Petronix Ignition Module

[RCR Z]

Hey All,

Installed the Petronix Ignition Module and 3.0 Ohm Blaster Coil from Motorsport Auto on my 70 240. Per instructions for the 3 ohm coil, the ballast resistor was eliminated and leads connected together. The car starts and runs better than before with points but noticed that above 1500 RPM the Tach no longer works??

Has anyone had a similar experience and resolution?

 

 

 

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[Zed Head]

What does it do?  Or not do?

One problem with the 240Z system is that all of the coil current runs through the tach.  So, when you get more current flow it can cause problems.  I've proposed this before, but I don't think that anyone has tried it.  You could split the circuit through the tach by adding a bypass wire.  This would cut current through the tach but let full current through the coil and module.  That assumes that the problem is too much current.  If it's too little current then you'd have to get a lower ohm coil.

I'm not sure which is the cause, since the Pertronix instructions try to get you close to the current you had before.  If your other coil was still good, you could put it and the ballast back on.  If you don't have the Pertronix II, there is no coil "power" increase.  It's just a points-elimination procedure.

 

[RCR Z]

What does it do?  Or not do?
One problem with the 240Z system is that all of the coil current runs through the tach.  So, when you get more current flow it can cause problems.  I've proposed this before, but I don't think that anyone has tried it.  You could split the circuit through the tach by adding a bypass wire.  This would cut current through the tach but let full current through the coil and module.  That assumes that the problem is too much current.  If it's too little current then you'd have to get a lower ohm coil.
I'm not sure which is the cause, since the Pertronix instructions try to get you close to the current you had before.  If your other coil was still good, you could put it and the ballast back on.  If you don't have the Pertronix II, there is no coil "power" increase.  It's just a points-elimination procedure.
 

At start up with full choke Tach reads zero. Once warmed up enough to decrease choke, the Tach begins reading at about 1500 RPM. I double checked the RPM with an external meter and they match closely.
I don’t believe it is the Petronix II but MSA said that the 3.0 ohm would produce a hotter spark??? They could not provide an explanation or fix as this was described as an anomaly. My typical luck!
I will take your suggestion this weekend a see what happens. I expect that it will return to normal.
Thanks for your help.
Dennis


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[Zed Head]

7 minutes ago, RCR Z said:

At start up with full choke Tach reads zero. Once warmed up enough to decrease choke, the Tach begins reading at about 1500 RPM. 

That is weird.  The guys that understand how the "choke" works might know more.  Can't see how a carb setting would affect electrical.

[RCR Z]

That is weird.  The guys that understand how the "choke" works might know more.  Can't see how a carb setting would affect electrical.

There is nothing electrical about the choke so I think it has to with the motor RPMs. To your point of current, as the rpms increase doesn’t the alternator current? The car is fitted with the MSA internal regulator alternator but don’t think that should make a difference?
Alejandro at MSA told me today they have no idea why this happens on some but not others? Not sure if the earlier models had different wiring with respect to the Tach than did later models?
More digging to find out but will let you know the outcome.


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[Zed Head]

You said that you had an external meter but never said what RPM was with the choke on.  

The Z's charging systems are known for bing weak at idle.  So, actually, even though it's not supposed to be the case, there might be lower current at lower RPM.  It should show up as voltage on your meter, since the two are related by Ohm's Law.  

So, if we knew what the idle RPM are with the choke on, something might become clear.  I don't think that what people commonly call the "choke" is actually a choke, so can't really guesstimate idle RPM.

[zKars]

Stupid ancient tachs. Good reason that all tachs from about that time on-ward went to coil negative terminal trigger. Much more predictable waveforms to trigger on and count accurately  

There may not be much you can do to get your tach to trigger properly. Anything you "add" to the + side of the coil will reduce voltage and/or current to the coil and reduce spark strength. MAYBE go get a 0.7 ohm blaster coil and put the stock ballast resistor back in line and see if that helps? Use the stock coil and ballast? 

All of the modern tach adapter modules are designed for the negative trigger mode. No way to predict how any one of them might help or hurt, mostly likely will make it worse, unless you find one that specifically designed to handle the + coil power supply counting tach issue.

Replacing your tach guts with a later 280z tach that uses the negative trigger is one common approach that tends to be reliable. 

Edited April 10, 2018 by zKars

[SteveJ]

It doesn't seem so weird to me that the problem is happening at higher RPM. It's likely a combination of the amount of current and frequency of the pulses.

Splitting the circuit as Zed suggested may not be enough. It may also require a small resistance in the tach side of the split. If I were to test, I would start with having a potentiometer in the tach side to see how much resistance is needed and replace the potentiometer with a fixed resistor of the value in the test.

ZKars does have a point about adapting the tach. It may be easiest to use a 260Z/280Z tach and the faceplate from the 240Z. Either way will require some wiring mods, though.

[Zed Head]

I just wanted to confirm that "choke on" is higher or lower RPM.  I have a 280Z so don't know what the choke does on these cars.  Just confirming, with numbers.  I love numbers.

I like the potentiometer idea.  I proposed it in a past thread, but it might have been for one of the MSD/tach problems.  Great minds!!!!   

Really, this problem crops up now and then, and it should be either too much or too little current.  There's a solution for either.  If I recall right, the stock system does not add up to 3 ohms of resistance, so there's no obvious answer.  We need numbers!

Edit - by the way, the quick way to bypass the tach would be to just tap in to the supply line and the return line, in th harness.  One inch of wire, in the harness, you don't need to even get up close to the tach.  You could put a potentiometer there to control resistance of the bypass.  If the pot in the bypass doesn't work, move it to the tach supply line.

Edited April 10, 2018 by Zed Head

[RCR Z]

Thanks guys!!! Will let you know what happens!


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[RCR Z]

Good news Guys, Ian at Pertronix had the answer! On the back of the Tach there is a white wire that is looped and fastened via a plastic block, metal bracket and 8mm nut.

Ian’s instructions were to unloop the wire one loop at a time. As you can see there was only one loop. So I pushed the wire with the black band through the plastic block to make the loop larger.

The Tach now revs through the entire range. At least up to 5200 where I decided to stop. Will have to wait for her to get on the road before checking it up to redline!!!

Thanks for all your input and I hope this info helps others!


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[Patcon]

@Captain Obvious

Why????