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I just happen to think of another source to check and since my Mach is a 4V it has a modulator and sensor attached to the system that has something to do with the speedo cable. The sensor is the cast female union in my cable. My car had a Pertronix set up when I bought it and the speedo has never worked. I know it has something to do with the dizzy modulator/sensor but since mine has been switched to Pertronix how do I make my speedo work. I hope this is clear enough so someone can make some sense out of this and tell me what to do. Maybe I switch to a regular speedo cable? I really don't want to do that unless I have to .
Thanks
Ken
 

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I'm 99.99999% sure that you're barking up the wrong tree. The system that you're refering to is the IMCO emision system for the 1970 mustang.
It consists of the following
1. speed sensor on the speedo cable
2. gray box conected to the sensor
3. ambient air sensor in the door jamb
4. vacuum hoses to the thermal switch on the water pump
5. a section of metal vacuum line on the intake
6. vacuum hoses to both sides of the vacuum advance

I'd start with making sure the gears are good in the transmisson. Next check the cable itself. Then the short cable for the IMCO. That leaves the speedo itself.
 
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