Coolant Temp Sensor

I have a '95 SL2 with two coolant temp sensors.

One is mounted to the engine block (I assume this is the one for the gauge)

The other is mounted in the coolant reservoir tank. ( I assume this is the
one for the aux. fan to turn on)

Am I correct?
If not what is the function of the one mounted in the coolant reservoir
tank?

Jerry

 

gauge)
And for the computer.

Nope. That's your coolant level detector.

 

I wondered about that.
Can you explain how that thing works?
Seems to be a glass enclosed microswitch.
No contact with the coolant, just sits in a chamber which extends into the
reservoir.
I really don't understand how it works.
Not really having problems with it, removed the tank to clean it out real
good and saw it and when I removed it the rubber "o" ring fell apart.
Doesn't seem to be a water seal as most of it was already missing. When I
reinstalled it I used some Teflon tape to seal it. I assume the "o" ring
seal is for dust & dirt.
No leak as far as I can tell.

Jerry

gauge)
And for the computer.

Nope. That's your coolant level detector.

 

I think I've got it. the sensor is a magnetic reed switch and there is a
float with a magnet on it so that when the coolant gets too low the magnet
activates the switch.
I think I'm correct.

Jerry


I wondered about that.
Can you explain how that thing works?
Seems to be a glass enclosed microswitch.
No contact with the coolant, just sits in a chamber which extends into the
reservoir.
I really don't understand how it works.
Not really having problems with it, removed the tank to clean it out real
good and saw it and when I removed it the rubber "o" ring fell apart.
Doesn't seem to be a water seal as most of it was already missing. When I
reinstalled it I used some Teflon tape to seal it. I assume the "o" ring
seal is for dust & dirt.
No leak as far as I can tell.

Jerry

gauge)
And for the computer.

Nope. That's your coolant level detector.

 

Both coolant temp sensors are located in the cylinder head. The one with 2
wires is the one for the computer and the one with a single wire is the one for
the gauge.

 

i have a 95 sl..which cts controls the fan? are they both the same sensors or are they diff part numbers,,?
my fan does not seem to work..unplug the sensor with 2 wires & start car &fan still wont wrk..hook fan up to jump pack & fan works

 

Two wire sensor is the ECTS that the PCM monitors. The one wire ECTS is the one that drives the dash gauge. 95 was the last year that they had two temperature sensors. After that it was just the two wire sensor.

I believe that is a 5 volt sensor, not a 12 volt.
Additionally, it receives its power from the PCM on one of the two wires, do not attempt to put additional voltage across the sensor as you are trying to drive current in the wrong direction into the PCM terminal.

Doing so Will also cause the PCM to read wrong because it is measuring how much of the 5 volts across it is being dropped across the resistance of the sensor. When you jack it up to 12 volts, it's doing the math wrong thinking it has five volts. It basically sees no resistance drop or the equivalent thereof. Low resistance on that sensor means higher temperature. It probably thinks the engine is insanely hot so it turns on the fan.

The only thing you have learned is that the fan motor itself is functional.

That fan only comes on in two situations.

1) The AC is on and the AC clutch is engaged with the compressor spinning. This is to keep the engine from overheating when the AC is on and your car is not moving, say in traffic. Note that if your AC is low on refrigerant and is non-functional, the fan can't come on because the AC system has a low pressure limit switch which will open the AC circuit to keep from damaging itself. So that is only a valid test when your AC system is fully functioning.

2) usually when the coolant temperature gets sufficiently hot, somewhere above the third tick on the coolant dash gauge, the fan will kick in and cool the coolant down a bit past the middle tick on the gauge.

I don't remember the exact temperature it kicks in but I think it is something like 225° F. So it will not run often. But it will continue to cycle on and off based on the temperature reading that the PCM infers from the ECTS.

Note that for the cooling fan there is a relay in the under hood junction box that the PCM tells to close/ground in order to send power to the fan to turn on. This relay can fail, thus the fan will not come on when it's supposed to if the engine is hot

There is an additional relay for the AC system. Both of those relays are supposed to close when the AC is turned on and fully functional. That way the compressor and AC clutch get their operating current and the fan by the radiator gets its operating current.

By far the most failure prone portion of all this is the two wire ECTS and/or the connector for it. The ECTS when it fails usually goes open circuit, infinite resistance. When that happens, the PCM calculates the coolant temperature to be -35 at all times. In the situation, the fan will never come on due to high engine temperature and the engine will overheat if it gets hot enough to do so.

The connector can also build up corrosion where it connects to the two pins of the ECTS. The wiring going through the connector cap commonly also fails/breaks, leading to an Open circuit and -35C reading at the PCM.

Before starting the car, measure the resistance of the ECTS. Be careful that your meter probes touch only the pins of the ECTS and that you don't have one probe across both of them. It's very easy to do because there's not a lot of space in there. Infinite resistance is obvious failure of the ECTS.

The rest of the wiring to the PCM can fray or break but this is not terribly common.

If your ECTS has failed, replace the ECTS and the connector which comes with about 6 in of wire coming off of each lead. Also known as a pigtail. Sensor harness, sensor connector, blah blah blah

Do not I repeat do not purchase one from AutoZone or advanced Auto. They do not supply the correct part and never will. Try to get a Delphi brand ECTS. If you cannot, get an ECTS from Napa. The replacement ECTS will have all brass construction. If it does not, do not buy it.

I would not suspect the relays until and unless you have replaced the ECTS and pigtail and the fan still does not work.

Please report back with your findings and ultimate solution.