New purchase of an old part

[S7MS427]

Kent,

I don't think that is  quite correct. Stock 1966, 1967 and 1968 tachs gets power from the 12 vdc ignition line before the resistor wire (I'm not sure about other years but I'm assuming that they are similar). That is, the tach is wired in series with the 12 vdc ignition lead and the resistor wire, which then goes directly to the coil. When I wire the PerTronix, I run a 12vdc line in parrallel with the resistor wire, only I'm taking power for that line ahead of the resistor wire so the PerTronix unit never sees the 9 vdc or so that goes to the coil. Instead it sees a full 12 vdc. I've got two cars wired this way and I have no problems with either the PerTronix units or the tachs.

[Bob Gaines]

Kent,

I don't think that is  quite correct. Stock 1966, 1967 and 1968 tachs gets power from the 12 vdc ignition line before the resistor wire (I'm not sure about other years but I'm assuming that they are similar). That is, the tach is wired in series with the 12 vdc ignition lead and the resistor wire, which then goes directly to the coil. When I wire the PerTronix, I run a 12vdc line in parrallel with the resistor wire, only I'm taking power for that line ahead of the resistor wire so the PerTronix unit never sees the 9 vdc or so that goes to the coil. Instead it sees a full 12 vdc. I've got two cars wired this way and I have no problems with either the PerTronix units or the tachs.

+1  . Roy's description is correct and consequently there is not 12V going to the tach but only going to the Pertronix module. Using the correct wiring that Roy described there is an extra wire with 12v that runs out to Pertronix module .

[TA Coupe]

Buy one of these or make up one of your own:

Pertronix 2001 Ignition Power Relay Kit https://a.co/d/0gTgkg8m

     Roy

[S7MS427]

+1  . Roy's description is correct and consequently there is not 12V going to the tach but only going to the Pertronix module. Using the correct wiring that Roy described there is an extra wire with 12v that runs out to Pertronix module .

Just to be clear, in this setup, the tach receives 12 VDC as it is wired in SERIES with the ignition switch and BEFORE the resistor wire. The coil receives 9 VDC (or so) as it is wired AFTER the resistor wire. The tach receives 12 VDC because it is wired between the switched ingnition wire and the resistor wire. And the PerTronix receives a full 12 VDC as the splice for that is AHEAD of the tach and the resistor wire. It is easy to get lost in this scheme as we are talking about two different voltages: 9 VDC to the coil, 12 VDC to the tach, and 12 VDC to the PerTronix unit. Splicing off of a new wire inserted in the circuit BETWEEN the switched ignition wire (which is 12 VDC) and the tach insures that the proper votages are routed to where they are supposed to go.

I hope this is all clear enough. If I can find my spare slice, I'll post a picture of what it is supposed to look like.

[Bob Gaines]

+1  . Roy's description is correct and consequently there is not 12V going to the tach but only going to the Pertronix module. Using the correct wiring that Roy described there is an extra wire with 12v that runs out to Pertronix module .

Just to be clear, in this setup, the tach receives 12 VDC as it is wired in SERIES with the ignition switch and BEFORE the resistor wire. The coil receives 9 VDC (or so) as it is wired AFTER the resistor wire. The tach receives 12 VDC because it is wired between the switched ingnition wire and the resistor wire. And the PerTronix receives a full 12 VDC as the splice for that is AHEAD of the tach and the resistor wire. It is easy to get lost in this scheme as we are talking about two different voltages: 9 VDC to the coil, 12 VDC to the tach, and 12 VDC to the PerTronix unit. Splicing off of a new wire inserted in the circuit BETWEEN the switched ignition wire (which is 12 VDC) and the tach insures that the proper votages are routed to where they are supposed to go.

I hope this is all clear enough. If I can find my spare slice, I'll post a picture of what it is supposed to look like.

I misspoke .You are correct about the 12V to the tach.

[Kent]

Thanks for the clarification, Roy and Bob! You're right—I got my wires crossed regarding the voltage.

Voltage (12V) is fine: As you pointed out, the stock '67/'68 tach is wired in series before the resistor wire. This means it has always seen 12V from the ignition switch. The voltage itself isn't what kills it.

The Real Killer (Amperage): The danger comes from current flow. Since the tach is in series, every bit of power going to the coil must pass through the tach's internal transformer/windings.
The "Frying" Scenario: If someone installs a high-performance coil with very low primary resistance and bypasses the pink resistor wire to get a "hotter" spark, the amperage jumping through the tach skyrockets. That's what melts the internals.

