Weird ignition voltage problem
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From: Minnesota
Weird ignition voltage problem
I'm having a few problems with the wiring on my truck. First off, I'm using the Red w/white stripe ignition wire to turn on my relays and also its my switched hot wire for the ignition. Well here is the problem. With the key on, engine off, its putting out 12 volts. But, when the truck is running, its putting out like 6.5 volts. I checked a couple other switched wires with the truck running and they are 12 volts like they are supposed to. I think that ended up screwing up my MSD box. (The reason I'm checking this is cuz something fried my MSD Digital 6 plus and HVC coil and I'm trying to figure out what). I'm not sure what is causing that one wire (original keyed wire for coil) to drop to under 7 volts when the engine is running. I'm gonna have to find a different wire that stays @ 12 volts when engine is running. I'm hoping that cures the problems. Anyone have any idea if that wire is right not?
Oh yeah, truck is a 1982 F100
Oh yeah, truck is a 1982 F100
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From: Virginia
That wire has a piece of resistance wire made into it to drop the voltage. The original ignition system required this.
I don't think this fried the MSD box, though check the instructions, because it does probably require a full 12 volts.
What may have fried the box is having too much inductive load on the same wire as the MSD box. What I mean by this is anything that is a coil of wire, and is powered by DC voltage, will have a "kick back" voltage when the power is removed. Most of the time this is not a problem, but if you have a large relay or many relays, and they are in close proximity to something sensitive to reverse voltage spikes, it may cause a problem.
I don't know if you ever noticed, but most all A/C clutches have a diode in the wiring harness near the clutch wiring. This diode clamps the reverse kick back voltage spike, keeping the spike from burning up the low pressure switch.
How many relays do you have on this line? I believe the solution is one you had suspected. You need to run a seperate line from the fuse box or directly from the ignition switch, to the MSD box. If you have many large relays on another line, you may want to install some diodes to clamp that line.
I don't think this fried the MSD box, though check the instructions, because it does probably require a full 12 volts.
What may have fried the box is having too much inductive load on the same wire as the MSD box. What I mean by this is anything that is a coil of wire, and is powered by DC voltage, will have a "kick back" voltage when the power is removed. Most of the time this is not a problem, but if you have a large relay or many relays, and they are in close proximity to something sensitive to reverse voltage spikes, it may cause a problem.
I don't know if you ever noticed, but most all A/C clutches have a diode in the wiring harness near the clutch wiring. This diode clamps the reverse kick back voltage spike, keeping the spike from burning up the low pressure switch.
How many relays do you have on this line? I believe the solution is one you had suspected. You need to run a seperate line from the fuse box or directly from the ignition switch, to the MSD box. If you have many large relays on another line, you may want to install some diodes to clamp that line.
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