone know of a different year mustang neutral safety switch that is a bolt in replacement for our Mustang II?

Rockauto lists the wrong switch entirely for our cars.

78 Mach1 · Sep 28, 2024

I also was able to troubleshoot my windshield wiper issue. I have voltage from the switch to the motor. So I figured the motor was bad. I’m too cheap to order a motor so I pulled the motor out and gave it a poor man’s rebuild. I cleaned the commutator and brushes and removed some debris I found on the stator. Reassembled it and then tested it under the dash not bolted in. Same scenario as before. Voltage to motor but not running. I started checking and realized I had 12 volts on the outer case of the motor. Referred back to the schematic and that shows the case being grounded to the car chassis. I loosely connected a ground wire to the outer case and then everything started working. Now I’m confused, how is this motor supposed to be grounded? The motor is bolted to the body using rubber grommets. Not going to get ground through there. I did not see any ground wires hanging around that would have went to the motor. So how in the world was this grounded from the factory? Does anyone know or remember when they had there’s out how the windshield wiper motor gets ground?

2Blue2 · Sep 28, 2024

78 Mach1 said:
So how in the world was this grounded from the factory?

2Blue2 said:
Chased it down to the bolt ground contact washer thing completely caked with white powder.

One of the rubber mounts had a brass piece to ground wiper motor chassis, had to clean it.

LILCBRA · Sep 28, 2024

78 Mach1 said:
Thanks lil cobra with your help I was able to quickly troubleshoot it down to the neutral safety/reverse switch at the shift lever on the transmission. In true MII ownership experience, all of my searches came up with switches for 65 through 73 Mustang and 79 and up Mustang. They all skip our years. So now my question is, does anyone know of a different year mustang neutral safety switch that is a bolt in replacement for our Mustang II?

Rockauto lists the wrong switch entirely for our cars.

You're welcome! As for locating a replacement switch - I'd think that they're all pretty much the same since they're attached at the lever like they are. But in case there are differences, you could try to locate the part number on yours and just Google that, maybe the one I highlighted in that screenshot is the same part.

LILCBRA · Sep 28, 2024

Wait..... you know what..... I have a C4 laying here that I never put into a car and probably never will. I have no idea anymore if the switch is for a II or if the transmission is from something else, but let me know if you want it. The connector is toast, but the wires are more or less intact.

Edit: didn't pay any attention to the casting number on the case until I typed this up - looks like it's from an 82 something or other.

Blackhawkxx · Sep 28, 2024

Just a couple of weeks ago I gave away an old (72 I think) C4 that had the switch with it. Too bad, could have compared them.

78 Mach1 · Sep 29, 2024

2Blue2 said:
One of the rubber mounts had a brass piece to ground wiper motor chassis, had to clean it.

lol. This car has humbled me on several occasions. I took the whole motor apart and cleaned it up. And it ended up being a brass washer on the rubber grommet was corroded. Oh boy.

It is fixed now thanks to 2blue2

78 Mach1 · Oct 1, 2024

LILCBRA said:
else, but let me know if you want it.

If you’re willing to ship it to me. I’ll take it. You can PM me and I will give you my address.

I pulled mine out and couldn’t find any part numbers anywhere on it.

I was able to confirm it is definitely this switch. I jumped across white and black wires and my reverse lights came on.

I am assuming most older C4 switches would be the same. If yours doesn’t work out, I was going to just order one for a 73 mustang with a C4. I’m sure the connector would be different but the switch itself should be the same. I can easily rewire my switch on it.

LILCBRA · Oct 1, 2024

Pm sent.

78 Mach1 · Oct 18, 2024

I ended up ordering a switch from CJ Pony parts. The one for the 1973 Mustang C4 looked like the same style connector. If it’s not plug and play. I can always splice my connector onto that switch.

welder4956 · Jan 24, 2025

I'm going through a similar issue with finding a replacement switch. Here are some pics of my old one and the connector.

Part number for my original switch was marked D6EP-7A247-AA, but D5FZ-7A247-A is listed for replacement, along with D52P-7A247-AA or D6EP-7A247-AA as alternates (must delete purple wire w/white hash marks).

None of the replacement switches for earlier or later model years have the rubber grommet where it passes through the floor and the connector is a 6 pin connector, even though there are only 4 wires. You can use the '73 switch, but you'll need to splice it onto the old connector and use the old rubber grommet.

Edit: The notes reference Motorcraft part SW-1263. If you Google that, you'll get some hits for NOS parts with a much higher price tag than the '73 switch. Here's one example: https://www.ebay.com/itm/3876036603...pid=5335821607&customid=&toolid=10001&mkevt=1

Wiring diagram

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will be trying this sex one when I can find it

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