Calibrating OBC temperature reading

[GregT53]

Can the OBC be calibrated to display an accurate temperature? Mine is off by about eight degrees. The sensor is new and is OE BMW.

[GazM3]

I guess. The sensor is only essentially a resistor that varies with temperature. Is it constantly 8deg too high or too low.
If it's high the sensor may be Getting some outside heat influence. Make sure the sensor is in the direct air path and there is not a broken duct or something else creating more heat

[shogun]

Did you just install the new one? If so, just be patient, takes a long time to go to the correct temperature after it was disconnected.

[GregT53]

The sensor was installed correctly two years ago and there are no air leaks. It consistently reads high. This is the second, new sensor that behaves this way. Wonder if changing the OBC would make a difference. Also, wondering if the S38 engine generates so much heat that this problem is unavoidable.

[shogun]

OBC adjustments are only for Speedo correction http://bmwe32.masscom.net/johan/obc_...c_secrets.html

[GregT53]

OK, thanks.

[GazM3]

The sensor was installed correctly two years ago and there are no air leaks. It consistently reads high. This is the second, new sensor that behaves this way. Wonder if changing the OBC would make a difference. Also, wondering if the S38 engine generates so much heat that this problem is unavoidable.

u are probably correct with the s38. My m88/3 on board temp gauge reads high also. The e23 with the m30 is more accurate.

Adding resostance may make it read higher. Unless u put the resistance on the incoming wire or even used a thicker gauge wire from the sensor to the obc. It's a lot of effort to get it reading right but I can understand the need to do so.

[GregT53]

I may try using some heat shielding material around the sensor to block the engine heat. My E34 525 displays accurate temperatures so I am lead to believe it is the S38/M5, engine heat/bumper configuration that is the root cause of the problem. If I discover a remedy then I'll post it up for the good of the BF.C order.

[GregT53]

Update:

I pulled the sensor from the bumper mount, letting it hang loose. Temperature reading is now spot on with weather service measurement.. Next step is to fabricate a heat shield to block engine heat from the mount area.

[dgoodsy]

When the car is started for the first time in a while (a day or more) is it 8 degrees high then? Or only once the car warms up? If it is 8 degrees high all the time I dont think it could be lack of engine bay heat control. My car reads high only after it is turned on after running and warmed up within the last few hours. After driving it will come down to accurate temp and stay there for the rest of the drive, even sitting in traffic. I have read (may or may not have been official bmw documentation or just heresay, I don't recall) that the obc will buffer a change in temp display if a significant temp change is sensed by the sensor when vehicle speed is low or stopped. This is to prevent heat from the engine bay to mess with the temp display in a slow moving vehicle.

My first thought is there is corrosion/resistance in the wires and connections. (does more resistance = higher obc temp reading? I thought that was how thermocouples worked, but disconnecting the sensor makes the obc read a large negative number doesn't it?)

[GregT53]

Mine reads correct temperature on start up and then reads higher after driving for awhile, especially in warmer conditions. Last summer, when it hit 103, the temperature registered at 117 on the OBC.

The fact that the reading is stable with the sensor unplugged from the bumper and dangling in open air leads me to conclude it is an engine heat problem. If the wires were corroded and increased resistance caused a higher than actual temperature to be displayed then it would have happened with the sensor unplugged.

[zkd]

IMO. and from experience the temp. is not 100% accurate, it gives you an approximate reading of the surrounding air temperature, there are many factors to take into account, if you are driving in traffic, the heat from surrounding cars will effect it, also the heat from the surround air when the car is standing still, and some other factors.

I notice that it is more accurate when driving at highway speed with little or no cars around. but in the city or traffic that's another matter entirely.

[dgoodsy]

Yes, if it reads fine with a cold engine and then gets higher from there, it isn't a wire resistance issue I don't think. It even does it when doing freeway speed for a while?

[zkd]

did you try to change the sensor?

[GregT53]

Sensor is new, BMW OE.

I am convinced that the S38 produces sufficient heat to cause an inaccurate outside temperature reading.

Evidence: 1) the temperature reading is accurate when letting the sensor hang in open air, unplugged from the bumper mount; and (2) my E34 525 using the same sensor does not have the problem.

I used 3M aluminum tape and a heat shield fabric to create a cover that fits over the mounted sensor. Not elegant, but it works!

[pauliescff]

They automatically adapt to the proper ambient temperature. It does take a while though.