My 260z has a Mallory Unilite distributor, promaster Coil (#29440), and a Mallory Ignition Active Power Filter. Starting this year, when I drive it a short distance the engine just dies. When I go through the Unilite diagnotics, it show the ignition module has failed the test where I measure the voltage between the negative side of the coil and a ground with both the ignition module LED blocked and open. I've tried replacing the ignition module but the problem persists. Mallory's tech suggested a better coil ground so it now has a dedicated ground wire going directly to the battery.

I can restart the car after it sits until the engine compartment cools down. The ignition module then reads good. It will run for just a couple of minutes but again shuts down.

I don't think the fuel tank is building up a vacuum because opening the cap doesn't result in a rush of air and doing so will not start the car up again.

I recently added a new radiator cooling fan to replace a fan that stalls and draws a high current and installed an under engine splash guard so I should be getting better air circulation. I got rid of the dead head fuel system a PO installed and the fuel now flows back to the tank to minimize vapor lock. The fuel pressure gauge shows adequate fuel pressure.

Should I try a new coil or power filter?

1 minute ago, Jeff Berk said:

it show the ignition module has failed the test where I measure the voltage between the negative side of the coil and a ground with both the ignition module LED blocked and open. I've tried replacing the ignition module but the problem persists. Mallory's tech suggested a better coil ground so it now has a dedicated ground wire going directly to the battery.

Maybe it needs a better module ground. And/or heat sink.

The module looks like the typical GM module that grounds through the mounting points, and sinks heat through the contact of the bottom with the mounting surface.

Edited Sunday at 06:59 PM1 day by Zed Head

The brown wire is the ground. The mounting plate is attached by two screws but thermal paste insulates the plate from the distributor. I'm tempted to make a heat shield for it but it hasn't had one for 50 years so why now?

Edited Sunday at 11:06 PM1 day by Jeff Berk

Have you measured resistance through the coil? Too much current through the module might be overheating it. Might have a bad coil.

And the ballast resistor might have a short. It's part of the total resistance on the primary circuit, that passes through the module to ground.

7 hours ago, Jeff Berk said:

it show the ignition module has failed the test where I measure the voltage between the negative side of the coil and a ground with both the ignition module LED blocked and open.

Is it failing closed or open? Some voltage or no voltage? The ballast resistor has a coil of wire that adds the resistance. Might be failing open somewhere. Probably at a connection.

Details of the test procedure might give some clues.

I'm a couple of hours away from the Z until Saturday so I need to check this list of potential problems this weekend.

I was using the following procedure to test the uniLite:

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://documents.holley.com/a605_test.pdf

I think the last time it failed with the voltage at 12 V with or without the LED blocked. Once it cooled, the voltage would drop and the car started right up.

From an internet search it looks like the Mallory coil should have the following values:

primary: 0.6ohms
secondary: 11.3kohmf

1 hour ago, Jeff Berk said:

I think the last time it failed with the voltage at 12 V with or without the LED blocked. Once it cooled, the voltage would drop and the car started right up.

The voltage that it dropped to after it cooled might be a clue. If you're passing too much current through the module it wil be less than the low limit of one volt. They don't describe intermittent problems, you're on your own here.

"2) If battery voltage is present, place a credit card, drivers license, business card or similar and block the photo optics of the module. The voltage should drop to 2 volts or less (1 to 2 volts). If this test is positive, then the module is working.

3) If the test results are as follows:

a) Voltage does not drop, module is open and must be replaced. This may have been caused by a power surge, high resistance in the plugs or plug wires, or improper ground. Possible charging system load dump.

b) Voltage always stays below 2. 0 volts. The module has been spiked by high voltage or amperage, lack of ballast resistor, or improperly wired.

c) Voltage only drops to 3-4 volts could produce a weak spark."

Could also be the not uncommon shifting of manufacturing to the cheapest supplier overseas. Might just be that Mallory is just a brand name now, quality has degraded.

I think that you could wire in a GM HEI module, even though it's not a VR system. People use them on the ZX ECCS ignition system that uses zero and five volts to trigger the module and they seem to work just fine even though the trigger signal never goes negative. That would allow you to get rid of the ballast resistor also.

OR - it might not be the module. It might be the hall effect sensor photo sensor. Isn't there a visible light where you're putting the credit card? Make sure the light is there.

Never mind, I see that they integrated the sensor with the module. None of the above will work. Although a missing light might mean something. Who knows what though.

Fun problem. Wish I had some broken stuff to fix.

Edited 56 minutes ago56 min by Zed Head