Ambient temp display issue

[UrS4Eben]

Hi guys I hope someone can help me out or at least point me in the right direction..
I had a dud ambient temp sensor in my UrS4 when I bought it which I then replaced with the correct spec siemens one from Maplin. What happens now is:

In the morning when I start the car up I get the correct temperature reading and it stays correct for about 30mins after which it goes down to -45... as if someone cut the sensor cables and there is an open circuit.... when i'm finished with work and start it up to go home, it works properly again for 30 mins and then seems to go open circuit again....

Can anyone shed any light on this? How would I trace the fault? ie. pinouts for the display in the console?

Does anyone have a spare I can try swappping out to see if its the display unit or the wiring?

Any help appreciated

Thanks

Eben

[Nuvo]

Sounds like the sensor is the wrong spec to me. NTC instead of PTC maybe? AFAIK Maplin do not offer a suitable equivalent.

Can you provide a link to the device you got from Maplin?

[UrS4Eben]

Thanks Nuvo, this is the one I have
HERE
I suppose that is ptc right? Any UK sources for what I need?

[UrS4Eben]

Jeez I've just checked now, do I actually have the wrong value?

[Nuvo]

Did you post this question?

Q) Is this NTC or PTC. i.e. does resistance go up or down with increased temperature? - R.M.
A) Resistance goes down with increase in temperature.

The answer given impies the device has an NTC, I would have expected a PTC to be used. The data provided on the spec sheet also agrees with this.

I have a spare ATA sensor I can test to confirm this although it won't be possible until the weekend. I'l have to find it first.

[UrS4Eben]

I got this from http://www.20v.org/climate.htm

Outside Temperature Sensors
(Fred L. DeRoos, Jim Fraser)
These cars use a pair of negative temperature coefficient thermistors to sense the outside temperature. One of them is located in the plenum where the climate control air intake is located (under the hood on the passenger side) and the other is attached to grille next to the driver's side headlight. Both thermistors are identical and are 1000 ohms at 25 deg C. They have an R value of 9.10 and can be purchased from Digikey Corp. in Thief River Falls, MN for approximately $2 each. The part number is KC016N-ND. This is actually a Keystone Thermometrics part. The K.T. Eng. P/N is RL2004-582-97-D1. The AC head unit reads both sensors, then displays the one that senses the lower of the two temperatures. This is done to prevent the display of an artificially high temperature due to engine or sun heating. The temp may still be a little high, but at least it would be the lower of the two. The problem is when one of these sensors goes bad, it typically increases in resistance. That means that your temp display will show low. If the climate control thinks the outside temp is really low, you'll always get heat from the vents at all settings, except "LO".

The display will operate with only one sensor, so you can do one of two things. Best choice is to check the resistance of each sensor at approximately 25 C and see which one is reading high, then replace it. Second approach is to unplug each sensor separately and see which one makes the temp read correctly (assuming you haven't been parked in the bright sun or just run the car and the engine is hot).

To replace the thermistor, you will need to remove the bad assembly, then remove the plastic tip from the holder. The original thermistor is soldered to the two wires and fits into a plastic tube. The new one is just slightly larger, so you will need to insert a screwdriver into the plastic tube and enlarge it approximately 0.02 -- 0.04 inches. Audi would be happy to sell you a replacement, however, they list for approximately $100.

By that I would imagine that it is NTC but the value might be wrong.. I have a 100K instead of a 1K one.... 1000Ohm is = 1KOhm right?

[Nuvo]

Just did some digging for you NTC is right after all, it should read 1000 ohms at 25 degress celsius which is 1K as you ask. Your are seeing a x100 scaling problem as a consequence.

There must be a source for the correct parts some place local.

[UrS4Eben]

Item 4450-020 HERE?

[Nuvo]

Hmmm. The 1K value is correct for 25C for zero power. The value of 90R at 100C I can't be sure about.
You can only make a linear approximation over a small temperature range.

If you can find a device that provides a graph or table of results from -10C to +60C then I could make the comparison for equivalence.

UPDATE: And these are disc type thermistors when you need a bead type.

Digi-key.com seem to have a fantatic selection, they are in the US, I'm not sure how happy they would be to send you 1 or 2 of these. Are you able to read the faulty device markings? If you can then see this data sheet on how to make sense of the marking to help you find exactly what you are looking for.

Also check out the Somerset address at the foot of the data sheet. They might be worth a call?

[UrS4Eben]

This one looks a bit better I think. Still not sure which is the R value.

[Nuvo]

Farnell do the very one you quoted above from 20v.org. Trouble is they want have a delivery charge of £15.95 per order. See here

This information could be helpful to you regarding R values and so on. I don't have the time to look at this at properly right now.

One more link which might help can be found here.

Good luck.

UPDATE: Okay I had a little play with the tools I provided the link to and order code B57891M0102J000 appears to be a good match to these figures as you can see from the attached print out from NTC R/T Calculation 3.0.

[Nuvo]

You can order them from here.

Be sure to order a handful and send me a couple while your at it please.

(Its the third one on the list.)

[johne]

I suggest you use part number 191-2140 from RScomponents, www.rswww.com, that's what I'm using. Cost is 66 pence + VAT