I've been wanting to replace the sensors for water/coolant just to make sure everything's running good. However as I've been reading more, I'm getting confused as to what everyone refers to and what's on the parts list.

My main question is, is the thermoswitch the same thing as the temp. sending unit or temp. sensor? In previous cars I've owned this has always been referred to as the water temp. sensor, but now through here there's several different names people use so I'm confused by it.

I know the fan switch is different than the thermoswitch, but I don't know if people just refer to the temp. sending unit as the thermoswitch, or if they're 2 different things?

I want to replace whichever sensor gives the water temp so that I can get an accurate reading of it on my powerfc. And I also want to replace the thermoswitch to the FC one which turns the fans on at a lower temp. So idk if those 2 are the same thing or different?

Another thing is that I remember reading somewhere that the ecu/powerfc gets the water temp readings from a different sensor than where the water temp. gauge on the dash gets it? so if this is true, Idk which is the correct sensor to replace?

Thanks in advance for any help!

 

There are two water temp sensors. The thermoswitch is the one on the front of the water pump housing under the filler cap. The second sensor is on the driver side of the motor and feeds input to the temp gauge in the cabin. I believe the PFC takes its signal from the thermoswitch or temp sensor under the filler cap. easy way to tell is unplug it and see if the signal disappears to the commander. In my signature is a link to the factory service manual in pdf searchable format. Free to download and will clarify this question with many other questions you may have in the future.

 

There are two sensors at the rear of the thermostat housing. The upper one with the green connector sends coolant temperature info to the PCME to help set A/F ratio. The lower one is the fan thermoswitch that closes and turns on the fans at 107°C. If replaced with an FC thermoswitch, the turn-on temperature will drop to 97°C.

For a photo of the two sensors, look at post #33 on this forum page.

(Sorry, I do not have a pic of the temperature gauge sensor.)

 

There are five temperature instruments on the engine:

Water temperature sending unit (meter) - located on back iron next to oil pressure sensor on drivers side. This sensor only operates the temperature gauge in the instrument cluster. It supplies a ground bias to the gauge.

Intake Air temperature sensor- located on the underside of the upper intake manifold. This sensor feeds a ground bias to pin 3L of the ECM and is used for engine fueling, idle, and spark controls.

Fuel thermosensor - located on the primary injector rail under the upper intake manifold. This sensor feeds a ground bias to pin 1U of the ECM and is used for operating the fuel pump speed relay amongst other engine fueling controls

Engine coolant temperature sensor - located on the engine coolant fill housing on the back side below the water thermoswitch sensor. This sensor feeds a ground bias to pin 3E of the ECM and is used for cooling fan normal speed operation (fan relays 2 and 4) and engine fueling / idle / and spark controls.

NOTE Fuel thermosensor and Engine coolant temperature sensor are the same part numbers and are interchangeable.

Water thermoswitch sensor - located on engine coolant fill housing on the back side above the engine coolant temperature sensor. This sensor feeds battery ground to fan relay 3 to set fan speed medium or high depending if AC is on or not. It is also a backup normal speed fan switch should a failure occur in the normal fan operating circuitry.

 

This may help:

Thermosensor & thermo sending unit = a variable resistor in a circuit... a temp sensing device whose resistance changes throughout a temperature range (output = variable signal)

Thermoswitch = on/off switch in a circuit... a temp sensing device that opens or closes at or near a certain temperature (output = OPEN, no signal or CLOSED, full signal)

ECU, PFC get their temp signal from a thermosensor located on the backside of the thermostat housing.

Temp gauge in cluster gets its signal from a thermosensor that is screwed into rear iron, near oil filter adapter.

Radiator fan thermoswitch is also located on the backside of the thermostat housing. Replacing the stock switch with the FC switch is a popular upgrade.

Ted

 

ttmott,

The "Engine Coolant Temperature Sensor" that connects to terminal 3E of the PCME is the upper unit (with green wire harness connector).

The "Water Thermoswitch" is the lower unit (with black connector), at least on our 1994 RX-7.

 

You are right - I stand corrected.

 

Awesome thanks for clearing everything up for me, really appreciate it!

 

the water temperature on my power Fc is usually around 80 to 90......recently, the reading are wrong, its nut going past 60, and is increasing in long interval from 44.....i am assuming some sensor is bad, any thoughts???

 

Which one will be the one that will turn on the low coolant light? The engine coolant temp sensor or the water thermosensor?

 

Good to know that the plug for both water and fuel thermosensor are the same. The way they come out of wiring harness could lead to confusion. The front one is for fuel, while the single out of harness is the water...

 

None of them.

The low coolant light is turned on by the sensor on the front of the filler neck below the cap.

-Geoff