Tachometer Accuracy

I’ve done some searching and haven’t found exactly the issue I’m seeing. My tach works, but it reads about 1/3 of the actual RPM. I have a Pertronix distributor and coil, so maybe that’s part of the issue? From what I’ve read, electronic ignition is either all or nothing where tachs are concerned, so I’m thinking the problem is internal to the tach. If I’m on track, is it possible to clean or adjust the tach, or do I need to find a replacement?

How do you know what the actual rpm is?
Is this a stock tach in the cluster or an aftermarket? If it’s an aftermarket, there should be a switch on the back to select 4, 6 or 8 cyl.

Thanks for the fast response! I’m having trouble with the factory tach in the dash. It’s pretty obvious that it’s reading low because it barely bounces off zero at idle and it reads a little over 1000 rpm on the highway. I also have a remote tach that I use when tinkering under the hood (setting timing, adjusting carb, etc.), so I’m able to compare.

If you followed the direction on installing the Pertronix unit the tach will not be affected. The coil needs to be the same resistance as stock in order to work properly. The factory tachometer reads current to the coil and translates that to RPM.

How exactly did you wire the ignition system?

A little background. The car had a no-name electronic distributor and matching coil on it when I bought it. The coil had issues and would start to fail as the temperature under the hood rose. That was a nightmare to diagnose! The tach had it’s accuracy problem when I bought it with this ignition set-up.

So, I replaced those with a Pertronix II distributor and Flamethrower coil. It’s wired per the instructions with the ignition wire (red w/green stripe) going to the coil positive and the distributor red and black wires to the coil pos. and neg.

I’ve had the gauge cluster out on a few occasions, and I can’t see that the wire from the tach to the coil has been messed with. However, I can’t see that it’s pink either. But I do know that I have 12V to the coil with the engine running. I have considered running a jumper from upstream of the tach directly to the coil, but I don’t see how that would improve the tach problem.

I’m fairly confident with the ignition wiring, so that’s why I suspect an issue inside the tach. Any suggestions are appreciated.

I’ve had your current setup (Pertronix, Flamethrower) in both '69’s for years with no tach issues…

Check your engine… those low readings sounds like it could be a Chevy… :rofl:

My engine’s blue. Has to be a Ford.

The Pertronix distributor needs a 12V source to function properly. If you have it hooked up to the factory coil (+) wire, then the resistance wire is only delivering around 7V when the engine is running.

The tachometer reads the pulses of the coil charging and discharging. With the Pertronix module hooked to the same power wire as the coil, it’s likely interfering with the tachometer reading. I would remove the Pertronix Red wire from the coil and power it temporarily with a jumper directly from the Battery (+) terminal and see if your tach reading is now correct. If not, then the tach will need to be serviced.

To my knowledge, the 68s don’t have 12V circuit that is hot in run and start, so you may need to run a relay to get 12V to the distributor.

Thanks Hemi. That should be an easy test. I’ll give it a try this afternoon and I’ll let you know how it goes.

Unfortunately, jumping the distributor positive didn’t make a difference to the tach. Looks like I’m either repairing or replacing the tach. Is servicing the tach a DIY operation, or is this something that should be left to the experts?

Grab an ohm meter and check the resistance in the wire from the tach (not through the tach) to the coil + post.
It should be 1.5 ohms. If more than that the tach will read low - less will make the tach read high.
For example: If someone added an external inline ‘Mopar style’ resistor (Accel ships these with their coils) than it will read 3.0 ohms and the tach will read 1/2 scale i.e. 2000 RPM would show as 1000 RPM.

The '68 XR-7 does have a circuit that is 12V + hot in run and start. It’s the red wire that goes from the ignition switch to the tachometer.

That sounds like a job that will require removal of the instrument cluster. I’ll give it a try next, but it might be a little while before I get to it. Thanks for the suggestion!

NOTE: If performing this test on an XR7 you will need to reach under the dash and unplug the tach and install a jumper (bypass the tach).

Per 1968 Ford Shop Manual (Group 9, Sec 12)

Resistance wire voltmeter test

  1. Connect the voltmeter leads as shown in Figure 12 (RED lead to ignition wire removed from coil BAT terminal, BLACK lead to Coil BAT terminal.):

Resistor wire test3

  1. Install a jumper wire from the DIST terminal of the coil to a good ground on the distributor housing.

  2. Turn all of the accessories and lights off.

  3. Turn the ignition switch on.

  4. If the voltmeter reading is between 6.6 and 4.5 volts the resistance wire is satisfactory.

  5. If the volt meter reading is greater than 6.6 volts, or less than 4.5 replace the resistance wire.

  6. Turn the ignition switch off. Disconnect the voltmeter leads. Remove the jumper wire connected to the coil DIST terminal and the distributor. Remove the jumper wire connected to the coil BAT terminal and the coil BAT lead. Connect the BAT lead to the BAT terminal and go on to the Coil to Ground test.

Thanks again. I’ll give this a try first opportunity.

You only need to remove the ashtray surround to access the tachometer wires. Removing the instrument cluster is not needed.

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The issue is in the tach. Points dist is a 2 wire, electronic is 3 wire and different guts. And yes the 68s have only 7 volts after starting in the run position and electronic dist need 12+ Found that out the hard way.

Rocketman has a nice underhood relay setup to give you 12 volts underhood without flying wires and drilling holes all over.

I have an 69 XR7 tach behaving erratically in my base convertible…. MSD plug and play diz… I haven’t been too worried as I have always wanted to replace with a Rocket Man classic tach. But cost of living, diving Aussie dollar and other expenses , I am starting to think I might have to debug …. Monitoring this thread… good luck