4.3L won't shut off with key, fuel pump runs constantly

[RyanSS]

The boat is a 2007 Searay 185 Sport
Engine is 4.3L Mercruiser TKS carb, 190 hp

I was cruising along the lake and the motor shut off. Got it towed back to the ramp and back home. Got to poking around and the positive wire of the alternator had rubbed on the ground bolt of the alternator and shorted out. This blew the 90amp fuse on the starter solenoid. I replaced the fuse and the boat will start and run now, but won't shut off with the key or the kill switch. I can turn the key off and pull it out but the boat keeps running. I can flip the kill switch and it also keeps running. To turn the motor off, I have to turn the key off and then either reset the 50amp breaker on top of the motor or pull the coil wire. Also, the fuel pump starts running and stays running as soon as I turn the battery switch on. It's not supposed to turn on until i start cranking. The oil pressure switch should only be sending power to the fuel pump if there is oil pressure (running or cranking).

So...I've read that the short could have fried the alternator diodes? Would this cause the motor to run without a key and for the fuel pump to have constant power?

 

[wrench 3]

Considering your original problem I suspect that the exciter circuit to the alternator is shorted to the battery wire to the alternator. Check for more damaged wiring near the alternator.

 

[Bt Doctur]

shorted diodes will let the alt backfeed . If you didnt touch tha alt wiring then suspect the diodes. if you DID touch the wiring you reversed the sense ans excite wires

 

[RyanSS]

Didn't mess with the alternator wiring other than removing the damaged section of the hot wire and putting a new terminal on the end. I did check the whole wire for damage and it's good. I suspect it's the alternator. If i take it to Autozone to be tested, would it show up faulty?

 

[Bt Doctur]

depends on how good they are at testing. you can test it by removing all the wires ,insulating them and running the motor. If everything acts normall except for the volt gauge then the alt is bad .

 

[RyanSS]

It doesn't have a volt gauge. So...if i understand what you are saying, I should remove all the wires from the alternator and insulate them, then start the motor. If everything works correctly (key shuts motor off, kill switch shuts motor off, and fuel pump runs when it's supposed to), then it's the ALT. That makes sense to me, I'm going to try that tomorrow.

 

[Bt Doctur]

It certainly does not have an ammeter gauge to tell you its charging ,so it should have a voltmeter gauge to tell you its charging.

 

[bruceb58]

deleted....Bt Doctor beat me to the test in post#5

 

[Fun Times]

Pretty good chance that the TKS diode is allowing voltage to back feed to the fuel pump. The diode mercruiser part number 83-865913A01 may be found as a red cover like the image below resting near the carb area. The following link offers more info about the diode but unfortunately the service bulletin is no longer working online. http://forums.iboats.com/search?q=TKS+Diode&searchJSON={"keywords":"TKS+Diode"}

 

[RyanSS]

It certainly does not have an ammeter gauge to tell you its charging ,so it should have a voltmeter gauge to tell you its charging.

Nope, nothing. I do have a Lowrance that displays Volts, so I'll use that. But the boat didn't come with a battery gauge of any kind.

 

[Bt Doctur]

he describes the original failure of the orange output lead grounding out on the bolt at the rear of the alt. this blew the 90A link ,
This is another of my modifications when working on a customers boat. the 90A is supposed to protect the wiring from the starter to the alt output lead.It also is connected to the Red main power wire. That main system is protected by the 50 A circuit breaker.
I cut the red wire away from the 90A block, extend it and connect it directly to the batt starter lug.
Should the 90A blow and not stop the motor, the alt is not connected to a battery and will run wild with overvoltage blowing the engine electronics and regular electronics, not to mention the panel bulbs.
With this modifcation ,the engine system is always connected to a battery reducing the chance for a large voltage spike.

 

[Fun Times]

he describes the original failure of the orange output lead grounding out on the bolt at the rear of the alt. this blew the 90A link.

There was a Mercruiser service bulletin number 2006-02 - about the orange Alternator Wire. I'm thinking the short may have damaged the diode too.

Possible Chafing of Orange Alternator Wire on 5.0L/5.7L/6.2L MPI Engines.
Situation
On the units identified above, an orange wire attached to the output terminal of the
alternator (the orange alternator wire) was routed by MerCruiser in one of two ways. One
way was correct, the other was incorrect.
The incorrect routing places the orange alternator wire between the oil dipstick tube and
the alternator. With that incorrect routing, it is possible that the insulating material will be
chafed by the dipstick tube or the alternator, which could cause the wire to fail. If the wire
fails, it could become an ignition source. See the "incorrect routing" photographs below.
The correct routing passes the orange alternator wire beneath the alternator, not in contact
with the dipstick tube, which eliminates the opportunity for chafing and failure identified
above. See the "correct routing" photograph following.
In addition to the two orange wire routings described above, many boatbuilders modify or
disconnect the orange wire to facilitate the installation of battery isolators. Boats that have
been so modified will not require the corrective action outlined below.

This usually will blow the 90 amp fuse on the engine starter.

 

[RyanSS]

depends on how good they are at testing. you can test it by removing all the wires ,insulating them and running the motor. If everything acts normall except for the volt gauge then the alt is bad .

If I do this, won't I be disconnecting the ALT from the battery and then running it? Isn't that bad for it?

 

[bruceb58]

If I do this, won't I be disconnecting the ALT from the battery and then running it? Isn't that bad for it?

Not if you also remove the excite wire.

 

[Bt Doctur]

you mis-understood. with an alt system you can NEVER remove the battery cable from the battery to see if the motor will still run. We did that with generators.
removing the output wire ,the sense and excite wire make the alt just a piece of aluminum and copper.
A four wire alternator has a black ground wire, orange output wire, purple sensor wire and a red with purple stripe excitation wire. These last two wires tell the alternator what the battery voltage is and when to start charging the battery.
Were all assuming the alt is damaged and back feeding the system thru the sense or excite wiring. So turning the key to off does not stop the motor because the sense or excite is feeding it.

 

[RyanSS]

Ok, I understand now

 

[RyanSS]

Alright...so I can't imagine how much money was blown on mechanics before the internet.

You were right about the alternator. I disconnected it and insulated the wires. Everything functions normally. The key works, kill switch works, fuel pump doesn't run with the boat off.

So now to shop for an alternator!

 

[Bt Doctur]

Good news there. I would still modify the wiring at the 90A block on the starter. An alt should only blow the alt fuse and not affect the ships power.

 

[Bt Doctur]

Just Imagine if we charged for correct answers too

 

[JustJason]

Just Imagine if we charged for correct answers too

I do