105C thermostat replacement

[Dimitris]

Hello all

I got myself a 1996 E38 735i back in August and I, as expected, run into some probs. I have seen the engine temperature dial, when cold, it was rising to maximum for a short period and then dropping back to normal after a few min. when the car was hot it was running OK.

What I have done, back in August when I got the car, was to remove the 105C thermostat and replace it with a non 88C Map thermostat. The electrical connector on the thermostat is not connected – free plug.

Will the non use of the MAP thermostat through a fault like the temperature dial raising to max when running cold?

 

[Greydog]

Good to have you back Dimitris

I would think there will be an internal fault recorded but if the Thermostat is working it will simply open earlier so I would think gauge wise you will see nothing unusual

Dave

[Dimitris]

I checked on the ODB and there is no fault recorded , apart from "map activation". The temp gauge behaviour is strange. Any idea why it's going up and down like when cold and then it settles down ?

[Greydog]

Morning

BMW use 2 temp sensors one is with the thermostat the other is at the bottom hose to the rad (check realoem for your model) the car uses an average reading across the two to decide if things are OK 

Check wiring to both it could be that as there is no signal from your new thermostat it's confused things

Dave

[Dimitris]

I have checked realoem and it gives me no temperature sensor option , which is strange. Also what I did was to remove the map thermostat and disconnected the resistance heating that is inside the thermostat and replace it with a 88C from an x5. Nothing else has been changed. The strange thing is that it run like that for 3 weeks in August.  When I start the car last week the temperature dial was going to max before dropping to normal value. And that happened only when cold. When warm the dial was showing the right temp. I also saw that the map for the thermostat was engaged when the car was running cold which made me think that there may be a connection between the thermostat dial and the map thermostat not being connected.  

 

[Greydog]

Not sure what engine your running but there should be a Temp sensor in the Hose from the Rad to the water pump then there should be  a Temp sensor in the housing for the Thermostat Both are shown in realoem for the 3.5 V8

I thought the V8 was the only one with dual use sensor on the Thermostat housing but stand to be corrected

Dave

[Dimitris]

You are absolutely right. The strange thing is that when I put the chassis number on the realoem it shows that the temp sensor is not applicable on my vehicle.  I will physically check for a sensor.

In the meantime, do you think that the removal of the MAP activated thermostat will effect the temp gauge on the dial instrument item 6?

I am tempted to add a dummy resistor , 11.5ohms - 13watts , to simulate the MAP thermostat.  

Your thoughts? 

[Greydog]

As I understand the wiring diagrams and operation Item 9 on the thermostat housing is the sensor which sends its signal to the ECU for interpretation with information from the map sensor. Information from item 12  (1st drawing ) is also used to provide average temp across the cooling system. As far as I am aware the Thermostst is independent of the sensors and operated by expanding wax material internally