Engine Gauge Feed 3 vs 4 wire

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Joined
Aug 8, 2018
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Location
California
My Car
72 Fastback - 351C-4V, Fitech EFI, T56 Magnum 6 Speed
Hello, preparing for the end of the world so cleaning up the wiring in my '72 formerly auto non AC car and ordered this new feed cable to go from the firewall to the sensors. 

https://www.cjponyparts.com/cj-classics-engine-gauge-feed-wiring-harness-usa-made-351c-1971/p/WGF6C/ 

My car has a 351C but didn't come with that originally and I didn't catch the difference before ordering. My current feed harness has 4 cables where this one only has 3. Of the 4 on the old harness 2 are actually cut off at the plug. The two remaining has one going to the oil pressure switch and the other powers the ignition. Not sure how that happened, but that's the way it is now. I have a wiring diagram, but not able to make sense of this.

I'm changing from lights to gauges so I replaced the sending units and figured I'd just restore the cabling to what it was and go from there. Looking at this closer, it looks like the wires work out if I just plug in the new 3 wire feed cable into the 4 wire connector with the power and oil pressure in the same locations. 

Not sure if that is wise or not so figured I'd check here and find out how this should be working. I don't care if it's factory accurate, but I want it to work well. I also stuck a meter on the connector at the firewall and I'm getting switched 12V on 3 pins and ground on the 4th. So now I'm more confused as I didn't think there should be power going to the sending units.

Hope that makes sense. But basically are the cables on that 3 wire harness in the same locations if I plug into that 4 pin connector at the firewall? Is 12V going to the senders normal? Is that where the ignition usually get's it's power?

And for bonus points, right next to that 4 pin connector at the firewall, there is a 2 pin connector hanging at the firewall that has switched 12V. Anyone know what this is for? 

Thanks as always for any help figuring this out. 

 
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What engine was originally in your car?

What are the wire color codes on the 4 original wires?

You should be able to make it work with 3 wires.

The sending units provide a variable resistance to ground with the positive feed on one side of the gauge. If you're installing aftermarket gauges you would see 12 volts on the sending unit side until it is connected to the sending unit, loading the circuit, and reducing the voltage reading on the sending unit side of the gauge. However, if you are installing original style gauges they should be connected through the instrument voltage regulator, and the voltage should be around 7 or 8 volts until it is loaded.

That 12 volt switched lead may have gone to the throttle position solenoid. What is the color code?

 
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Mustang went from 3 to 4 wires for their engine gauge feed harness in 72 by adding a RUN-only line for the carb solenoid anti-stall device. The other 3 wires are oil pressure, water temperature, and coil.

A 3 wire engine gauge feed harness can work with the 4 wire plug, ignoring the outermost female bullet on the firewall side of the plug.

 
Thanks for the great feedback so quickly. I think the car had a 302 originally. The dash is currently setup for idiot lights and not gauges. Working my way to under the dash here soon, hopefully as soon as tomorrow. So the senders get sent voltage?

Boy I really misunderstood how those work. I really thought they just varied resistance to ground and no the actual voltage happened at the sender. I have a little solid state voltage regulator that I was planning on using on the gauges so I didn't need to deal with the old style voltage regulator. Now I'm a little confused on what I need to do to convert from lights to gauges. Sounds like I need to send regulated voltage out to the sending units?

Not sure about colors at the moment as everything is black / dirty, I did verify that on the 3 pin and 3 pin colors match and are the same as my wiring diagram for water and oil.

Thanks

 
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There is already an electro-mechanical instrument voltage regulator on the back of your main instrument cluster, that just supplies reduced voltage to the fuel gauge. An original gauge wiring harness taps into that same regulator for the oil pressure and temperature gauges.

 
OK, than any thoughts on why I'm getting 12v at the plug? The gas gauge works as does the oil pressure light. Well at least the light goes on and then turns off as it starts. And stays on if disconnected from the sender.

Sent from my SM-N975U1 using Tapatalk

 
The idiot lights work on full 12 volts. The circuit on them is designed so that the light comes on until the engine starts, whether connected to the sender or not, as a test so you know the light is functional, and not burned out.

EDIT: I am proficient at reading wiring diagrams and wiring vehicles, but if I were converting from idiot lights to OEM gauges I would likely get new the correct harness from Midlife, much cleaner installation, and no additional splices to worry about, and matching color codes, and pretending I'm a contortionist working under the dash.

 
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Sorry still not sure I'm getting this. Is that why I'm seeing 12V at that plug on the wires that go to the sending units? Will that drop down once the engine starts or something like that?

 
I believe that the wires you are measuring the voltages on are still connected to the idiot light circuits under the dash. I believe you need the new harness that runs from the sending units to the gauges. Just installing gauges does not change the wiring runs. and locations. If you can't reroute the wires yourself, I would suggest that you contact Midlife and get the correct wiring harness.

 
The only difference between gauge and lamp engine gauge feed harnesses is the length of wires and their end connection points, if (and I don't know this for a fact) how the oil pressure sending unit plug is configured. Various engines have different lengths as well. Electrically, the engine gauge feed harnesses are identical.

 
OK perfect. Sounds like the 12V is normal until I rework the wiring behind the dash. And yes, that engine side harness looks exactly right other than the plug being incorrect, but will still work.

Thanks again for all the help.

 
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