Mercruiser Main Power Relay 8.1L

[CaptRonn]

Has anyone had any problems with one of these?

My boat will start and run normal, but after a while the engine will stop as if the ignition has been cut off (engine stop, gauges go dead). If I use the emergency power switch, the engine starts normal and runs normal, as long as I keep emergency power switch engaged. Once I release the emergency switch, the engine shuts off and loses power to the gauges.

And when I return to the dock, an hour later the engine starts working normal.

It's been an intermittent problem for a while now. I've traced out all of the wires, replaced the battery, replaced the ignition switch, replaced the ignition circuit breaker. Still happens. And I've cleaned all of the grounds and also checked the battery cables.

These engines have a MPR (Main Power Relay). I'm beginning to wonder if the MPR is going faulty and causing this. However, while it explains the sudden loss of 12vdc, it doesn't explain why switching batteries via emergency start brings the system back online and works uninterrupted.

Mercruiser is being no help. I tried their technical support and was told to go find a local Mercruiser dealer.

 

[Jimmy Buoy]

I had that happen once to me as I left the dock. Like yours, the only way to start the dead engine was with the emergency power switch engaged and held that way.

Upon checking the breakers, I found the one engine ignition breaker had tripped for some reason. Once "off" it would of course not start with its own ignition switch.

You've replace that breaker though... Did you disconnect the engine ground cable, clean, and reinstall?

 

[techmitch]

Check connections at both alt. and starter. 8.1s in a 99 I'm guessing repower, so the relay connectors of that era are suspect also.

 

[CaptRonn]

I’ve replaced the ignition CB’s.

I’ve also cleaned the grounds, twice.

Here’s a thought. I lose the ignition circuit, this shitting down the engine. To restore, I engage the emergency which just ties the port and starboard battery buss together and we are back running.

Looking at the diagram, the engine gets its 12vdc power via the battery cable at the starter terminal, a separate red line goes through the 50amp CB and a orange line takes power to the alternator. The alternator also ties in the 12vdc feed (red/purple) to power various engine functions.

Remove that 12vdc and you shut down the engine and also kill the alternator. Which is what I’m seeing.

Now, when I tie the port battery to the starboard, I have power again.

With that being said, I’m beginning to suspect a cable issue from the strbd battery positive terminal to the battery switch, or a cable problem from the switch to the emergency start solenoid.

Think about it, all I’m doing when I engage emergency is adding 12vdc at the emergency start solenoid opposite terminal.

The cable from there to the starter is fine.

So with saying all that, I wonder if in one of the two cables I have corrosion building inside the crimp connectors?

Finally, what’s so odd about this is it will never fail in the dock. Never. I’ve ran the engines for over an hour trying to induce a failure at the dock, and it just won’t do it. But if I leave and go 20 minutes away, shut down and anchor, when I go to restart, no ignition power till I apply the ES.

 

[Mike sandor]

Had a similar problem last year. Would only stay running if I held in the emergency start button. When released, motor died as did the smart craft display. It turned out to the be the 50 amp breaker located on the top of the engine. It would trip and reset but not pass power. Verified with a meter. Replaced and all is good.

 

[CaptRonn]

Had a similar problem last year. Would only stay running if I held in the emergency start button. When released, motor died as did the smart craft display. It turned out to the be the 50 amp breaker located on the top of the engine. It would trip and reset but not pass power. Verified with a meter. Replaced and all is good.

Thanks. I've checked mine and it appears ok.

While my problem is similar to yours, mine gets weird as after an hour or so it starts working normal. And to add, I have the SC5000 tachs as well, and the same thing happens.

I'll believe I will go ahead and order a new one today and just replace it just to be on the safe side. Ya never know. I'm also going to replace the MPR as well.

 

[CaptRonn]

I went ahead and ordered a new 50amp CB.

This whole problem started when I blew the 90amp fuse, so logic would have it that the 50amp downstream may have been damaged. Not sure why adding the opposite battery bank gets it to work again, but my problem does act as if something gets hot, slightly opens and kills current, then once it cools starts working again.

I’m also replacing the MPR as a precaution as well. This weekend I’m going to go through the engine harness and see if I can spot any problems.

Before the battery/blown fuse issue this system worked flawlessly. I have to believe it’s something very inconspicuous creating the problem.

 

[Wanderer 290]

Have you checked battery voltage and alternator output?

 

[CaptRonn]

Have you checked battery voltage and alternator output?

Yes. New batteries (ok) and alternator output (normal)

 

[Fun Times]

What design is the Circuit Breaker? Red push button or yellow drop down lever? I recall some issues with the yellow drop down lever type enough that Merc had put out a service bulletin to go back to the red push button type.
88-11178A01 BREAKER, Circuit - 50 Ampere 865827T...., Genuine Mercury & Mercruiser parts. 88-865827T SST 11178A01 Genuine OEM Mercury Marine-Mercruiser part

 

[CaptRonn]

I have the red push button type.

According to my research, these too can be problematic and others have reported intermittent failures similar to what I’m experiencing.

 

[CaptRonn]

I’ve replaced the 50 amp CB. The old one shows continuity, but the button looks a little canted to one side (very slight). Put on the bench and heated it up try to induce a failure, but still had continuity.

Replaced the MPR as well. Old one looked good, no discoloration or burned terminals.

Finally, replaced the 90amp fuse.

So far in the system I’ve replaced the above, plus the ignition CB, the battery and the ignition switch. The only component not replaced is the alternator, which appears to work correctly.

I’ll see the next time I take the boat out if this solves the mystery. I still believe the 50amp CB may have been the culprit, I’m going to open it up later and look at it.

And no thanks to Mercruiser customer support. I asked (via email) to speak to a tech rep on a few questions I had concerning this issue. I got a reply email telling me to go to a Mercruiser repair shop as they will not assist me.

Needless to say, I will never purchase another Mercruiser product. Great way to chase away customers.

 

[CaptRonn]

Did an autopsy of the old Klixon 50 amp CB I removed.

First, when removed I noticed the red button was canted very very slightly to one side. It still showed continuity between the two post.

The points are extremely small and only a tiny point on either side to make contact.

Here's the spring and interior parts

My overall impression this isn't much of a CB. The red reset cap can easily cant from one side or another, and the spring is very weak.

I'm use to aircraft grade type pop out CB's which are more robust, and it's very evident when one pops and it's easy to see when it resets.

 

[sejensen737]

Did replacing that 50A CB solve yout problem?