Temperature Sensors Driving Me Crazy

I had a problem with the engine over heating on my '94 SL2. I traced
it to the coolant/fan switch sensor which appeared to be doing
nothing. I bought an after-market sensor and installed it. After
starting the engine, the temp light came on steady, and the fan began
to run immediately. I pulled the sensor again, checked the resistance
across the pins and got a dead short, (Yes, I was very careful, being
a trained electronics tech). Presuming it was bad, I went and got
another one with the exact same results. Bad, too, I presume. I
checked the connector and got good +5 volts on one socket, and good
ground on the other socket. So, the PCM seems to be working fine.
Thinking that I must be the victim of cheap Taiwan parts, I bit the
bullet and got one from the Saturn dealer, and even asked him to have
a tech check it, which he claims to have done. This time, I did not
screw it into the block, but carefully attached the connector and left
it loose. Started the engine and the SAME D**N symptoms. Yanked the
sensor, checked it at the pins and again read DEAD SHORT CIRCUIT.
O.K. says I, I run to my parts box, pull out a 2500 Ohm resistor, and
a 250 Ohm resistor. These represent the top and bottom of the
resistance range of the sensor which is supposed to go from around
5000 Ohms to 200 Ohms in range. I plugged the 2500 Ohm resistor into
the socket and started the engine. All normal, no fan, richer fuel, no
idiot light, so the PCM is doing what it's supposed to do. Repeated
the experiment with the 250 Ohm resistor and sure enough, the fan
comes on as it should, and no idiot light. So, gentle reader, WHATS
WRONG????? Can all these sensors be bad? There seems to be nothing
else left.

Larry

 

Go to a different autoparts store, And make sure they are gettign the
correct sensor.. Not one that says it "should" work

Screw it in and have a go at it..

 

Presuming it was bad, I went and got

Did you check any of the sensors for resistance before you put them in the car?

 

If the Saturn dealer doesn't know which one it is, who should? Not
trying to be a smart a*s, but the one they gave me looked exactly like
the others.

 

Yes, I went to a NAPA store, and checked the one they offered me. Got
about 2.5 Ohms that went to zero Ohms. The sensor should be robust
enough that it can handle the miniscule amount of current from a
multimeter. The ECM is not capable of putting out much current either.
The thought that the ECM was somehow burning them out crossed my mind,
but I don't see how that's possible. THe only other scenario I can
imagine is that somehow a bunch of faulty knockoff parts has gotten
into the system. I don't know how likely that is, but from what I
hear, it's not impossible these days.

Larry

 

Yes, I went to a NAPA store, and checked the one they offered me. Got

Another thought;

Why don't you try using the IAT(intake air temp) sensor in the place of the
CTS? Its the exact same sensor and you know the one you have should work.....

 

Another thought;

That's a good idea. I know the IAT sensor is good. I'll try that and
post the results.

Larry

 

Here's the scoop, folks. THERE ARE SCADS OF BAD SENSORS IN THE
AFTERMARKET BEING SOLD AS GOOD BEWARE! I went you one better and took
a trip to the local salvage yard. I relieved three dead Saturns of
their sensors, both fan switch and air temperature (the same). I took
them home and checked them all out with a multimeter. All read from
4300 ohms to 3400 ohms, all except one that read dead short. I put one
of the good ones in my car and fired up the engine. It worked like a
champ. On examining the one that was shorted, I noticed that the
construction of it was identical to the two bad ones I bought at
Autozone, and the one from a local Saturn dealer. That was the one
that the tech supposedly checked. The good sensors have a deep
depression in the plastic between the pins located at the bottom of
the socket well. The bad ones have none, and simply stick up from the
flat bottom of the socket well. My suspicion is that these are
counterfit parts, or a huge run of bad ones from the manufacturer.
BEWARE.

Larry