Thoughts on blown fuse

[timfives]

Gents,

I'm working to get my 3.0 mercruiser running, It had been running and now nothing happens when i when the key is turned. Well almost nothing.

There is a 20amp fuse in the dash that continues to blow every time they key is turned. Curious if anyone has a simple answer.

We've done a lot of trouble shooting here. There doesn't seem to be any shorts in the system.

there is a 3.5 amp draw on the battery to the alternator,

The fuse still blows even if the purple wire from the alternator is unplugged

I'm in the process of testing the Alternator, Slave Starter Solenoid and the main starting solenoid, though i think the issue is on the alternator side of the power flow.


Hopefully this is the last issue to getting it in the water


Any thoughts and or suggestions are welcome.

Tim

 

[Howard Sterndrive]

Re: Thoughts on blown fuse

have a look at the blown fuse. Is the glass all black - looks like a major flare out of the ribbon?

that's something shorting to ground or to very low resistance quickly- the rapid current flow makes that blackened smoked look.
look for a wire touching ground somewhere

if it is not black.... just looks like the fuse very gently melted... then you are looking for a continuous overload - some device drawing more than designed current.

 

[timfives]

Re: Thoughts on blown fuse

Howard,

Thanks for the tip, the fuses aren't black just burnt through, you may be helping to paint the picture that there is something with a significant draw.

Thanks

T

have a look at the blown fuse. Is the glass all black - looks like a major flare out of the ribbon?

that's something shorting to ground or to very low resistance quickly- the rapid current flow makes that blackened smoked look.
look for a wire touching ground somewhere

if it is not black.... just looks like the fuse very gently melted... then you are looking for a continuous overload - some device drawing more than designed current.

 

[stonyloam]

Re: Thoughts on blown fuse

When you turn to "on" or when you hit "start"? If on: I would suspect that there is a short to ground somewhere in the purple wire from the ignition switch. Measure with an ohmmeter from the purple wire at the back of the ignition switch to ground. If it reads close to 0, start looking for a short somewhere there. If "start" a short in the slave solenoid, control handle neutral start switch wiring would be the place to look. Good luck.

 

[timfives]

Re: Thoughts on blown fuse

Thanks,

Went to check the starting (slave) solenoid, odd thing, when i did the test i connected 12v to it, and could hear the audible click of the solenoid switching, but when i hit the other poles with the volt meter i got a something like 4 millivolts, these would be the lugs sending power to the starting solenoid. I was expecting to see 12 volts with the volt meter.

My assumption is that this slave solenoid is bad??

T

When you turn to "on" or when you hit "start"? If on: I would suspect that there is a short to ground somewhere in the purple wire from the ignition switch. Measure with an ohmmeter from the purple wire at the back of the ignition switch to ground. If it reads close to 0, start looking for a short somewhere there. If "start" a short in the slave solenoid, control handle neutral start switch wiring would be the place to look. Good luck.

 

[stonyloam]

Re: Thoughts on blown fuse

Dug out my trusty 3.0 wiring diagram: Pretty sure you have a standard type solenoid, so you have two small terminals on the front, two large on top, by wire color: small black, solenoid ground, 0V, small y/r 0V, or 12V from key in start position, large r/p 12V from battery, large y/r 0V, or 12V to starter when key in start. If you jumper from the large r/p to the small y/r the solenoid should activate and the starter should turn. If you jumper across the large terminals you bypass the solenoid and the starter should turn. Good luck.

 

[timfives]

Re: Thoughts on blown fuse

Stony,

Yep, you are correct, so i can get the engine to crank, so i think the solenoids are okay. The interesting thing is that it won't run, because I don' think the ignition coil is getting power (purple wire)

I also tested the shift cut out switch in the throttle assembly, and that seems to be okay, i have continuity when it is in neutral and no continuity when it is in gear, so the only other thing to test is the lanyard switch.

Damn this is frustrating!

Thanks for the help

T

Dug out my trusty 3.0 wiring diagram: Pretty sure you have a standard type solenoid, so you have two small terminals on the front, two large on top, by wire color: small black, solenoid ground, 0V, small y/r 0V, or 12V from key in start position, large r/p 12V from battery, large y/r 0V, or 12V to starter when key in start. If you jumper from the large r/p to the small y/r the solenoid should activate and the starter should turn. If you jumper across the large terminals you bypass the solenoid and the starter should turn. Good luck.

