Mercruiser water pressure sensor

Devil_Inside

Petty Officer 3rd Class
Joined
Jun 17, 2007
Messages
76
It appears that my water pressure sensor is faulty, which I understand is a pretty common problem. I found an older thread on this forum discussing possible alternatives for the "OEM" sensor, which costs about $140, and New Eagle was mentioned as possibly being the sensor manufacturer. I was wondering has anyone been able to figure out how to order one and from where?

Alternatively, does anyone know how to successfully bypass this sensor? Again I found multiple discussions in older threads, but thus far I haven't been able to find the correct resistor value... If anyone can shed some light I would really appreciate it.

This sensor cause my engine to go into Guardian mode yesterday at the worst possible time (crossing a choppy inlet) and it wouldn't let me go above 1200RPM.... I really don't want this happening again, and if these sensors are known to flake out, I would much rather bypass the sensor altogether and rely on my temperature gauges (analog and VesselView).

P.S. Since you guys are probably going to ask, the engine is a 2005 Horizon MPI 350 Bravo, serial 0W3285##
 

Devil_Inside

Petty Officer 3rd Class
Joined
Jun 17, 2007
Messages
76
Thanks, Bt Doctur. After experiencing firsthand what happens when this sensor fails, I am not sure I can trust the Chinese knock offs. Heck, I am not even sure I would trust the overpriced OEM version.

I would prefer to bypass this thing altogether with a resistor package, but I haven't been able to find a basic diagram nor have I been able to find the correct resistor values - do you know where I might be able to find this information?
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,092
Its a 3 wire system. 5 V , Gnd, and a wiper. Havent got around to building a test jig yet. If you can hold off a few days....................
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,092
some of my old notes suggest 2 15,000ohm resistors in series with the center tap as the wiper
click to enlarge
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
Would be worth a try to get a 30 to 50K POT and do some slow adjusting to find the sweet spot
 

metsfan3197843

Petty Officer 2nd Class
Joined
May 2, 2011
Messages
156
I was lead to believe if this sensor kicked into guardian mode in a jam a guy could unplug it and with an open guardian mode would only stop you from exceeding 90 percent. Anyone confirm this?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
...2005 Horizon MPI 350 Bravo, serial 0W3285##

That serial number isn't showing up anywhere. If you're going to put a serial number, we need all of it.

That may be in the era of the first generating sensors. They were suspect to static discharge. Merc changed the sensor and the way it was grounded on later engines.

If you look up the sensor (using the full serial number), the new one is the revised sensor. Shouldn't give you any further trouble.

If you do try to bridge it out, the ECU will detect it. As it will be expecting a 0 value when the engine isn't running and a 'dumb' bridge won't be giving that... It will flag the sensor as fauty and put you in guardian mode immediately...

Chris....
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
I was lead to believe if this sensor kicked into guardian mode in a jam a guy could unplug it and with an open guardian mode would only stop you from exceeding 90 percent. Anyone confirm this?

Yes. You are correct.. (see Guardian mode chart extract below)

Chris.......

SWPlo.PNG
 

metsfan3197843

Petty Officer 2nd Class
Joined
May 2, 2011
Messages
156
Thanks. Something to keep filed away in a hairy situation

sorry for the hijack but good info for emergencies
 

Devil_Inside

Petty Officer 3rd Class
Joined
Jun 17, 2007
Messages
76
Thanks. Something to keep filed away in a hairy situation

sorry for the hijack but good info for emergencies

Not a hijack at all, that was actually super helpful. If that's accurate then I am not going to rush replacing the sensor right away and instead I will try cleaning the connectors and the hole it screw into since that's the way it's grounded.

More importantly, is that true for the rest of the sensors - temp, oil pressure, etc? I dont believe those are as prone to failure, but if push comes to shove I want to be able to disable the Guardian system in a hairy situation.
 

Devil_Inside

Petty Officer 3rd Class
Joined
Jun 17, 2007
Messages
76
That serial number isn't showing up anywhere. If you're going to put a serial number, we need all of it.

That may be in the era of the first generating sensors. They were suspect to static discharge. Merc changed the sensor and the way it was grounded on later engines.

If you look up the sensor (using the full serial number), the new one is the revised sensor. Shouldn't give you any further trouble.

