No spark

[sms986]

The boat I am working on is a 1989 mercruiser alpha one with a 5.7 and thunderbolt iv ignition. I am at WITS END with this thing. I have had issue after issue and every time I fix one another pops up. Today's issue:no spark. I did a compression test before I left and stupid me forgot to ground the plugs when doing the test. Well, I got back today, a few weeks later, and tried to start the boat. No spark. I replaced the coil with the one on my car (same coil, works for sure), replaced the cap and rotor, and tested the old coil. Old coil was fine but I left the other one on for now. I have no power to the coil. Should have 12 volts. Tried doing diagnostics based on the forums, because I am not an electrical guy whatsoever. The only evident issue I have is that there is a radio that is spliced into to one of the power wires from the coming from the fuse box (one of the access fuses) and grounding to the back of the tach. The wire going to the radio has a fuse on it. This fuse blows every time I turn the main battery switch on. None of the fuses under the dash were blown and this radio has always worked fine. Other than that, I don't know where to start looking. The wiring behind the dash is just a swamp. Does anyone have any methods for further testing?

 

[achris]

.... The only evident issue I have is that there is a radio that is spliced into to one of the power wires from the coming from the fuse box (one of the access fuses) and grounding to the back of the tach.

This is usually a very bad sign.. When things just get 'tacked on' it almost always ends in tears...

The wire going to the radio has a fuse on it. This fuse blows every time I turn the main battery switch on. None of the fuses under the dash were blown and this radio has always worked fine.

Everything always works fine, until it doesn't... If the fuse to the radio is blowing, it's no longer 'fine', it's junk, toss it...
And when you wire in the new one, start with the right philosophy... Run a heavy cable pair back from the battery switch to a red and a black terminal block and run everything that is not 'engine' from there. Sounder, lights, radios, GPS, bilge pumps, blowers, etc... The only thing that should be on the Mercruiser engine circuits is Mercruiser engine circuit. Everything else should be isolated form the engine,

Other than that, I don't know where to start looking. The wiring behind the dash is just a swamp. Does anyone have any methods for further testing?

So, start with the basics. Read my description of how the TB IV system works (in the stickies), check for 12 volts at coil + with the key on.... If you have no power to the coil, check you have 12 Volts on the purple wire coming out of the key switch... If you have power there, the problem lays between the key and the coil. Check the 10 pin engine harness connector... Check the 'kill switch' hasn't accidentally been flipped.

Chris......

 

[sms986]

Ok,so I will whole-heartedly admit to my mistake. A long time ago I accidentally crossed terminals on a battery. My buddy and I decided to paint a big red spot on the + side of every battery we had. Somehow this battery was switched... So, I had the battery crossed the entire time. Switched it over correctly, and it fired up. Hard to start at first, but I got it going and now it idles fine. Alternator, all lights, and accessories all work. My question now is, what damage could this have done, if any. I have no blown fuses or anything. I am mainly concerned with turning the engine over backward multiple times.

 

[achris]

The engine now runs, the alternator is charging and everything is good? Consider yourself VERY VERY VERY lucky...

Turning the engine backward may have flipped a blade over on the impeller, but I would be bothering too much about that, it'll flip back when you use the boat, and you'll be servicing it soon enough (part of your winterizing process I would hope).