Mercruiser Thunderbolt IV Troubleshooting

S

seachunk2

Member
Joined
Apr 16, 2006
RO Number
20886
Messages
131
I'm troubleshooting the ignition on a 4.3L Mercruiser with 4Bl Weber, Thunderbolt IV ignition and an electric fuel pump. I referenced this link for the procedure.
http://www.ebasicpower.com/faq/MERCSB92-11.htm
The procedure leads me to replacing the sensor.
I'm OK with that but I have some hesitation because prior to referencing the link above, I connected an inductive timing light to the ignition coil wire and got a nice bight steady flicker of light when I cranked the engine. I then checked several individual ignition plug wires and also got a nice flicker though it was less frequent. So from this observation I would think the ignition is working.
So my question boils down to....could there still be spark running through the ignition wires if the sensor in the distributor is bad? If yes, then I'll be more confident about replacing the sensor. If no, then I'll have to dig further.

Some back ground info:
I took the boat out yesterday for the 1st time this season. it started fine but would bog down a bit as I tried to plane off. I'd ease up on the throttle and slowly reapply throttle and it planed off and cruised OK. I came off throttle after a 10 minute cruise and tried to plane off again and things got worst. Eventually, I couldn't start the engine. It would only crank with no indication whatsoever of any sort or firing up. Not even a cough. I shot a squirt of starting fluid and still nothing. Fuel system seems OK. Boat is a 1995 with a 4bl Weber, electric fuel pump.
 
I have had both sensors, module and coil failures aggravated by heat. your history of the symptoms sound like mine.

Best thing I ever did was replace it all with a DUI (Davis Unified Ignition) system. Got mine from Summit.
 
Have you tried using a known good spark plug in the ignition coil wire and or one of the normal plug wires yet? If so, did you have spark or not?......D.........
 
"I connected an inductive timing light to the ignition coil wire and got a nice bight steady flicker of light when I cranked the engine. I then checked several individual ignition plug wires and also got a nice flicker though it was less frequent."

I have no vaguely-recent experience with Merc ignitions, but keep in mind all the tests in your link assume there really is NO spark, but your timing light seems to indicate there ....might...be.

Besides the coil and module, the rotor and dist. cap , wires and spark plugs all have to be in good shape to allow that coil secondary circuit to distributor pulse to translate to a good reliable path to engine ground.

If your inductive timing light still shows a pulse, you might try another shot of starting fluid before cranking with throttle wide open in N , with NO pumping in case the plugs are very wet and fuel is bad. Even if fuel is not bad the float valve could possibly be stuck open a bit.
And of course Dave's suggestion can demonstrate spark or no.

Does the fuel pump run? Does the carb throat smell strong gassy, or stale?

Does this system have a shift interrupter switch? I'm probably wrong, but I think I have heard if that goes bad it may ground the ignition just enough to prevent strong spark but some pulse might be stil be evident on a timing light. Not sure if that makes sense or not.
 
A shift interrupt switch does just that, interrupt the primary side of the ignition...so no spark.

If you have spark and squirt fuel down the carb and the engine doesn't start, the engine must be broken. The only thing that would allow this to happen is if the timing is so far off, it's firing at the wrong time
 
Thanks for the replies. I'm going to take a closer look at things later today after work. I want to go through the troubleshooting procedure once again. With regards to the step where I test the white/red wire at the distributor terminal, am I supposed to disconnect the snap on wire connector and probe the male pin inside the distributor module? I noticed the white/red wire (and white green wire) has a bullet connector about 6 inches from the snap on connector that attaches to the module. Can I test for the voltage at by separating the bullet connector and probing there since that is a lot easier than trying to probe the male white/red pin within the distributor module due to accessibility reasons. Hope I explained myself well.
 
You must log in or register to reply here.
Back
Top