>I've been searching archives and looking at web sites, and haven't found a
>good explanation - would someone talk about the engine temperature 
sensors?
>what each does, what happens when one is bad, how to test, etc....

>John Pirtle
>87 Auto


From: Kurt Rottner [kurt.rottner@gmx.net]
Sent: Sunday, July 23, 2000 10:09 AM
To: 928
Subject: [928] RE: temp sensors

Hi John

I haven't seen any response to your questions so far so I'll try to answer 
them:

The engine has the following temp sensors/switches : NTC II measuring 
engine temp (=water temp) , the sender for the temp gauge in the dash and 
the coolant temp sensor in the radiator which controls the fans and louvres 
and finally the air temperature switch in the intake manifold which (among 
others) causes the fans to keep running when the engine is shut off.

Only the NTC II is used for the engine management. NTC stands for Negative 
Temperature Coefficient, which means that the resitance drops with higher 
temperature.

The brain uses the input from NTC II to enrich the mixture under cold start 
conditions and for the ignition timing the engine temp is also factored in. 
The NTC II sensor is located where the right hand side cooling water hose 
enters the block. Since both LH- and EZK brains want to know about the 
engine temp, there are two sensors integrated in the same housing. Both 
work in the same way: Typical values: engine cold (around room temp) : 2-3 
kOhms, engine hot: 200-300 Ohms. Resistance measured between the contact 
and ground as each contact pin goes to one temp sensor.

Don't mix the NTC II up with the water temp sender for the gauge, they look 
almost the same but the gauge sender is located on the left hand side.
If NTC II doesn't work (shorted or open loop), the brains disregard the 
signal und act as if the car would be warmed up. Thus you should see 
problems during cold start. The worst thing that can happen is a bad 
connection to the sensor ( as happened to me some time ago). With an 
additional resistance due to bad contacts, the brains think the engine is 
cold and pump in additional fuel causing the engine to run rich - with the 
usual consequences.

The name NTC II implies that there is a NTC I as well - this was a 
necessary feature for K and L jets, which used vains or plates to determine 
the air flow. The temperature was needed to calculate the air mass. In the 
LH jets the air mass sensors have an integrated air temp measurement 
circuit, so the LH doesn't need the additional NTC I.

Thus, the air temp switch in the intake manifold is not NTC I but only a 
switch which is used for the coolong flap/fan control unit.

The switch closes with rising temperature at 87 degrees C and (with falling 
temp) opens again at 82.7 degreesC. If the switch closes after the engine 
has shut down, the cooling fans run at 50% power.

The coolant temp sensor also has a NTC characteristic, the resistance 
varies between 4kOhm at 60 degrees C 1 kOhm at 100 degree C. If this 
doesn't work correctly, your fans and louvres won't operate correctly: 
 either running permanently or at not at all - with the obvious 
consequences

Hope that helps

Kurt
90 S4 Auto
schwarzmetallic
928 OC, PCA Germany Region



From: Kurt Rottner [kurt.rottner@gmx.net]
Sent: Monday, July 24, 2000 4:33 PM
To: 'Pirtle, John'
Subject: RE: temp sensors

Hi John,

the problem you describe could indeed be due to faulty temperature 
measurement.

About the location you're absolutely correct: NTC II is to the left if you 
stand in front of the car, or on the passenger side of the car (if you have 
a LHD)

If you suspect the NTC II (or the connection to it) to be faulty, just take 
the connector off. The brains recognize "implausible" signals and act as if 
the engine was warm. If you problem goes away then, you have found the 
culprit.

To check if one NTC in the housing is faulty, compare the resistance values 
between the two connector pins (vs ground) they should be the same,

You can check the connection to the sensor by disconnecting both main plugs 
to the brain in the passenger side footwell and measure the resistance 
between PIN 13 to ground (PIN 17) of the LH connector. You should get 
around 2 kOhms for engine cold and 200 Ohms for engine warm. While 
measuring, get someone to wiggle the cables around the connectors a bit and 
check that the resistance doesn't change (otherwise you have contact 
problems)

The pin numbering starts with 1 where the cable enters the connector and 
the row with the 18 pins (vs 17 on the other side) is the one Pin 1 - 18
so it's:

              1  2  3  ...  18
cable ->
              19  20 21 ... 35

Good luck, hope that helps
Let me know how it works out

Kurt