Help Please Mercruiser fault codes Overspeed and Blk Press Low

I have twin Mercruiser 8.1 HO

I have twin Mercruiser 8.1 HO inboards and experience an intermittent problem in my starboard engine. Intermittently (and with no apparent pattern) my guardian alarm sounds a continuous tone just as I exceed 3500 RPM. All gauges read normally and my Rinda scan computer shows no fault codes or evidence of the guardian system in memory (other issues from the past do show up in the memory however so there seems to be a memory function that works). Under this circumstance the guardian system does not cut power either. I finally was in a position to hook up the scan tool while the alarm was sounding and it shows the following codes (though once again none were retained in memory):

19 Overspeed
9 Guardian
3 Blk Pres (Low)

Both engines appear to run perfectly without any other incident. Shutting the ignition off and restarting often cures the problem for hours. Any hints would be greatly appreciated and thanks so much in advance for help…this is driving me slightly crazy!

"A Low Block Pressure fault ca

"A Low Block Pressure fault can cause a hard alarm and cause the engine to go into Gaurdian Mode. When this fault is set a reduction of Available Power is made based on engine and manifold temps and engine speed. The more critical the situation, the more Available Power is reduced. This is one possible explaination.

Another possibilty is that you have a problem with the Shift Switch. If the ECM thinks the engine is in neutral, it will reduce the rev limit to 3500RPM, and will set an overspeed fault and hard alarm when this RPM is exceeded."

"Dave,

I think you may well

"Dave,

I think you may well have the answer here!! I occassionally (here comes that dreaded intermittent situation again!)have a situation when starting the engines where I have to move my shift levers back and forth as the neutral switch has stuck indicating the engine is in gear when it is not. I will check this although I would be lying to say I know off hand where the switch is located...at the lever or on the transmission? I am also unsure as to exactly what a low block pressure fault indicates i.e. is this referring to water pressure, oil pressure, exhaust manifold pressure etc and where the sender is approximately located for this particular alarm, because there sure seem to be quite a few of them on these engines.

I hate to sound so ignorant (or to admit I am as ignorant as I am to be more honest) as I have restored cars for 30 years and maintain my boat to a similar standard but am still learning the intricacies of modern marine engines every day. (My skills with a "one lung Palmer" don't get me too far any more.)

I am really grateful for your input and continue to be amazed by the amazing communities that exist on line and the willingness and kindness shown by people within them. Thanks so much!-Michael"

"I learned something too! Well

"I learned something too! Well done, Dave N."

"Dave N.
Thanks for a very in

"Dave N.
Thanks for a very interesting rundown on Michael's issue. You are obviously well versed on Merc EFI systems.

Got a question for you. I have a 6.2 MPI with an Alpha Gen II in my boat. The engine was installed last year. When installed we removed the Bravo shift cable bracket with the shift switch that came with it and replaced it with an Alpha bracket with an interrupt switch only; no shift switch. I have been wondering since what implication this may have.

The engine seems to run very well, and certainly exceeds 3500 rpm without any apparent issues. I have not had any alarms at all. Is there any thing I should do with the open plug conector that the shift switch is supposed to plug into (such as jumper it)? Will it harm anything to leave it as is? Should I retrofit the shift switch to the Alpha bracket and connect it up?

Thanks,
Rod"

"Mike,
Block Pressure refers

"Mike,
Block Pressure refers to seawater pump system pressure. Meaning you may have a restricted intake or bad impellor or something. The sensor is located on the transmission cooler at the back of the engine. Normal pressure at idle as 1 to 3 PSI, and you should get about 15 PSI at WOT.

I’m not as familiar with the 8.1 (inboard) as I am with the 496 (sterndrive), and I'm not sure if the ECM uses the neutral safety switch or if it has a seperate switch on the transmission, but it shouldn't be too hard to figure out. You should be able to test by disconnecting the switch, which would tell the computer that it's in gear.

Rod,
There shouldn't be any issues with not having the shift switch connected, the early 496/8.1's didn't have them at all, it was added later. What is your interrupt switch connected to?

Dave"

"Dave,

Once again my most s

"Dave,

Once again my most sincere thanks. You have more than made my day!

Michael"

Dave;

My interrupt switch i

Dave;

My interrupt switch is an NC plunger type (new style) that is wired in series into the pink/wht wire from the ECM to the coil trigger circuit.
It seems to work quite well.

Rod

2006 350 mpi with overspeed as well 9-19 codes.
I change water pressure valve fuel pressure fine Techmate pro says overspeed 2850 rpm ?? anything would be helpful