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Author Topic:   Voltage Regulator Failure on 1992 Mercury 175XRi
rj12 posted 01-01-2009 11:37 PM ET (US)   Profile for rj12   Send Email to rj12  
[The spelling of the word "battery" or "batteries" has been changed throughout this article to conform to the standard spelling.--jimh.]

During recent outings I have noticed my starting and trolling [batteries] have been draining much faster than normal. After a few trips the problem seemed to get worse. Thought I was in for a new set of [batteries] until I performed a close inspection of my 1992 Mercury 175XRi motor.

I found both voltage regulators had failed catastrophically. They looked as if they both shorted internally. The top one had a crack in the potting compound and a bead of solder on the outside. The bottom one splattered and dripped burned potting compound. In addition, the bullet connectors on the twp long yellow wires and one of the two long red wires to the bottom regulator looked as if they had been hot, discoloring the clear rubber insulator over the connection. I disconnected then checked these connections; they still appeared to be intact.

Naturally I checked the stator per the service manual and it checked out good on resistance between two short then two long yellow wires: 0.4 ohms each. Next, short yellow then long yellow to ground: no continuity. I didn't bother with resistance measuremen of the regulators since they were visibly cooked. Wires to the starting relay positive terminal all looked good.

--What other condition could have caused this kind of failure?

--Any other checks that should be done before slapping in new regulators? NOTE: I had my livewell drain plumbing fixed just prior to experiencing abnormal battery consumption. Mechanics had to remove [batteries] to access the drain.

--Do you think they could have caused this by hooking up my [batteries] wrong?

--Would the regulators fail instantly and violently if the [batteries] were temporarily hooked up wrong?

I could really use some advice. At $100 a pop I don't want to overlook something and end up bank fishing this spring. Any suggestions?

jimh posted 01-02-2009 08:43 AM ET (US)     Profile for jimh  Send Email to jimh     
A reversal of the polarity of the battery connections will cause the diodes in your rectifier assembly to fail in a catastrophic manner. Let me explain the mechanism:

In the rectifier assembly there typically are four diodes. The battery is directly connected to these diodes. In normal operation, the voltage of the battery causes these diodes to be reverse-biased, and no current flows. When the voltage from the alternator (the stator) is applied, and it exceeds the battery voltage, the diodes become forward biased, and current flows from the alternator into the battery, charging it.

When a battery is connected with the polarity reversed, at least two and perhaps all four diodes in the rectifier assembly become forward biased, and current flows from the battery, through the forward biased diodes. Typically the path is very short: battery positive, diode, battery negative. In other words, the diodes act as a direct short circuit across the battery terminals. Current flow is only limited by the resistance in the battery cables and connectors. A very high current flows for a very short time, until some component of the circuit melts from the high temperature. From your description this sounds like it may have occurred on your boat.

It is also possible that a diode spontaneously fails, perhaps from high temperatures, and begins to allow current to flow through it in a reverse direction. This could occur at a lower current flow, but, left for days or weeks, it could also produce damage. This would account for the slow draining of the battery as described.

rj12 posted 01-02-2009 10:09 AM ET (US)     Profile for rj12  Send Email to rj12     
Thanks Jim,

That clears up alot about what goes on under the potting compound. I don't want to believe the mechanics would have left me high and dry like this but...I just wonder if someone wasn't paying attention for a moment when reconnecting the batteries. That's why I wanted to know how quick the failure could happen. My guess is that if this did occur they probably had it hooked up wrong for a very short time then corrected the connection. Anyhow I guess what's done is done.
BTW: I researched parts for days and found three models of voltage regulators available Merc 883072T, CDI 1945279, and Sierra Marine 18-5743. After reading up on all I ordered two of the Sierra Marine model at $181.96 total with shipping. Their specs said the regulator has a built in capacitor to prevent damage from voltage spikes (from the stator) as may be caused by switching batteries with ingnition on...not that I'm going to test the theory though.
I decided against the CDI because I read about people who had alot of problems with overcharging when using AGM batteries (mine are currently maint free lead acid not AGM). Furthermore, the installation sheet from CDI also stated "Do not use a Maintenance Free, AGM or Dry Cell Battery with this regulator/rectifier". That made me a little nervous. And last the Mercury one was just more expensive.