 

[Silvertip]

Re: Thoughts on blown fuse

Grin!!!! He isn't listening (or reading).

 

[timfives]

Re: Thoughts on blown fuse

Care to elaborate?

Grin!!!! He isn't listening (or reading).

 

[stonyloam]

Re: Thoughts on blown fuse

If you have a conventional coil (not est), during starting the coil gets its power from a p/y wire coming from the starter solenoid. This applies battery voltage directly to the coil and bypasses the purple resistance wire. Check that wire for 12V (or so) when the engine is cranking.

 

[timfives]

Re: Thoughts on blown fuse

Okay,

check out page 4 of this link, that is the wiring diagram that i'm using. EST ignition.
http://www.boatfix.com/merc/Servmanl/13/13E4R2.PDF

Thanks Man!

T

If you have a conventional coil (not est), during starting the coil gets its power from a p/y wire coming from the starter solenoid. This applies battery voltage directly to the coil and bypasses the purple resistance wire. Check that wire for 12V (or so) when the engine is cranking.

 

[sethjon]

Re: Thoughts on blown fuse

Replace the switch. The short is probably internal.

 

[Titanty]

Re: Thoughts on blown fuse

Stony,


I also tested the shift cut out switch in the throttle assembly, and that seems to be okay, i have continuity when it is in neutral and no continuity when it is in gear, so the only other thing to test is the lanyard switch.

\

T

How did you test it? We had a similar situation...completely disconnected the shift unterupt switch (mounted on the motor) and BAM started right up. Drove back to the ramp and took it home for a real fix.

 

[timfives]

Re: Thoughts on blown fuse

Thanks,

First thanks to all for the suggestions, Here is what i did to isolate then correct the problem.

First to isolate where the issue was. I disconnected the harness from the engine and put power to the proper lugs, and still got the blown fuse. Which made me realize that it had to be between the dash and the motor.

I then decided to work front to back, starting with the switch. I disconnect the purple and yellow / red wires to see if it still blew. Since it didn't i figured it had to be in either of those wire I disconnected. It was in the yellow red. Then i was getting someplace. I then disconnected the yellow / red from the slave started solenoid, and it blew again. Now i had it narrowed down between the dash and the engine, and now isolated the wire.

The only thing left in that leg was the shift interrupt switch (in the throttle control). Sure enough once i got it apart, i found that when the marina put it in last year they never bothered to wrap the connectors with e-tape and it went to ground on the aluminum panel.

I isolated those connectors, and boom the engine ran as soon as i bumped the key.

While frustrating a little methodical searching got it narrowed down. The other item that i did that was misleading was a continuity test on that circuit, it checked out, which is what you would expect as all the wires were still connected..

The silver lining is that i have a good idea how this boat is wired!

Thanks to everyone for your help

T

How did you test it? We had a similar situation...completely disconnected the shift unterupt switch (mounted on the motor) and BAM started right up. Drove back to the ramp and took it home for a real fix.

View attachment 61760

 

[jpx8080]

Re: Thoughts on blown fuse

I'm having a similar problem. When I went from "clutched in" to neutral my fuse blew 3 times on the water the other day.

 

[wire2]

Re: Thoughts on blown fuse

Here's a tip to save repeated trips to the auto parts store for more fuses when searching for a short;

Connect a 12v headlight lamp in place of the fuse. The lamp will glow brightly on a dead short, dim on a lesser load. It will not harm the wiring, the max draw will be 5 amps (60 watts).

To make it work, cut 2 pieces (length is not critical) of 14 or 16 ga insulated wire, crimp ?" female spade terminals on 1 end to fit the lamp, crimp ?" male spade terminals on the other end. Trim the male ends to mimic the size/shape of a fuse prong, then plug the ends into the fuse block in place of the fuse.

Now you can undo connections in the harness, etc, and observe the glow of the lamp to pinpoint the fault.

It works in a house also, but then use a 115v lamp, of course.