250 EFI Continuous Alarm & Guardian Mode

[TideRider]

Hey guys, long time reader and learner of this forum. First post, hoping you guys can shed some light on what direction I should trouble shoot next or if I should take the boat in to see if any codes can be pulled.
Any helps is greatly appreciated. Thank you

S# 0T556597
2002 CXXL
Mounted on jack plate
Max RPM 5790
Prop; Mercury Marine Offshore Series 23P
Idle water Pressure 3-4psi WOT 12-14psi
Fuel Pressure at idle 41PSI,
Analog Instruments & Vessel Mobile View

I do NOT have a DDT

Issue is constant beep of alarm and guardian mode with RPM reduction to 1800-2000RPM. Once I bring the throttle back to clutch alarm and Gmode go away immediately. Issue first arose 11/11/22 when I was testing the new hi pressure fuel pump I had installed out on the water. OEM fuel pump works great and engine starts, idle and comes up to speed really well. Just get the continuous alarm in the upper 3K rpm range. Got the alarm and Gmode when I was on plane so idled back to the ramp and headed home for some forum reading and testing.
Vessel Mobile View shows engine is not overheating, sitting at 125-132F, verified with infrared heatgun.
Port Bank digital temp sensor 3.73k Ohm at 129F measured off the Tstat housing
Port Bank analog temp sensor 183 Ohm at 130F measured off the Tstat housing
Stbd Bank digital temp sensor 3.5k Ohm at 129F measured off the Tstat housing

Tested boat again in the water
1^st test all sensors plugged in alarm and Gmode activated when rpm got to upper 3K range, throttled back down and alarm ceased
2^nd test unplugged analog temp sensor (tan wire), alarm and Gmode activated in upper 3K range. throttled back down and alarm ceased
3^rd test plug analog sensor back in and disconnect port bank digital temp sensor, alarm and GM activated at upper 3k rpm range. throttled back down and alarm ceased
4^th test plug port bank digital sensor in and disconnect stbd bank digital sensor, alarm and GM activated upper 3K range. throttled back down and alarm ceased
5th test unplug all 3 heat sensors, Vessel mobile view showed engine temp of 34F, alarm and GM activated upper 3k range. throttled back down and alarm ceased
6^th test plug temp sensors back in and unplugged oil reservoir float switch (blue wires with blue tracer), alarm and GM activated at upper 3K range. throttled back down and alarm ceased
7^th test Plug oil tank float switch back in. Rev in neutral to upper 3K RPM alarm and GM activated. throttled back down and alarm ceased

Tested Crank shaft position sensor (358.6ohm) when i got back to the house.

Motors both have their own fuel pick ups and lines from one fuel tank. I Just had the VST apart and it was still clean and nothing offensive in the fuel/water separator.

[JOE54]

suggest have all faults removed return to water for test create warning one time and return to dealer have fault scanned then youll know for sure , possible TPS problem

[TideRider]

Roger that. Thank you

[JOE54]

that way you and he knows its the only recorded fault stored in freeze frames

[EuropeanAM]

+1 on printouts.

You've either PM'd or emailed this same question as well. Rather than re-type the entire response, check the shift position switch also.

[TideRider]

Problem Solved.
Took the boat to have it scanned and showed hi voltage in the RPM range where the alarm was activated. Suspected bad alternator (aftermarket one not OEM).
I swapped the alternators and took the boat out for test run. Problem with same exact symptoms followed over to the STBD motor.
Ordered and installed new OEM alternator and took the boat out again yesterday. Everything worked and ran excellent.
Interesting part is the bad alternator was putting out 14-.14.2V at idle or low speed so I never suspected it. There fore never looked at the voltage on the Vessel View Mobile as i was concerned about overheating.
The VVM did show the hi voltage when I tested the bad alternator on the STBD motor.

Lesson learned really have to check all the basics even through the RPM range if possible.

Don, I'm sorry about double messaging and being inconsiderate of your time. I can get a little overly enthusiastic when I start searching for answers.

[EuropeanAM]

That fault info can be quite helpful in many cases.