The Proper Setup (The Roy Simkins Method):
By running a separate 12V line to the PerTronix unit from a source before the tach (or at the ignition switch), the PerTronix gets the clean 12V signal it needs to "trigger," but the heavy lifting (the coil power) still follows the original path. This keeps the load on the tach within factory specs.

The MSD Confusion:
I mentioned the MSD Tach Adapter because MSD ignition boxes (like the 6AL) use a capacitive discharge system that sends 400V+ to the coil. If you hooked a stock Ford tach to that line, it would be gone in a microsecond. But for a simple PerTronix swap, following Roy's wiring keeps everything safe without an adapter.

To clarify further, even with Roy's wiring, there is a hidden danger when upgrading to a PerTronix III with a low-resistance coil (like the Flame-Thrower III).
The PerTronix III system is designed for very low primary resistance (around 0.32 Ohms). Because the '67/'68 tach is wired in series, the massive increase in amperage required by that high-output coil still has to pass through the delicate internal windings of the tach.
Even if the PerTronix module itself is getting a separate 12V feed, the main 'load' for the coil is still pulling through the tach. If you run a high-output coil, you are pushing much more current through those 55-year-old internals than Ford ever intended. To be 100% safe with a PerTronix III, using a relay (as Roy suggested) is the best way to trigger the coil without melting the tach's transformer.

Horsepower: Switching to PerTronix 1 or 3 on a street engine won't give you a massive HP boost (maybe 1–2 HP). Peak power remains similar if points were well-adjusted.

Driveability: This is where the win is. PerTronix III offers Multi-Spark at low RPM for a smoother idle, better cold starts, and cleaner combustion.

The Dual Points: Dual Points were designed to prevent "point bounce" at high RPM (5,000+). However, modern replacement points often lack the heavy-duty spring tension of original racing parts.

Reliability: Points wear down and the timing drifts. PerTronix offers rock-solid timing that never changes and includes a built-in Rev-Limiter (on the III), which is an engine-saver if you ever miss a shift.

Conclusion: You don't switch to PerTronix for "racing power" alone; you do it for reliability, a smoother idle, and to stop the maintenance headache—all while keeping your original tach safe via a relay.

[TA Coupe]

This is the schematic for the Pertronix 2001 Ignition Power Relay Kit. Hopefully it helps.

      Roy

[jimhyc]

My 69 GT500 is running a Petronix 1 with the 1.6 ohm blaster coil. According to the notes from the previous owner it was installed in the 90's (have to read the notes for the exact year). It's wired directly into the original engine wiring harness. I know the tach is accurate since comparing to a diagnostic test. I've driven at least 1,000 miles last season and a couple thousand since the 90's.  With a Pertronix 1 is this something I should worry about?
Also thinking about just converting back to points and not have to worry about it.  But is there a good source for quality points these days? All I hear is that they are all Chinesium junk. Trying to make the car as correct as possible and the last thing I want to do is fry the tach. 

[Bob Gaines]

My 69 GT500 is running a Petronix 1 with the 1.6 ohm blaster coil. According to the notes from the previous owner it was installed in the 90's (have to read the notes for the exact year). It's wired directly into the original engine wiring harness. I know the tach is accurate since comparing to a diagnostic test. I've driven at least 1,000 miles last season and a couple thousand since the 90's.  With a Pertronix 1 is this something I should worry about?
Also thinking about just converting back to points and not have to worry about it.  But is there a good source for quality points these days? All I hear is that they are all Chinesium junk. Trying to make the car as correct as possible and the last thing I want to do is fry the tach. 

If you don't see a separate wire out to the Pertronix module for its power and instead getting its power through the factory harness at the coil then a high likelihood of problems at some point.

[jimhyc]

My 69 GT500 is running a Petronix 1 with the 1.6 ohm blaster coil. According to the notes from the previous owner it was installed in the 90's (have to read the notes for the exact year). It's wired directly into the original engine wiring harness. I know the tach is accurate since comparing to a diagnostic test. I've driven at least 1,000 miles last season and a couple thousand since the 90's.  With a Pertronix 1 is this something I should worry about?
Also thinking about just converting back to points and not have to worry about it.  But is there a good source for quality points these days? All I hear is that they are all Chinesium junk. Trying to make the car as correct as possible and the last thing I want to do is fry the tach. 

If you don't see a separate wire out to the Pertronix module for its power and instead getting its power through the factory harness at the coil then a high likelihood of problems at some point.

No separate wire.  I'm just going to remove it and go back to stock with points and a yellow top coil. Now to find a good set of points. 

[Kent]

I also think about to convert to points so if someone knows reliable dual points just let us know.