Expansion Tank Coolant Pressure alarm

Check coolant level, then put the cap on and find the shrader valve and pump it up to 16 psi. See if the problem persists. I've had that happen on your style MAN's and it turned out that the alarm board was bad and about $2500. We had to change the board after a while because you couldn't reset it anymore and it would make the engine go into Limp Mode and restrict RPM's to around 1600rpms. If it goes off again, then try unplugging the sensor while the alarm is on and see if it goes off (I'm not sure if this will work.) The only other thing you could do is swap sensors from engine to engine and see if the issue persists.

Depending on the installation, you can always get to one of the caps. Yes, they are different size also (L vs R).
On the sensor, With a med/small point Phillips screw driver, you can remove the retaining screw that allows the plug/cap from the sensor. This cap can open up more revealing the wire connects to the plug. Check them over well, spray with wd-40 and reassemble. This will clear up about 25% sensor problems. More than 50% more of the time it's the sensor. Another 15% the engine harness connects or the computer box connect. That should leave 5% for connects and board problems inside of the box. That last 5% is left for FM.
Never found anything in the manuals about precharging the coolant system (only diagnostic procedures). It has been mentioned in these forums before.
The caps blow at lower pressures so the system can't hold 16psi and I would be scared of pumping any block up that much.

You will get a solid sensor alarm if unplugged. A bad connect just makes noise and bugs the computer.

What is the exact quote from the display when this error comes up. There are a few pressure sensors on the V8-900 crm.

I may have a sensor in my kit.

I have the .126 sensor in hand (pressure after pump). I can get you a .125 but you have to promise to take a heart pill before I can quote it.
Let's check all connects before we go there, including swapping between mains.

They have 2 different caps for purging air out of the system when you drain all of the coolant for servicing. Mans have to be bled/filled a certain way with both caps to get the air out. You should only open the one on the expansion tank (larger one) for checking and filling.

From the manual, other than diagnostic work;

Note:
Do not open the cooling system when the engine is at operating temperature. This causes a pressure loss in the cooling system.
If the cooling system has been opened when the engine is at operating temperature this can lead to the alarm “pressure in the expansion tank” when the engine is then put into operation and to a reduction in the engine output.
Coolant pressure in the expansion tank is only built up again when the engine has cooled down. The cooling system must therefore only be filled up when the engine is cold.

If the expansion tank needs to be filled up while the engine is at operating temperature, the must be observed:
To operate the hot engine without alarm again after the end cover has been opened, a prepressure of 0.7 bar must be present in the cooling
system
To achieve this, a pressure valve has been fitted on the expansion tank; a commercially available air pump can be connected to this. Use this to pump the system up to 0.7 bar

I was just reminded of a call last year. A leaking cap caused the same error you have. The manual states to replace the caps every 2 years.

Try cleaning and rotating the caps between the COLD engines.

If you read the last post. Service the caps and coolant level only when cold. The computer watches the pressure come up with the temperature.

BTW 0.7bar = 10.15psi

Yes, I have also seen the leaking caps cause the alarm as well. Change them all. MAN does recommend every 2 years on them.

Glad I found this thread. A buddy of mine is moored up at Coinjock NC and alarm, on a MANS 1550 says"Cool.press exp.tank" but the actual eng temp is ok. If anybody has any advice please call me asap (dont want to give out his number) at four34 2nine4 seven7twozero. Thanks folks.

Yes; Search all past MAN post and call a real MAN tech out.
The reason I recommend searching all MAN post, most issues were solved with a real MAN tech.
It could be a sensor, one of two caps gone bad, then the list get s deeper.
You want to protect a very expensive engine, Call a REAL MAN tech.

What is the man engine display reading for coolant pressure at expansion tank? What does the other one read? I’ve had these go bad before. The sensor is usually located at the bottom of the expansion tank.

Where is the tank on a princess 61?

Coolant expansion tank is on the front, top of the motor on MAN's, the pressure sensor is usually upside down on the bottom of that tank.