77 Merc 115 battery charging ?

Can someone explain how the charging system works on a 77 Mercury 115 horse motor?

Can this motor run with a dead battery if it was manually(rope pull) cranked?

Re: 77 Merc 115 battery charging ?

I have this same motor. You probally don't want to rope start a 6 cyl. Would be a hard pull, and these engines are cold blooded. U need to choke the heck out of them.



I would check all or your wires and connections. Its old..so wires get brittle and connections get weak.

I had to replace some wires. Its still a work in progress, but you will get there

Spend the extra money to get a good Marine Battery. You will need it.

Re: 77 Merc 115 battery charging ?

the charging system works from a stator under the flywheel, it produces AC power, has 2 yellows from it to a rectifier whic coverts the power to 12 volts DC. this rectifier is most often the problem. it gets burnt out from battery cables being disconneted while engine is running. also dirty connections, also battery cables connected wrong.. this charing system when work doesn't produce much amperage.. I have found a good fully charged battery will outlast my money to buy the gas to run the battery down. gotta have a battery to run

Re: 77 Merc 115 battery charging ?

This is straight from the Book.

The stator is located under, and protected by, the flywheel. When left undisturbed this sheltered position often protects the stator from damage and it should be noted that the stator is seldom the cause of a problem with the charging circuit.

However, these tight quarters can be hazardous to the wiring harness if disturbed or installed incorrectly. A wire which protrudes enough to be contacted by the flywheel will quickly short to ground, possibly destroying the stator coil winding with it.

When stator problems DO occur, a wiring harness is often to blame.

Meanwhile, the most common problems with the charging system can be traced to the rectifier or to the battery.

Regardless, if either the stator or the rectifier fails the troubleshooting tests, the defective unit cannot be repaired, it MUST be replaced.

Re: 77 Merc 115 battery charging ?

That motor has a battery operated CDI Ignition system. If you pull started it because of a dead battery, it would not run.

Re: 77 Merc 115 battery charging ?

Thank you all for the replies. I will check the rectifier as that does seem to be the main cause for concern.

When I got this boat, the wiring harness was rotted and many wires shorted...this is probably what fired it. I made a new wiring harness and all seemed fine but one thing I forgot to check for was battery charging.

Also, the info about the ignition system requiring battery power to run was my assumption as well although I was told the stator should produce enough juice to run the motor. But then again, even if it did, a bad rectifier would also cause it to not run, right?

Re: 77 Merc 115 battery charging ?

emilime75 said:

Thank you all for the replies. I will check the rectifier as that does seem to be the main cause for concern.

When I got this boat, the wiring harness was rotted and many wires shorted...this is probably what fired it. I made a new wiring harness and all seemed fine but one thing I forgot to check for was battery charging.

Also, the info about the ignition system requiring battery power to run was my assumption as well although I was told the stator should produce enough juice to run the motor. But then again, even if it did, a bad rectifier would also cause it to not run, right?

Click to expand...

You need a charging circuit that is working, and at least a dead battery on it or you will fry ignition electronics.

The stock murkery rectifier is quite deficient. Yer onto a good replacement on another thread. Best bet for you would be to get your charging system including the battery in good shape before you rely on it.

hope it helps
John