Mercruiser 90A Fuse Blown

B

Brian R

New Member
May 28, 2019
5
Boat Info
2005 Sea Ray 300 Sundancer
Engines
5.0L Mercruiser w/Bravo II Outdrives
I was replacing the alternator on my port engine, forgot to disconnect the batteries and shorted out the 90 amp fuse at the starter. It took a while to figure out the problem, as I didn't realize it happened, but I had no electrical power to the the engine. Looking at the wiring diagrams, it seemed like that was the only culprit, and we were able to confirm that by checking continuity between the battery and fuse (continuity), and the battery and the hot wire for the alternator (no continuity). The alternator is on the other side of the fuse. I wasn't able to reach the fuse, other than to touch a lead to battery side, so almost gave up and called a mechanic. According to the wiring diagram there are two things coming off that fuse: The orange 8 AWG wire to alternator, and a red wire that goes to a circuit breaker that I believe goes to the control systems and ignition. To solve the problem, we installed a 90 amp fuse at the bus for the port bank, and ran an 8 AWG wire and tied into the orange wire at the alternator. Electrically, I believe this is basically the same. The only difference I can see is that the power to the system before the engine is running is going through the orange wire from the alternator. Is there an issue with this approach? Why is this fuse 90 amps, when the alternator is limited to much less?
 
The fuse is there for the exact reason you experienced.
Remember fuses and breakers are not there to protect the device, but to protect the wiring from over heating and catching fire. Had the fuse not been there you would have smoked that alternator feed wire and possible started it burning. This is why Merc added that fuse directly at the starter battery terminal, that is where the wires change to smaller size(then the battery cables)
How you have re-wired should be ok, and easier to service in the long term.
 
Thanks for the response. That's what I was thinking, but it seemed like a smaller fuse would have been warranted to prevent a long duration of a lower than 90 amp current from burning up the wire. I suppose that a short blows the fuse quickly enough that the wire doesn't have time to heat up. It really should have been designed so that fuse is in a place that can be reached. I spoke to a mechanic that said they can do it, but I don't think I could do it without removing at least the fuel cooler and water pump. I dropped the water pump to touch a lead to it and realizing that I still couldn't reach both sides before getting frustrated and deciding that we could bypass it.
 
I located the 90 to the back of the alternator and ran the heavy red from starter to 50 amp breaker. This has been discussed before and I know Pat doesn't like this but with single engine I won't get stranded.
 
It sounds like you're approach is very similar to mine, perhaps a little better, because I have an additional six feet of 8 AWG wire between the fuse and the alternator, and you used "heavy red". But I believe that's within the spec of 8 AWG wire. I think it's actually better this way because the current path to the engine systems and charging the battery from the alternator are now separated. That was why I felt going with the same gauge wire as what existing on the alternator was acceptable.
 
As long as it still gets isolated from the batteries with the main battery disconnect switch in the off position, should not be an issue, IMO.
 
hughespat57 said:
The fuse is there for the exact reason you experienced.
Remember fuses and breakers are not there to protect the device, but to protect the wiring from over heating and catching fire. Had the fuse not been there you would have smoked that alternator feed wire and possible started it burning. This is why Merc added that fuse directly at the starter battery terminal, that is where the wires change to smaller size(then the battery cables)
How you have re-wired should be ok, and easier to service in the long term.
Click to expand...
Hi There. I was reading this post and it's kind of related to a similar question I have about this mystery orange wire. Our port engine is out of the boat and we're doing some maintenance before we put it back. When we were disconnecting everything in prep for pulling the engine, we noted that the orange wire from the alternator does not go directly to the starter as per wiring diagram. Rather it is spliced into a red and blue (or purple) wire that then goes to a harness. Do you guys have any experience with this? I'm assuming that at some point we end up back at the starter, I don't want to assume. Also I haven't seen this 90amp fuse anywhere, unless you are referring to the circuit breaker. Haven't had a problem so far, but very curious. My only guess is maybe it has to do with the fact that we have two engines and there is cross-pollination within the starting system?
 
Bill Goetz said:
Hi There. I was reading this post and it's kind of related to a similar question I have about this mystery orange wire. Our port engine is out of the boat and we're doing some maintenance before we put it back. When we were disconnecting everything in prep for pulling the engine, we noted that the orange wire from the alternator does not go directly to the starter as per wiring diagram. Rather it is spliced into a red and blue (or purple) wire that then goes to a harness. Do you guys have any experience with this? I'm assuming that at some point we end up back at the starter, I don't want to assume. Also I haven't seen this 90amp fuse anywhere, unless you are referring to the circuit breaker. Haven't had a problem so far, but very curious. My only guess is maybe it has to do with the fact that we have two engines and there is cross-pollination within the starting system?
Click to expand...
scoflaw said:
Somebody hacked it, orange wire comes off the 90 amp fuse.
Click to expand...
I would first thoroughly inspect what the other engine has.
 
Same on the port side engine. Mercruiser wiring diagram shows orange going directly to the hot wire post on the starter. They also show a circuit breaker wired in to the same post. Geez!
 
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