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Old October 20th, 2009, 04:15 PM   #1
joe_jeep
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Default 79 f150 400m no spark

we swapped motors between 2 trucks. a 78 bronco and 79 f150. both motors ran good b4 the swap. now the f150 has no spark. bronco runs fine.
we changed the coil and module(brain box). they work fine on the bronco, but not he f150. we got out the test light and started poking around.
it appears to have power to both wires to the coil on the f150, but not on the bronco. we are stumped.
any ideas??
thx
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Old October 20th, 2009, 05:01 PM   #2
Yota Bill
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your test light shows positive voltage on both coil wires on the pickup because it has pos on one side, then through the coil, and pos coming back out...I'm betting your test light showed pos voltage only on one wire on the running bronco...the other wire was grounded on the bronco, but on the f series it is not....

the coil has a constant pos wire, and the ground is switched through the ignition control box, which is a common part to fail (common enough to keep a spare one in the glove box) of course, your test with a test light does not prove anything, since the ground is switched on and off, so a good working system will show pos at both wires at different times....connect your test light between the ground terminal at the coil and battery positive and see if it flashes when the engine is cranked, do this while everything is connected properly. If it does not, then connect the test light between the coil pos and battery ground, and repeat...if the test light goes out at all during that test, then check the constant pos wire. If the pos feed to the coil is fine, but no flash is seen on the ground side, then suspect the ignition module or wiring, pay close attention to the connector at the ignition box, known to corode/possibly melt

since you just swapped motors, I would first double check all wiring connections, especially grounds..you could also try hooking a jumper cable between the battery neg and teh engine block to verify that it is properly grounded
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Old October 21st, 2009, 02:42 PM   #3
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ill check it out. thx for the ideas.
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Old October 22nd, 2009, 01:48 PM   #4
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it was the plug to the distributor form the harness. was clean and tight but just would not trigger the distributor. new plug will fix it right up.
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