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My XR7 died or won't start

 
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TheRktmn
Original Curbster


Joined: 22 Jun 2005
Age: 57
Posts: 8072
Location: TX, USA

1969 Mercury Cougar XR7

PostPosted: Tue Dec 06, 2016 5:45 pm    Post subject:  My XR7 died or won't start Reply with quote

A bad tach is the most common cause of the XR7 not starting. This is because the tachometer is wired in series between the ignition switch and the ignition coil.

In the first picture we see a typical Ford ignition circuit. The voltage comes from the ignition switch through the 'pink' resistor wire to the positive (also called the + or ign) side of the coil. Pretty simple stuff.
<i> Note: that resistor wire is there for a reason and we will get into that in another thread.</i>

<center><img src="http://www.thecurb.us/Tech/ignition-std.jpg"></img></center>

In the second picture we have added a Ford factory style tachometer to the circuit. These tachometers are wired "in series" which means that the voltage from the ignition switch has to go through the tachometer before it gets through the resistor wire and to the coil. If the tachometer fails the circuit is broken and the car cannot run.

<center><img src="http://www.thecurb.us/Tech/ignition-xr7.jpg"></img></center>

If your XR7 'tries to run' with the key in the start position but dies when you release the key to the run position chances are you have a bad tachometer. You can unplug the tachometer under the dash and put a jumper wire across the wiring harness (not the tachometer) connector, thus bypassing the tachometer and completing the ignition circuit. That is the easiest way to check for a bad tach.

<b>67-68</b> The tachometer has two bullet connectors, one male and one female. The folks at Ford were nice enough to include a built-in jumper on these years, It is about 3 inches long, unplug it from the tach side and plug it into the harness.

<b>69+</b> These tachometers have one two wire connector with a male and female bullet. You will need to make your own jumper for these years.

By the way; If the car runs and the tach does not work chances are it is already bypassed. Another possibility is that the movement portion of the tachometer is bad but the electrical portion is still good.
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