Rectifier

mustangman

Cadet
Joined
Jul 21, 2008
Messages
13
I have an 87 Mercury 90hp 3 cyl. The last two outings I was registering up to 16.5v while under throttle. Normally I had been getting around 13.5. I had found a dead battery before the last 2 outings and replaced it and now I have the higher voltage. It is also registering the same voltage on my fishfinder and with a voltmeter at the battery. My question is I do not know how to test a 3 prong rectifier to see if it's bad. I am getting 12.5v at the positive terminal of the rectifier with the motor off and the same at both of the other terminals. Is there supposed to be voltage there with the motor off? If not what should I be looking for? Thanks
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: Rectifier

Do you have the 9 or 16 amp system? The 9amp will have the small square rectifier and the 16 will have the regulator/rectifier on top of air cover...
 

mustangman

Cadet
Joined
Jul 21, 2008
Messages
13
Re: Rectifier

I have a 16 amp system with a square 3 post rectifier. A Red positive and two yellow wires with a grey wire on one of them.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589
Re: Rectifier

That the 9 amp system as the 16amp does not use the small three post rectifier on this model. Some models came with a optional voltage regulator. If you have this installed, here is how to test it.
Testing:
1. Check battery voltage at battery with engine running.
2. If battery voltage is above 14.5 volts, replace voltage regulator. Check condition of battery as overcharging may have damaged battery.
3. If battery voltage is below 14.5 volts, charge battery. If battery can NOT be satisfactorily
charged, replace battery.
4. If battery accepts a satisfactory charge, check battery voltage while cranking engine; If
cranking voltage is not acceptable, replace battery.
5. If cranking voltage is acceptable, disconnect end of RED wire (located between rectifier (+) terminal and starter solenoid) from rectifier. Secure RED wire (from voltage regulator) to rectifier (+) terminal with hex nut.
6. Connect RED (+) ammeter lead to (+) terminal of rectifier and BLACK (?) ammeter lead to RED wire (disconnected in last step).IMPORTANT: For accurate test results the voltage
at battery with engine running, in next step, must be 13.5 volts or less. It may be necessary to operate electrical accessories to drop voltage to 13.5 volts or less.
7. Run engine at 3000 RPM.
8. Meter should read between 7 - 9 amperes.
9. If meter reads 7 - 9 amperes, this indicates the charging system is functioning properly and the
battery is being discharged because the amperage draw on the system is greater than the amperage output of the system.
NOTE: With engine running at the following RPM?S, the ammeter should indicate the following approximate amperes:
RPM AMPERES
IDLE 1
1000 4
2000 8
3000 9
10. If the meter reads less than 7 amperes, test the stator; refer to ?Standard Stator (Alternator Coils) Amperes Output?, following. If stator tests OK, replace voltage regulator.
 

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: Rectifier

The rectifier is fine. The 9 amp system is what we would call a current source system, where the load determines the voltage. Open circuit (no load) it would go to about 90 volts.

Now, that said, either the battery is weak, or connections are weak, or the meter is a bit off on the high side.

The old battery registered lower voltage because it had a shorted cell. That's why it wouldn't crank the engine. Yer meter is suspect because I would expect a shorted cell to hold the voltage down to around 12.5V

It's probably acting perfectly normal. Just make sure the battery is maintained, as it will normally use a little water with this simple system. At any rate, there's nothing wrong with the rectifier.

hope it helps
John
 

mustangman

Cadet
Joined
Jul 21, 2008
Messages
13
Re: Rectifier

Thanks Admiral John and Admiral Faztbullet. A little more information I should have posted earlier. I have a red stator which I was told is the 16v stator and 2 batteries. 1 was bad and replaced and 1 was good. The overcharging started after I replaced the bad battery with the good. I replaced the battery in my meter also just in case. I got the reading of 16.5v at my dash voltmeter, my fishfinder voltage and my test meter at the battery while running. I was told that the rectifier dissipates excess voltage as heat and I just installed a thermostat that had been removed before I bought the motor. I was wondering if since it was running warmer with the thermostat, would that be too hot for the rectifier to dissipate the excess heat as well. I live in Florida and the water temp was 85.5 when I was on the water the last 2 times. Would it be a good idea to replace the rectifier with a voltage regulator instead and keep the voltage more constant?.
 

mustangman

Cadet
Joined
Jul 21, 2008
Messages
13
Re: Rectifier

Well here is an update. I removed the rectifier and replaced it with a voltage regulator. It now runs with a steady 13.5v. I did keep the t stat since it was designed that way and it seems to run better. I was going to take her out for a full fledged test run today but now it's supposed to rain for the next 3 or 4 days. Soon as it clears I will let her rip. Thank you all for your suggestions and assistance.
 

mustangman

Cadet
Joined
Jul 21, 2008
Messages
13
Re: Rectifier

I finally got out and the motor ran great. Started out with 12.5 volts and then increased to 13. Going to take it on a long river trip with the grandkids tomorrow and give it it's final stress test. Thanks again for your help.
 
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