79 Merc 90 Thunderbolt Charging problem

[Ben.Skelton]

Hello all,

Had trouble finding info on this issue.

Ever since I got my boat it seems to have charged at about 17 volts. (seems high to me) Then one day the motor died on me out on the sound, and couldn't get more than a click out of the motor. Kicker home, run some tests on it. Check the battery, and find loose leads. So I tighten them up, and she fires right up if a bit sluggishly. Now the voltmeter says only 12 volts. So, I do more troubleshooting and find that I have a bad rectifier... order and install new one. Still same problem.

So, before I go off and tear into this thing again; Are there any tests I could or should run on the alternator? I figure I should test the new rectifier to see if it is working properly. Any other thoughts?

 

[verado7]

Re: 79 Merc 90 Thunderbolt Charging problem

are you using a handheld voltmeter or one installed on the boat ?

i'd first use an independant tester - but the magneto could be bad ( under the flywheel )

 

[Ben.Skelton]

Re: 79 Merc 90 Thunderbolt Charging problem

Originally was using the voltmeter in my Lowrance fishfinder. Then when it went bad I checked it with a handheld against the fishfinder and it concurred.

What are the signs of a bad magneto/ alternator? How do I test it?

Ben

 

[Chris1956]

Re: 79 Merc 90 Thunderbolt Charging problem

Ben, Does that motor have distributor ignition or ADI? ADI will have 6 coils.

You need to test the stator charging coil. On the dist ign model, the alternator is two yellow wires. Test them for continuity. More often broken wires are the culprit.

On the ADI ign, the same test applies. Let us know what resistance readings you get.

Rectifiers (especially Merc ones) can go bad pretty easy. One loose connection on the ground or +12VDC (red) wire will pop one of the diodes.

 

[Ben.Skelton]

Re: 79 Merc 90 Thunderbolt Charging problem

It is distributer type ignition.

Do I need to take the flywheel off to check for continuity on the yellow wires? or shoult I just bridge the 2 leads with my ohmeter and check for continuity? Repair manual in the mail, but I really need to get this thing back on the water before it's due to show up. I will recheck the Rectifier, and will try to check the stator for continuity.

Ben

 

[AllAboard]

Re: 79 Merc 90 Thunderbolt Charging problem

It is distributer type ignition.

Do I need to take the flywheel off to check for continuity on the yellow wires? or shoult I just bridge the 2 leads with my ohmeter and check for continuity? Repair manual in the mail, but I really need to get this thing back on the water before it's due to show up. I will recheck the Rectifier, and will try to check the stator for continuity.

Ben

You do not need to the flywheel off. The 2 yellow wires should be on the side opposite of the distributor. You can remove the wires from the junction block to be sure you're measuring only the alternator winding. Should be very little resistance.