Wiring Problems
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Wiring Problems
Hey I'm new to the forums. And well I got a 1976 F150 with a 300 six in it. I bought this truck not running and was hoping it be easy to get running but the ignition wiring is giving me troubles. Everything is new on the ignition: switch, relay, coil, distributor, cap, wires, plug, module. And still no spark. When I turn the key on I get 8v at the coil and relay. Isnt there spouse to be 12v there?? Also if I put my DVM on continuity and put a lead on ground and touch the Ignition wire at the relay and at the coil it beeps. Doesn't this indicate a short to ground somewhere?? Please help this truck is driving me nuts
Hotshot
Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
The coil recieves power through a resistor wire in the harness, you won't see 12v at the coil except when cranking.
Check the pick up coil in the distributor. Did you replace that too?
Check also, there should be 12v at the red wire on the module.
Check the pick up coil in the distributor. Did you replace that too?
Check also, there should be 12v at the red wire on the module.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
First question: as Mike said, this is expected because of the ballast resistor upstream of the coil primary winding.
Second question: Was the key in RUN when you did this? A meter will still beep even if there are several ohms of resistance between the leads. If you had the key in RUN, the "I" terminal of the starter solenoid is technically tied to ground through the coil primary winding resistance, which is less than 2 ohms and will beep the meter.
Second question: Was the key in RUN when you did this? A meter will still beep even if there are several ohms of resistance between the leads. If you had the key in RUN, the "I" terminal of the starter solenoid is technically tied to ground through the coil primary winding resistance, which is less than 2 ohms and will beep the meter.
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I didn't know about the resistance wire. And I did more research on how to test it and the ignition wiring to the coil is good. And the I found out when i was checking continuity at the relay and unplugged the connector at the coil continuity stopped. So it was going through the coil and then to ground that's why it was beeping.... Ok well now I got that figured out I got some more questions. I unplugged the wire from the coil at the disturbitor cap and put a spark tester in and every now and then it would get a faint spark. And I started looking at the wires going to the coil and found where someone splice connector on the green and yellow that was corroded so I took it out cleaned the wire and put a butt connector in and heat shrink. The I checked the resistance of the green and yellow wire from the coil to the ignition module and it was 23ohms, Shouldn't it be like 2 or 3 ohms?? And if so then the wire has high resistance in it and can I just run a wire from the coil over to the ignition module? Is there anything hidden on this circuit I don't know about? Like how my wiring diagram doesn't tell you about a resistance wire
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
There should be no resistance along the GREEN wire that goes from the negative terminal of the coil to the ignition module. You should only measure the lead resistance of the meter. Touch the two leads of your meter together to make sure the meter reads less than an ohm. Also make sure you're getting good contact on the circuit you're testing; the meter probes must make full contact on clean metal.
Any resistance here will keep the coil from firing. You should repair this section of wiring to eliminate any resistance if the measurement is indeed correct.
Any resistance here will keep the coil from firing. You should repair this section of wiring to eliminate any resistance if the measurement is indeed correct.
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Ok well I got that wire fixed and I was checking for spark and I thought I saw a little bit of one so I waited till it was dark out to see it better and you can start cranking it and as soon as you let off the key it sparks one time. What would cuase this to happen??
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The module is not firing the coil, either because the module is bad, the module is not getting power, or the module is not getting a trigger from the pickup in the distributor.
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Well the module was new but I went ahead and rechecked all the wiring and the distributor and all tested good. We have an old 1984 E250 with a 351 that was parked about 10 years ago and i decided to look under the hood and saw that it's ignition module had the same conectors but different shape as the 76 so I plugged up just to see what it would do and now it has a good spark!!! so I'm going to return the module I bought. Thanks for yall's help though. I'm glad I finally got this worked out.
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