Crank but no fire - Mercruiser 4.3L ignition trouble

Good evening all!


Am hoping someone might be able to point me in the direction of a fix before my boat becomes a permanent fixture at the bottom of some lake.


The Setup: 1992 Bayliner 2050 with a Mercruiser 4.3L Gen II Alpha One, Thunderbolt IV ignition, carbureted (2BBL). Motor serial number is D701474. Compression tests from late last year (when it was still running) were good although I can't find the exact numbers.


The Problem: Over the winter I replaced the wiring harness, ignition coil and ICM trying to address an intermittent surge/backfire/cut out situation that has plagued me since I picked this thing up a few years ago (plus the existing components were third hand when I got the thing and were pretty tired...). When I went to de-winterize and test fire a couple of weeks back the engine cranked fine but wouldn't fire.


What I have tried: I verified fuel getting to the carb so am thinking no spark. Went back through all my connections (just to make sure I didn't miss something easy) and found what I think could be problem straight off. With key in the Run position I am getting +/- 11.85V at both the positive and negative terminal of the ignition coil which seems odd. Following the power to the ICM i read 11.85V heading in on the purple lead but get almost nothing coming out of the red/white back to the ignition sensor. Following the troubleshooting guide it would seem to indicate a bad ICM (which would suck as this is a brand new unit!) but when I tested again with the original ICM (which was working before the harness swap) I get the same exact condition. I also tested with the original ignition coil (same results) and tested with the tach lead off (same results). SO either both ICM's are toasted in the exact same way OR (fingers crossed) I have a problem somewhere before the ignition coil that is causing it to ground itself. In addition I have ensured that the shift cutout switch is NOT engaged.


Am wondering if anyone else has run into this issue and could possibly point me towards whatever solution they found. Haven't been able to find anything in any forums re: the positive voltage at the negative terminal and am really hoping that has something to do with it. Am really hoping to avoid putting this thing back in the shop and paying my mechanic to go fishing for the issue, so any guidance is appreciated!

Could also be the pickup sensor in the distributor.

Search this forum as there have been many posting of the factory troubleshooting procedure...following it will lead you to the item needing attention...the only other easy thing is to check the ignition kill switch on the control box, if you have one..

The coil has a very very low resistance so reading 12 volts on both + AND - is normal.... dont read into it...



You must follow the test proceedure EXACTLY. the reason you are not sure is becaus of this.

Your test results do not reflect the test procedure.

try this and report back.


Tests for Thunderbolt Ignition:


W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!

#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts is present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced.

No voltage at the white red wire. ( The wire comes out of the distributor and to a small box on the back side of the distributor.) Test taken at the connectors that appear to allow you to remove the distributor without having to disconnect wires from the small black box.
No spark from coil wire.

Turn on Ignition switch, put a spark plug on the HTcoil lead and ground it. Get a jumper wire and hook it to the negative of the coil, watch the spark plug while tapping the other side of the jumper on a good ground. If you have ignition spark, the coil and amplifier are fine. If not replace the coil and test again.

If you have ignition spark when you first test chances are the Dist pick up is bad, I change these when I do a Tune up, Especially if it is still a two wire pick up.