I'm afraid I know the answer - but?

[Tom7227]

I have a 1984 90 hp 6 cyl Mariner outboard. I checked last night and there was no change in the voltage when I tested with the motor both on and off. Rectifier? Any idea what one would cost? Is it a do it yourself deal? Should I bother? I live on a lake and only use the boat there. It's not a big deal to re-charge the battery. In fact I have a solar panel on order to take care of that for me.

Thanks for any advice.

Tom

[boatfixer]

You should have higher voltage with the motor running, but no more than around 14.5 What kind of change are you talking about Tom? Have you been having charging problems?

[marine_man]

The rectifier replacement isn't real difficult on your motor. Do you have a tachometer? Is that working? Usually rectifier and tachometer problems on mercs go hand in hand.

marine_man

[Tom7227]

Tach hasn't worked since I got the rig 4 years ago. The voltmeter was 12.68 with the motor off and 12.65 with it running - and I figure I used up the .03 getting it started.

[marine_man]

Has it charged in the last 4 years?

Sure sounds like a rectifier or stator and rectifier.

You won't do any damage by not replacing it, but, as you already pointed out, you won't be charging the battery. So, on a longer trip you may want to have a spare battery with you in the boat, unless you like pull starting it with the little rope they give you.

marine_man

[boatfixer]

Ya I see there must have been a typo. That motor should have 7-9 amps coming from the stator. If you cannot check the amperage, then a simple way, but not as accurate way, of checking the stator output is to use a good quality test light at the rectifier between the yellow leads still connected to the rectifier. If the light gets brighter as rpms increase, then the stator is producing some energy.

Rectifier testing and troubleshooting

CAUTION: Make sure the battery is disconnected prior to servicing.

Here are the steps:

Ensure all wires have been removed from rectifier and battery has been disconnected.

There are 3 terminals on the rectifier. There is one marked with "+" this will be "B", moving clock wise the next will be "A" the next "C"

1. Place/set your multimeter to the 1000 ohmmeter reading (RX1K, R X 1000, etc). See picture below.

2. Place the RED lead to the ground (G) and place the BLACK lead to A and C alternating. You should show continuity.

3. Place the BLACK lead to the ground (G) and place the RED lead to A and C alternating. You should NOT show continuity.

4. Place the BLACK lead to the terminal () and place the RED lead to A and C alternating. You should show continuity.

5. Place the RED lead to the terminal () and place the BLACK lead to A and C alternating. You should NOT show continuity.

6. If your results differ, then the rectifier should be replaced.

[eurolarva]

are you sure about the 1 k ohm setting. Seems to me to test a rectifier you need to be on a diode testing mode which on most meters is 2K ohms.

[boatfixer]

I suppose if you have a meter that has diode mode that would be the case. But all you are doing is checking continuity so a simple ohms tester will work just fine just by reversing the leads. I know with the 2 different testers I have, I test that way all the time.

[eurolarva]

A diode will only pass voltage one way. The ohm meter uses DC voltage even on the ohms setting. That is why you only read resistance one way. By reversing the leads the diode will not allow voltage to flow so it shows infinite ohms. A meter that has a diode setting will use a voltage higher then .7 volts which is the voltage that must be overcome for a diode to turn on if it is silicon and .3 volts if it is germanium. Sorry to be a nerd here. but some meters will not work without a diode tester built in. I have a radio shack one that wont work and I have a fluke meter that will only work on the diode range. I have never tested a marine rectifier only diodes in electronic applications so maybe they are configured different.

[boatfixer]

Nope, not being a nerd, I'm learning a few things. All I know is how to test 'em and so far it's been working. I have one digital that has 200, 2k, 20k, 200k, 2m, 20m. Then my DVA tester, the one Mercury Marine has endorsed and sold for years. has x1, x10, x100, x1k. Trust me Euro, I'm not trying to argue with you, you seem to know more about meters than I do. But thats the meter I've tested with for years on the 1k scale. But you have taught me a thing or two, thanks. Heck, maybe my meter has the diode mode built right in, and I don't even know it.

[eurolarva]

Usually the meter has a picture of diode on it on the 2K range. If a meter has this it is dangerous to use on some circuits cause it can blow up some chips that are real sensitive. I have read your posts and FM is lucky to have you as a member. I can fix a radio but here on fishing minnesota it is much better to know how to fix a motor and a boat.

[Tom7227]

Bruce,

My son came over with his fanch tester and followed your instructions and it is clear that the rectifier is kaput. I'm going to try and track one down and put it in and hopefully all will be working. There's even a chance that the tach will start to function.

Thank you for your help on this. I certainly wouldn't have been able to solve the problem without your taking the time to put those detailed instructions. I appreciate your effort.

Tom

[boatfixer]

Glad I could help Tom. You shouldn't have too much trouble finding one. The Sierra # is 18-5707 (aftermarket). The merc number should be 816770T, but if you go that route, make sure to give the dealer the model and serial number.