Low ignition voltage?

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69 Rustang

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96 Cobra Convertible
Been working on the ‘72 wrapping up various things to finish it.  This week I wanted to swap the carb base for another Holley base that included the automatic transmission kick down as the current one does not.  So I did the swap, put it back together and the car ran terrible.  The bases are virtually identical other than two small things.  But, different is different, so I reversed it all and put the working base back on.  Guess, what, car still runs terrible.  After fiddling with it and thinking about it I remember something odd—a weird electrical issue.

When the car was running, the tach wasn’t reading properly.  It stayed below about 1200 RPM no matter how much I revved the engine and barely registered at idle.  So I checked some voltage readings.  At the battery off, nearly 13 volts.  Running, closer to 15 volts.  Good ground to chassis, ground continuity to coil is good.  However, voltage at the batt side of the coil was only 5.9ish volts.  Also, voltage at the solenoid terminal (Photo with finger pointing) also only about 5.9 volts.  I do have a pertronix, but the pertronix instructions say there should be 12 volts.  Why can’t things be easy?  Can someone make this easy for me with some solid advice?

If I get this fixed and it is electrical, I am going back in on the carb base swap!

1F7F8CAE-2430-4E06-B261-7D74CA10BE6B.jpeg

 

midlife

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When running, the ignition voltage goes through a resistor wire out to the headlight to underdash junction, behind the fuse box.  From there, it goes to the engine gauge feed plug and another line goes out to the I post on the starter solenoid.  When cranking, the I post puts out battery voltage which goes to the gauge feed plug and out to the coil, bypassing the resistor wire.  When running, the I post (without anything installed) has zero volts, and the wire going to it has the same voltage as what the coil is seeing.  You should see something like 9.5 to 10.5 V while running. 

To trouble-shoot, remove line from coil, and with key in RUN (not engine running), test for voltage at that line: you should see 12V or so as there is no load on the resistor wire.  If less than that, something is amiss.  With a potentially bad tach, you can further test things by bypassing the tach (jumper the male and female leads from the underdash harness) and test again.  My guess is that the tach is bad and/or you have bad connections somewhere.  The engine gauge feed harness would be a prime suspect.

 

69 Rustang

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When running, the ignition voltage goes through a resistor wire out to the headlight to underdash junction, behind the fuse box.
So do you mean from the resister all the way to the headlights then back to the fuse box OR just to the headlight switch, then to the fuse box?  I think the latter, but I know this is your area of expertise.  Thanks!

 

midlife

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No...the resistor wire is entirely in the underdash harness, and it ends at the junction where the underdash and headlight harness mate, back of the fuse box.  It has nothing to do with the headlight switch.

 

SteveO_71

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At the battery off, nearly 13 volts.  Running, closer to 15 volts.
Looks like the voltage regulator is not working. Shouldnt the running voltage be more around 14v?

 

Don C

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Ford specs for the regulator is 13.5 to 15.3 volts.

 

69 Rustang

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Is there some way to test the tach?  It is definitely barely registering.  Tested running voltage at the batt side of coil and the solenoid and it is a little over 8 volts.  I haven’t tested disconnected like Midlife suggested, I have to do that later because I accidentally killed the battery.  I may try jumping the tach too after this.  Isn’t there someone on here that repairs/rebuilds tachs?  

 
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69 Rustang

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69 Mach 1 - 351C
72 Mach 1 - 351C
96 Cobra Convertible
I used to have a choke wire routed incorrectly—it was going to the terminal on the solenoid as shown with my finger in the picture.  It is now connected to the stator terminal.  The tach is working again, but the car still runs like it has a miss or something.  Short of swapping the carb off my ‘69 as a test, not sure what to look at as I hadn’t done anything to the ignition prior to the problem surfacing.  The carb on the ‘69 is a 780cfm, so it will be a bit much for this car, but I suppose it would be ok as a test of carb or ignition.

 
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