If you have a 1989 US delivery Volvo, you do not have a thermo time switch. Thermo time switches were used on CIS and possibly other earlier styles of Bosch fuel injection. They had heating coils and a bimetallic switch and directly controlled the fifth injector. Your car has LH 2.4 Jetronic, if US delivery. It has a coolant temperature sensor. In the LH 2.4, the coolant temperature sensor is actually two sensors in one. It is an input to the fuel injection and the ignition. The coolant temperature sensor is a variable resistor based on temperature. Its input to the ECU controls the mixture. The fifth injector on your car should be disconnected, as I remember reading that Volvo has a service bulletin out to do such. A bad coolant temperature sensor will lead to poor starting, poor economy, rough idle, and poor power. The place to check the resistance of the coolant temperature sensor is at terminal 13 of the ECU. I don't have resistance values for the coolant temperature sensor for the LH 2.4, but for the LH 2.2 it is between 2,280 and 2,720 ohms at 68 degrees F. I think it is similar for the LH 2.4. There is additional information in the 700/900 FAQ.