Ignition control module
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New User
Joined: Jan 2005
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From: Idaho
Ignition control module
I have a 79 400. Newly rebuilt with high comp flat top pistons and head work. It is 9.3 comp ratio. I am running the xe256 cam. After setting the timing I disconnected the timing light from the neg side of the coil and the engine quit. It would turn over but not start. After a few minutes it fired right up. I again tried the same thing and this time it quit when I connected the timing light to the neg side of the coil. It runs ok and has not quit while drving. The power is not what it should be. The coil is new...the duraspark ignition is 26 years old now. My question is this. When the ignition goes bad does it do it gradually meaning less volts or ohms, or does it completely fail with no power at all? Also, if I need a new ignition module would a generic replacement be fine or should I go with the MSD or something to help with this motor configuration. Thanks.
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Joined: May 2005
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I had a problem with my 351m losing power also. I am curious why you hooked up the timeing light to your coil? I hook mine up to + and - ON THE BAT,and to no. 1 spark plug wire. If you are losing power at mid range,try hooking up the timing light like I do and hook your pickup wire to each one of the plug wires one at a time and reving up the engine. If your timing light goes out at around 2500 rpm or so on any of the wires, then your duraspark box needs replaced. I think the cutoff point between years is 1978. In other words, a 1978 box wont work on a 79 and vice versa
Posting Legend
Joined: Sep 2001
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The Duraspark distributor is OK but depending on what you have done to the motor the vacuum advance may need to be modified. There are specs for the distributor here in the 335 engine forum. I am with 78COBRAII, -why did you connect to the negative side of the coil??? You may have damaged the ignition box as well as the primary circuit ballast resistor. Take the ignition box to a parts store and have it tested. There are specs in the repair manual for testing the ignition system resistor. Replace the ignition module with a Motorcraft replacement part. The other brands fail often. A replacement guarantee is not worth the wasted fuel and your time. The Duraspark system has advantages over other aftermarket units that are worth it unless you are racing and operating the engine at 6000+ RPMs (which is NOT where you operate a 351M/400).
Last edited by Torque1st; Dec 29, 2005 at 06:25 PM.
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From: Idaho
Well...according to a manual...it said to connect the neg (black) from the timing light to the neg side of the coil. The pos to the battery pos. I did connect the plug lead to the #1 cylinder. I think I need a more modern book. The distributor is new. And most definitely not for RPM above 5000. I guess I will give it a try on the test and go with the stock motorcraft. Thanks again.
Posting Legend
Joined: Sep 2001
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In my 40 years of working on vehicles I have not seen one either and I have used a few different types...
The powered units always connect to the +/- battery terminals.
If the instructions do say to connect to the negative side of the coil take it back to where you purchased it along with the repair bill.
The powered units always connect to the +/- battery terminals.
If the instructions do say to connect to the negative side of the coil take it back to where you purchased it along with the repair bill.
Last edited by Torque1st; Dec 29, 2005 at 10:17 PM.
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back about 25 years ago, I had a very cheap timing light that hooked to the tach terminal on the coil instead of the #1 plug wire, but it was a piece of trash, and went right into the trash... For power, it still connected to the +/- on the battery, though...
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Posting Guru
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From: PacNW
Huh...interesting. I've never seen a timing light ground through the coil either. I always hook mine up with the red to the positive battery terminal and the black to ground, usually the engine. Way back when I did have a cheapo timing light that had a remote starter switch built in that could be hooked up to the solenoid. The trigger was actually the starter switch and the light was on all the time when power was hooked to it. Seemed like a good idea except that when you're used to pulling the trigger to make the light work you end up engaging the starter with the engine running. Bad noises... 
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