Outside temp sender fault, reading very low. FIXED

[infrasilver]

I have had an issue for a couple of months where the outside temp on the dash is very low -38 up to -2 and is continuously fluctuating between the two even though it has been +5 to +13 degrees outside recently.

I have lived with it but a couple of weeks ago I ordered a new temp sender and fitted it yesterday but unfortunately that didn't cure the problem, I am still getting the same very low dash readings. I've checked the wiring to the plug and that all looks fine, so I am now wondering if the plug itself that the temp sender goes into may be at fault or worse it could be a dash issue?

Anyone had a similar issue and what was the cure or are there any tests I can perform?

 

[Alex]

Is it possible to read the temp with an obdII reader? I can read my temp on the HVAC but you can only hack that on a gen 1.

If you can read it from an obd reader and its correct, it's probably your binnacle.

 

[infrasilver]

Good shout, I'll get the laptop out tomorrow

 

[infrasilver]

I plugged the Durametric into it this afternoon and the live values option follows what it shows on the dash, it started off at -40 and the Durametric showed 39.75 and as it dropped on the dash the values followed, so going by your thought, the wiring or plug looks to be at fault?

It looks like i'll be removing the bumper in the next couple of days.

I could do with knowing what voltage I should be getting at the sender plug ofr testing purposes?

 

[Alex]

Might be worth checking the reading with it disconnected. Sounds to me like you may have a short somewhere and it's altering the resistance. One other thing worth noting - your aircon won't work if the reading seen by the HVAC is in minus figures (clutch won't engage on the compressor).

 

[infrasilver]

The AC not working has been noticed and one reason I decided to get a move on with it as the weather is starting to get a little warmer.

I found the end dangling and the sensor not seated in its holder a month or so ago and had thought the wiring had been cut on the metal grille but it seems fine, ie the the wires sheathing hasn't been cut through but I think it could be internally damaged? Because of the grilles I need to remove the bumper to get at the wiring to have a better look.

 

[infrasilver]

I've just had another mess around with this, if I remove the sender I get the failure message on the dash and once you clear that you get three dashes, ie

--,-C

I used my multi meter to see if any voltage is coming from the wiring and I got nothing but I'm unsure if the probes are going fully into the connector so I'll have another go at this with some thinner probes.

I have also looped the connector back on itself with a wire and the temp gradually moved up to its highest temp of 1.5 degrees and held steady-ish there. This can be a bypass if I need to get the AC working at some point

 

[P9XX]

My Tuppence worth - I'd start with the sensor disconnected from the car.
I don't have any experience with automotive temperature sensors but do have with Building Management Systems (BMS).

A temperature sensor is very often a "Thermistor".
Normally, an NTC or Negative Temperature Coefficient solid state resistor.
Precision thermistors for BMS sensors are normally 10 kOhm @ 25°c.

Being NTC, its resistance would be higher at low temperature and lower at high temperature.

Measure its resistance at two different temperatures.
Say -20°c from a freezer and room temperature +20°c.
If the resistance changes significantly, the sensor is probably good, suspect wiring loom or connector plug.
If resistance is infinity or OL on my multimeter, thermistor is N/G.

If the above is true, your low dashbord readings could be due to high resistance, i.e. caused by either a failed open circuit thermistor or a break in the supply wiring.

The following is a quote from the internet :-

"What can cause a thermistor to fail?
The cause of such failures are usually due to mechanical separation between the resistor element and the lead material, caused by handling damage, excessive heat, thermal mismatch, etc. The second most common failure mode is drift in resistance value as the thermistor ages, or parameter change"

 

[infrasilver]

We also use thermistors at work in UPS and data systems but I don't know them in the same detail as yourself of how they work, I just know they do and tell me if there is an issue

I do suspect the wiring now as one sender is brand new and the original one is showing the same readings on the dash as the new one so I suspect both are working fine?

 

[infrasilver]

I have measured 3v supply at the grey plug connector, I'm unsure if this is correct, if anyone has the time could you stick a multi meter on yours to check if that is correct or not?

 

[infrasilver]

I managed to fix this issue a couple of weeks ago but it wasn't straight forward after a bit of twisting cable to the original cable and fitting the sensor I got it to work at the correct temperature so knew I had got back to some decent copper wire.

For fun I did price up the front end wiring loom, I got a price back of £7500 and I think that was + VAT.



I found that the cabling was dark inside the sheathing most of the way up the short loom so I cut it back and stripped, then cleaned it up as best I could, soldered new wire in and heat shrink over that.
The issue came when trying to find the correct Molex pins to fit the sensor. I originally went to RS Components to ask them for help and after spending around 30 minutes with the sale girl I couldn't match what I already had. I ended up just ordering 3 different types and going to collect them, luckily I did get one of them to match so I got a refund for the two that didn't match.

This is the part number from RS



After getting them home I realised the Molex crimp tool I have was to big to crimp these pins and then I set about trying to find a pair that would fit. I found a pair that went down to 0.25 which would work, so had to wait for them to be delivered.



Once I put everything back together I got a correct reading temp on my dash once again.

While I had the bumper off I changed the high tone horn as that wasn't working, I got a pair of BMW horns as they are exactly the same fitting and half the price.
I also noticed while doing this that the pins for the horns are exactly the same as what I used for the temp sender, which may help someone in the future as all this cabling is in the direct firing line of the weather and more will corrode as they get older.

 

[Alex]

Good work sir!

Wished I'd have known about the horn before, just changed my high last month. Although I did manage to do it without taking the bumper off.......which was a god send.

 

[infrasilver]

You did well to change the horn without removing the bumper, I'm sure I can't even see my horns from underneath as the underside goes quite a way back and with grilles at the front it makes it impossible for me to do without removing it, although I can remove the bumper in around 15 minutes so it didn't really add to much time and made life easier