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5th gen sub fuse failure – why?


vfr800_red

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Hi All,

 

My 5th gen recently died when I was slowly pulling away from a stop in first gear, slowly letting out the clutch. The starter motor turned over fine, but it would not restart. I soon realized the fuel pumps were not energizing when they key was turned to ignition. The fuel pump fuse was fine. I followed the procedure in 5-58 of the factory service manual, discovered the fuel pump was not getting battery voltage, and traced it to a blown sub fuse. I replaced the sub fuse and everything is fixed, the bike starts and runs great and has had numerous restarts. However, I know that blown fuses are often a symptom of a more serious electrical problem. Any thoughts why the sub fuse would suddenly blow? I’ve never had this problem before. I noticed I didn’t have headlights when the sub fuse was blown, that is, it “shares” a circuit with the fuel pump. Thanks.

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As I've recently had a meltdown of main fuse B all things elektrikery seem to interest me again. Sorry i cant help but there are some great people on here who know their stuff for that kind of thing. 

Did you check yours yet?

20220722_141356.jpg

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8 hours ago, vfr800_red said:

Hi All,

 

My 5th gen recently died when I was slowly pulling away from a stop in first gear, slowly letting out the clutch. The starter motor turned over fine, but it would not restart. I soon realized the fuel pumps were not energizing when they key was turned to ignition. The fuel pump fuse was fine. I followed the procedure in 5-58 of the factory service manual, discovered the fuel pump was not getting battery voltage, and traced it to a blown sub fuse. I replaced the sub fuse and everything is fixed, the bike starts and runs great and has had numerous restarts. However, I know that blown fuses are often a symptom of a more serious electrical problem. Any thoughts why the sub fuse would suddenly blow? I’ve never had this problem before. I noticed I didn’t have headlights when the sub fuse was blown, that is, it “shares” a circuit with the fuel pump. Thanks.

Hi vfr800_red.

I think you have confused the Sub Fuses with the two Main Fuses.

The Sub Fuses are all the smaller ones located in the Fuse Box. To add a little more confusion, Sub Fuse B in the fuse box is a dedicated 20amp EFI fuse that is fed by Main Fuse B.!

 

The two Main Fuses 30amp A and B are located in the Starter Relay and next to the Starter Relay.

 

On a fifth gen (according to the drawing I have) Main Fuse B 30amp provides power to All your EFI stuff via the Sub Fuse B 20amp AND, unlike the 6gen, it also provides your Headlight power via Sub Fuse F.

So blowing Main Fuse B explains why your bike died instantly And you had no headlights. You were able to crank the engine because the Starter Relay power comes off the Main Fuse A circuit.

 

Your bike is now 24 years old, fuses can become a little more prone to blowing with age. However, Main Fuse B is a known problem child largely because of its poor quality fuse holder, the current draw it deals with, moisture/dirt ingress, and a poor quality inline joiner for the main Red/White wire. Ultimately high resistance develops producing heat further exacerbating the problem. The fuse, fuse holder or wiring will fail eventually in this condition, Not necessarily caused by an electrical short down the line.

So the bottom line is to have a very close inspection of Main Fuse B 30amp and it local wiring, fuse holder, and the inline connector. Check for any signs of discoloration or heat stress.

 

Also adding any electrical  accessories (depending on current draw) should not be wired into these Main Fuses, especially Main Fuse B.

 

Hope this answers your questions?

 

Cheers.:fing02:

 

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Hi Grum,

 

It is the same fuse in Bren’s picture. However, my fuse was 20A, and it looks like Bren’s was also 20A. If that is main fuse B, according to the wiring diagram, it should be 30A. Should my replacement fuse be 30A?

 

Thankfully, there is no corrosion at the connector and all other fuse connector are clean. But there is some gunk at the stater relay connectors so I will clean those up. And I’ll take a closer look at other nearby wiring.

 

I’d never replaced that fuse, and I’ve had the bike for 19 years. It was probably original, and I like the explanation that fuses don’t last forever. For that reason, I will be replacing all the fuses.

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2 hours ago, vfr800_red said:

Hi Grum,

 

It is the same fuse in Bren’s picture. However, my fuse was 20A, and it looks like Bren’s was also 20A. If that is main fuse B, according to the wiring diagram, it should be 30A. Should my replacement fuse be 30A?

 

Thankfully, there is no corrosion at the connector and all other fuse connector are clean. But there is some gunk at the stater relay connectors so I will clean those up. And I’ll take a closer look at other nearby wiring.

 

I’d never replaced that fuse, and I’ve had the bike for 19 years. It was probably original, and I like the explanation that fuses don’t last forever. For that reason, I will be replacing all the fuses.

 

Yes definitely fit a 30amp fuse for both Main Fuses.

 

This will still provide good wiring protection and will also give better fuse reliability. The 6gen is definitely 30amp for both of the main fuses, no reason why the 5th gen shouldn't be the same.

 

A good thing to do with both Main Fuses would be to apply a good coating of Ox-Gard to the fuse legs.

OX-100B_PKG_01.jpg

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My 20a fuse was from a long ago rewire attempt to keep the original rectifier going before the fh020 came along. Should have changed it out long ago but life and everything else......I forgot.

 

Fuse B should be a 30a and I too will be putting in new fuses everywhere as some are definitely original ie. 24 years old.

 

What a great bike. I have to do a write up in winter of what I've done the last years. May entertain, may not. YMMV.

 

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