Overheating, low speend fan not grounding?

[blefevre]

Background: Car overheated into the red this weekend. My wife got out and noticed the fan wasn't on. She said soon after the fan turned on and cooled down enough to bring the temp down.

I checked it out that night and the cooling fan wouldn't turn on (it works with the AC). The car got to 3/4th after light reving and the fan wouldn't turn on so I shut it off.

The car always ran just a bit hot so I decided to send it to a local bmw shop. I had a waterpump/thermostat laying around so I had them put it in. I asked them to look it over and also check the fan switch.

What they found....
They said the fan switch was working just fine. However, it wouldn't turn on the low speed fan. They said the ECU wasn't getting/receiving a ground signal for the fan.

Questions:
1. When should the high speed mode kick on, I think it should be before 3/4 temp.

2. What would be causing the fan to not work if the fan switch apparently is fine?

Thanks for the help, they are going to look at it tomorrow some more but if the fan doesn't come on it is just going to overheat again. Any help would be great!

[Bluebimma]

Coolant temperature sensor? After so long, they become inaccurate as well as dis/non-functional causing issues with the engine ranging from hard/no starts, incorrect DME readings for optimum fuel/igntion mapping, as well as incorrect display of temperature via the cluster. Faulty wiring can also cause improper fan run time and settings, be sure to check over the wiring as well as the relay.

[dguzzardo22]

http://www.pelicanparts.com/cgi-bin/...i%20M%34%32%20

[blefevre]

I know that. But like I stated, the shop tested that and it was working perfectly fine.

[Bluebimma]

Shops are not always correct and parts may test fine one minute and bad the next

[blefevre]

Good point. I might just bring it home and replace the temp sensor, and both fan relays.

[xxxJohnBoyxxx]

Low fan relay is good if the fan turns on with the A/C. Also the grounding that turns the fan on for cooling the radiator happens in that sensor. The sensor grounds the center pole to one outside pole for normal fan and the center to the other outside pole for high fan. I do not believe this grounding sensor on the radiator that runs the fan goes through the DME. It is a separate curcuit not controlled by the DME.