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Petronix question

Posted: Sun Sep 13, 2020 9:36 pm
by m15256
Today I took some time to install a 24V Petronix unit in my otherwise stock TGB111. In the process I made an assumption that the relay and the disk shaped object just in front of the mounted coil in the engine compartment made up an ignition relay and ballast resistor. Petronix specifies that the black module wire go to coil negative post and red wire go to a switched 24 volt source prior to the ballast resistor. So after getting everything clean and installed I used 24V off the battery as the switched source to start the engine and see how it ran. It was cold, started right up, ran a little rough for about 6 seconds and died. It won't fire now which I assume means I compromised (destroyed) the module.

So my question is: Was my assumption about that metal disk mounted on the side of the engine bay correct? It is the original ballast resistor? Its kind of sealed up tight and challenging to get a meter on. I'd like to figure this out since those modules aren't cheap.

I'll try to post a picture.

Thanks, Rick

Re: Petronix question

Posted: Sun Sep 13, 2020 9:59 pm
by Garrycol
Petronix is very sensitive to the resistance of the coil - the module will burn out. So you need to check the resistance of the coil and any resistors used as a circuit - not individually - make sure this combined resistance is within specs for the unit.

On my haflinger I had a wasted spark coil and it was outside Pertronix specs so I had to change the coil back to a normal style coil of the required resistance.

So check the combined resistance of your coil and any resistors as a combined unit.


Re: Petronix question

Posted: Mon Sep 14, 2020 6:55 am
by m15256
Attached photo of relay and Ballast (?)
Coil itself has 5.7 ohms of resistance. Installation instructions recommend at least 3 ohms.

Re: Petronix question

Posted: Mon Sep 14, 2020 9:08 am
by rmel
From the installation instructions for a 24V 6-cyl conversion -->

24-Volt Negative Ground Instructions
For Part Number: 2863N24
Before installing, please read the following important information....
2. The Ignitor is designed for 24-volt negative ground systems.
3. The Ignitor is compatible only with a 3.0 ohm “resisted style” coil. You can purchase our 3.0 ohm
Flamethrower coil.

Re: Petronix question

Posted: Mon Sep 14, 2020 9:17 pm
by m15256
Vince posted a wiring diagram Jan 15, 2019 which is the only version I have seen showing the relay and resistor. It's under the Body and Electrical group. All the other wire diagrams I've come across are for 12 volt civilian C303.

Is anyone else still using this original set up which seems more complex than necessary? Why not just run the hot ignition wire off the key switch to a ballast resistor then coil?

Is there a written description for the relay function? It doesn't appear in the shop manual I've seen copied.



Re: Petronix question

Posted: Thu Sep 17, 2020 4:58 pm
by Jim Molloy
To my understanding, in the EOM configuration, the relay is designed to temporarily allow unresisted 24VDC directly from the ignition switch to the coil at start-up. Shortly thereafter, the ballast resistor comes into play for normal coil operation. Seems more complicated than necessary but the Volvo engineers must have had good reason to include that.

I have operating a prototype Power Arc Ignitions system on one of our TGB1314A trucks for the last two months. It requires switched 12VDC to its tigger module (distributor) and three coils. From the ignition switch, the green wire travels back and splits with one wire going to the ballast resistor. I simply routed a wire from that junction to a 40A (total overkill) 24VDC to 12VDC converted which now provides switched 12VDC power to the ignition.

I am currently waiting for a second prototype to install in our AluZn TGB11. In the meantime, I was considering using an older Pertronix Ignitor. I am actually most curious about the Laubtec (Germany) conversion of stock B30A distributors to electronically triggered units. Think Pertronix Ignitor on steroids.

Please keep us informed as to what comes with your system.

Take care.
Jim Molloy
Waldersee Farm

Re: Petronix question

Posted: Thu Sep 17, 2020 8:07 pm
by rmel
Same with the Pinz, the ballast resistor is bypassed with the push to start button
providing more oomph for a spark to start. Ballast resistor is in the circuit
otherwise. Done with a switch rather than through a relay.