Coolant Pressure Sensor on Man CR900

Did you pump up the shrader valves (on the top of the expansion tank) to 16 psi with a bicycle pump after filling with coolant and water? You need to do this anytime you drop the coolant, then get them up to temperature, let it cool off, then check the level and pump it up one more time. Also MAN requires both coolant caps replaced every 2 years, this is important also.

If this doesn't solve the problem, it's a bad sensor....I've had them go bad a few times on 1100 common rails.....the one for an 1100 Common rail was $412.14 (part # 51.27421.0258) in 2013 .....I wouldn't swap, as you could easily break the other sensor. The sensors/senders seem to go bad on the port 1100 Common rail, but never seem to on the stbd for some reason.

You're supposed to pump up the coolant system anytime you drop the coolant. Try it, it might be an air bubble that is causing it to show the low pressure for a second. But generally, from what I've seen on the MANs, it's never a connection issue but the sensor itself and generally how they behave when they start going bad.

hopefully that's all it was. When the one on the 1100 was going bad it would sometimes work fine and sometimes do what yours is doing......time will only tell. It's a shame the MAN sensors are so darn expensive, a similar sensor for a CAT is like $60. Have you had to replace your alarm monitoring boards yet? We had to do both of them and was around $4500 (need the dealer to program the new ones included in that). Went to the boat one day and both hour meters read 99,999 hours. It's a known problem because MAN had the data stored in the EPROM instead of the RAM on the circuit board or vice versa and the new ones fixed the problem and are recorded in the right section.

Skip,

FWIW, we've got the same sensors. It's right behind the heat exchanger on the starboard side of the engine if we're talking about the same thing. It takes a deep fairly large socket.

I had one that was exhibiting the same symptoms you've outlined. I was convinced it was a connector. Right before our big exumas trip it acted up and I swapped it with the other motor. The problem moved to the other motor. I bought a new sensor. IIRC, it was around $150. If there is a common sensor failure on these this seems to be one. I've run my boat a lot and this is the only sensor issue I've had. It took about 15 minutes to fix.

There are many opinions on how to pressurize the tank. Everything from 16lbs to nothing. Having spent a lot of time with the MAN tech I've arrived at the following:
1. Easy after a refill to get some pockets of air that come through when the tstats open. Just let it warm slowly. Same when cooling it off. I pressurize to 8psi if coolant has been removed or lost.
2. If I do #1 I don't worry about pressurizing the tank and I never have an issue.
3. Starting at 0PSI mine will be at 4psi as soon as the temp comes up at all.
4. I'm about 10-12psi at 183 degrees.
5. caps are new and I keep the coolant about 2" below the neck or it pees out.

Again, you're on top of your motors but just passing on what I see and what's worked based on input from the MAN tech.

Tom

192-194F is fine and does not hurt anything for them to run at that temperature. They might not even be running there and the senders could be off a few degrees. Some of the common rails will always run at that temp. I ran a set of 1100 common rails brand new, and they ran at 192F at 80% load in a 45' Cabo at the time and all of the way from Fort Lauderdale to Savannah.

IIRC - if I recall correctly.
I bought the sensor at Scott marine in FTL.
I don't think the tank has much volume and not a lot of pressure. just a quick pffft undoing the valve and some pressure is lost.

You may have just had some air pockets working their way through. I did when I did the HE's. I went 200miles to key west bringing the boat back and it was throwing alarms when I got in. I was still in the virgin MAN paranoid state so I was convinced the earth was ending. Just the sensor since then.

Fwiw, Scot told me it's not worth buying spare sensors. They don't fail often and they can go bad bouncing around in a tool box.

Unfortunately, it is what is. I used to justify it when the Euro was $1.50-$1.60, that it was the exchange rate that caused the high German parts prices.....BUT the Euro has been down for quite some time and about even and the prices are still high.

When you change it, take the cap off the heat exchanger to let any pressure out, put the cap back on, if you put a rubbermaid or something under the sender, you can usually change them without losing but a solo cup of coolant (have the new one ready to screw right in) that you can pour back in.