Ignition Problem.. - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 07-03-2008
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Ignition Problem..

I am having problems with the boat. Each time I take it to a mechanic, it seems to come back with a new problem so I'd like to see if I can fix this myself.

The boat is a 1996 21' Wellcraft with a Mercruiser 4.3 LX AlphaOne. Thus far this week we had the starter replaced, and we replaced the blower and the horn. We tried to take the boat out again after this, and it started fine, worked perfectly for aprox. 45 mins after which the 20 amp ignition fuse blew. We replaced the fuse several times, and each time the fuse would blow again right away (within two minutes).

So we brought the boat back in, and had a look at it and here's what we've found:

First, we started the boat succesfully twice, but each time it died shortly thereafter. After dying the second time the boat will no longer start; it still rolls over, it just won't catch.

Second, there seems to be a short in the purple wire going to the ignition. This wire has a 12V load on it when the key is in "accessory on" position, and a variable load of 10-11 volts while attempting to start it. It heats up VERY quickly, and gets smoking hot. The circuit is as follows: It comes from the engine, daisy chains to several gauges (the tach among others), then goes to the kill switch, then to the ignition. We tried removing the kill-switch and all the gauges from the circuit, but the wire would still heat up while trying to start it.

Any assistance or insight anyone can provide would be greatly appreciated.

Thanks,

Kyoufu

Last edited by kyoufu; 07-03-2008 at 02:10 AM.
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Old 07-03-2008
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ky...

Disable the wire that is heating up. Trace its exact path. Disconnect it at both ends, and tape the ends up. Now put a multimeter across the two live terminals and I am fairly certain you will have a voltage difference between the terminals and your wire is shorting it and heating up.

It is difficult to know from here. But any wire without a load in it should not have a voltage between the terminals of it.

Somewhere you are running a live to a ground without a load in between.
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Here's a guess (without looking at it)...

If the purple wire goes to the alternator it's probably the field wire. The alternator could be bad, pulling a ton of current through the field wire draining your batteries and also not charging anything. If that's where the wire goes you most likely need a new alternator. You could pull it off and have it tested if you're not sure.
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Old 07-03-2008
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One more thing. Dirty connectors cause resistance. Resistance cuts down on the voltage to the load that is being run. The load demands more amps to make up the difference which can also cause fuses to blow and wires to heat up.
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I would guess the last person to work on it did something. Maybe when the put in the new horn they accidently pulled a wire off and wasn't sure where it went or pinched a wire while hooking up the new stuff.

It is possible that it is the alternator too. Make sure to have it checked. Just take it off and carry it to Auto Zone or Napa, some place like that.

Does the wire get hot while the ignition is on or just when trying to crank?
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You are looking at the wrong end of the wire when looking at the dash/boat end. Look at the engine side and trace the wire. As I remember there is a "black box" in the ignition that fails on those motors (4.3/5.7) and will draw too much current and will also take out the coil. This is a common failure point with the original unit. Also look for micro switches in the throttle linkage in that circuit as on some motors the manufacturer added momentary "kill switches" to allow for softer gear engaugement.

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When the key is in "Accessory on" position, the wire has 12 volts on it, but it does not heat up. Only when the engine is trying to crank. Since I can't start the boat I don't know if it heats up when the boat is running, but since it was blowing fuses it's likely.

Also, the reason I've been looking at the ignition end is that is where the heat seems to be most extreme.
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If it only heats up while the engine is turning that points back to the alternator. If the alternator has a short at some point(s) of rotation it will draw major current while turning, and may not while stopped.
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Hrmm, well I tried splicing in a wire as near to the engine as I could and ran it directly to the ignition, and it still heats up (and still doesnt start). So there is no short on the wire itself, and its not connectors. I would assume this means its nothing to do with the throttle either.

I noticed one strange thing when I had the voltage meter on it; as I said before it drops from 12 volts to 9-10 volts while its cranking, but then when i stopped it jumped back up to 11.9 or so. Then it climbed back 11.91, 11.92, 11.93... back to 12.04 and its during this time the wire seemed to heat up. Seemed like an odd behaviour, hoped it was somehow indicative.

Oh and ignore where I said the heat was emanating from in my previous post. Incorrect info.
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I tried disconnecting the alternator, I found the purple wire going to the alternator and disconnected that. Doing this had no effect. So I then disconnected the red wire going to the alternator (These were labelled purple ALT B, and red alt A or some such). Anyway, even with both these wires disconnected the short was still there, the wire was still getting very hot very quickly right after I try starting it.
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