Two temperature sensors?

[Bjaspud]

Hi, I'm new to 122's. It appears my car, which has lots of non-stock parts, has two temperature sensors. One is plumbed into the water pump and the other is bolted into the rear of the cylinder head. Can someone tell me why the car has two different sensors / senders? How is the system supposed to work?

[Derek UK]

The one on the top rear of the head is connected to the gauge in the speedometer. The one at the front/water pump may be an electrical contact for an another gauge perhaps fitted under the dash. The one at the rear of the head works using a bourdon tube filled with ether. These are frequently broken when trying to remove the sender from the head. The tube doesn't like being twisted. That may be the reason for the extra gauge. Original gauge can be repaired by a specialist or bought as a replacement. Neither option is cheap. Again a reason for the alternate one. With an adaptor an electric gauge can be fitted to the original rear position on the head. Sender must match the gauge electrically.

[142 Guy]

Plumbed into the water pump, as in the water pump inlet pipe has a fitting to accept a sensor like in the attached photo? That is a Volvo OEM part associated with a Volvo electric radiator fan kit (probably part of the air conditioning fit kit). It is listed in the 140 parts manual, I can't advise on the Amazon.

If it is wired up do you have an electric radiator fan? If not, perhaps as suggested, a sensor for a replacement dash temperature gauge.

The top back of the head is the normal location for the sensor for the dash temperature gauge. In the 140 the gauge is electrically operated and you can find electric aftermarket gauges which will work with a suitable adapter to fit in the threaded hole.

The pump inlet / lower rad hose is not a good place for a sensor to measure engine temperature because has the lowest temperature in the whole cooling circuit.

[jimbo1593]

Good knowledge...solves the question of where to put temp sender for elec fan too.

[Ron Kwas]

142guy;

"pump inlet / lower rad hose ...has the lowest temperature in the whole cooling circuit"...agreed!...so in my thinking, when that gets hot, it is the indicator that Cooling Sys needs some help, and its time to turn ON the Elec Cooling Fan...an ECF control could of course be characterized and function fine for either configuration, so we don't even have to disagree on this...it's just two ways which lead to the same result!

Cheers

[simonvolvo]

Originally Posted by 142 Guy

The pump inlet / lower rad hose is not a good place for a sensor to measure engine temperature because has the lowest temperature in the whole cooling circuit.

I agree, but only in part.

I agree that the pump inlet / lower rad hose is not a good place for a sensor to measure engine temperature. But not for the reason you give.

As far as I am concerned, the reason, simply, is that location of a temperature sensitive element somewhere at the bottom of the radiator is awkward from the point of view of accessibility, and bad in terms of levels of dirt and grime. All else being equal, much better, to have it somewhere at the top of the engine bay.

I once experimented with both locations (high and low) and found no difference in the way the system worked.

What is different (stands to reason): when the temperature sensitive element is located at the top the set point (temperature at which the switch switches) has to be adjusted to a higher level than when it is located at the bottom. But that does not, in principle, affect the way it works.

When the temperature of the engine rises, the temperatures at all points in the cooling circuit (top as much as bottom) rise.

IMHO (backed up by experiment).

[142 Guy]

What is different (stands to reason): when the temperature sensitive element is located at the top the set point (temperature at which the switch switches) has to be adjusted to a higher level than when it is located at the bottom. But that does not, in principle, affect the way it works.

When the temperature of the engine rises, the temperatures at all points in the cooling circuit (top as much as bottom) rise.

I don't dispute your observation that you can make the lower hose location work by offsetting the operating point for the temperature sensor and you can experiment to find a suitable offset. However, "the temperatures at all points in the cooling circuit (top as much as bottom) rise" is not the complete story. The exit temperature from the radiator may rise more or less than a change in the radiator inlet temperature.

On my B20E the thermostat goes fully open at 89 C. There is no point starting the fan until after the thermostat is fully open so the on set point for the fan control is 93 C and I use 2 C of hysteresis so it turns off at 91 C which insures that the thermostat is not cycling open - close. Knowing the target coolant exit temperature from the top of the block makes this a 2 minute exercise to set up and I am good to go.

The temperature drop through the radiator is variable depending on the air mass flow through the radiator and the ambient air temperature. You might start with a coolant temperature of 93 C at the top hose; but, the radiator outlet temperature is going to be all over the place for the same inlet temperature depending on air mass flow rate and air temperature. You are going to have to experiment to find out what a worst case delta T is through the radiator and you can set the fan operating point to be safe under the worst case conditions (steep climb at low speeds with 40C ambient); but, that will result in the fan coming on during conditions when it is unnecessary.

You can make this work; but, why would you bother trying? I expect Volvo used the lower inlet pipe location because the electric fan was an add on for the air conditioning (or perhaps really hot climate markets) and it saved them having to do any significant re design to accommodate the additional sensor.