2002 Civic Si – Evaporator temperature sensor

[Charles5654]

My understanding is that the TXV is controlled by a sensing bulb that sits at the exit of the evaporator to sense the superheat.
What is the purpose of the evaporator temperature sensor that connects into the electrical system of my Civic?
Thanks

[JohnHere]

My understanding is that the TXV is controlled by a sensing bulb that sits at the exit of the evaporator to sense the superheat.

That's true with a TXV that uses a sensing bulb. Self-contained H-block TXV's do essentially the same thing but have internal sensing. However, superheat (and subcooling) are not typically used in mobile vehicle A/C systems due to their constantly changing conditions, as pointed out in other posts. The refrigerant amount in a MVAC system is charged exclusively by weight.

What is the purpose of the evaporator temperature sensor that connects into the electrical system of my Civic?

Unlike a (mechanical) TXV that regulates refrigerant flow, the (electrical) evaporator sensor or thermistor monitors evaporator temperature, turning the compressor clutch on and off at certain points so that the evaporator doesn't freeze up and block airflow.

[Charles5654]

Thanks for explaining the function of the electrical evaporator sensor.
As far as the type of TXV in my 2002 Civic SI, I am not too sure at this time. I plan on taking a picture of the TXV the next time I have the panels that cover the TXV off.
From the attached 3 photos of a 2000 Civic (that I would expect to be similar to my car with regard to the AC system) it appears that the TXV has a sensing bulb. So my car set-up should be similar to the set-up of a residential unit.
Could you please clarify your statement ‘The refrigerant amount in a MVAC system is charged exclusively by weight’? Could you please explain the function of the equalizer (?) line between the valve and the evaporator discharge line in Photo3?
Many thanks

[JohnHere]

Could you please clarify your statement ‘The refrigerant amount in a MVAC system is charged exclusively by weight’?

I don't work on residential or commercial A/C systems. But the way I understand it, refrigerant is charged into such systems until the optimum superheat and subcooling temperatures are reached. In MVAC, refrigerant is introduced into a system until the specified weight is reached using an accurate refrigerant scale.

Could you please explain the function of the equalizer (?) line between the valve and the evaporator discharge line in Photo3?

Ahh...now you're entering into the realm of refrigeration theory.

Essentially, a TXV, which meters the flow of liquid refrigerant into the evaporator, is all about pressure relationships at three different points internally. The three pressure points interact with one another to achieve a correct balance which, as you'll see, is very important.

Without getting into a detailed discussion of superheat, the TXV needs to meter the flow of liquid refrigerant into the evaporator at the same rate as the refrigerant is changing state from a liquid to a gas caused by the heat load. The three aforementioned pressure points, working together, cause an internal needle valve to allow more or less refrigerant into the evaporator so that the refrigerant "evaporates" within the evaporator, thereby creating the desired refrigeration effect and cooling the cabin when air is blown across it.

For the externally equalized TXV in your pictures, the equalization tube compensates for a slight pressure drop across the evaporator by providing "equalization" pressure from the evaporator suction line to the bottom of the TXV diaphragm. An internal superheat spring also acts on the bottom of the diaphragm. Both of these forces coordinate with the pressure at the top of the diaphragm generated by the sensing-bulb tube, thus controlling the needle-valve's throttling action.

This rather complex pressure relationship ensures that the correct amount of liquid refrigerant always enters the evaporator via the internal needle valve at any given time.

[Charles5654]

JohnHere,

Many thanks for your detailed answers to my questions!

[Tim]

Charles,

Thank you for the site donation.

[Charles5654]

AI9,

Thank you for the clarification on the type of valve installed in my car and all the additional interesting information provided in your post.