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Expansion Tank Coolant Pressure alarm

Discussion in 'Cabo Yacht' started by CSkipR, Feb 2, 2014.

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  1. CSkipR

    CSkipR Member

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    Man V8 Engines CR900 Alarm
    Couple of times the expansion tank coolant pressure alarm on starboard engine has gone off. Went off again yesterday. I see no signs on the sb engine instruments that are concerning except on the expansion tank coolant pressure reading it shows no pressure. The other shows 9 psi at idle. Pretty sure it's probably the sensor but what should I check for besides coolant in tank and loose wire on sensor. No work has been performed on the engines for the past 7 months.
  2. Capt J

    Capt J Senior Member

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    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.
  3. CSkipR

    CSkipR Member

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    Expansion Tank Coolant Pressure

    Capt J,
    Why does each engine have two heat exchanger caps? One on each end of heat exchanger?
    Skip
  4. Capt Ralph

    Capt Ralph Senior Member

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    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.
  5. CSkipR

    CSkipR Member

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    RC,
    Yes, your correct disconnected the sensor and the alarm started. What comes up on the screen is "Coolant Pressure Expansion Tank". The number on the sensor is 51.27421.0125. I will take the sensor connection apart and check for loose wire connection. Found that issue once before where one of the wires was loose.
    Skip
  6. Capt Ralph

    Capt Ralph Senior Member

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    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.
  7. CSkipR

    CSkipR Member

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    CP Exp Tank

    Agree need to check wires inside before ordering. Yea I know how much these **** sensors cost because I've replaced two. Ridiculous! $350-500 I'm sure BB over at MLD has one, well maybe.
  8. Capt J

    Capt J Senior Member

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    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.
  9. Capt Ralph

    Capt Ralph Senior Member

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    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
  10. Capt Ralph

    Capt Ralph Senior Member

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    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.
  11. Capt Ralph

    Capt Ralph Senior Member

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    BTW 0.7bar = 10.15psi
  12. Capt J

    Capt J Senior Member

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

    CSkipR Member

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    Coolant Pressure Exp Tank

    The caps were all replaced last year in March when I had the 1000 hr service completed. Will switch them around to see if anything happens. Never no one could be defective.
    Later today will check the wire connections on the sensor.
  14. CSkipR

    CSkipR Member

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    CP Exp Tank

    Was not able to get to the inside of the plug to check wiring. Not enough slack in wire. Checked water/antifreeze and when I removed cap (eng cold) there was a pressure drop so pretty sure cap is good. Will take the boat out again maybe this week and see if it alarm goes off again.
  15. cnkale80

    cnkale80 New Member

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    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.
  16. Capt Ralph

    Capt Ralph Senior Member

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    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.
  17. Capt J

    Capt J Senior Member

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    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.
  18. Jim Davidson

    Jim Davidson New Member

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    Where is the tank on a princess 61?
  19. Capt J

    Capt J Senior Member

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    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.
  20. Voodoo

    Voodoo New Member

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