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

[Scubasteve67c]

Please! Please! I have a 96 C1500 auto with a swapped L29 7.4l engine. It had a L31 prior. I have read multiple threads and scoured the web to no avail. I cannot get this thing to start. Here is what is going on:

Turns over (cranks) over just fine
ICM has 12v power, ground and ECM signal
Have tach signal via scanner
55psi cranking at service port
Harber freight spark checker lights up from coil when cranking
Adjustable spark check shows weak spark out of coil - won't jump much of a gap. Nothing ever out of distributor to plugs.
Swapped out coil, ICM, crank sensor, new distributor, wires and plugs. Also swapped out the ECM. No change.
Shows no codes when cranking
Rotor turns when cranking as well.
Has oil pressure when cranking.

Help!

 

[Caman96]

Maybe replacement coil is bad. What did you replace it with? Did you test it?

 

[Schurkey]

How much voltage at the coil + terminal?

 

[Scubasteve67c]

Maybe replacement coil is bad. What did you replace it with? Did you test it?

A few New from parts store and known good from old 5.7l. No change. I did ohm the old good one.

 

[Scubasteve67c]

How much voltage at the coil + terminal?

12 volts without the battery charger on. 13.4v with it on. 10.4v while cranking. Thanks for the reply.

 

[Spepe]

Update: I had it towed to a classic car repair/restoration shop and spoke with the lead tech & the service advisor there. They apparently traced it down to the flashed ECM & contacted the vendor. Unfortunately, the vendow was little help, but I did verify that the flash was for the proper engine & the VIN used matches the chassis. After a week of "diagnosis", I will be towing it back home not running. It stll has a weak spark from the coil & does not have spark out of the distributor. I'm at a bit of a loss on my next step & need to regroup. It will sit in my driveway as a reminder that I should have let a sleeping dog lie.

 

[Schurkey]

Quotes got kinda screwed-up as I rearranged stuff.

96 C1500 auto with a swapped L29 7.4l engine. It had a L31 prior.

Both engines use essentially the same ignition system. Vortec distributor, spark timing set by the crank sensor, ECM receives crank sensor signal, triggers remote-mounted ignition module, which triggers remote-mounted coil. Distributor does nothing but route spark to correct plug wire, and supply a camshaft position signal.

Turns over (cranks) over just fine
Have tach signal via scanner
55psi cranking at service port

Shows no codes when cranking
Rotor turns when cranking as well.
Has oil pressure when cranking.

^^^ All that is good. Are the injectors spraying as you crank it? Fouled plugs? Excess fuel in the cylinders washing the oil off the cylinder walls?

ICM has 12v power, ground and ECM signal

How GOOD is the ground? How many ohms between module ground and intake manifold--or between module ground and battery negative terminal?

12 volts without the battery charger on. 13.4v with it on. 10.4v while cranking.

I'm really suspicious of the wire harness between module and coil, and quality of the circuit between module and ground. Both coil and module receive "system voltage" which varies with starter motor operation, alternator operation, etc. The module grounds the coil to create current flow in the coil windings, and then stops current flow to collapse the magnetic field, producing a spark. I'm wondering if the harness between coil negative and module "C" terminal is damaged--broken wires inside the insulation, that limit the current flow and therefore the coil output power. Same for module ground.

Is the battery fully-charged? 12.6--12.7 volts with no load?

Does this vehicle have a tach on the instrument cluster?

Harber freight spark checker lights up from coil when cranking

Garbage. Any "spark tester" that has a light-bulb is not to be trusted.

Adjustable spark check shows weak spark out of coil - won't jump much of a gap. Nothing ever out of distributor to plugs.

But you HAVE spark. That validates pretty-much all of the triggering, ECM, and ignition module circuitry.

Swapped out coil, ICM, crank sensor, new distributor, wires and plugs. Also swapped out the ECM. No change.

Thus my suspicion of something wrong that you HAVEN'T replaced. And you've replaced pretty-much everything except wire harnesses.

