Factory tach and center gauges without changing harness?

[TheDude]

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

 

[Don C]

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.

 

[TheDude]

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

 

[Don C]

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



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.

 

[TheDude]

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.

 

[Don C]

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.

 

[midlife]

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.

 

[NOT A T5]

I couldn't agree more with what Don said, Rocketman and midlife are the way to go!

 

[eddyw]

I couldn't agree more with what Don said, Rocketman and midlife are the way to go!

+1

 

[TheDude]

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

 

[Mister 4x4]

The tach itself wouldn't that big of a deal, except that it's constructed with Ford proprietary wiring - which is why they guys are steering you the direction they are.  If it were an aftermarket tach, it wouldn't be tough at all to simply run your own lines and following the manufacturer's instructions for installation, as you're thinking... but Ford engineered these things to work off specific voltages and resistance, so your results might turn out a lot differently than you're hoping for.

As for the center gauge cluster, I'd probably just replace the gauges with aftermarket units and follow their directions.  You'll have 'almost' the factory look, but won't have to mess with the factory harness.  Member jpaz has a thread going right now about swapping in some aftermarket gauges that look pretty awesome.  https://www.7173mustangs.com/thread-center-gauges--33369

There are also some other threads floating around where others have done the same with excellent results.

I'm with the guys on this - digging into the factory harness looking for something that's not there isn't the best way forward, IMHO.  Tach and 3-gauge cluster cars came wired with those circuits from the factory.  I also bought a tach cluster and harness from a friend on another site a long time ago, thinking I could just swap things and it turned out to be very much NOT what my car needed (tach cluster, sure... but not so much the 3-gauge cluster, and the fact that it was from a '73 where mine's a '71).  So I gave up on that idea and went with an RCCI clock/tach conversion and haven't looked back since.

Not saying you can't do this, or trying to discourage you in the least.  Just echoing the other guys in saying there's an easier, possibly safer, way to do what you're wanting.

Hope this helps!

 

[Carolina_Mountain_Mustangs]

One of your questions was about the actual plug going into the cluster. Yes it is different between gauges and idiot lights. Gauges have a smaller / shorter plug. One of my cars had the harness all hacked up and yes I did change the whole thing from headlights, engine and under dash.

It is difficult to find the exact harness you need. Each harness is different depending on what options your car has, Air, vert, gauges, tach, etc. 71 & 72 is also different in a few places than 73. I hate wiring.

If you do change out the harness it is easier to pull the whole dash out.  If you do that then go ahead and replace your heater core, lol. One of those snow ball deals.

Couple pics of back side of dash and the hacked up harness PO had done that I removed. Note the spliced wires going to the plug for speedo.

 

[Hemikiller]

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

It's not really an issue, more that the guys are trying to steer you to a more robust and reliable setup. 

I have done the non-gages to gages conversion with the harness in place and it's kind of a PITA. You'd need to source the correct tach cluster connector, as well as the dash harness side connector for the center gauges. You then need to swap the connectors on the speedo cluster, easiest way is to remove the pins one at a time and move them to the correct locations on the new connector. The wires for the center gauges will be left and you'd need connect them to the connector. Regardless, the ammeter will never work unless you rewire as midlife noted previously. At that point, it would be smarter to have Rocketman do the voltmeter conversion on that, as it's more accurate anyway. 

https://www.rccinnovations.com/index.php?show=menu-volt-all

For the tach, you need to dig into the harness and connect it inline before the ignition power wire mates tot he engine compartment harness at the fuse box.

To be honest, if I were to do it again, I'd leave the idiot light harness intact, get a Rocketman tach conversion on the warning light face and also run aftermarket gauges that fit in the factory center pod. Idiot lights are more in your face when something happens, than a gauge that's totally out of your field of view. 

https://www.rccinnovations.com/index.php?show=menu-tach-all

 

[SteveO_71]

Dude,

Here is an article that may or may not help. As for the ammeter you may want to put a voltmeter in there instead. 

http://www.mustangsandmore.com/ubb/RichPajzerGauges.html

Here is rocketmans converted ammeter:

https://www.rccinnovations.com/index.php?show=menu-volt-all

 

[TheDude]

Dude,

Here is an article that may or may not help. As for the ammeter you may want to put a voltmeter in there instead. 

http://www.mustangsandmore.com/ubb/RichPajzerGauges.html

Here is rocketmans converted ammeter:

https://www.rccinnovations.com/index.php?show=menu-volt-all

Thanks! That is exactly what I was looking for.

 

[Hemikiller]

Oh jeez, Pazjer's hack-n-whack conversion how to lives on....

 

[midlife]

Oh jeez, Pazjer's hack-n-whack conversion how to lives on....

+1.