No spark, troubleshooting Denso ignition

[BayRose]

2000 Mercruiser MCM 5.0L Alpha with Denso 131800-7071 ignition module. There is absolutely no spark from the coil when cranking.

But here is a clue, although I don't know exactly what it means: There are two wires running from the distributor ignition sensor to the Denso module, WHT/RED & WHT/GRN. I disconnected them both and used my 12 V test light to check for presence of 12 V coming from the module on either one. As soon as I touched the grounded test light to one (I forget which) there was a nice 1" blue spark from the coil wire to the engine block.

I'm wondering how to determine if I have a bad ignition sensor or bad ignition module. I'm thinking it's the sensor since bypassing it produced a spark. Can I use an ohmmeter to check the sensor while cranking as if there were points?

Thanks for your expert help!

 

[Don S]

Re: No spark, troubleshooting Denso ignition

with Denso 131800-7071 ignition module

That is just a part number and manufacturer of the part, if you need help, post a SN of your engine.
I will assume here for a minute and guess you have a carbed engine with a Thunderbolt V ignition system. So lets start with this chart for troubleshooting.
First, make sure your kill switch lanyard in in place when you check for spark. That has bit many people.

 

[BayRose]

Re: No spark, troubleshooting Denso ignition

Thank you Don S. Your flow chart appears to be exactly what I need. I'll try it out today. You are great!

 

[BayRose]

Re: No spark, troubleshooting Denso ignition

The kill switch is not the problem, it is working correctly. But there is no 12 V on the WHT/RED wire from the module. The flow chart indicates a bad module. I'm confused though, because before I had the chart to follow, I had already tried grounding the WHT/GRN wire from the module and that generated a spark from the coil, which according to the chart indicates a bad sensor. Not sure now which is bad.

Engine SN: L634079
Another number off of the block: VAU1292A

Yes, it is carbureted. Don't know what ignition system it has.

 

[proxyx]

Re: No spark, troubleshooting Denso ignition

Since you have a spark with grounded wht/grn your module seems to be ok. Your problem may be wht/red wire not connecting to the module. Clean ALL module connectors/check wires. The voltage on wht/red will be anywhere from 1-12V. In my case 4.5V (4.3 Merc).


Your issue may also be an interrupter switch (second wht/grn wire), disconnect it from the distributor or the other side of interrupter from the ground. Check switch operation (on/off) with the meter. Make sure it's wheel is in the valley (switch plate) and in off position.

Now the sensor issue... which was somehow never addressed in this forum... With disconnected interrupter (so it doesn't interfere) and wht/grn and wht/red wires connected to the distributor (just like normal operation) position the rotor (turn the engine by hand) so that one of the rotor tabs is between the sensor's magnet and the actuator - in the motion sensor valley. Pull off the rotor rapidly. You should get the spark on the coil. Try it several times. Has to be a rapid movement. Another way... connect voltmeter to the sensor terminals (harness disconnected), meter on lowest volt setting (mV, uV) and pull the rotor in the same fashion. You'll see meter needle action. If that occurs sensor is sending a pulse. Means OK. If you have a little play on a distributor gear allowing for little left/right rotor movement... with proper rotor positioning (in my case 1/3rd of the tab inside the sensor) you can trigger the spark by rapidly turning the rotor even within this very small margin of movement. Any of these 3 methods will show whether your magnetic pickup is working or not. The principal is that any metallic object (with similar material properties as rotor tabs) moving inside the sensor will trigger it.
Do that with disconnected tach wire from coil (2nd gry wire), interupter switch and choke wire. All 3 may interfere here. If succesfull reconnect them one at a time and trigger the coil to verify.

My guess: wht/red wire connection issue or interrupter switch.
Don't throw your module away yet!

See here:
http://forums.iboats.com/showthread.php?t=348673

PS. don't know how Denso is different from standard Merc ignition harness (probably the same) but adjust your testing method according to possible differences.

 

[BayRose]

Re: No spark, troubleshooting Denso ignition

Problem solved definitely! And I can explain it fully now. Thanks to all the help, especially the tip that the voltage from the module may vary from 1-12V--that told me it was a weak voltage source, probably too weak to drive a test light bulb. So there I learned the first lesson, to use a real voltmeter on these newer electronic-fangled machines. In fact there was a full 12 V there, but when the test light was connected, it dropped to 2 V, at first making it seem to me there was no module voltage.

So the ignition module passes all the tests. As for the ignition sensor, I was blessed to have the guts to tear it open. Inside was a small PC board with about 4-5 tiny (tiny) components on it. I could to identify one resistor and luckily read its value, 100-Ohms. I tested it and it was truly bad, completely opened. I had a good friend install a new resistor that can handle more power. When all the wires were re-connected, the spark triggers from the sensor now.

And I didn't even have to go through all the nasty trouble to check the dreaded GRY tachometer wire for a short to ground (by the way it measured about 35 ohms to ground--which I guess is normal for the actual gauge element. Never did find anything resembling a interupter either.

I still can't believe all the help you folks offered. Thanks a hundred!

 

[proxyx]

Re: No spark, troubleshooting Denso ignition

Glad you're done, I like it better that way then the trip to the store to get a new one, makes you understand things better, have you by any chance taken any pics of your sensor guts? Troubleshooting sensors is more of a voodoo science. If it works don't worry about your tach. Enjoy the fruits of your work.

 

[BayRose]

Re: No spark, troubleshooting Denso ignition

No--I did not think to take a photo of the dis-assembled sensor assembly. Now I wish I had for the benefit of the forum. But I did sketch out the schematic of the little sensor board, and I do have the defective resistor as a souvenir--it's downright amazing how a tiny part can kill a big boat dead in the water. I can try to photograph the resistor and upload the schematic for future reference.