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 resistance 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
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 vanes 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 the "NTC I" but only a
switch which is used for the cooling flap/fan control unit.
The switch closes with rising temperature at 87 degrees C and (with falling
temp) opens again at 82.7 degrees C. 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
Hope that helps
I presume that your car and my '87 S4 are similar in this area. If so,
you can check the operation of the sensor using an DVM.
Go to the control unit under the panel outboard of the Passenger seat.
The control unit plugs are #1 for the outboard plug and #2 for the inboard plug.
Per the shop manual, the temperature sensor should be read between points pin 1 of plug #2 and pin 7 of plug #1.
Values converted are-
4012 - 3712 ohms @ 140 degrees F
1636 - 1528 " @ 185 "
1003 - 931 " @ 212 "
I checked mine at rest and when warmed up (don't know temperature) but I got values which seem to be consistent with those in the manual.
'87 S4 Auto
Question: the "air temp sender" that is located on top of the intake manifold,
When it's on it's way out, does it just die or can it fail slowly and cause intermittent hot start problems? Is there an ohm test that can be performed to see if it's still with in specs. Or what other method is used to check to see if it's functioning properly. I've already lubed the ISV, it seems to be working ok.
Its just a thermal switch - normal condition should be open-circuit. If it is closed circuit when cold it has failed. This one on top of the intake manifold/plenum has no real effect on hot starting only on the cooling fan operation.
1. The air temp sensor (switch) located in the intake manifold towards the rear is used to turn on the fans. It will activate the fans after the engine is turned off.
2. The sensor that affects the running of the engine is called the Temp II sensor and is located on the water bridge near the front of the intake manifold towards the passenger side. It has two sensors built into a single probe.
3. There is also a temperature switch located on the water bridge near the front towards the drivers side, which is used for the dashboard indicator.
4. There is an additional coolant temperature sensor on the lower front left hand (driver side US) corner of the radiator that provides fan control input.
the S4 does not have an air intake temperature sensor like the earlier car do, you might be thinking of the plug on the air filter housing I have one on my 84 Euro, this measures air temp for the intake air.
The S4 does measure the air temperature by way of the MAF this is done with a heated wire and the air cools the wire and so then the density and temperature can be figured by the MAF.
On the S4 there are 3 sensors as has been noted the one on top is part of the cooling fan circuit, the other 2 sensors are on the water bridge as Bill has noted the sensor with the Bosch connector (square ) is the Temp 2 this sends coolant temps to the LH and EZK .
the other sensor with the female plugs is for the coolant temp gauge and light. There are detailed test procedures for testing all of these sensors in the WSM.