False temperature alarm on Volvo MD22

[Wavelength]

On my MD22LA I have no temperature gauge, only the standard panel.
Problem: when the engine is started a few instants later (max. 1 minute) the temperature alarm is activated.
Actions done up to now:
checked temperatures with infrared thermometer (all OK!)
replaced the temperature switch (9) and the temperature sending unit (6) with new (expensive) volvo ones.
Suggestion of a volvo engineer: replace the complete electronic print
(costs 250 to 300 Euro) but no guaranties if it will eliminate the false alarm.
The alarm test on the panel works fine and I can switch off the soundbeep after starting by pressing the alarm test button once but the temperature alarm light stays flashing and what is the worst that I am not well informed if a real problem with engine temperature should occur.
Anybody with same experiences or suggestions to help me?
Looking forward to reactions

[GordMay]

Greetings and welcome aboard the CF, Wavelength.

[Cavalier]

Hi Wavelength,

Welcome to CF.

As an additional debug, have you tried seeing what's happening with the interaction between the sender and the main PCB? Many temperature sender units are resistance based, and on that assumption:

1. Make sure the engine is cold. Disconnect the sender unit from the main PCB terminal. Using a multimeter measure the resistance across the cable's terminals using a digital multimeter (record this value) then reconnect.
2. Start the engine and allow to warm up until your temperature alarm goes off.
3. Disconnect the sender unit connector from the main PCB again. Measure the resistance across the terminal pins and compare this with the reading from before - there should be some hundred ohms difference between the two readings.

If there's no difference then the problem is the callibration on the main PCB. The chances are that the output from temperature sensor is correct, but something has happened to the linear calibration of the instrumentation inputs on the PCB - so the PCB believes the temperature to be much higher than it actually is: this could be the result of a cracked solder terminal on the PCB, for example.

Sanity check:
From a cold engine, start the engine. Allow it to run for 5-10 seconds and disconnect the sender unit from the PCB. This should result in the temperature alarm going off as the 'measured' resistance goes to infinity. If the alarm doesn't go off at all, even at the time when it would go off, then the problem definitely lays with the temperature calibration on the board - and a new PCB will be your only choice. rationale: the input from the sender unit is drifting as a result of something wrong on the PCB. no input means there's no value available to 'drift' so that the alarm limits aren't reached on the board.

Alternatively, if you have a half decent microscope or magnifying glass you can inspect the PCB following the lines from the sender-unit's input terminal and back to see if there's anything 'wrong' with any of the solder joints..

Phil

[Wavelength]

Thanks a lot Phil for your fast reply.
When i am back on Wavelength next week I will try to follow your suggestions and give feedback.
Cheers,
Bob

[Cavalier]

Originally Posted by Wavelength

Thanks a lot Phil for your fast reply.
When i am back on Wavelength next week I will try to follow your suggestions and give feedback.
Cheers,
Bob

Let's hope it's that simple.

btw: If the sender has 2 wires as output then it's probably resistance based. If it has 3 then it's probably some odd thermopile type device which'll mean that requires a supply voltage (which you'll work out from the output on the PCB) and one of the 3 lines will return another smaller voltage. If that ends up being the case then the best way you can analyse this is to slide the connector off the PCB terminal just enough for you to get your multimeter probes in to the pins without breaking the connection.

Good luck!

[Cavalier]

Originally Posted by Cavalier

If there's no difference then the problem is the callibration on the main PCB. The chances are that the output from temperature sensor is correct, but something has happened to the linear calibration of the instrumentation inputs on the PCB - so the PCB believes the temperature to be much higher than it actually is: this could be the result of a cracked solder terminal on the PCB, for example.

Sorry Wavelength,

The above paragraph should have read:

If there's no difference then it's the sensor (you can let the engine run for 5 minutes to see if there's any change at all), if there's only a small difference then the problem is the callibration on the main PCB. The chances are that the output from temperature sensor is correct, but something has happened to the linear calibration of the instrumentation inputs on the PCB - so the PCB believes the temperature to be much higher than it actually is: this could be the result of a cracked solder terminal on the PCB, for example. The sensor shouldn't be reading so much more/less than the cold reading after only 30 seconds.

Phil

[Cotemar]

Wavelength,

This setup may be useful in the long run.
Wireless Remote Engine Temperature Monitor

http://www.cruisersforum.com/forums/...tml#post654749

[Wavelength]

the problem is solved.
it was the electronic box (containing a PCB with a few diodes and one transistor), placed behind the instrument panel, under the tachometer, what had to be completely replaced. (the inside of the box is not accessible).
I first borrowed a used box from a volvo dealer in Harlingen. which gave oil pressure alarm and not temperature alarm (so was broken too).
Then I ordered a new one (costs 300 euro!) and now problem is solved.
Still have problems with runned-hours-meter,(works only from time to time) but this seems to be normal for Volvo Penta engines...
The volvo dealer in Harlingen was the only dealer I found in the Netherlands who was prepared to take back the unit if this was not the good solution for my problem.