Coolant Temp Sensor

[ahlem]

Can someone describe what happens when your coolant temp sensor doesn't work? 1990 535im. Which one is it, where is it located? How does it affect things at upper RPM? I seem to have a rev limiter condition at acceleration and wonder if I need a CTS. I have to pull the cap and check it and the rotor also. It's finally warm enough to keep from getting numb fingers. My mileage has slipped from 20 on the road with more acceleration to 15 and bogging. Conforti chip present. Thanks.

[genphreak]

Can someone describe what happens when your coolant temp sensor doesn't work? 1990 535im. Which one is it, where is it located? How does it affect things at upper RPM? I seem to have a rev limiter condition at acceleration and wonder if I need a CTS. I have to pull the cap and check it and the rotor also. It's finally warm enough to keep from getting numb fingers. My mileage has slipped from 20 on the road with more acceleration to 15 and bogging. Conforti chip present. Thanks.The voltage output decreases or becomes erratic or zero causing the ECU to 'think' the engien is at a different temp. Will cause all kinds of trouble. Get a voltage meter on them to test- the specs are in the Bently nad the sensors on the water neck. There is one in the AFM also, but it is more reliable.

[pingu]

If the coolant temp sensor (CTS) is broken then the ECU might think that the engine is cold - if the ECU thinks that the engine is cold then (even if the engine is warm) the ECU will put the "choke" on - i.e. it will inject extra fuel. Might be what is knackering your mpg...

[ahlem]

Sounds like I'm heading in the right direction. I don't have a Bentley Manual, only the sketchy Haynes. There are two sensors on the water neck. Which one would cause the most problem. Anyone have a link to the specs or testing procedure? Thanks.

[Paul in NZ]

Sounds like I'm heading in the right direction. I don't have a Bentley Manual, only the sketchy Haynes. There are two sensors on the water neck. Which one would cause the most problem. Anyone have a link to the specs or testing procedure? Thanks.
one is the temp sensor for your gauge the other for the ecu.one is brown,one is blue. i dont know which is which!

[Anton CH.]

one is the temp sensor for your gauge the other for the ecu.one is brown,one is blue. i dont know which is which!
Doesn't really matter which one is which. You can check the sensor by switching the temp sensors and watch if guage displays correctly. More often the contacts could be corroded in which case the problem would "fix" itself after you perform the switch.

[Paul in NZ]

true...i once had a dodgy temp gauge which after i removed the terminal checked wire es et,and reconnect worked fine

[shogun]

I don't have a Bentley Manual, only the sketchy Haynes
http://luuk.xs4all.nl/bmwboeken/

[Dave M]

Doesn't really matter which one is which. You can check the sensor by switching the temp sensors and watch if guage displays correctly.

Curious:

Why would we need two separate sensors with two different parts #s if they perform the same job? Why not send the message from one sensor to both the guage and the DME?

In trying to track down an engine stumble, I swapped the engine temp sensor and coolant temp sensor wiring (new sensors as well). The temp guage needle sat at the 1/4 mark at operating temp when connected to the engine temp sensor. Do I indeed have a bum engine temp sensor???

Dave M

[Anton CH.]

Curious:

Why would we need two separate sensors with two different parts #s if they perform the same job? Why not send the message from one sensor to both the guage and the DME?

In trying to track down an engine stumble, I swapped the engine temp sensor and coolant temp sensor wiring (new sensors as well). The temp guage needle sat at the 1/4 mark at operating temp when connected to the engine temp sensor. Do I indeed have a bum engine temp sensor???

Dave M
Not sure, when I did my HG job. I think I accidentally swapped them and it still worked the same. You can't really share sensors. So you would need to send one signal to DME and then DME would send it to Guage, I guess historically it came that way, don't know.
The sensor is just a resistor which I can't see how it can go bad. But anything is possible I guess? Try to clean up contacts to see if it helps.

[Jon K]

Dave/Anton -

You can't share a sensor between two devices because your will offset the resistance by hooking up the devices. So, if you have a specific resistance at 80F, that resistance will change if you hang more connections off of the sensor, so that doesn't work.

Its more than just a resistor - yes it changes resistance, but its a thermistor inside - its a small element with very little mass that changes in different temperature. Like any resistor, they get gradually and gradually less accurate as you heat them up - too many cold/hot cycles will cause the variance to go out of spec, hence failed CLT sensor.

[pingu]

I don't think that's quite true - it's perfectly possible to connect loads of devices (temperature gauge, DME etc) to a single temperature sensor (i.e. thermistor) - for example by making the DME act as a constant current source (and having the DME measure the terminal voltage of the thermistor) while using a high impedance (e.g. buffered by an op-amp) input for the temperature gauge, so that the temperature gauge merely 'sniffs' the voltage of the thermistor.

I'm similarly mystified as to why BMW have two sensors with apparently two different part numbers. The only reason that I can think of is that the the two sensors have different functions and thus for improved accuracy (given the exponential response of a thermistor - see http://en.wikipedia.org/wiki/Thermistor ), two sensors are used. I'm guessing here, but it seems that one of the thermistors is used by the DME to monitor the engine temperature with regard to the 'choke' and fuel injection (with sensitivity being required at perhaps about 30 deg C ?), while the other thermistor is used for the temperature gauge and the overheating check control alarm (with sensitivity being required at a different temperature, about 100 deg C). Anyone else?

[genphreak]

One is head temp, one coolant temp. The head temp one is for the ECU, it allows the ECU to monitor head temp directly and get the combustion stoichiometry right. The other drive the gauge and reports cooling system temp.

They are close together as they are only on opposite sides of the thermostat.