72 Mercury 800, Motor dies when Pos. lead is disconnected

[EWade]

I have a 72 Mercury 800 with new stator, rectrifier, enternal & external wiring harness. The motor will fire off ok, but when I disconnect the positive lead off the battery, the motor immediately dies. Also, on the water, the boat will plane off but as soon as it reaches 30 mph, it will loss power and only reach 5-10 mph.

 

[WillyBWright]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

You're going to need another rectifier. Disconnecting the battery while the motor is running is a really bad idea. Electricity made in the stator needs a place to go. If you disrupt that flow, electricity that has already been made will find a place to go. Usually thru one or more diodes in the rectifier. I replace dozens every year. Most due to corroded and/or loose battery connections. I'd bet that accounts for at least 90% of rectifier failures.

You have amplified battery ignition. By disconnecting the battery, you're depriving the system of power for the amp. So you are basically shutting off the spark. I'm curious, why are you disconnecting the battery lead in the first place? Seems kinds like playing Russian Roulette to me.

With a failed rectifier, your charging system won't charge. The battery will lose voltage. And with increased demand of higher RPMs and sparkplugs firing more often, it would be logical to expect the system not to be able to keep up. Or you are losing fuel pressure and your motor is starving for fuel. Flip a coin. Check both. Fuel pump kits are cheap. Rectifiers not so cheap.

 

[EWade]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

You're going to need another rectifier. Disconnecting the battery while the motor is running is a really bad idea. Electricity made in the stator needs a place to go. If you disrupt that flow, electricity that has already been made will find a place to go. Usually thru one or more diodes in the rectifier. I replace dozens every year. Most due to corroded and/or loose battery connections. I'd bet that accounts for at least 90% of rectifier failures.

You have amplified battery ignition. By disconnecting the battery, you're depriving the system of power for the amp. So you are basically shutting off the spark. I'm curious, why are you disconnecting the battery lead in the first place? Seems kinds like playing Russian Roulette to me.

With a failed rectifier, your charging system won't charge. The battery will lose voltage. And with increased demand of higher RPMs and sparkplugs firing more often, it would be logical to expect the system not to be able to keep up. Or you are losing fuel pressure and your motor is starving for fuel. Flip a coin. Check both. Fuel pump kits are cheap. Rectifiers not so cheap.

I thought it might be like testing an alternator on a car. By disconnecting the battery, the motor should continue to run.

 

[Clams Canino]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

Disconnecting the battery running is also a good way to kill the rectifier. Forget cars.

-W

 

[j_martin]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

I recently bought a bass boat with a 1988 150 horse V6 on it. It wouldn't keep the battery up. Checked the rectifier and found it was bad. Took it off and crossed the numbers on the bottom, and found it was a cheap Mexican 6 amp rectifier encapsulated in a lot of crap that would even de-rate it more.

I replaced the rectifier with a Radio Shack 25 amp 200 volt bridge rectifier. It was a lot of work to make it fit, but I don't expect to ever make that repair again.

I am amazed that Mercury builds expensive motors with such a sophomoric charging system on it. The battery is used to limit the output to a safe level, thus if a battery connection is bad, voltage goes ballistic, and something expensive usually gives. (Maybe I just found the reason for the design.)

Never, never start or run a Mercury engine, even rope start it, without a battery connected.

hope it helps
John

 

[WillyBWright]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

I looked up your V-6 and it has a separate regulator. Newer evolutions use a pair of regulator/rectifiers. They've been updated a few times. and the newest ones seem to hold-up better, but they'll still fail with corroded/loose battery connections. I suspect your stator has a small amperage output. The newer ones are stronger, but still not Earth-shattering by any means.

 

[EWade]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

Thanks alot for the information. Its has really been useful.

 

[EWade]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

The stator is putting out 10.8 amps @ 2000 rpms. Rectifier is putting out 13.8 V.

 

[ricksrster]

Re: 72 Mercury 800, Motor dies when Pos. lead is disconnected

Adjust the angle of you engine a little at a time to see if this helps your speed. You may be too high or too low.