bad outside temperature sensor or wiring ?

[radu1976]

I have been owing my 1992 525i since 2013.
From the beginning my outside - yellow one - temp. sensor didn't work. It was always showing 73-77F mostly of the time. In many trips the temp was almost constant varying only a few F degrees.
A few months ago I got a used sensor from scrape yard from another E34. After installing the new used sensor doesn't seem to be better as it's indicating 40-50C mostly of the time. The variations are much larger , yesterday I took the car for a long trip and at a moment the sensor was showing 10-11C, then it started to gradually go up to 15C, then it decreased again around 11C but at the end of the trip it was showing 40.5C.
The sensor and my headlights splashers are the only things not working in my 525i.
Is it worth buying a brand new E36/E34 sensor or I more likely have a wiring problem somewhere so the first choice would be nothing than a waste of money ?

[shogun]

First check the wiring and the plug in the front where the sensor goes in. Maybe contact problems. In my case the problem was caused by a corroded plug.

[moroza]

Could be wiring, could be the sensor. These work by measuring the resistance in the circuit, which should be near-zero in the wiring and varies inversely with temperature in the sensor. Hot = low resistance; cold = high resistance. Typical failure is reading low all the time (-37) because of an open circuit (very high resistance), not like this situation. Unplug the sensor and see what happens. Reads low? Wiring ok, sensor bad. No change? Wiring is bad (specifically, a partial short-circuit) and the sensor might be ok (easy to test with an ohmmeter).

I don't know what a new one costs but these are common junkyard items. I might have a couple laying around and I'm sure Andy (south42) and others would have good used ones as well.

The sensor and my headlights splashers are the only things not working in my 525i.

You're either lying, ignorant, lucky, or an excellent maintainer of your vehicle.

[GregT53]

You're either lying, ignorant, lucky, or an excellent maintainer of your vehicle.

Ha! Well some of us are lucky enough to be excellent maintainers of our E34s because we're not ignorant about what it takes to stay on top of these vehicles, no lie!

[radu1976]

When sensor is removed temp shown is -37.
I think that my new used sensor bought from junkyard 2 months ago is pretty good : this weekend it was showing 23-24C when outside it was around 15C. After I removed and I installed it the temp started to go up from 9C with 0.5C degree every a few secs until it reached 24C or so. I don't know if a sensor can be 'pretty good' or just good or bad. But as I mentioned it's usually showing around 48C.
The older one though is always showing around 78F so around 20C all the time - summer and winter - Still have it, didn't throw it away.

Until last year even the headlights nozzles were working - not the fogs ones as they were disconnected as they were broken - but I had a paint job done on the bumper and other parts and since then they don't work anymore and I couldn't fix that.
I am not an excellent maintainer of my car , it's just the way it is, i guess I was lucky. Everything is 100% functional. It had many small issue when bought it - 5 years ago when it had 167,000 km - , they were fixed by me, my father and a BMW mechanic and now , at 194,000km nothing is wrong except those 2 things.
There was always a sweating in the oil pan area - and differential area as well - but I haven't seen anything dripped on my parking spot during those years.
Until 2 years ago the engine used to eat a bit of oil - I understood that this is normal for older BMWs - but since I have installed a new ECU - a german well reviewed ECU which provides an extra 15hp - the car doesn't seem to be eating any oil at all.

Could be wiring, could be the sensor. These work by measuring the resistance in the circuit, which should be near-zero in the wiring and varies inversely with temperature in the sensor. Hot = low resistance; cold = high resistance. Typical failure is reading low all the time (-37) because of an open circuit (very high resistance), not like this situation. Unplug the sensor and see what happens. Reads low? Wiring ok, sensor bad. No change? Wiring is bad (specifically, a partial short-circuit) and the sensor might be ok (easy to test with an ohmmeter).

I don't know what a new one costs but these are common junkyard items. I might have a couple laying around and I'm sure Andy (south42) and others would have good used ones as well.



You're either lying, ignorant, lucky, or an excellent maintainer of your vehicle.

[moroza]

When sensor is removed temp shown is -37.
I think that my new used sensor bought from junkyard 2 months ago is pretty good : this weekend it was showing 23-24C when outside it was around 15C. After I removed and I installed it the temp started to go up from 9C with 0.5C degree every a few secs until it reached 24C or so. I don't know if a sensor can be 'pretty good' or just good or bad. But as I mentioned it's usually showing around 48C.
The older one though is always showing around 78F so around 20C all the time - summer and winter - Still have it, didn't throw it away.

Your wiring is ok. It's also possible that the OBC is bad. All good E34 temp sensors I've seen are quite accurate, so if yours is showing Shirtless Weather when it's actually Longsleeves Weather, it's not right.

It had many small issue when bought it - 5 years ago when it had 167,000 km - , they were fixed by me, my father and a BMW mechanic and now , at 194,000km nothing is wrong except those 2 things.

Well, that's good maintenance.

[radu1976]

OBC would be my on board computer, correct ?
It works pretty good when it's about the time - always good - autonomy according to the fuel remained in gas tank or fuel economy - real fuel economy was 1.5% better than what OBC is showing during a long trip. I know that as I did fill up many times to see the real fuel economy. So a very small error -
On contrast my 2002 530i is showing a fuel economy which is actually 1% better than the real one, so the real one is 1% less than OBC is showing - On the other side the E39 temperature sensor is right on spot !
Thank you for the hints, I will try to check both my sensor with an ohmmeter - great touch here ! - . I do have a multimeter but I have no idea if or how can I check if those 2 sensors are good ... will go to an electronic shop.

[QUOTE=moroza;30091417]Your wiring is ok. It's also possible that the OBC is bad

[moroza]

Yes, On Board Computer. I don't have the resistance values handy for the sensor; search for those or just plug in another sensor (E32 and E36 are the same, and possibly others) and see what happens.

There's a fuel economy trim setting. On my 544 it was reading some 8% too high. Search for "E34 OBC unlock" (or something to that effect) for details on how to use it. There might even be a trim setting for the temp sensor.