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1969 AMX Tach - Driving me crazy

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musictoyz View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote musictoyz Quote  Post ReplyReply Direct Link To This Post Posted: Jun/19/2018 at 2:08pm
I am still working on this....it has to do with DI and DV tach input....I cant believe no one on the internet has a work around for Pertronics and Series Tach
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Jun/19/2018 at 3:08pm
The Series or VDO tachometer states that a diode may be required to solve erratic tachometer problems - technical bulletin.
I'd phone the tach company up and ask for your specific Pertronics distributor, tachometer combination and find out what they say.
My VDO had to be wired to the MSD6 box output as it would not pick up off the coil with the Motorcraft distributor.  
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musictoyz View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote musictoyz Quote  Post ReplyReply Direct Link To This Post Posted: Jun/19/2018 at 3:32pm
The Stock AMC Tachometer is in Series with the Coil. That's why I said Series Tach....my bad if confusing...


I have done a lot of searching, this is nothing new, there are posts in every auto forum on this same subject...and no one has posted a fix. I mean 20 to 30 forums....

I did find why it happens in a Spitfire forum, as a stock points distributor to Pertronixs pointless changes the type of distributor from DI to DV...and that is the culprit....

Distributor guys want 299$ to change or mod it...but I imagine its a diode or a cap or something not that hard.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 6:22am
Have a look at this site for the Pertronics distributor with the Factory tach (GTO):
From what I read the Pertronics signal is too fast and too weak for older factory tachometers. The wiring diagram has the MSD 8920 used as an amplifier for the factory tachometer.
Don't know if this is a solution for you.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote musictoyz Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 10:38am
That is a start Trader.....it is definitely a Pulse Electric problem...the tach bounces or doesn't work..because the Pertronix doesn't put out a clean square wave any longer.

It puts out a multi signal that is giving an old tach headache...like my old brain
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Post Options Post Options   Thanks (0) Thanks(0)   Quote S Curry Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 11:13am
My 70 factory tach w/Pertronix doesn't work too bad up to 3k. Then it just kind of floats. Compared to another dwell/tach I can hook up under the hood, they compare about the same at idle up to about 1500.I question how accurate. I am following and investigating for an answer also.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote musictoyz Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 11:24am
I am in contact with a guy in Euro that has the answers and also makes an attachment for Euro vintage cars with Pertronixs….its Volt vs Current...

Guy is pretty sharp....stay tuned
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/20/2018 at 6:03pm
Originally posted by musictoyz musictoyz wrote:

That is a start Trader.....it is definitely a Pulse Electric problem...the tach bounces or doesn't work..because the Pertronix doesn't put out a clean square wave any longer.

It puts out a multi signal that is giving an old tach headache...like my old brain
Your OE tach is not measuring a pulse signal output from the distributor. It is measuring the current flowing through the primary circuit of the coil. If anything, the Pertonix should cause the tach to read high because of the increased current due to adjusted dwell times and higher voltage coil.

My understanding of the Pertronix conversion:

Ignitor I replaces the mechanical switch with a transistor switch. Dwell is a fixed, similar to points
Ignitor II has a variable dwell over 3000 RPM, but operates like an Ignitor I under 3000
Ignitor III  has more smarts in it which controls the dwell at all speeds and provides a rev limiter.

Reading Pertronix's FAQ, it appears the biggest issue with current sensing tachs is re-calibration. The example they use has a calibration screw on the back of the tach. The OE AMC tach does not.

I have an Ignitor I in my Javelin and the tach is working, though I have not checked the calibration (yet). My guess is there is a problem with the meter movement in your tach. It might just need a bit of lubrication.


69 Javelin SST BBO 390 T10
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Jun/22/2018 at 9:23pm
It also could be the caps going bad. Lots of them have a finite lifespan.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Jun/23/2018 at 9:09am
Originally posted by musictoyz musictoyz wrote:

I am in contact with a guy in Euro that has the answers and also makes an attachment for Euro vintage cars with Pertronixs….its Volt vs Current...

Guy is pretty sharp....stay tuned


This would be helpful.  I just cruised through some Ford forums (Ford used a series tach also) and they are having the same problem with the Pertronixs retrofits.
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

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