The dreaded 4000 rpm tach cut off

[grannyknot]

I know this has been a recurring  subject over the years but so far I haven't found a definitive fix for it in the archives.

1970 240Z, same year tach, Pertronix ignition. Tach will accurately show RPM up to 4000 then it cuts out and the needle drops back down until the revs come below 4000 when it will start working again.

Any advise welcome,

thanks,

Chris

[Jarvo2]

The writeup on zhome.com seems to be the best out there (thanks Carl!).  Here's a link to the page directly:  http://www.zhome.com/ZCMnL/tech/ZXPertronix.htm

I'm running this setup in my '72 without any issues.

[Zed Head]

I've seen threads where people talk about adding a loop to the trigger and I've seen some that imply that the tachs don't like the high current w/o the ballast.

If it were mine, I'd probably try running the tach on a parallel circuit with the module, with the ballast resistor and extra resistance to mimic the old system, instead of in series.  Actually, I'd just wire in a potentiometer in a parallel circuit and turn up the current until it started working right.

The ignition module does the current limiting for coil control on the grounding side.  Just split out that tach circuit and let the module control it also.  The coil get its full current and the tach gets its limited current.  The tach counts the module activity now instead of the coil.  Basically, two separate circuits controlled by the ignition module, and separately controlled.  .  I haven't thought it completely through but that's what I'd be focusing on.  Make the combined coil and tach problem two separate ones.

Some the above is redundant but I'm afraid I'll lose something if I edit.  The tach just needs to see when current is flowing through the coil, and it neds a certain signal strength.  It doesn't necessarily need to see the actual current though.

[Zed Head]

With a few seconds of pondering I realize that coil probably does need to be in the circuit.  Otherwise too much current for the module.  But I think that the parallel circuit concept still has potential.  Just with both the coil and module at the end.

[Zed Head]

It might look like this, but in place of ballast, there's a potentiometer.  The coil and tach get all the power they need, the tach gets lowered current.  Actually, circuit-wise, just splitting the two with no ballast should cut current
(by half during Start) on the tach line.  So you might actually have to put the potentiometer on the other circuit to increase current through the tach.

Where's the guys that do circuit math in their heads?

[grannyknot]

9 hours ago, Jarvo2 said:

The writeup on zhome.com seems to be the best out there (thanks Carl!).  Here's a link to the page directly:  http://www.zhome.com/ZCMnL/tech/ZXPertronix.htm

I'm running this setup in my '72 without any issues.

Jarvo, I guess you have a 1.5 ohm coil and a ballast resistor?  I have a 3 ohm coil and no resistor.

4 hours ago, Zed Head said:

It might look like this, but in place of ballast, there's a potentiometer.  The coil and tach get all the power they need, the tach gets lowered current.  Actually, circuit-wise, just splitting the two with no ballast should cut current
(by half during Start) on the tach line.  So you might actually have to put the potentiometer on the other circuit to increase current through the tach.

Where's the guys that do circuit math in their heads?

Zed, do you know what the specs on that pot would need to be ?

Thanks guys,

Chris

[Zed Head]

Actually, in your case, looking back at your specs. I don't know if my suggestion offers anything.  Running the extra line to coil positive will reduce current through your tachometer trigger loop.  Seems like you need more current, or an extra loop (but I don't know for sure).  If my guess is right, the extra line will make your problem worse.  It will drop current on the tachometer circuit.  Shouldn't hurt anything though, it still runs power through the coil and Pertronix module.  I might connect the line first and see if the problem gets worse.  The tach might quit working completely due to low current.  If it does, that would imply that you need less overall resistance, maybe a lower resistance coil.  If the tach works better that's a clue that it was over-currented at high RPM.  Maybe overheating.  A way to run more current through the tach loop is to use a lower ohm coil.  That would be on the same path as an extra loop on the tachometer.  Might be a worthwhile experiment.

The Pertronix situation is interesting because with the ballast and tach in series as they recommend, the overall resistance is higher than just the coil resistance.  So they recommend a 3 ohm system but don't really address the ballast and tach effect.

Ponder the ways to increase or decrease current through the tachometer circuit.  It's hard to get a complete logic path to a solution because you don't know how much resistance the Pertronix module has when grounded, plus the ballast resistor is actually a variable resistance device.  It heats up at low RPM since dwell time is longer.

SteveJ probably has the answers and could critique my suggestions.  He's a tach guy.

Short answer is "I don't know", I'm just brainstorming.

[Zed Head]

Forgot to say, one of the main weaknesses with the Pertronix system (not bashing, just pointing out [obscure pun!]), compared to other electronic options, is that it doesn't do current-limiting.  That's why you have to balance the coil and ballast, and I think it's why the tachs often don't work right.  The tach is expecting a certain quantity of current.  That's what the game is, finding that quantity.  I think.

Edited March 24, 2016 by Zed Head