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Author
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Topic: 71 Factory Tach and MSD?
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MrXerox
Gearhead Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000
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posted 07-30-2001 01:01 AM
   
I screwed up the factory tach in my 71 Mach trying to get it to work with the MSD 6AL and no tach adapter...now that I have another one, I am scared to mess with it until I know for sure what will work. Are 71 factory tachs "negative" triggered? I have already bought a Tach Adapter that I believe I need but would like to hear from someone who has a 71-73 with MSD and factory tach to see how they have it wired up...
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Mike_R
Gearhead Posts: 176
From: Indianapolis, IN 46237
Registered: Feb 2001
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posted 07-30-2001 01:27 PM
Does your 6AL have a Tachometer output on the side of the box? Is this where you connected your tach when it didn't work?
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MrXerox
Gearhead Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000
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posted 07-30-2001 03:19 PM
Yes, thats where I had it...Do you have a 71-73 factory tach wired to it?
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Mike_R
Gearhead Posts: 176
From: Indianapolis, IN 46237
Registered: Feb 2001
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posted 07-30-2001 04:26 PM
No, I don't. I just noticed you hadn't had any responses so I thought I might take a stab at it. I was just curious about how you were hooking it up. I've used a couple of universal type tachometers with mine where the green wire normally connects to the negative side of the coil with the standard ignition. With the MSD I've just run that green wire to the tach output. I'm assuming then that your factory tach somehow operates differently than a universal tach.
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mach72
Gearhead Posts: 129
From: Bixby, Ok. USA
Registered: Jun 99
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posted 07-31-2001 11:21 PM
The 71-73 factory tach is a current triggered tach that connects between the + coil terminal and the ignition switch. When I converted my mach to a Crane Hi-6 spark box, I decided to just replace the stock tach with an autogage/autometer shift lite tach. It costs about the same as the adaptor, and I like the shift lite. I did this after Alex the great (moneymaker) told me that the stock tach can be off as much as 10%. At 6000+ rpm 10% is too much(5400-6600). If you use the stock Ford point distributor or a petronix upgrade, then you need part #8910. If you use a duraspark or msd distributor, you need part #8920. Another note about the stock 71-73 tach is that if the tach goes out and creates an open circuit, then the car will not run. This is because the stock tach is wired in series between the coil+ and the ignition switch. If this happens you must remove the wires from the back of the tach and hook them together, to bypass the tach and restore continuity to the circuit. Good luck, Tom
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