Need Help Please!!! Cranks low spark out of coil.

[Scubasteve67c]

I will definitely check the grounds thoroughly when I get the truck back. It seems to always be a ground

 

[Scubasteve67c]

I got the truck back and poked a little bit yesterday. I noticed that the white coil wire has 8.6v. My understanding is that the white wire is connected to white/black wire and goes to the tach. I thought that should be a negative signal from the ICM to fire the coil.

What am I missing here? I don't think I should have power on the white tach wire.

Thoughts?

 

[Schurkey]

I noticed that the white coil wire has 8.6v. My understanding is that the white wire is connected to white/black wire and goes to the tach. I thought that should be a negative signal from the ICM to fire the coil.

What am I missing here? I don't think I should have power on the white tach wire.

The "Tach" output is connected to the grounded (ignition module) side of the coil primary windings.

The windings have "12 volts" applied to the other end (actually 14+ volts with alternator charging.) The ignition module grounds the "Tach" end to cause current flow, and then interrupts the ground to stop current flow, causing the magnetic field to collapse and create a spark.

The "Tach" end has a pulsed-ground when the engine is running. With no ground, you have system voltage there, when grounded you'd have little- or no-voltage.

Your meter is probably averaging 14 volts and 0 volts, and coming up with 8-ish volts on the display.

 

[Scubasteve67c]

The "Tach" output is connected to the grounded (ignition module) side of the coil primary windings.

The windings have "12 volts" applied to the other end (actually 14+ volts with alternator charging.) The ignition module grounds the "Tach" end to cause current flow, and then interrupts the ground to stop current flow, causing the magnetic field to collapse and create a spark.

The "Tach" end has a pulsed-ground when the engine is running. With no ground, you have system voltage there, when grounded you'd have little- or no-voltage.

Your meter is probably averaging 14 volts and 0 volts, and coming up with 8-ish volts on the display.

Thanks for the explanation.

So with key on should I see voltage to the white wire with B-? The truck is currently a crank/ no start condition.

I appreciate the help

 

[Schurkey]

with key on should I see voltage to the white wire with B-? The truck is currently a crank/ no start condition.

I don't understand "B-".

You'd have voltage at both the B (+ Red) and C (- White, Tach) terminals, the two ends of the coil primary windings, if the engine has ignition power but isn't running.

 

[Scubasteve67c]

I don't understand "B-".

You'd have voltage at both the B (+ Red) and C (- White, Tach) terminals, the two ends of the coil primary windings, if the engine has ignition power but isn't running.

Sorry Battery negative (B-)

 

[Scubasteve67c]

Sorry Battery negative (B-)

Solved!!!

I cut the white tach wire off of the 3 wire coil plug and it lit right up! It must have been getting power from somewhere in the harness causing the coil not to fire.

Thanks for everyone's help!

 

[Schurkey]

Faulty tach?