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I was measuring 2 voltages today an a train air system. The system is a pressure transducer which takes in 24v supply and outputs a respective voltage dependant on pressure. Only 2 wires in circuit I referenced my voltage reading to earth, battery return and the negative supply from the control unit of the transducer but every time I connected the probes the voltage decreased exponentially to 0v in say 30seconds. Why?

It seems that your voltmeter provides an unexpected path (in the mega-ohm range resistor) which perturbate the working of the circuit (discharge of a polarized capacitor used to bias an AC signal, as example).

In general, the OUTPUT of the transducer is identified and measured in relation to its own GROUND, also identified, but this can be dependent of the circuit. If not identified, you are on your own, I am afraid, since the amplification can be through an OpAmp, or even with a single BJT, you have three possible configurations!

It is also possible that the transducer communicates the pressure through a protocol, and not with an analogical voltage. In such case, you have to use the equivalent decoder, not a voltmeter. It can even be a wireless protocol!

Thanks for replying. Was thinking capacitor as acts like discharging a pure capacitive circuut . I'll put the schematic on tomorrow but there no Information about how the sensor detects pressure. Funnily enough the one i had a fault on the voltage measurement acted as expected when there was a fault on it as in a supply voltage of 24v that was readable and a return voltage again readable. No system negative on return from the sensor. Just a feed and signal