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65-65 GT350 Tach Wiring Question

7664 Views 7 Replies 5 Participants Last post by  Duke123
Hello All,

I'm new to the site and need your help. I'm trying to wire an original Shelby GT350 tach to my 1966 Mustang Shelby clone and can't seem to find a good switched ignition source. I have tried the following with no luck - coil, starter solenoid, heater sending unit (just pegs the temp gauge) and yellow female plug under the dash which has power with switched ignition (but it keeps blowing the fuse). I do not have an accessory terminal in the fuse box. The tach is properly grounded and works when hooked to the temp sending unit, the light in the tach also works.

Any ideas?? Thanks in advance for your help!!

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yellow connector is the right spot. need to find out why you blow fuses. im thinking there might be a short in the tach. my 2 cts.
Hello. :) If this actually is an original tach from a GT-350, then it does not get wired up the same way as a modern tach. The original tachs were wired inline with ignition.The only thing needing a keyed power source is the light. If you locate where the red wire with the green stripe comes out of the back of the ignition switch and plugs into the pink resistor wire and unplug those two, that's where you would plug the two wires that go through the tach. :)
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...tach is properly grounded and works when hooked to the temp sending unit...
The temp sending unit is not connected directly to +12V but only connected to the non-CVR side of the temp gauge. You are stealing current from the temp gauge/CVR and more current through any of the gauges makes their needle move to the right. Each gauge needle pegs at about 1/4 Amp. Drawing more than 1/4 Amp by such a connection, continuously, could well damage the temp gauge. You also are limited to a max of 6V by connecting to any of the senders.

Although the triple, yellow female plug under the dash has switched +12V power it is not likely the correct place to get power for your tach light. The yellow plug was used for radio, backup lights, etc. I don't know exactly how Shelby hooked up their tach but the factory Rally Pac (tach and clock) got its light bulb power by connecting to the triple red, female connector closer to the ignition switch that supplies power to the radio light through a special harness 15A006. That Ford number means 'clock power wiring' since there was another wire in the same harness that connected to the courtesy light power near the driver's door for continuous +12V to run the clock. Using the red connector means the tach light, like the other instrument lights, only burns when you also have your outside lights turned on.

Any +12V the tach needed for its operation (if any?) was pulled from the above-mentioned ignition switch red wire that was originally plugged into only the pink ballast resistor wire that runs to the coil.
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Does your car have a resistor wire?
When I hooked up my Rally Pac I had to re route the resistor wire into the tach.
The gauge light actually taps into the door courtesy light circuit at the switch on the A pillar and then back to the gauges.
If I can find the wiring diagram I will post a picture later.
Not 100% sure if the 65 -66 models are the same.
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I think yours is like my '66 which would say that detail didn't change for 64/5/6. I doubt the light power comes from the courtesy light switch; if it did, then the light would stay on 24/7 in your tach and clock. That was where they got power for the clock which needed power 24/7.

From the factory all of the cars had the pink resistor wire. 6s, 8s even HiPos all used the same coil, same pink ballast wire, etc.

The '66 and '65 diagram are the same WRT how the Rally Pac was wired. The '64 says the same thing but in a more complicated fashion. The connections for both the clock light (to [red] plug shown by speaker) and clock power (to lower courtesy light) are shown with their intermediate harness although no mention is made that those extra jumpers shown are actually wrapped together with electrical tape nor that they have a specially assigned wiring part number; still C5ZB15A006A for late 1966. Even the ignition ballast wiring detour through the tach (also shown on drawing) is taped into that same bundle. Although a '66 Shelby used the same basic Mustang wiring I don't know that the tach connections would have been exactly the same.


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Here is the installation diagram for the Rally Pac.
Note loom A runs to the door courtesy light.


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Thank you to everyone for your responses! I plan on working on the car tomorrow, so I will let everyone know if I am able to get the issue resolved.
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