Mercruiser 7.4L MPI Alarm won't stop

PetewilliamsNS

Recruit
Joined
Oct 5, 2022
Messages
1
Wondering if anyone can help.


I’ve got a 1998 7.4L MPI/MEFI3 Mercruiser. Last year it was running fine, but was badly corroded and required a rebuild, which was completed recently.

When I tried to start her up, after about 10-15 seconds, I get an alarm. 1 Second on, 3 seconds off. Goes on forever.

The Mercruiser computer has been connected to it and at first was showing alarms from all sensors, then after a while, only showed alarm #14, low coolant temp. Very odd, as it would always be cold at start-up. Have run it up to temp in idle-1,700. Alarm stays on.

I’ve tried a new sensor, and have checked wire resistance is OK.

Any ideas???
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,348
If it’s not the sensor you’ve replaced…the odds on favourites otherwise are the gear oil header tank level low or it’s sensor at least. The IAC and sw pressure sensor is common too. How’s the oil pressure ?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,115
This is what's called a soft alarm. Certain MEFI-3 will set a soft alarm below 3000 RPM and it will go to a hard alarm (constant sound) above 3000 RPM.

The MEFI-3 engine’s Audio Warning system will sound the alarm differently and it has more
items that will cause it to sound.
There is what is called a ‘soft’ and a ‘hard’ alarm.
1. Soft Alarm = Below 3000 rpm. Horn on for 1 second, off for 3 seconds, on for 1 second,
off for 3 seconds, etc.
2. Hard Alarm = Above 3000 rpm. Horn sounds all the time.
Audio alarm sounds for the same malfunctions as MEFI-1 and -2;
1. MCM - Low lube level in Gear Lube Bottle. [General 1] or MIE - High Transmission fluid
temperature [General 2].
2. MCM/MIE - Low engine oil pressure.
3. MCM/MIE - Too high engine coolant temperature.
Early production and service ECMs had the following items that will sound the
alarm;
1. MCM/MIE – Horn will sound if there is an active engine Diagnostic Trouble Code [DTC]
that is occurring while the engine is running. The horn will not sound a stored DTC.
Correcting the DTC malfunction will stop the horn.
2. MCM/MIE – Horn will sound if the battery voltage to the ECM is low, [less than 9v for at
least 5 seconds]. Using the throttle lever to increase engine rpm to get the alternator to
put out more voltage will correct most low voltage problems.
 
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