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Factory tach and center gauges without changing harness?
#1
My '72 Fastback came with the factory idiot light cluster rather than a tach. I manged to pick up an original tach gauge cluster and the center dash gauge pod with the 3 gauges. I really don't want to get into replacing the complete harness for this. Is there a reasonable way to get these gauges working without replacing the whole harness? Not sure if sensors need to be changed and new wiring need to be run or what exactly. This isn't going for a restoration or show car, so I just want the wiring to be functional and clean, so not worried about originality behind the dash. 

I also wouldn't mind replacing that AMP gauge with a volt meter if there is a way to do that and have it look like the other gauges. 

Thanks
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#2
It can be done, but you had better know how to read wiring diagrams and know how to wire a car.

You would have been better off buying Rocketman's tachometer conversion for the tach. The OEM tachometer wiring connects in line into the wire that runs from the ignition switch to the coil. Rocketman's connects like a regular aftermarket tach.
https://www.rccinnovations.com/

The center gauges (oil and temperature) get their positive voltage from the instrument voltage regulator, not that easy to do without the proper OEM harness. It would have been easier to do by putting aftermarket gauges into the gauge center console, several forum members have done it with excellent results. The ammeter is even more difficult, as it works from a voltage shunt, reducing the actual current through it. Without the correct under hood harness it is not easy to do, you have to have a real understanding of electrical theory. I have constructed my own shunt circuits, but without the knowledge a person is likely to burn up the car. Rocketman also makes a voltmeter conversion for the ammeter, which is what I would do, rather than trying to wire in a shunt circuit.

The oil and temperature gauge sending units are just on/off switches for the warning lights. They have to be replaced with variable resistance sending units.

If you want to continue with the installation of the OEM tach and gauges I would strongly recommend that you get the correct wiring harnesses from Midlife.
EDIT: updated website address:
http://midlifeharness.com/

Bob (Rocketman) and Randy (Midlife) are both site supporters, Mustang aficionados, and forum members and know their stuff.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#3
Thanks. I had looked at the rocketman conversions before. I got the gauges bundled in with a bunch of other stuff so it was pretty cheap. On my mile long list of things to do, replacing the whole harness just isn't something I'm going to get to anytime soon.

I haven't opened it up before, but the back of the tach enclosure looks like it has a couple wires going to it that are separate from the circuit board thing. Is that the source for the tach? If so that looks like it might hook up like any other tach?

Same for the center gauges as they are all just a couple of wires. Seems like I could wire from new oil and temp sensors to those gauges without dealing with the harness, much like if I installed after market gauges.

Is that not the case?

Thanks
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#4
The tachometers used in our cars are current sensing, meaning that the power for the coil/distributor runs through the tachometer. They do not connect the same as an aftermarket or RCCI conversion tach. Here is an old diagram that I believe was from Mallory
[Image: Current-Sensing-Tach-Wiring.jpg]

The Ford gauges are designed to work from the instrument voltage regulator, which puts out around 5 volts, meaning they must be connected to the instrument voltage regulator and cannot be connected directly to 12 volts. They need to be connected to the one on the back of the instrument cluster or you need to wire one in to feed the positive side of the gauges.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#5
OK thanks. From that drawing it would seem to look like I could just add a small resistor between the side of the coil and the tach and that would get me there on the tach. I'll try and dig up more info and maybe just test under the hood to before ripping the dash apart.
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#6
There is already a resistor wire, pink, that runs from the ignition switch to a connector on the firewall. I would suggest that you download the wiring diagrams for your car from the Mustang Data tab at the upper right of this page.

You will have to run a wire from the ignition switch to the tachometer and then connect the pink (resistor) wire after the tachometer. You will need to get bullet style plugs and sockets to connect to the tachometer wiring.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
  Reply
#7
(12-27-2018, 03:01 PM)Don C Wrote: It can be done, but you had better know how to read wiring diagrams and know how to wire a car.

You would have been better off buying Rocketman's tachometer conversion for the tach. The OEM tachometer wiring connects in line into the wire that runs from the ignition switch to the coil. Rocketman's connects like a regular aftermarket tach.
https://www.rccinnovations.com/

The center gauges (oil and temperature) get their positive voltage from the instrument voltage regulator, not that easy to do without the proper OEM harness. It would have been easier to do by putting aftermarket gauges into the gauge center console, several forum members have done it with excellent results. The ammeter is even more difficult, as it works from a voltage shunt, reducing the actual current through it. Without the correct under hood harness it is not easy to do, you have to have a real understanding of electrical theory. I have constructed my own shunt circuits, but without the knowledge a person is likely to burn up the car. Rocketman also makes a voltmeter conversion for the ammeter, which is what I would do, rather than trying to wire in a shunt circuit.

The oil and temperature gauge sending units are just on/off switches for the warning lights. They have to be replaced with variable resistance sending units.

If you want to continue with the installation of the OEM tach and gauges I would strongly recommend that you get the correct wiring harnesses from Midlife.
http://midlife66.com/harnesses/

Bob (Rocketman) and Randy (Midlife) are both site supporters and forum members and know their stuff.
Please use http://midlifeharness.com instead of that older site.

There's more involved in the wiring: you have to remove the wiring for the alternator indicator lamp, including a resistor wire.  That circuit feeds the voltage regulator, and you'll need to replace the alternator harness with one that is tach-compatible (actually, one for an ammeter).  That signal feeding the VR serves a different function with an alternator indicator lamp or an ammeter/voltmeter.  I do these conversions/refurbishment all the time for $225, and a 2 year warranty.  It is not a trivial task.

Let me check your shorts!
http://midlifeharness.com

[Image: Flamicon2.jpg]


[Image: oldfart.png]
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#8
runninpony 
I couldn't agree more with what Don said, Rocketman and midlife are the way to go!

71-73 Mustangs never die, they just go faster!
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#9
(12-28-2018, 05:38 AM)NOT A T5 Wrote: I couldn't agree more with what Don said, Rocketman and midlife are the way to go!

+1
  Reply
#10
Sorry, I must be thick headed, but I'm not really getting what the issues are with just running wire from the coil and sensors under the hood to the back of the tach and other gauges.

I actually do have a complete wiring harness as well from the donor car, I just am having a hard time understanding why I would need to use it other than to make things look factory. What's the issue with just bringing up those cables from the engine compartment to the dash?

Is the connector on the back for the speedo / tach pod substantially different? Obviously it would have the extra wires in it for the idiot lights and none for the tach. Is the actual plug different though?

Thanks
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