50 amp fuse tripping at start-up

[JDClark]

Hello everyone! Ok my 50 amp engine breaker keeps tripping while trying to start engine. The engine does turn over slightly and then the breaker trips every time. I have inspected and replaced all the main battery cables. I did a full tune up on engine plugs, plug wires, cap, and rotor button. Only thing I haven't checked real close is the wires in the main harness because they are covered and wraped up. What could be some of the things causing the breaker to trip? It's a mercruiser 454 with thunderbolt ignition.

 

[alldodge]

Hello everyone! Ok my 50 amp engine breaker keeps tripping while trying to start engine. The engine does turn over slightly and then the breaker trips every time. I have inspected and replaced all the main battery cables. I did a full tune up on engine plugs, plug wires, cap, and rotor button. Only thing I haven't checked real close is the wires in the main harness because they are covered and wraped up. What could be some of the things causing the breaker to trip? It's a mercruiser 454 with thunderbolt ignition.

If and I mean "IF" your wiring has not been changed and the breaker is tripping, my guess would be your 50 amp breaker is bad.

Start by disconnecting all wire off of the alternator, make sure all three are disconnected and taped so they don't ground out.

Take a short (maybe 3 to 4 inches) piece of 12AWG bare wire and put terminal lugs on them, then connect them across both terminals of the breaker. Have someone watch the jumper wire when you go to crank to see if it starts to get hot. If it does start to get hot, turn key off and find out what is shorted.

 

[JDClark]

Ok will do AllDodge thanks for the info. If there is a short should I check all the wires in the engine harness first for the short? All my grounds check out good.

 

[JDClark]

What wiring are you referring to when you say if "your wiring has not been changed"?
Thanks again

 

[alldodge]

What wiring are you referring to when you say if "your wiring has not been changed"?
Thanks again

People do some really weird stuff, so if you have owned the boat since new then no problem. But if you just got to boat then a prior owner may have changed something, that's all. It could also very well be your starter solenoid is shorted.

If there is a short then something else will be getting hot, just need to start tracing. The pic below is for another engine but the wiring is the same. Item 7 is the 50 amp breaker. Could get a better diagram if I knew what year your boat is

 

[JDClark]

Ok I understand thanks. Not really sure on year of motor. The boat is a 92 Monterey and title says engine is 97 but the metal tag on engine is gone. I bought the boat last year and had it out 4-5 times with very little problems but did have a few no-start or hard to start situations and just trying to go through everything before getting back on the water. I know for sure it's not original engine. Thanks for the diagram.

 

[gm280]

The still could be a really good case for a bad starter that has some shorted windings causing excessive current flow... JMHO!

 

[JDClark]

Ok gm280 I will keep that in mind. Last year a few of the no-start situations I got just a click and then after a few attempts it would fire up but the breaker never tripped last season.

 

[Bt Doctur]

Couple of things to try.
1 pull the engine harness plug and run a jumper from the battery to the + side of the coil and jump the small lug with the yellow/red wire at the solenoid to the batt cable lug. motor should start and run
2 remove jumper ,motor stops, locate slave relay touch jumper to the small yellow/red wire. solenoid should trip and spin the motor and 50A should not trip. Y,N

 

[JDClark]

Ok will do btdoctur thanks for your input. Sadly it's pouring rain here and won't be able to try anything today.

 

[JDClark]

Well AllDodge I did what you said and the wire was getting warm so I inspected the wiring to the starter and solenoid and the yellow and red wire to the regulator was rubbing the orange wire from the solenoid. So I fixed that and the fuse isn't tripping anymore. But now when I turn key I can hear the starter turn but it seems like it's not engaging and not turning the engine. You think the short could have messed up the starter and solenoid? It was engaging before and the engine was turning over slightly before the breaker would trip. New starter and solenoid I guess?

 

[alldodge]

Well AllDodge I did what you said and the wire was getting warm so I inspected the wiring to the starter and solenoid and the yellow and red wire to the regulator was rubbing the orange wire from the solenoid. So I fixed that and the fuse isn't tripping anymore. But now when I turn key I can hear the starter turn but it seems like it's not engaging and not turning the engine. You think the short could have messed up the starter and solenoid? It was engaging before and the engine was turning over slightly before the breaker would trip. New starter and solenoid I guess?

I would recommend pulling the starter and have it tested (Autozone, O'Reilly's). If you have a starter/generator repair shop locally I would take it there. Many can be fixed for half the cost of a rebuilt starter.

 

[JDClark]

How difficult is it to remove? Is it just two hex bolts mounted to the the sides that hold it in? And it looks like the solenoid is attached to the starter right?

 

[alldodge]

How difficult is it to remove? Is it just two hex bolts mounted to the the sides that hold it in? And it looks like the solenoid is attached to the starter right?

Your probably ringht but you haven't mentioned what year your motor is. If it is between 1988 and 1992 then it is like this one
http://www.mercruiserparts.com/Show...inbr=961&bnbr=10&bdesc=STARTER+AND+ALTERNATOR

 

[JDClark]

No Title

I think this is it but will have to look it over again and pull it to make sure. Will hopefully get a part # off it. Thanks again AllDodge!

 

[Maclin]

I think I would take out all the spark plugs and then try to turn the engine by hand, or even with the starter, to make sure it is not hydrolocked or has some other problem keeping the engine from being able to turn over.

 

[JDClark]

Just to follow up I got a new marine delco starter and put it in and she fired right up. I bench tested the old one and sure enough it was spinning but not engaging. I went ahead and bought the newer version with the 11 splines instead of the old 9 spline starter and seems the engine fires up quicker.
Thanks for the help!