Temperature gauge Volvo Penta MD31A

[corvus]

I have a Volvo Penta MD31A from about 1990.



For the last year or so I am trying to figure out why my temperature gauge is not showing anything any more. It used to work perfectly and then for some reason that is beyond my understanding it stopped working.


This is what I have done so far in an attemt to fix it:

I replaced the temperature sensor.
I tested the temperature sensor (the new and the old one) in boiling water; the Ohm reduced nicely from somewhat 300 Ohm down to 30 Ohm; I rigged the gauge with a 12 V supply and connected the temperature sensor that was in boiling water and it nicely showed me some 90 Degrees Celsius. So I figured both the sensor and the gauge must be working ok and it must be something in the cabling.



I tested the signal cable from the temperature sensor to where the gauge is and there was almost no resistance (0.2 Ohm).
I tested the connection between temperature sensor housing and the engine on various points and always found nice conductivitly.
Here is the weird part that I do not understand: if I test the conductivitly between the temperature housing and the minus on the controll panel where the gauge is I find no resistance and the same resistance between sensor signal cable and gauge minus as I have between sensor and sensor housing. However, when I turn the ignition key on (before starting position) that connection between temperature housing or engine ground and the panel is "lost" and I have huge resistance. I believe that is also the reason why the gauge isn't showing anything because it simply converts huge resistance into "no temperature".


I also tried to shorten the temperature signal cable at the sensor with the housing and then the gauge will nicely move to its max temperature position.


I can't figure out why the gauge isn't showing any temperature any more. I put it down to the fact that there seems to be no connection between the engine ground and the panel when the ignition key is on.
So I took the ignition key apart and looked if there is any shorting or corrosion; it is all looking in good order.


I also thought it might be a relay that is causing a problem with the ground. But there are only two relays and they shoudl only come on in starting position and not before.



Would anyone of you engine experts have any idea what to look for to get this conundrum solved?


Many thanks in advance!
Stefan

[Wotname]

Originally Posted by corvus

I have a Volvo Penta MD31A from about 1990.



............
Here is the weird part that I do not understand: if I test the conductivitly between the temperature housing and the minus on the controll panel where the gauge is I find no resistance and the same resistance between sensor signal cable and gauge minus as I have between sensor and sensor housing. However, when I turn the ignition key on (before starting position) that connection between temperature housing or engine ground and the panel is "lost" and I have huge resistance. I believe that is also the reason why the gauge isn't showing anything because it simply converts huge resistance into "no temperature".


I also tried to shorten the temperature signal cable at the sensor with the housing and then the gauge will nicely move to its max temperature position.


I can't figure out why the gauge isn't showing any temperature any more. I put it down to the fact that there seems to be no connection between the engine ground and the panel when the ignition key is on.
So I took the ignition key apart and looked if there is any shorting or corrosion; it is all looking in good order.


I also thought it might be a relay that is causing a problem with the ground. But there are only two relays and they shoudl only come on in starting position and not before.



Would anyone of you engine experts have any idea what to look for to get this conundrum solved?


Many thanks in advance!
Stefan

OK, you have carried out a lot of good diagnostic work so far, so keep going to get to the problem!

The emphasised section above suggests to me that you are trying to measure the resistance of the sensor wire while the circuit is active - you can't do this.
Resistance measurements using the an ohmmeter must be made with the component (the wire in this case) inactive and preferably disconnected at one end.

Although I not familiar with the ignition key switch on the MD31A, other marine diesel engines that I am familiar with switch positive with the key on/start switch, not negative.

I suggest you make sure you have 12V at the gauge when the key is turned ON and then measure the voltage (to negative) on the sensor wire at the gauge end.
Measure it with the far end (i.e. sensor end) disconnected and also with far end shorted to the engine block. Disconnected it should not read any volts and connected it should read zero volts. Reconnect the wire to the sensor and it should read "some" volts (but less than 12V). The actual volts will depend on the engine temperature. Investigate any variation to these readings.

Good luck.

With any electrical trouble shooting, any unusual or unexplainable results normally means one doesn't understand the circuit or measuring techniques. Put another way - electricity always obeys ohms law and when it appears not to, we are wrong in our understanding!

[GordMay]

Basic Engine Gauge Theory and Testing
➥ http://www.cruisersforum.com/forums/...sting-645.html

[jimbunyard]

There are known issues with Volvo wiring harnesses of that era, usually regarding, in my opinion, undersized wiring, crimped internal pins, and grounds and power wires running through 16 pin screw-to-connect connectors.

Any where you have one of these



consider it suspect until you've tested it under load. They are very prone to corrosion-induced heating and cracking, which can lead to all kinds of electrical gremlins.

They are often hidden, and are sometimes inaccessible when multiple cables are required to span long runs.

[jamhass]

Ditto what Jimbunyard said. Best in the long run to replace them with good terminal strips.

If you really want to replace or fix the existing connectors, they are an Amphenol unit and bodies, pins and sockets are available at good electronics outlets, such as Allied. Look at what you have closely and you will find Amplenol part numbers.

Also if you still have the volvo "multi-fuse" block, replace it with a good sealed fuse holder. Volvo thought the multi-fuse would make it easy to replace a blown one just by moving the connector, but the "unused" fuses tend to corrode and become useless.

[skipperpete]

Hi Corvus
Have you checked the actual operating temperature of the engine with a thermometer? My Volvo temp gauge starts at 40°C and operating temp is only a few notches above that at 75°. Maybe one of the thermostats is stuck open and the engine is not getting hot enough to move the gauge.....just a thought[emoji848]