99 4.0 With no spark

JCross

JCross

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Jan 9, 2021
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Michigan
My sons 99 4.0 AT has no spark.
It only gets a split second of 12v pos to the coil when the key is turned on, then nothing while cranking.
I ran a jumper wire straight to the DK GR/LT GR wire from the battery and still nothing.
I have replaced the cap/rotor, plugs and wires and the coil.

I am at a loss and need to get this thing running so he can get to work.

Any help would be great!
 
The coil only gets +12 volts when the ASD (auto shutdown) relay is energized which only happens for 1.8 seconds after turning the ignition on or while the engine is being cranked.

With +12v on its positive side, the coil only fires when it gets its negative side grounded by the PCM. The PCM provides a pulsed ground connection to fire the coil when it's happy with other inputs like from the crankshaft position sensor.

Since your coil isn't producing spark even with the +side of the coil receiving a constant +12 volts from your jumper wire, it's because the PCM isn't happy with something so it's not providing the pulsed grounds to the - side of the ignition coil. As above could be from a bad crankshaft position sensor but it's hard to tell without being there.
 
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Check all the pins in the ECU receptacle and the clips in the (4) 32 pin harness connectors for oxidation and ensure the harness connectors are firmly inserted into the ECU receptacles.
Make sure the wiring harness that is routed behind the rear of the valve cover has not come in contact with the head stud and rubbed thru the harness shorting ECU signals to ground.
 
Thanks Charles,
I checked them all, terminals are all clean and it doesn't look like it's rubbing on the valve cover.
 
Jerry Bransford said:
The coil only gets +12 volts when the ASD (auto shutdown) relay is energized which only happens for 1.8 seconds after turning the ignition on or while the engine is being cranked.

With +12v on its positive side, the coil only fires when it gets its negative side grounded by the PCM. The PCM provides a pulsed ground connection to fire the coil when it's happy with other inputs like from the crankshaft position sensor.

Since your coil isn't producing spark even with the +side of the coil receiving a constant +12 volts from your jumper wire, it's because the PCM isn't happy with something so it's not providing the pulsed grounds to the - side of the ignition coil. As above could be from a bad crankshaft position sensor but it's hard to tell without being there.
Click to expand...
Thanks Jerry!
I know a lot of folks on here say to get Mopar only on certain things. Is this one of those, or will an AutoZone crankshaft position sensor be ok?
 
JCross said:
Thanks Jerry!
I know a lot of folks on here say to get Mopar only on certain things. Is this one of those, or will an AutoZone crankshaft position sensor be ok?
Click to expand...
Never go with store brands for critical components like sensors, radiators, water pumps, starters, etc. Autozone's Duralast brand is probably the worst. NAPA usually carries better quality but if you go there get their best quality.

No guarantee your problem is caused by the CPS but it is certainly capable of causing it.
 
Jerry Bransford said:
Never go with store brands for critical components like sensors, radiators, water pumps, starters, etc. Autozone's Duralast brand is probably the worst. NAPA usually carries better quality but if you go there get their best quality.

No guarantee your problem is caused by the CPS but it is certainly capable of causing it.
Click to expand...
It was the CPS! My son just replaced it and it fired right up! Thanks for the help!!!
 
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