If you do try to bridge it out, the ECU will detect it. As it will be expecting a 0 value when the engine isn't running and a 'dumb' bridge won't be giving that... It will flag the sensor as fauty and put you in guardian mode immediately...

Chris....

Hi Chris,

I hadn't thought about the 0 value when the engine isn't running so you make a great point. I'd be tempted to test this, but for now I just want to get back on the water and enjoy the rest of the season, which is quickly coming to an end. The reason why I was even considering bypassing it in the first place was not to save money, but rather to avoid another potentially dangerous situation down the road, should the replacement sensor flake out as well.

As far as the serial number is concerned, simply replace ## with any two numbers you like, for example "01" and it would bring up my engine configuration. I prefer not to post my actual serial number for the entire World to see.
I can tell you is that I believe that I have one of the earlier sensors because it is WHITE in color, where the revised version is Black - is that correct?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
.... I prefer not to post my actual serial number for the entire World to see....

Why not? They can't do anything with it, except look up parts... And by not including it, you're just making it harder for the people who want to help you...

Chris.......
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Agree with AC Chris here. Just get another sensor. Problem solved. Lasted years in mine and hundreds of hours since.
 

Devil_Inside

Petty Officer 3rd Class
Joined
Jun 17, 2007
Messages
76
OK, so the plot thickens - I replaced the sensor with an OEM Mercruiser sensor, and my pressure readings are still kinda low. According to VesselView, I am only getting 0.4psi to 0.7psi at idle, about 2psi @1000rpm, 4-5psi around 1500rpm, etc. No alarms or Guardian mode this time around, but I only ran the boat for a short period of time, at the slip.
I went to Home Depot and I bought a cheap 0 to 15psi gauge, and a couple of fittings, and I installed it in place of the sensor. My readings went up ~1psi, so at idle I was getting a reading of 1.4psi to 1.7psi, about 3psi @1000rpm, 6psi @1500rpm and 8psi @2000rpm.
I don't know how accurate the cheap $10 gauge is, nor do I know how accurate the new pressure sensor is, but even the higher values shown by the gauge are kind of low aren't they?

This motor is new to me and thus far I've only ran it for about 2 hours total (everything from idle to WOT), during which time the temperatures stayed normal according to both the analog gauge and VesselView. This engine replaced a raw water cooled 1995 Mercruiser 5.7 TBI/EFI, which I ran for 6 years and over 800hrs, and it too never had any overheating issues so I don't think the problem is "Bravoitis". I am embarrassed to admit that I never checked what's behind the water outlet while I had the motor out, but at the time I hadn't even heard about "Bravoitis".
The impeller and wear plate are brand new, and so is the water distribution housing, and water hose (transom assembly to raw water pump). While I had the raw water pump apart I didn't notice any excessive scoring inside, but then again I probably wasn't paying as close attention as I should have as I wasn't aware of that known issue either....

I am really not sure what to make of this, but I think I should do a flow test as a next step. I recall reading about a target value of ~7.5qts water for 15 seconds of runtime as a baseline - does that sound about right?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Pressure is merely a resistance to flow. If you have no resistance (like the engine, manifolds and elbows are clear of obstructions) then you could have a high flow, 60 litres/minute, and still show little on a pressure gauge. You could also have the opposite, high resistance and low flow. That would show as a high pressure. Without knowing the flow for any given pressure, the numbers are almost meaningless.

The flow is more important then pressure.

Chris........

Extract from the Merc service manual..

pump output test.PNG
 
Last edited:

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,581
If your in fresh water Bravoits may not be a concern, but would be easy enough to check, if you can get at it. Remove the hose fitting on transom assembly

Has the exhaust elbow gasket been replaced, and if so which ones?
 

Devil_Inside

Petty Officer 3rd Class
Joined
Jun 17, 2007
Messages
76
Pressure is merely a resistance to flow. If you have no resistance (like the engine, manifolds and elbows are clear of obstructions) then you could have a high flow, 60 litres/minute, and still show little on a pressure gauge. You could also have the opposite, high resistance and low flow. That would show as a high pressure. Without knowing the flow for any given pressure, the numbers are almost meaningless.

The flow is more important then pressure.

Chris........

Chris, your comment is once again spot-on. Thank you for sharing the proper procedure for measuring the flow - I will perform the test as soon as possible, and I will report my results.
 
Top