Logo
FULL SITE MENU

4 stroke 40 HP Mercury

R

Rpar4

New member
Good morning,

I have a 40 hp, 4 stroke mercury, tiller handle on my g3. It has electric trim and tilt and electric start.

For some reason, it drains my starter battery. Today, I started the motor, and then it all of a sudden totally died. I was under the impression that once started, the motor had an alternator that would maintain or charge the starting battery.

Where do I start in diagnosing the problem? I have noticed that if I have the motor trimmed up, the motor will automatically trim down if I apply a lot of throttle. This must be the issue.
 
Rpar4 said:
Good morning,

I have a 40 hp, 4 stroke mercury, tiller handle on my g3. It has electric trim and tilt and electric start.

For some reason, it drains my starter battery. Today, I started the motor, and then it all of a sudden totally died. I was under the impression that once started, the motor had an alternator that would maintain or charge the starting battery.

Where do I start in diagnosing the problem? I have noticed that if I have the motor trimmed up, the motor will automatically trim down if I apply a lot of throttle. This must be the issue.
Click to expand...
These charge the battery via a stator under the flywheel, which has a magnetic strip on the inner circumference of the outer edge. The voltage regulator or stator may be bad but the best way to diagnose this is by finding a service manual- it shows what to look for, with voltage and resistance measurements.

Start by measuring the current draw after fully charging the battery- you shouldn't see any, really. Using a multi-meter set to measure Current (a test light may indicate fairly high current, but you need to see a number), disconnect one of the battery cables and connect one of the meter's leads to the cable and the other to the battery post. All other accessories must be turned off- if you have GPS/Fish finder, VHF radio or anything else installed, make sure they have been turned off.

You're assuming that it's a charging issue and it may be, but you (or someone else) will need to find the cause- it's not always possible to see one symptom and know unless these have a tendency to do this- I haven't worked on enough of these recently to know that. If any accessories were added, inspect the wiring.

The trim sounds like the cylinder seals may be leaking- does it struggle to raise the motor? Accelerating forces the lower gear case forward and if the trim cylinders are leaking or if the reservoir is low, you may see foamy oil. If it's low, add trim fluid and cycle it up and down several times- the pump will push any foamy fluid through the cylinders and into the reservoir, so it should improve. If that doesn't help, the cylinders will need to be serviced.
 
Wow, thank you for the super quick, in depth response.

I disconnected the positive battery cable and touched the meter to the battery post and cable as you suggested.

This is the reading I got. I'm not sure what it means, but it seems like there's current, and you said there shouldn't be.
 

Attachments

  • IMG_0253.png
    IMG_0253.png
    938.2 KB · Views: 2
Rpar4 said:
Wow, thank you for the super quick, in depth response.

I disconnected the positive battery cable and touched the meter to the battery post and cable as you suggested.

This is the reading I got. I'm not sure what it means, but it seems like there's current, and you said there shouldn't be.
Click to expand...
That shows 1 mA, which is very little. How long does it take to kill the battery?

Leave the meter connected and trim the motor up and down but reattach the battery cable and connect the meter so it measures Voltage- if you have a large current draw, the meter won't survive if it doesn't have a fuse- it's maximum current is only 1/4 of an Amp.

Measure Voltage while the motor is trimmed up and down- if the voltage drops significantly, you have short and that will require checking the harness where it bends or where it enters the motor shroud.
 
Back
Top