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Old 12-14-2010, 03:54 PM
rdl rdl is offline
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Location: Ontario, Canada
Join Date: Jun 2010
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Mein Auto: 530i 2003
Originally Posted by bluebee View Post
Two questions about that since, in my cooling system autopsy, I only replaced one temperature sensor and it wasn't anywhere near the rear of the engine ...

Q1: If the coolant temperature sensor is near the rear of the engine, what is this coolant temperature switch doing that is in the front lower radiator hose?

Q2: If we want to test a bad gauge, would it help the OP to read his temperature off the high cluster?
From WDS for post MY 99
Engine Coolant Double Temperature Sensor
The temperature sensor is screwed into the cylinder head. A varistor (NTC = Negative Temperature Coefficient) serves as the measure for the coolant temperature.
Among other things, the coolant temperature serves as a measurement variable for following functions:
•Start volume calculation
•Injection volume calculation
•Set idle speed
•Characteristic Map Cooling
In the event of the engine coolant temperature sensor failing, a fault code is entered in the DME control unit and a substitute value is calculated from the value of the intake air temperature sensor with the aid of a temperature model.
The housing of the coolant temperature sensor contains two sensors (double temperature sensor). The one sensor sends the signal to the DME control unit while the other sensor is connected to the instrument cluster.

Coolant Temperature Sensor at Radiator Outlet
This temperature sensor is fitted in the outlet hose of the radiator and registers the coolant temperature after it flows out of the radiator.
The coolant temperature at the radiator outlet serves the purpose of driving the electric fan.

A much more complex question, or answer I should say

Prossibly would help. Presumably the reading in the check control cluster is coming from the DME. But, @#$%^& the WDS for the pre-99 ('98 528) models describes:
Coolant temperature display
Since 5/97 an instrument cluster has been phased-in which features the plugged-on instrument cluster electronics (IKE ) integrated in the display unit (AE ). This instrument cluster can replace older instrument cluster versions.
The instrument cluster determines the current coolant temperature by way of the coolant temperature sensor (NTC resistor). The coolant temperature sensor is connected with a separate ground (analog ground) and a sensor line (analog positive) to the instrument cluster electronics (IKE ).
In the case of the integrated version of the instrument cluster, phased-in as from 5/97, the coding data defines whether the coolant temperature signal is routed via the CAN bus from the engine control unit or via a separate line from the coolant temperature sensor to the instrument cluster.
The instrument cluster electronics (IKE ) transfers the "coolant temperature" information via the instrument bus (I-bus ) and body bus (K-bus ).
So, we wouldn't be sure if we're seeing two different sensor readings or the same reading twice. Confusingly, all the diagrams for pre 99 show a dual temp sensor, which would be redundant for some of these cases above.

If both sides of the dual temp sensor are being used, at least one would know what the DME thinks the temperature is when the gauge says "hotter than normal" and from a different sensor in the same device/location. Better, the two circuits are independent, they don't share a ground. (unless the connector is damaged).

Assuming that coolant level OK, cooling system bleeding is good, air flow not obstructed, o/heats at random not only on hot days, only when I drive hard, etc.

It seems from the OP's description, that this is an intermittent problem, e.g. gauge shows over normal temp but then not at repair shop. I start thinking electrical faults such as:
1 chaffed wire on gauge circuit - NTC sensor goes to low resistance as temp increases, so a ground or partial ground on this circuit would drive the gauge high
2 intermittent in electric fan control circuit or fan motor on it's way out
3 faulty gauge
4 damaged or corroded connector somewhere
5 CAN-bus, I-bus or K-bus problem
6 ???

A more positive suggestion
The next time the gauge shows hot, quickly check that both upper and lower rad hoses are very hot to the touch and that the electric fan in front of the radiator is running fast. Then shut off the engine, or go directly to repair shop while still reading high.
If truly overtemp, all should be "yes." Although, all yes doesn't mean must be overtemp, say if gauge is bad. If any are "no" you have a place to start looking.
BTW, if the gauge goes to red, I personally wouldn't risk the checking. I'd shut down immediately and wait for next time.

Re head gasket
No oil in coolant or vice versa isn't conclusive - it depends on what & where failed.
A good repair shop can test for combustion gas in the coolant as a check for bad head gasket or cracked head. This seems unlikely to me - I've never heard of one intermittent. They start slow, persist and get worse. Overtemp usually worse under heavy engine load.

Didn't mean to be so longwinded.
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