2005 4.3 liter Mercruiser ignition trouble

kepruno · Aug 14, 2025

I’ve been searching threads and found the helpful flow chart for trouble shooting the thunderbolt ignition system. I am up to the point where I have a spark tester on the coil and the other end connected to ground ( I have it going to a spark plug) and I get a spark when I strike the green / white wire to ground.
According to the flow chart I should replace the sensor in the distributor which I did (along with new cap and rotor) but the boat still will not start.
I’ve seen some things to unplug around the dash for trouble shooting but I’m unsure where or what these are. Is it possible I got a bad ignition sensor?

Something to note; several times now I’ve gone to start the boat, most recently after the replacement parts install, and the boat wants to start. It does actually but only very briefly. Then just cranks with nothing. After this happens installing a spark tester in line with the plug will yield a spark or 2 every time I reset the key to off then nothing.

alldodge · Aug 15, 2025

Welcome
Sorry to say it wouldn't be the first time that a new dizzy sensor is bad out of the box.
First is the sensor you bought a Merc one or aftermarket?
Next does the new sensor have 2 or 3 wires?

Put a voltmeter on the Plus side of the coil and crank, do you see 12V while cranking?

kepruno · Aug 15, 2025

alldodge said:
Welcome
Sorry to say it wouldn't be the first time that a new dizzy sensor is bad out of the box.
First is the sensor you bought a Merc one or aftermarket?
Next does the new sensor have 2 or 3 wires?

Put a voltmeter on the Plus side of the coil and crank, do you see 12V while cranking?

It is a Quicksilver with 3 wires. Looks identical to the one I took out. I will check the voltage.

kepruno · Aug 16, 2025

kepruno said:
It is a Quicksilver with 3 wires. Looks identical to the one I took out. I will check the voltage.

Very significant voltage drop at the + terminal of the coil when cranking. Good for a second or 2 then falls to about 4 volts. I don’t understand the system well enough to know what that means. Bad coil I would guess. I did same test on the battery and did not see voltage drop off like that.

Pmt133 · Aug 16, 2025

If it's dropping that low you are getting a voltage drop on the ignition side of the electrical system... either bad ground or bad connection. Check purple at ignition switch while cranking to see if you see the same issue. From there you can at least determine if its an ignition switch issue or something else and work your way to where you are losing voltage.

alldodge · Aug 16, 2025

Agree
Start measuring and cleaning connections. Might start with the key switch, than with the 10 pin engine connector

kepruno · Aug 16, 2025

This all started when the boat was running. Everything was going great until it wasn’t. Still think ignition switch? It’s easy enough to check and I will just thought that info might help point a direction. It was like I ran out of gas.

Jeff J · Aug 16, 2025

Is the 2005 fuel injected? We had a newer one do exactly like you described out on the lake. The fuel pump was failing. It was only providing 30-35 PSI to the rail. I could pump it up with a primer bulb I added for troubleshooting and it would run fine for a few minutes and could be kept running with the bulb. It would act like it was trying to start but it would not start without pumping the primer up first.

kepruno · Aug 16, 2025

No fuel injection on this model but I think it was available at the time. My issue is for sure spark not fuel.

kepruno · Sunday at 8:07 AM

12 volts at purple wire with key on goes down to nothing while cranking. Seems like a problem.

Jeff J · Sunday at 8:12 AM

Have you load tested the battery to make sure the voltage drop doesn’t start there?

kepruno · Sunday at 8:27 AM

I did check battery with volt meter while cranking but no load test other than that. I only have an analog volt meter but it did not go below 11 volts

Jeff J · Sunday at 8:32 AM

Should be good then. I can’t think of a better way to check it without a dedicated tester.

nola mike · Sunday at 12:23 PM

so now you need to find out where you're losing your ignition voltage. you get 12v at the coil with ignition on? you can check resistance across connections but better off doing a voltage drop test

kepruno · Sunday at 1:23 PM

I checked at the coil already. I thought that was why they had me check the ignition switch. 12V with key on 4V while cranking. What is the resistance across connections supposed to be?

nola mike · Sunday at 1:45 PM

you should have zero resistance across the connections. zero from red/pur wire at the 50a fuse, zero across the cannon plug, zero to the ignition switch, etc. you're losing voltage when cranking, but not when the ignition is on, so you might have zero resistance across connections but still be have a significant resistance somewhere once current starts flowing. which is why a voltage drop test is better. it will tell you where the drop is while under a load.

kepruno · Sunday at 2:16 PM

What’s the voltage drop test that I haven’t done. Where else am I checking it? Is there something between the ignition switch and the coil? I was assuming it was the same purple wire between ignition switch and coil which was telling me the switch was bad.

alldodge · Sunday at 3:01 PM

kepruno said:
This all started when the boat was running. Everything was going great until it wasn’t. Still think ignition switch? It’s easy enough to check and I will just thought that info might help point a direction. It was like I ran out of gas.

kepruno said:
I checked at the coil already. I thought that was why they had me check the ignition switch. 12V with key on 4V while cranking. What is the resistance across connections supposed to be?

alldodge said:
Agree
Start measuring and cleaning connections. Might start with the key switch, than with the 10 pin engine connector

kepruno · Sunday at 5:30 PM

10 pin engine connector visually looks immaculate. Not sure how or what to check with volt meter.

kepruno · Sunday at 5:36 PM

Anyone have a diagram of this? I need a map

2005 4.3 liter Mercruiser ignition trouble

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