Question about 90 amp fuse on starter

DanD34

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I have an old 1987 3.0 alpha one with breaker point ignition. There is no 90 amp fuse on the starter. The parts book shows the fuse, however the wiring diagram from Mercruiser shows nothing of it at all. I understand it will protect the orange wire between the starter and alternator and the short end of red wire that goes to the circuit breaker in the event of a short circuit. So my question is, should I add it for extra safety or forget about it since the wiring diagram doesn't show it? I've also read that at different times both the red and orange wires were connected to the fuse and at other times they came with only the orange connected. What's best practice here. I'll attach the pertinent part of the wiring diagram for reference.
 

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alldodge

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Adding for safety is a good thing, but if there is never a short, then it wasn't needed. Something like, if you have a fire extinguisher in your house and never need it then it might have been a waste of money :unsure:
 

DanD34

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Adding for safety is a good thing, but if there is never a short, then it wasn't needed. Something like, if you have a fire extinguisher in your house and never need it then it might have been a waste of money :unsure:
Well using that line of thought then car insurance, medical insurance and a breaker box in my house might all be a waste of money too.
 

Bt Doctur

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instead of paying and excessive amount for that 90A you can also use the Maxi-Fuse type for the alt protection the 50A one covers everything else
 

DanD34

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instead of paying and excessive amount for that 90A you can also use the Maxi-Fuse type for the alt protection the 50A one covers everything else
I guess I was wondering too if it is truly the case that they used to not have it at all and if it was added in later production if there really was an issue or more to keep the legal department or coast guard happy.
 

Bondo

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I guess I was wondering too if it is truly the case that they used to not have it at all and if it was added in later production if there really was an issue or more to keep the legal department or coast guard happy.
Ayuh,...... Two reasons,..... The legal dept, 'n to protect sensitive electronics when Efi / Mpi came out,.....

You don't need one,.....
 

Grub54891

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As Bondo said. But my 1983 and my 84, and 85 3.0has them on there. Never blew one yet.
 

nola mike

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The older models didn't have them. I find it odd that it's there when there's no protection from the battery to the starter. I think it's there mostly to protect the alternator.
 

alldodge

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I go the other way, used to protect the wiring. If ALT shorts the ALT (orange) wire is inside with other harness wires and will melt the harness.

Could put a breaker or fuse between Bat and Starter but would need to be somewhere between 200 and 400 amps
 

nola mike

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I go the other way, used to protect the wiring. If ALT shorts the ALT (orange) wire is inside with other harness wires and will melt the harness.

Could put a breaker or fuse between Bat and Starter but would need to be somewhere between 200 and 400 amps
Don't have the diagram in front of me, but harness should be protected by the 50a fuse ...
Or do you mean if the orange physically melts and takes out other wires?
 

alldodge

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Melt and takes out the others
Even if it wasn't in the harness the amount of current which could be drawn could also blow the battery, but that's a maybe
 

dubs283

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The 90/100 amp fusible link/fuse is in place on newer oem mercruiser engines to protect the internal regulator/rectifier in the alternator in case of accidental reversed polarity when connecting the battery

This came about when many boat owners installing batteries reversed polarity and blew the regulator/rectifier in the alternator thus causing a need for a new/rebuilt alternator and possibly a new battery due to melting one of the posts

Mercnet allows owner input/questions and comments regarding their products and came up with this simple yet somewhat expensive fail-safe. Cheaper than a new alternator and battery but the cause of much baffled troubleshooting for some
 
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