OBC Temp drop when headlights on

[MFritts]

This is driving me crazy - the outside temp display will drop 10-20 degrees whenever I flip on the park and/or headlamps. I've cleaned 3 ground connectors (2 on drivers side front fender, 1 on pass. side front fender), all wire insulation looks intact (that I can see without tearing things apart). As long as the headlights aren't on, the temp display is accurate.

I've searched the forum with no luck, any suggestions?

Thanks,

Mike
1989 525i 214K

[Ferret]

Hmm, this does sound like an earth problem, but it might equally be a v+ problem, I'd check your wiring and all localised connectors as well as the earth points.

[Ross]

It's cooler at night!
Ferret's low voltage idea sounds on the right track, the OBC is powered by #20 fuse in your car the light switch is connected to #s 4&5.
Have a look at voltages there lights on/off, maybe find something. Keep us posted.

[MFritts]

It's cooler at night!

True, but I only use the headlamps during the day, and wear my sunglasses at night:p

I'll check the voltages this evening if I have the chance. About 80 degrees F here in No. Va. right now, let's see how low I can go - got it down to 7 degrees last night.

[MFritts]

Ok, checked voltage at all fuses, everything looked normal with key in acc. pos, "on" position, and car running with the exception of a -.2 voltage (compared to other fuses) on #'s 10 & 11.

Even swapped out the Sylvania Silverstar bulbs I put into the low beams last fall with the old ones that I got with the car.

If anything, the temp drop seems to be getting worse the more I futz with it! Got it down to -32F last night...and the temp drop is progressive - mess around, test, lights on...turn lights off...mess around, turn lights back on, and temp drops another 20 degrees...argghh!

[Ferret]

Ok, checked voltage at all fuses, everything looked normal with key in acc. pos, "on" position, and car running with the exception of a -.2 voltage (compared to other fuses) on #'s 10 & 11.

Even swapped out the Sylvania Silverstar bulbs I put into the low beams last fall with the old ones that I got with the car.

If anything, the temp drop seems to be getting worse the more I futz with it! Got it down to -32F last night...and the temp drop is progressive - mess around, test, lights on...turn lights off...mess around, turn lights back on, and temp drops another 20 degrees...argghh!

Ah! Your temp sensor has a total failure or something going on - the way it's creeping lower like that - there may be a break or something in the cable that only shows up when the engines running & causing vibrations.

[MFritts]

the temp drop only occurs when the headlights are turned on, otherwise the display is steady and accurate.

Will do more exploring on that issue later, tonight I need to replace the plastic fuel door hinge - hopefully that will be quick and easy.

[MFritts]

...the worse it gets! Cleaned the sensor itself, checked the yellow wire connection and insulation, but still have problems

Found this link in the archives about a -30 degree fix, but it appears to be dead now - http://bmwe32.student.utwente.nl/

Anybody know where I can find the procedure or an updated link?

Thanks,

Mike

[winfred]

i agree, sounds like a ground issue, grounding problems can do some weird **** as so many systems can share the same ground point, it can backtrack all over the car turnning stuff on or half way on looking for it's ground, theres several spots under the hood where the harness bolts to the body, check them for tight and clean


Hmm, this does sound like an earth problem, but it might equally be a v+ problem, I'd check your wiring and all localised connectors as well as the earth points.