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Factory tach with HEI |
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Ralph G.
AMC Apprentice Joined: Apr/06/2016 Location: Cincinnati Status: Offline Points: 185 |
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Posted: Nov/22/2016 at 6:02pm |
Is there a way to use the factory tach in my '68 AMX with an HEI distributor? My understanding is that the tach, wired in series, acts as the ballast resistor for the ignition points in the stock configuration. The HEI needs a full 12v to trigger it so it can't be wired the same way. Is that correct? Thanks!
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WesternRed
AMC Addicted Joined: Aug/03/2010 Location: Australia Status: Offline Points: 5808 |
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Mine works fine.
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304-dude
AMC Addicted Joined: Sep/29/2008 Location: Central Illinoi Status: Offline Points: 9082 |
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I have yet to hook up my tach with my HEI. It will be some time... from what I gather, the tach output on HEI is inverted. Meaning instead of ground pulses from a points distributor, the HEI produces pulsed positive voltages. So, depending on how your tach is made, some older tach don't play well. But it is an easy fix, as there are inverter plug and play devices to patch between HEI and tach. It does not hurt to give your tach a try, before making a decision to buy a HEI tach signal inverter.
Edited by 304-dude - Nov/22/2016 at 6:56pm |
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71 Javelin SST body
390 69 crank, 70 block & heads NASCAR SB2 rods & pistons 78 Jeep TH400 w/ 2.76 Low 50/50 Ford-AMC Suspension 79 F150 rear & 8.8 axles Ford Racing 3.25 gears & 9" /w Detroit locker |
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uncljohn
AMC Addicted Joined: Jan/03/2013 Location: Peoria AZ Status: Offline Points: 5394 |
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The early AMC products used a tachometer that was "in series" with the points (or as stated the ignition) but did not act as a ballast but as an AMMETER to measure the current flow through the points as the engine ran and calibrated to read RPM rather than AMPS. Tachometers from early up through believer 1974 for the V8 cars were that way and became pulse triggered tachometers after 1974. The I-6 cars remained of that type through end of production.
My 1974 TSM indicates the Javelin was a pulse type tachometer with all others being in series thus the pulse triggered tachometer for Javelins might have changed with the hump fendered models of 71,2,3 and 4 Thus if you are planning on going with HEI look into the adapter that is available to continue the use of the factory installed tachometer IF using a V8 74 except hump fender Javelins or an I6 car with tachometer up through end of production. That said this comes from memory of messing with this subject some time ago and I think I am correct. The V8 of course was no longer used from 1980 on in the cars so a V8 Hornet/Concord or Spirit based AMX would be the question mark and probably in series type. Edited by uncljohn - Nov/23/2016 at 2:03am |
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70 390 5spd Donohue
74 Hornet In restoration 76 Hornet, 5.7L Mercury Marine Power 80 Fuel Injected I6 Spirit 74 232 I-6, 4bbl, 270HL Isky Cam |
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White70JavelinSST
Supporter of TheAMCForum Joined: Aug/08/2012 Location: Minnesota Status: Offline Points: 4867 |
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The White70JavelinSST has an MSD Ready To Run Billet Distributor.
To enable the factory tach I had to swap a few wires around a bit, and it worked. No need for a tach adapter. I'm not sure how that works with the HEI distributors and the Pertronix units but MSD has my vote for reliability, simplicity and function. Check out the MSD tach adapter, it may be what you need for that HEI distributor to work with the 68 tach. I believe there are several varieties. Calling their tech line might benefit you. |
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70 Javelin SST, second owner, purchased 1972
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Ralph G.
AMC Apprentice Joined: Apr/06/2016 Location: Cincinnati Status: Offline Points: 185 |
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WesternRed, how is your tach wired? My understanding is that the factory tach was wired in series in the ignition system, and that it replaced the ballast resistor wire that was used in cars that did not have a tach. (Isn't that why the pink wire at the starter solenoid was left disconnected on factory tach cars?)If I wire the tach into the 12V ignition feed wouldn't the tach's resistance value prevent the full 12V from reaching the distributor? Or am I totally off in left field (again!) Any insight would be greatly appreciated.
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304-dude
AMC Addicted Joined: Sep/29/2008 Location: Central Illinoi Status: Offline Points: 9082 |
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No, it is parallel... ignition resistance wire is bypassed by replacing it with a jumper from I terminal on solinoid to the coil's + terminal. The tach lead is added to the coil's + terminal. So with factory coil connections intact, there should be 4 wires total. Unless you replace the factory resistance wire with a 16 awg wire, then you will have 3 wires at the coil's + terminal.
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71 Javelin SST body
390 69 crank, 70 block & heads NASCAR SB2 rods & pistons 78 Jeep TH400 w/ 2.76 Low 50/50 Ford-AMC Suspension 79 F150 rear & 8.8 axles Ford Racing 3.25 gears & 9" /w Detroit locker |
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Ralph G.
AMC Apprentice Joined: Apr/06/2016 Location: Cincinnati Status: Offline Points: 185 |
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Thanks for all of the responses. I'll play with the tach and wiring a bit and see what happens.
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Capt.America!
AMC Fan Joined: Sep/10/2013 Location: Lake Stevens Status: Offline Points: 24 |
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White70JavelinSST, what wiring did you switch around to run the factory tach? I am trying to hook up my factory tach with a DUI HEI distributor and would rather not have to get an adapter to make it work.. i know te an old thread but if anyone could help that'd be great!
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WesternRed
AMC Addicted Joined: Aug/03/2010 Location: Australia Status: Offline Points: 5808 |
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Mine is wired in series, the same as factory and works fine, I did remove 2 separate sections of resistor wire from the circuit. |
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