-Is the Sierra Marine part a good choice, do they have a good reputation or lower failure rate, how do they compare?
-Do you think the capacitor is a good feature?
-Do they all have the capacitor but just don't advertise it?
-Do you know if the the Sierra regulator can be used on AGM, Maint Free and/or Dry cell (was thinking about getting Optimas in the future)?
-Do you have any other recommendations?

jimh posted 01-02-2009 10:58 AM ET (US)     Profile for jimh  Send Email to jimh     
My interpretation of the advice recommending against using a particular type of battery with the battery charging system of your motor is as follows:

The battery types listed, Maintenance Free, AGM, are both sealed lead-acid batteries. If these batteries are given an excessive charging voltage they, like all lead-acid batteries, will bubble out gas from the electrolyte. Since these batteries are sealed, this causes a pressure build up. If the pressure becomes excessive a regulator valve opens, venting the gas. This results in a loss of electrolyte. Because of the sealed design, more electrolyte cannot be added. Operating in this manner eventually leads to battery failure.

With a battery charging system like this you should use a conventional flooded cell lead-acid battery. It will be more tolerant of higher charger voltage, and if electrolyte is lost you can replenish it by adding water and re-charging.

rj12 posted 01-02-2009 01:18 PM ET (US)     Profile for rj12  Send Email to rj12     
Thanks for the info Jim.

If anyone has any input (good or bad) on the Sierra Marine and/or the CDI voltage regulators mentioned previously please let me know. Just want to make sure I got a good product.

bviperj posted 01-30-2009 10:32 AM ET (US)     Profile for bviperj  Send Email to bviperj     
I have twin 1994 200 HP merc 2.5L outboards. There are two batteries, the port engine is hooked up to one battery, the starboard engine is hooked up to the other. The starboard engine also has the load for all the electronics. This summer, the battery didn't seem to be getting a full charge, and consequently I had trouble turning over the starboard engine. I put a multimeter on the starboard alternator and it registered 12.5 volts apprx. The gauge on the console also reads slightly above 12, but with electronics and a turn of the key it drops to 10. I think it is the rectifier/stator that is failing, but have no idea how to fix this? are there any pics to help identify things?? or how to go about replacing them??


phatwhaler posted 01-30-2009 03:17 PM ET (US)     Profile for phatwhaler  Send Email to phatwhaler     
I put some CDI Rapair regulators on my last 150 EFI. They worked great. I paid around $160 for two of them as a kit. They nice thing about CDI is their tech department. If you have any questions they are pretty good about helping you.

I have no idea why yours failed other than they may have gotten too hot? I think all of these mercury regulators are junk. I'd be suprised if there are any 90's models origianl regulators still being used.

phatwhaler posted 01-30-2009 03:21 PM ET (US)     Profile for phatwhaler  Send Email to phatwhaler     

Do both of your motors put out only 12 volts? If not then you could use the good engine to diagnose the bad one. I would double check your voltage at the starter solenoid.

By the way, if you have an alternator on your motor it must be newer than a 1994. 1994's had stators. The windings are stationary.

jimh posted 01-31-2009 10:47 AM ET (US)     Profile for jimh  Send Email to jimh     
The term "alternator" applies to any device that generates alternating current, without regard to whether there are rotating coils. In a permanent magnet alternator the usual design seen in outboard motors is to have a fixed coil (called a stator coil) and rotating magnets. In non-permanent magnet alternators (which most people are familiar with from their extensive use in automobiles), a rotating coil (called a rotor coil) is revolved in a magnetic field of a stationary coil (called a stator coil).

Both the permanent magnet and non-permanent magnet devices are alternators. Their alternating current output is rectified to a pulsating direct current by a rectifier or diode assembly.

For more discussion about how outboard motors generate electrical power, see

Boat Electrical Power Generation

For a discussion of permanent magnet alternators, see
Permanent Magnet Alternators

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