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This weekend I did some monitoring of the ECT in my 93 245 LH 2.4 and wanted to share some observations I made. I back probed into terminal 13 of the ECU with the harness attached and monitored voltage and resistance both while the engine was off and while it was running. My understanding of how the circuit works is that when the key is on, the ECU is supplied with a variable voltage. It doesn't get a constant voltage because of intermittant loads on the system such as the fan, lights etc. I guess it's too difficult to design a circuit that supplies the ECU with a standardized voltage. So, the ECU checks the voltage and then sends it to the ECT and interprets the voltage it gets back in terms of a temperature. Because of this, the ECT's resistance, and hence the temperature, cannot be accurately measured at the ECU while the engine is running since the circuit is powered. If you want to observe resistance while the engine is running, you have to disconnect the ECT and probe it directly. The ECT in a running engine must be observed in terms of a voltage. Since this voltage changes due to the car's changing electrical load, the graph in the Volvo fuel injection manual shows a band (or range) of voltages corresponding to temperature instead of a curve.
By observing the voltage change, it's amazing to see how the ECT quickly responds. I can move the heater control lever to hot (without turning on the fan) and see the ECT voltage instantly increase.
An interesting thing is that the ECU remains powered for exactly 3 minutes after the key is removed. It appears that the ECT circuit has about 0.4 volts on it during this time. This time corresponds the the time during which the fuel pump is not energized when the key is moved to Pos II. In other words, you may have noticed that if you restart your car (LH 2.4) after shutting it down, you have to wait at least 3 minutes before the fuel pump will repressurize the fuel rail when the key is moved to Pos II. I guess the assumption is that the fuel pressure in the fuel rail hasn't bled off until at least 3 minutes after shutdown, so there's no use in repressurizing it until the engine starts.
I'm going to make a breakout box so I can do more of this a little easier than back probing, and maybe extend that to some kind of datalogging using either a dedicated datalogger or a PCMCIA card. If anyone wants to discuss this let me know.
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