4.3 blows 15a fuse when you throttle up

[RGatt3030]

My 2007 Tahoe boat has a 4.3 Mercruiser carbureted engine. The engine is newer, maybe 6-7 years old and the harness is brand new. The old harness had a breaker wired in where this one has a fuse. The fuse is a 15a and is on the red wire with purple stripe on the top of the engine. Is this the ignition fuse?


The boat also has a brand new starter because at the end of last year we had lost keyed power from the solenoid to the fuel pump. We began this year with a new starter and harness. The boat starts and idles fine. You can cruise at slow speed. When you punch the throttle the fuse blows. Even slow acceleration eventually blows the fuse. There is no change in voltage or temp when this happens.

Have been towed twice this year, in two rides!

Rob

 

[dubs283]

the breaker in an oem harness at the red/pur wire is 50 amp

curious as to why the engine harness was replaced?

 

[RGatt3030]

Its not the main 50a breaker. Its one of those watertight fuse holders in line. The harness is oem from Mercury as was the starter we replaced. From the factory it contains a 15a fuse. On the old wiring harness, which someone obviously messed with to deal with this gremiln, they patched in a little breaker which would Blow and reset itself after it cooled. The old harness had been messed with so bad I had my mechanic put a new one on when the engine was out to replace the starter. Any ideas what would surge that line? Wires to this fuse are red w purple stripe. Power goes to main breaker first because if this fuse is removed you still get power at the main breaker if that helps. But if this fuse blows engine will not turn over at all. Battery cables and all accessory stuff is fine. When I get home I can post a pic of the fuse if it helps.

 

[dubs283]

you mentioned in your op, that a 15 amp fuse was in place of the breaker?

a fuse will blow if there is a short to ground or the current in the circuit exceeds the rating of the fuse, my guess is the latter in your case. could be due to corrosion, loose connection, etc...

i can't recall ever seeing a 15A fuse in line on the red/pur wire on an engine harness, only a 20A fuse at the helm

if the old harness was oem and now the new one is, post your engine serial number and someone with access to the correct wiring diagram can be of further help

perhaps your mechanic that installed the harness could assist better?

 

[poconojoe]

And for safety sake and to be more specific, the fuse protects the wiring from burning up. The fuse is rated for the gauge of wire it protects. That is the purpose of the fuse.

The wire size is rated for amount of amperage the load will draw and the fuse is rated to protect that specific gauge wire.
Just thought I'd add that.

 

[RGatt3030]

Here is the fuse and the serial number. The harness was purchased from a dealer who matched it to the engine serial number. My mechanic is very well respected and I believe will figure it out but its one of those frustrating trial and error issues. He thinks the alternator may be overcharging but I don’t see any fluctuation on the dashboard voltage display right before it blows. Engine serial number OW653401

 

[alldodge]

But if this fuse blows engine will not turn over at all.

What color is the wire on the other side of the Red/Purple?

 

[bman440440]

my gut says the 15 amp fuse is to the TKS system

 

[alldodge]

The TKS uses a 20 amp, and if that fuses blows the motor doesn't stop

 

[RGatt3030]

Reading on tks now. Maybe that diode is bad. There is a service bulletin on that. Reading on it now......

 

[alldodge]

This is the simplified TKS diagram. Note if diode shorts the motor will not shut off because power back feeds to the ignition. If it opens the TKS module never opens up until the temp switch closes



Wiring connections

 

[RGatt3030]

Here are better pics both wires appear to be red with purple stripe. This is the closest fuse to the multi pin plug

 

[alldodge]

Since there the same, and the motor dies when its goes, it has to be ignition.

Look under the helm for a inline fuse on the red/purple feeding the key. My guess is its a 20. If it is, I would replace the fuse with a 20.

Newer motor using different ignition components and alternators only need 15. Yours i would bet it came with a 20

 

[alldodge]

Using your serial number the old fuse part number and new show a 15 and 20 amp. So I jumped the gun thinking yours may be a 20.

Now were back to ignition components or ALT, The coil will pull more amps as rpms come up, but not that much more

 

[RGatt3030]

Going to borrow a set of muffs and test alternator on the trailer sometime in next couple days. My mechanic thinks that is what to test next as well. The reason I will do it first in the driveway is that our house is both 15 miles from our mechanic and the lake!

 

[RGatt3030]

But one other thought on the TKS that was brought up before.... this boat would “diesel” after you turn the key off most of last year. We would have to wait 20 sec or so before the kids could jump in the water for fear the motor wasn’t completely off. I wonder if the TKS diode is going bad as well. Either way, I think the TKS system has black and white wires going to it and plugs directly into the carb. I also think the problem we have discussed at length here is ignition related.

 

[alldodge]

Dieseling is caused by to much fuel, incorrect timing, idle to high or getting to hot.

If the diode was shorted, you would turn the key OFF, but the motor would not shut off. Only way to kill the motor is to pull the diode or the coil wire. When the diode shorts it will also drain the battery over time

 

[RGatt3030]

So here are some electrical readings I took. May not be helpful at all but i figured I would post them. The voltage over the alternator posts is a consistent 14.4v when running even with increased rpms. I measured amps over the 15 amp fuse and it went from around 4 to a little over 6 as we went from 1000 to 3000 rpm. For Chits and giggles i measured the impedance of the tks plug that goes i to the back of the carb assuming that is the way to measure the diode. It measures 40.2 ohms one way and if you switch your pos and neg it measures 29. The wiring harness side runs 13.7 v with engine running. Battery measures 12.97 v at rest with nothing running. Measures 14.4 just like the alternator when running. The meter is fun to use so let me know what to measure tomorrow and ill have at it weather permitting!

 

[RGatt3030]

Consequently the fuse would not blow in the driveway at full rpm with muffs on.
Thanks for everyones help!

 

[Paintman1960]

Full RPM ? What do you mean by that?