Mercury 150 HP in Guardian Mode

Hello My son has a 2005 Mercury 150 HP that is apparently going to a Guardian mode, put different props on boat and also goes to 4,900 rpm and levels off at that rpm, no matter what pitch prop is on boat speed changes with prop but not rpm. I see that it can have a bunch of different sensors helping control the Guardian system. Does the Guardian automatic reset itself ? Can much be checked without the Mercury Scan tool. Would that year motor have a code stored that a scan would read. Thanks for any advice Jon

2 stroke, Optimax or 4 stroke. All were produced in 2005 and all have slightly different guardian systems.

In general the guardian is sensor driven.

When you say "apparently going into guardian mode" what do you mean.

Guardian has a horn and (at least a feed for) a light/indicator - those will go off/illuminate for certain conditions.

There is also a (component) that will restrict rpms - out of oil, oil pressure, over heat etc.

If it restricts rpms it's really thumps them down - for some things - oil pressure alarm/restricts rpms to half of max, low oil pressure (4s)/10% of max rpms - Opti's, 3000 rpm, low battery (4 stroke)/75% of max rpms, low battery (Optimax)/3000 rpms max, overheat 50%.

And no, without a DDT it is difficult to troubleshoot either a sensor or engine computer issue.

If you "unplug" one of the sensors the ECU will "pretend" it's there and use a "canned set" of parameters that it figures it would have got from that sensor had it actually been there.

And to your other question - each time you key up (turn the key from off to on), the guardian resets and runs it's check. So if it's going off it's because a condition "exists right now", not like the check engine in your car that can stay on for multiple cycles even if the fault has cleared.

But like your car, even if the problem clears it will be stored and can be retrieved with the code reader.

Hi Graham, Thanks for the quick help. The engine runs great but runs right at about 90 % of listed rpm (5,600) that's why I think might be in some kind of a Guardian Mode. When prop was short boat ran 51 mph @ 4900 rpm then bigger prop boat ran 64 mph @ 4900 rpm. Also we think tach is right off prop pitch calculation's, also when this prop went on another identical boat with 200 HP Mercury boat ran 72 MPH @ 5,400 RPM. Does have a light and horn can't see or hear because of wind and sun. Will find a Mercury Shop to plug in and get reading. Any of the Guardian modes put it to 90 % Thanks again Jon

Yes, low battery on a 4 stroke model - can't think of any else off the top of my head.

Sounds more like a bad tach or feed from PCM as usually any guardian alarm will reduce power and rpm to about 3000 rpm or less.

We have two identical boats, one with Mercury 200 HP with a 1.87 gear ratio with 28 pitch prop runs on GPS 72 MPH @ 5,400 RPM Next boat Mercury 150 HP / 1.87 gear ratio, no matter what prop is installed peak is 4,900 RPM when the same 28 pitch off other boat is installed this boat runs 64 MPH @ 4,900 RPM so we are sure tach is very close or dead on, engine just stops at 4,900 Motors are 2 strokes. Next week will have it hooked up and will let you guys know what is up. If find the RPM will end up with a 26 Pitch Prop. Thanks Jon

If it's a 2 stroke then there is no guardian mode that would stop it at 4900 rpms, "unless" the over rev module is gone flooey and it thinks you are running higher rpms than what you are.

It's job is to kick in and reduce rpms to a safe level if it detects rpms above the top end of the wot range.

Thanks again Graham, Will the Mercury scanner tool be able to tell us that, also will it tell hours on the 2005 model. Could we for a trial run disconnect rev module. What ever the issue is engine always levels out at the 4,900 Has great compression, starts and runs great. Was a used motor so new to us. Thanks again for the quick help. Jon

The event switch that tells the PCM that the lower is in gear or neutral will limit the rpm's below 5000. This is not a neutral safety switch. Disconnect it to test if a diagnostic scan tool is not available.

Thanks Chris, Will report back. Jon

Chris can you tell where to find this switch, Motor is a 2005 150HP L XR6 Thanks again to all. Jon

You dont have one as unit is not a EFI/DFI motor.

faztbullet said:

You dont have one as unit is not a EFI/DFI motor.

Click to expand...

That was my next question.

Looks like it only has a small box on it, no plug ins at all. Must have a rpm limiter built into box. Will change and see if that went bad.

Mercury V-6 carbed engines do not have rev limiters, swap tachs as it sounds like bad tach...

Ok. Will try right away. Thanks again. Jon

Ok new program, this engine 150 HP Merc, 2 stroke with carbs. apparently has no rev limiter at all, runs great, good compression, starts great, but will only rev to 4,900 RPM no matter what prop is on it wont go past 4,900 rpm tach is right on, what could cause it not to go up to say 5,500 rpm.

Stupid question, does your throttle open all the way?

No that is not a stupid question, yes it does, at this point any reply is fine trying to find another 600 to 700 RPMs. Thanks Jon

Looking in the Mercury trouble shooting section I see the timing bumps up 6 Degrees at 5,000 RPM and at 5,600 RPM drops back the 6 degrees, may change the small electric box with another one to see if that helps.