Engine keeps running when tach connected

[Bufus]

My car is a 73 Mach 1 with a 351C. Just installed a Pertronix Ignitor 2 conversion in my Autolite distributor. Also put in Ignitor 2 coil (0.6 Ohm) with relay to supply 12 V to it. If I leave the tach wire connected to the coil, the engine won't shut off. How can I use my tach without causing this problem? Thanks!

 

[midlife]

I'd examine your relay you are using to supply 12V to the ignitor: it should be RUN-only and that should shut off both the engine and the tach should stop as well. 

 

[SteveO_71]

Keep the pink wire connected from the tach to the coil and use a 1.5 ohm Ignitor coil. To low of an ohm coil will damage the tach.

Use the relay to supply the run-only 12v to the Pertronix module in the dizzy.

 

[Don C]

You cannot connect both the tachometer and the relay to the coil, it sounds like the relay is being self energized. Like Steve said, use the relay to supply voltage to the module, not the coil. If you want to keep the 0.6 ohm coil you'll need to get the MSD Tachometer adapter (Part No. 8920) and disconnect the factory wire from the positive side of the coil, connect it to the relay and then use the MSD adapter to provide the pulses for the tachometer.

Or, the cleanest way would be to get Rocketman (a site supporter and advertiser) to convert your tachometer to a 3-wire.

 

[Bufus]

I'm confused about the MSD tach adapter. It is for converting MSD box outputs to a usable signal. My signal isn't changing.  What good does the box do for me? Also, one of the main reasons I want 12 V to the coil is to increase the energy to my plugs. I had a miss on all cylinders probably because the tach wire only has about 4.5 V while the engine is running. The new coil fixed that problem.  Can I put additional resistance in line on the tach wire to stop the problem and protect the tach at the same time? Still waiting on response from Pertronix support on this problem.

 

[Don C]

That's about half of the voltage you should get with the original wiring. Did someone also add in a ballast resistor, in addition to the stock resistance wire?

Have you measured the running voltage with the coil you're now using? I suspect your old coil was shorting out internally, if you don't have an added ballast resistor. The higher the load on a resistance circuit, the lower the voltage will be.

Pertronix has 1.5 ohm coils that are designed to work with their modules and with stock (resistance wire) circuits, providing you supply the PII with full battery voltage with one of their relays (part no. 2001).

Using a 0.6 ohm coil doubles the amperage in the circuit, and the load through the tachometer.

The MSD adapter would allow you to use the 0.6 ohm coil and PII at battery voltage and still have a functioning tachometer.

 

[bdennis]

Interesting discussion.

I have a non tach dash in my 72 Mach1 H code. I added the P series 1 electronic points (black box) to mine and found that after about an hour of driving it would start to miss and carry on. Mine still has / had the original 1972 yellow top/Black coil. So have now replaced it with a after market coil (non Pertronix). The coil is one that needs an external resistor and measures 1.0 ohms across the +ve and -ve when not connected. I contacted the local supplier and as the factory wiring has the 1.3~1.4 Ohm resistor in series then all should be ok. The Pertronix optimum coil resistance is 1.5 ohms. So I effectively have 2.3ohms if I count the in line resistor.

My question is.. If im still seeing issues and want to put in a genuine Pertronix coil then I need to either add a relay to avoid the in line 1.3ohm resistor or remove the resistor.. If I opt to remove the resistor, where exactly is it? Or is it just a special length of resistance wire?

Of note, I have a small after market tach connected to the coil also. This may also be causing issues. I will monitor it and disconnect if I start to see problems and re test.

Any advice would be appreciated.

Thanks. 

 

[TheRktmn]

My car is a 73 Mach 1 with a 351C. Just installed a Pertronix Ignitor 2 conversion in my Autolite distributor. Also put in Ignitor 2 coil (0.6 Ohm) with relay to supply 12 V to it. If I leave the tach wire connected to the coil, the engine won't shut off. How can I use my tach without causing this problem? Thanks!

I have seen this happen when using a relay in the ignition circuit:

 

[Bufus]

Thanks for the info. I already tried a diode in line with the relay coil and across the relay coil. Neither of those worked. I will try it in line with the regulator and post my results.

 

[Bufus]

Bdennis, from what I understand,  the resistance is in the pink wire that runs to the coil. As far as my problem, turns out there is no connection to the I terminal on the regulator in my mustang because it has an ammeter instead of a charging lamp. Is it possible that a diode in series with the S terminal on the regulator would solve this run-on problem?

 

[Don C]

I believe that you have created a self-energizing circuit, where the relay output is backfeeding through the ignition circuit, which keeps the relay engaged.

 

[midlife]

Bdennis, from what I understand,  the resistance is in the pink wire that runs to the coil. As far as my problem, turns out there is no connection to the I terminal on the regulator in my mustang because it has an ammeter instead of a charging lamp. Is it possible that a diode in series with the S terminal on the regulator would solve this run-on problem?

Negative, Pilgrim.  The S terminal on the starter solenoid (and not the regulator) is for the START the starter motor.  The ammeter/charging lamp has nothing to do with the starter solenoid.