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70 AMX Tach with Electronic Ignition |
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PHAT69AMX
AMC Addicted Joined: Jul/07/2007 Location: West Virginia Status: Offline Points: 5903 |
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HEI will not work the original 1969 Tach, nor will any other "electronic" HAL Effect Distributor.
The stock AMC Tachometer is like an early Mustang, those are both 'Current Triggered'. Understanding is the HEI, HAL Effect, Magnetic units are 'Voltage Triggered'. Apples and oranges. Something is needed to generate a 'Current Triggered' signal to run the stock tachometer from the 'Voltage Triggered' signal generated by the HEI.
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JerryB
AMC Apprentice Joined: Dec/19/2018 Location: Aiken, SC Status: Offline Points: 38 |
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I have sent my tach to the Tachman in Greer, SC. He is going to make it a "three-wire" tach to make it work. He is also lubing the speedometer and making sure it is calibrated. I should get it back in the next week or so. I'll have a better understanding then and I will give an update.
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mbwicz
Supporter of TheAMCForum Joined: Feb/20/2019 Location: Buffalo, NY Status: Offline Points: 1979 |
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I'm in the same boat, I just reached out to Tachman to update mine. I've seen several threads where he does good work. I look forward to your feedback.
Mike
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1970 AMX, one step forward, one step back. Both steps cost time and money.
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AMX2020
AMC Apprentice Joined: Feb/12/2020 Location: Canada Status: Offline Points: 81 |
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Looking at the Tachman website I see that they service the 69-74 AMX/Javelin tach. Is the 68 tach of a different design?
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Ralph G.
AMC Apprentice Joined: Apr/06/2016 Location: Cincinnati Status: Offline Points: 185 |
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It's a different size than the '69 but the same basic design. Jeeps of the 1970's -80's used the same basic design as well. I believe Faria made the Jeep units.
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bigbad69
Supporter of TheAMCForum Joined: Jul/02/2007 Location: Ottawa, Ont. Status: Offline Points: 6603 |
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For the 69 AMX, the tach was combined with the 140 speedo. The 69 Javelin had the same smaller tach, fit in the clock pod, that the 68 AMX used. This was a dealer installed option for the Javelin.
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69 Javelin SST BBO 390 T10
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AMX2020
AMC Apprentice Joined: Feb/12/2020 Location: Canada Status: Offline Points: 81 |
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So if I understand correctly, the Tachman can modify the OEM tach to work with voltage rather than current?
Also would the $100 device from MSD fix the issue (rather than sending to the Tachman) or is that device only relevant if you are using a MSD CD ignition box? My AMX is using a GM large cap HEI, no MSD box. The car is wired using Ron Francis wiring. I can see the tach wire attached to the Tach output on the distributor. At this point I do not know if is connected to the tach itself (but I assume so) or whether the tach itself is non-functional. There is no light in the tach pod but it may not be related to the tach not working.
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JerryB
AMC Apprentice Joined: Dec/19/2018 Location: Aiken, SC Status: Offline Points: 38 |
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Hopefully I will be able to answer some of your questions this weekend. Originally two wires came off the back of my tach. In the mix was a yellow resistance wire that was there to prevent my points from burning up in the OEM setup. The MSD won't work with my Ignitor 3 coil. I am not an electrical person, but I don't understand how it could run off voltage. I would think that would be constant. I am learning this stuff as I go along.
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bigbad69
Supporter of TheAMCForum Joined: Jul/02/2007 Location: Ottawa, Ont. Status: Offline Points: 6603 |
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The voltage in the ignition primary circuit is not constant. The breaker points (or transistor in an electronic ignition) are constantly opening and closing the circuit. The negative terminal on the coil has a pulsating DC waveform. i.e it alternates between 12V (~9V if the resistance wire is in place) and 0V. A voltage triggered tach senses those voltage transitions to calculate the engine RPM.
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69 Javelin SST BBO 390 T10
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