Distributor Ignition Sensor Mercruiser 5.7

[davebuc]

I apologize for not starting my own thread earlier.
Help please:
I have an issue with my ignition system. It follows a reassembly of a Mercruiser 5.7, Alpha one, Thunderbolt iV igniton with new heads (distributor has to come out to remove intake manifold).

I have used the Troubleshooting Thunderbolt IV ignition Flowchart (thank you, very helpful).
With it, I concluded my Distributor Ignition Sensor was bad. I replaced it, started right up in driveway. I put the boat in the lake and ran it about 25 minutes with the new ignition sensor. Later in these 25 minutes I had a popping sound coming out the carburetor. I believed I could track down the cylinder creating the popping by unplugging the spark plug wires from the top of the Distributor Cap one at a time. I got it on my first guess by unplugging #2 cyl plug wire. When I unplugged it, the popping stopped immediately. Within 15 seconds of unplugging the wire, the engine stalled. It would not turn over following the stall.

In case it matters, the nut on the rocker assembly for #2 cylinder worked itself loose, causing #2 exhaust valve not to open, which I believe is what was producing the popping sound at carburetor.

Ignition:
I notice when I crank the engine by turning the ign key that sparks jump from the top of the coil to the negative post on the ignition coil.

I followed the ignition system troubleshooting flowchart, which indicates I have to replace the ignition sensor in the distributor again. I have spark at the coil. I do not get spark out of the plug wires. I have not used a spark tester in the wire from the coil to the distributor cap, but I have substituted a spark plug in the distributor end of this wire, and grounded it, and it sparks strong and plentifully.

Did I fry the sensor by unplugging the plug wire from the top of the cap?

If not, it seems like I have a bad plug wire?
or the wire that goes from coil to cap?
Or is the rotor shorting to the shaft in the distributor?
It fits tightly and does not contact the sensor as it rotates.

All of these components are new:
Ignition Coil
Distributor Cap
Rotor
Spark Plugs

I have since added these 2 items to my list of possible causes:
1. ignition module and distributor ignition sensor should be grounded to the same post on the riser. They are not. Is it better for the ring connectors to be installed right up against the riser, (between it and the plastic spacer) OR grounded on the plate to which the ignition control module is mounted (which is separated from the riser by 1/2" spacer)?

2. When I put the distributor in, I put a paper gasket between it and my intake manifold. I don't think there was one there when I disassembled it.

Thanks for reading,
Dave

 

[dubs283]

Re: Distributor Ignition Sensor Mercruiser 5.7

ground the ignition sensor to the same stud on the block as the negative battery cable

check to be sure you have a good, clean ground for the module - do not connect to the same ground as the sensor

install a new set of plug wires, including coil wire

gasket for the distributor is there for sealing against oil leaks

there is no way the rotor is gounding through the distributor, unless the plastic is damaged

 

[wire2]

Re: Distributor Ignition Sensor Mercruiser 5.7

If you're getting a good spark from the coil to an open spark plug, it seems like the sensor is working. Have you checked the spark plug wires? Use an ohmeter set for Rx1000, you should see ~ 1000 Ω per inch of wire.

FYI, a coil will produce a very high voltage if there's no gapped path to ground (like through the distributer to a spark plug). In that case, the voltage will arc down the coil tower to the neg post, or positive. If it does it long enough, the arc will leave a carbon track.

 

[davebuc]

Re: Distributor Ignition Sensor Mercruiser 5.7

FYI, a coil will produce a very high voltage if there's no gapped path to ground (like through the distributer to a spark plug). In that case, the voltage will arc down the coil tower to the neg post, or positive. If it does it long enough, the arc will leave a carbon track.[/QUOTE]

The arcing has already has left this carbon track from the top of the ignition coil to the negative post. Is this a problem?

 

[wire2]

Re: Distributor Ignition Sensor Mercruiser 5.7

The arcing has already has left this carbon track from the top of the ignition coil to the negative post. Is this a problem?

Yes, It will tend to arc there easily from now on. Sand the carbon off with fine sandpaper, then polish with compound, or sand and apply nail polish. Or replace coil.

 

[davebuc]

Re: Distributor Ignition Sensor Mercruiser 5.7

Update:
Success!!!
Here is how I got there:
1. I sanded the carbon off the coil, between the top of it and the negative post. I then wiped it clean with a towel, and painted it with 2 coats of nail polish. I also wrapped electrical tape around the negative post on the coil to hopefully insulate it better.

2. I replaced all the plug wires including the one from the top of the coil to the distributor.

3. I did NOT replace my distributor ignition sensor.

4. I attached the ground wire for my distributor ignition sensor to the same spot that my battery grounds on the block.

When I turned the key it backfired (yay, this is better than no ignition) and with a minor timing adjustment, it then started right up every time.

Great Job 2 wire and dubs283.
Thanks, Much Appreciated!!!!
Dave

Unfortunately my lake test was cut short because a few of my stinkin' rocker arm nuts keep working themselves loose. I am just going to buy all new ones. Peace of mind is worth $0.89 each