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fuse blows in wiring harness

T

tross

New member
long time OMC guy but the wife got the bug for a new(er) boat, so went over to the dark side. Bought a very clean dealer serviced 2007 Bayliner with a 4.3 (GM) Mercruser/Alpha1 (2bbl carb) with the vaunted TKS. So last weekend the kids are water skiing, tubing and having a great time. Boat is working fine, suddenly dies. Tow it back to our dock and spend the next 5-6 hours tracing with a multimeter. Finally find a 15amp mini fuse buried in the harness behind the distributor. Ok, change the fuse start the engine, boat starts, all gauges normal, runs good for 2-3 minutes, fuse blows. The red w/ purple stripe wire from the fuse runs into the harness and ends up at the ignition switch. I'm guessing that there is a device ( sensor, switch, relay, etc.) somewhere between the fuse and ignition switch that has crapped out, but I can't find a wiring diagram that shows where the red/purple goes after leaving the fuse, and I'm reluctant to tear apart the wire looms and the wrap to find where the wire goes. (not to mention trying to cram my fat a** into the space behind the engine)
So anyone have any thoughts???
 
Red Purple is switched power to engine harness. So, there's something that needs juice on the other side of that fuse. Not a sensor.
 
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Look for a chafed wire in the harness that's shorting out on you.

A good trick, with the motor running, is to wiggle the harness and see if you can get the motor to miss.


Jeff
 
Thanks for the input guys, but the fuse doesn't blow immediately but after the motor runs for 2-3 minutes. It doesn't blow with the key switch on, but only after the motor runs, so I suspect something is changing state as the motor warms up and switches power. Yes the fuse is in the factory harness,. I hate to take it to the dealer because they'll probably put junior mech on it and he will do what I'm trying to avoid by tearing the harness apart looking for a fault. The carb has the TKS system on it, meaning Merc eliminated the electric choke and put a plunger style solenoid on the carb to add gas when starting, and added some additional temp sensors that are NO/NC depending on engine temp and a diode to keep things in line etc., etc. This was probably done to keep the EPA happy. (you know, the people that mandated the "new" spill-a-matic gas cans). Without a good engine wiring diagram and a clear sequence of who does what when, it's just sound shots into the bushes. I do have a Clymer MerCruiser stern drive shop manual but it barely mentions the TKS and the Mercury site isn't any better.
 
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So you gotta ask, what would kill the engine if it loses keyed power? Yup, ignition. So look for fault there first.
 
o2batsea said:
So you gotta ask, what would kill the engine if it loses keyed power? Yup, ignition. So look for fault there first.
Click to expand...


you could always put external power to the circuit and look for smoke:D


disclaimer, not serious about this, highly unsafe to do but its been done
 
Problem solved, the temperature switch, part of the TKS system failed. When it did the 20 amp fuse as well as the 15 amp fuse in the wiring harness blew as well as the Diode for the TKS module. Which is nothing more than a blob of bee’s wax surrounded by a 12 volt heating coil pushing out a plunger to close off the carb enrichment circuit. Replaced fuses switch and diode, boat runs great.
 
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