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Old 29 June 2022, 23:58   #1
Country: USA
Town: San Diego
Boat name: my boat or the rib
Make: ab
Length: 4m +
Engine: Merc 40hp 4str efi
Join Date: Dec 2018
Posts: 2
Merc 40hp voltage rectifier fuse

Somehow I just wrote a novel. Skip to TL;dr if you want.

I have a Mercury 40hp four stroke (2012 i think) that I got a couple years ago. It had spent its whole life in fresh water til I got it. Now it's ocean-only and I've put 30ish hours on it in the ocean. It has been great so far until now.

It has always started up instantly on first try. However, on the outing before last, it didn't start up on the first try at the dock, but it started fine on the second try. Later that day when pulling anchor to leave, it again failed once and started on second try, but it was very weak. Like mehhh mehhhhh mehhhhhhhhhhhvrooom. Phew! Nothing like the sound of the engine starting when the suns going down

Responsible seafarer that I am, I thought "I need to make a note to fix that before I go out again."

A couple days later I saw my note and crossed out "Sort out electrical" and wrote "CONFIRM JUMP STARTER IS PACKED AND CHARGED".

And that brings us to my last trip. Another weak start at the dock, but it was on the first try. After a few hours anchored, no start. Not even a sound hardly. BUT I MADE A NOTE ABOUT THIS. Jump start, fist pump, congratulate self for conquering the ocean and mastering boating.

Once started, I disconnected the jump starter and drove 15+ minutes at 4500 to 5000 rpm. (Only accessories are Nav lights, bilge pump, both off). I got to the 5mph zone (2000ish rpm) and as soon as I backed off the throttle it wanted to stall. Had to throw it up to 3000 for a couple seconds, shout "sorry, throttle stuck!" at the glaring cops on the police dock 20 yards away, then let it creep back down toward 2000, stutter stutter, gun it for two seconds, repeat 5 times in 5 minutes. Note: passing the police dock, I noticed my RPM gauge had stopped working (pegged at zero). Not sure when it failed, but it was definitely the same day.

It finally settled down at 2200ish and after 140 seconds of problem-free yachting, I was thinking "phew, made it. i even have time to turn on the bilge pump before I get to the D'OHHH!!!!!!!!!!!!!!!!!"

Jump start, dock. Assuming I had a dead battery, I left the engine running so it could power the tilt. The moment I pressed the tilt up button, engine died. Just like the bilge pump. The moment either button is pressed, engine dies.

I discovered that if I left the jump starter connected to the battery while engine was running, bilge pump and tilt both worked like normal and didn't kill engine. But clearly the accessories were just draining the jump starter. Even for me, that's too stupid of a permanent fix.

Sorry for being long winded but just trying to remember everything I did that could be relevant.


TL;dr Here's the symptoms:

*Battery "suddenly" (over the course of 5 days) unable to start engine whereas previously it always started instantly.

*Jump started engine ran fine at full throttle for 10 or 15 minutes, but immediately thereafter wanted to stall below 2300rpm.

*While putting along at 2300rpm, turning on the bilge pump or tilt instantly killed engine.

*RPM gauge dead, even when jump starter was left connected to running engine.

*Battery was 11 something volts when I put it on charger. There was still fluid in the battery, but just barely enough to keep all the plates covered when level.


I read about the voltage rectifier problems these engines have, and it sure sounds like that's my issue. But the rectifier box looked pristine, as did all the wires and connections. So I followed the wires to a fuse box and the 25amp fuse was blown.

Battery is charging now, and I will add distilled water when done. However, two of the plates in the battery have cracks or sulphur or something. I assume that is from lack of water? See picture here. With charger off, battery is now at 12.6v. Will leave it charging overnight.


*Buy a box of 25 amp fuses (done!)
*Charge battery and cross fingers (done!)


1.) Is my battery trashed? It's only 2 years old, and this is the only time it's been discharged this low. The responsible "smart" solution is obviously just buy a new battery, but I'd rather buy gas.

2.) Why did the fuse blow? Most likely culprit? What tests can I do? I will look for bad connections, corroded wires, etc., but is there anything in particular to look for?

3.) Is the voltage rectifier box likely still good? Likely not good? How to test? (I do have a Merc service manual, so it probably has procedure). It looks fine - absolutely no signs of corrosion, water, or burns.

4.) This may go in the stupid fix category, but would a bigger fuse be safe if the 25amp keeps blowing? How much bigger?

5.) Possibly related. I have a habit of fiddling with the engine trim a lot, always trying to make it better for every speed/conditions. Is this a no no? For example, is it ok to tilt the engine up (button press for 1-2 seconds max) while at full throttle? If this is a no no, could this be related to the fuse?

6.) Once I saw the fuse was blown, I didn't want to start it again. Will it damage anything to run the engine without the fuse?

7.) If jump starting, and assuming no other problems, is the best practice to remove the jump starter immediately? Or should it be left connected at least while the presumably dead battery is being charged? (rectified? alternated?)

I'm totally embarrassed by the length of this post so now I shut up. THANK YOU FOR ANY ADVICE GOOD OR BAD.

wetchorizo is offline   Reply With Quote
Old 01 July 2022, 11:22   #2
Country: UK - England
Town: Leicester
Length: 5m +
Engine: 135hp Mercury
Join Date: Sep 2013
Posts: 1,270
A battery showing under 12 volts is flat & charging may not recover it.
After fully charging the battery disconnect the charger & leave it overnight to allow it to settle & see if it holds voltage or has dropped.
If the latter it's new battery time - and as your battery has visible damage you already know it needs to be replaced!
If you want to carry on messing with it be prepared for damage to the engine charging system - if it hasn't already.

Dead/dying/low charged batteries are the cause of many problems - not just on boats & modern cars can throw hissy fits & fault codes if the battery isn't good.
Fitting a known good & fully charged battery often sorts them & if nothing else will eliminate the battery as the problem.

A lot of stops & starts/short journeys & using a lot of other electrical equipment will cause the level of charge to fall & the charging circuit isn't designed to charge flat/deeply discharged batteries with the likelihood that the overload will damage the alternator.
Worth a read, ignore the red top blue top etc sales pitch:

A very rough guide to battery charge state:

Is the charging circuit operating?
With a fully charged & known good battery connected and the engine off connect a multimeter set to DC across the battery terminals.
Note the reading.
Start the engine.
What does the meter do?

DO NOT replace a correct blown fuse with one of a higher rating. The whole point of fuses is protection. If you replace it with a higher rating you risk damage or fire.
paintman is offline   Reply With Quote

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