Mercruiser 4.3L Thunderbolt V ignition issue

[Ken_S]

Hi,

I've poured through the forums and done the diagnostic test which, so far, point to needing a new ICM. I am still in denial due to the cost of those things! So here is my story.

Went out for a ride one day and the engine seemed to want to stall. if I pumped the throttle it would stay running. Eventually it just plain died and we had to get towed in. After checking some things i am now going down the ignition path even though it appeared like fuel before. There could still be a fuel problem but i discovered the coil wire was open and the coil itself was bad (no resistance). The top connector of the coil was all black and ugly. So...I replaced those (Sierra parts) and still no spark on coil wire. I then found the very nice diagnostic diagram on this forum for the Thunderbolt V ignition. Here are my findings.

Tachometer not grounded - ok... so i tried unplugging the connector at the back of the tach and disconnecting the single gray wire from the coil. Is there another way to do this step?

with Key in Run position check for 12 V at Coil - I have 12 v

Unplug white/red and check for 12 v from module - I have 10.5 volts, not 12 <- is this an issue?

Spark test at coil with white/green quickly against ground - no spark

so with the above results and a new coil + coil wire does this positively point to the $600 ICM Module?

Mercruiser Model MCM 4.3L 262 CID sn 0M685713

Thunderbolt V
Distributor cap/rotor kit 815407Q Sierra 18-5396 <- not installed yet
Coil 8M0079202 Sierra 18-5438 <- installed
Wire Kit 816761Q16 <- installed only coil wire, others "look" ok

ICM Module 861252T1 <- hoping to not have to purchase

 

[alldodge]

Unplug white/red and check for 12 v from module - I have 10.5 volts, not 12 <- is this an issue?

It was on the lead coming from the module, not coming from the distributor side, correct?

You probably did, don't see voltage coming from the dissy
Disconnect connector from module and check for 12V on the purple wire going into the module

 

[Scott Danforth]

many times the stock ICM trigger pickup goes and not the amplifier. the trigger assembly is under $45

follow the troubleshooting tree.

 

[Ken_S]

Just measured

voltage at big circuit breaker is 12.3 volts

voltage at dist is 11.2 volts

white/red from module is 10.5 volts

purple into module is 11.3 volts

 

[alldodge]

With that and no spark on the White/Green then I would replace the distributor module. The newer style module has an additional ground wire (3 total) to help. If yours is original, it will only have 2 wires

 

[Ken_S]

so you mean the ICM...right? For my engine ICM Module 861252T1

 

[achris]

so you mean the ICM...right? For my engine ICM Module 861252T1

No, he's referring to the sensor inside the distributor...

https://www.mercruiserparts.com/892150q02-sensor-assy-ign

 

[poconojoe]

The ignition sensor under the distributor cap (as suggested) was the cause of my no spark condition. Maybe this is your problem also. Mine is a Mercruiser 4.3.
My old one was the original from 2003. A two wire setup and the exposed circuit board was all corroded. The new unit is hermetically sealed and has a dedicated ground wire, as another poster stated. It cost me $46.

 

[achris]

The ignition sensor under the distributor cap (as suggested) was the cause of my no spark condition. Maybe this is your problem also. Mine is a Mercruiser 4.3.
My old one was the original from 2003. A two wire setup and the exposed circuit board was all corroded. The new unit is hermetically sealed and has a dedicated ground wire, as another poster stated. It cost me $46.

Thunderbolt (4 and 5) use the same distributor and sensor for all engines, V6, SBC and BBC. The modules are different...

Have you read my write-up on the system? http://forums.iboats.com/forum/engi...-mercruiser-thunderbolt-ignition-systems-work

Chris.........

 

[Scott Danforth]

The original pickup trigger sensor in the dizzy is not sealed or conformal coated. Its a failure waiting to happen.

The replacement with the extra ground wire are all sealed

the troubleshootung tree coveres testing this

 

[Ken_S]

OK finally got the ignition sensor installed. Getting the old rotor off was no fun. Now it has a new rotor and cap too. The engine started like its old self. Needed pumping and just above idle to stay running. The speed was surging up and down and never really settled. After shutting down and checking some things it won't start. Spark tester at coil shows a few sparks at first then nothing. So I went back to the troubleshooting flowchart. Now I have a stupid question. How can I check if the tach wire is grounded? I see only one gray wire on the coil and it appears to go to the ICM. I haven't torn open all of the wiring harness to see if or where it splits off to the Gage. Ugh. Lost again...

 

[alldodge]

only one gray wire on the coil

Remove the wire off the coil and measure to ground.

 

[Ken_S]

Thanks! I will try that, hopefully, later today.

Could that have caused the coil and coil wire to go bad? I'd eventually like to find out the root cause of this issue.

Thanks again!

 

[alldodge]

Plug and coil wires do wear out. If they start arcing it starts burning the insulation jacket

 

[Ken_S]

OK. This must be it. There is about 1500 KOhms from grey wire (disconnected from coil) to engine ground. That can't be good. Where could it ground out? The engine and wiring harness look new. Very few hours on this motor. I hate to cut all the tape off the harness for no reason. Is it more likely closer to the gauge?

for what it's worth the tach has been erratic for a couple years. I thought the gauge was bad.

 

[alldodge]

I see no issue with 1500 Kohms (equals 1.5Meg ohms), 1500 ohms is less but not a short.

Is the tach disconnected?

 

[Ken_S]

Tach was still connected. So I'm only seeing the resistance thru the tach. No smoking gun...

 

[achris]

Nope.