After a week of "diagnosis", I will be towing it back home not running. It stll has a weak spark from the coil & does not have spark out of the distributor.

Bummer that the "Pros" can't help you. At least they didn't charge you anything, since they didn't DO anything useful...right?

 

[GoToGuy]

Ohm checking a coil will tell if it's bad but won't tell how good it is. Coil check lights, but won't jump coil gap jump test. The kind of tester that you set at the spark plug gap?

 

[BeXtreme]

I'm going with fuel here. If they just flashed the L31 tune from the original VIN, you won't be getting the right fuel delivery. You could always do an 0411 swap and then be able to actually live tune everything yourself

 

[Scubasteve67c]

Quotes got kinda screwed-up as I rearranged stuff.


Both engines use essentially the same ignition system. Vortec distributor, spark timing set by the crank sensor, ECM receives crank sensor signal, triggers remote-mounted ignition module, which triggers remote-mounted coil. Distributor does nothing but route spark to correct plug wire, and supply a camshaft position signal.


^^^ All that is good. Are the injectors spraying as you crank it? Fouled plugs? Excess fuel in the cylinders washing the oil off the cylinder walls?


How GOOD is the ground? How many ohms between module ground and intake manifold--or between module ground and battery negative terminal?


I'm really suspicious of the wire harness between module and coil, and quality of the circuit between module and ground. Both coil and module receive "system voltage" which varies with starter motor operation, alternator operation, etc. The module grounds the coil to create current flow in the coil windings, and then stops current flow to collapse the magnetic field, producing a spark. I'm wondering if the harness between coil negative and module "C" terminal is damaged--broken wires inside the insulation, that limit the current flow and therefore the coil output power. Same for module ground.

Is the battery fully-charged? 12.6--12.7 volts with no load?

Does this vehicle have a tach on the instrument cluster?


Garbage. Any "spark tester" that has a light-bulb is not to be trusted.


But you HAVE spark. That validates pretty-much all of the triggering, ECM, and ignition module circuitry.


Thus my suspicion of something wrong that you HAVEN'T replaced. And you've replaced pretty-much everything except wire harnesses.


Bummer that the "Pros" can't help you. At least they didn't charge you anything, since they didn't DO anything useful...right?

Quotes got kinda screwed-up as I rearranged stuff.


Both engines use essentially the same ignition system. Vortec distributor, spark timing set by the crank sensor, ECM receives crank sensor signal, triggers remote-mounted ignition module, which triggers remote-mounted coil. Distributor does nothing but route spark to correct plug wire, and supply a camshaft position signal.


^^^ All that is good. Are the injectors spraying as you crank it? Fouled plugs? Excess fuel in the cylinders washing the oil off the cylinder walls?


How GOOD is the ground? How many ohms between module ground and intake manifold--or between module ground and battery negative terminal?


I'm really suspicious of the wire harness between module and coil, and quality of the circuit between module and ground. Both coil and module receive "system voltage" which varies with starter motor operation, alternator operation, etc. The module grounds the coil to create current flow in the coil windings, and then stops current flow to collapse the magnetic field, producing a spark. I'm wondering if the harness between coil negative and module "C" terminal is damaged--broken wires inside the insulation, that limit the current flow and therefore the coil output power. Same for module ground.

Is the battery fully-charged? 12.6--12.7 volts with no load?

Does this vehicle have a tach on the instrument cluster?


Garbage. Any "spark tester" that has a light-bulb is not to be trusted.


But you HAVE spark. That validates pretty-much all of the triggering, ECM, and ignition module circuitry.


Thus my suspicion of something wrong that you HAVEN'T replaced. And you've replaced pretty-much everything except wire harnesses.


Bummer that the "Pros" can't help you. At least they didn't charge you anything, since they didn't DO anything useful...right?

The 'pros" don't work for free. I agreed to a minimum check out time. Unfortunately, I have nothing to show for it. Replace the engine harness and ECM with an aftermarket one was their recommendation....with no guarantees.