I have a question regarding the coolant temperature sensor on my 89 740 Turbo. My car has the following:

1) dead temperature gauge

2) the idle drops to approx. 500 rpm before returning to 900 rpm after I rev up the engine and let it drop back to idle.

Can these two problems be related ? Also does the 740 uses similar compensation board on the back side of the instrument panel as 240 ? I am thinking about cleaning the board as I did with my 240, but I am not sure if Volvo did not use a more reliable piece on 740....

Thanks !!!

The coolant sensor for the ECU and the gauge are different (the latter is the one toward the firewall). For the idle problem, have you cleaned the throttle body?

The coolant temperature sensor is only an input to the fuel injection system. It is under intake manifold port three. The temperature gauge sensor is under intake manifold port two.

As to the idle problem, clean the throttle body and adjust the idle postion throttle switch (if it needs it), and go to the 700/900 FAQ and set the basic idle speed.

Ok, I confused: which sensor is under intake port three and which is under port 2. It looks like John has the ECU sensor under port 3 but steve has it under port 2. My gas mileage goes to heck in the cold weather and I was thinking of checking this sensor which is why I am asking.

The Coolant Temperature Sensor is under port three. It gives the signal to the ECU.

OK...thanks for the quick replies ! I will try the throttle...