Blue Temp Sensor Test Results

Post Mon Jul 21, 2014 1:02 am

I managed to test the blue temp sensor over the weekend after some tips on here and got some readings at both hot and stone cold, at the ECU connector and at the sensor itself. The cold reading was taken on a stone cold engine which was left overnight, and the hot reading was taken when the engine got up to it's normal running temperature.

Firstly, I tested at the ECU connector. The results were:

Cold = 2500 ohms
Warm = 250 ohms

Then, I tested at the actual sensor itself. The results were:

Cold = 2500 ohms
Warm = 255 ohms

From what I was told on here, the readings measured are a lot lower than what I was expecting for both hot and cold. I tested multiple times to make sure I wasn't testing incorrectly and got very similar readings at each re-test.

Is the blue temp sensor on it's way out?

Most importantly, can these readings be a reason for my low idle? Sometimes when trying to start the car when it's at operating temperature, it takes a while to start. ICV has been cleaned out, and the TPS switch is operating as it should.

What's the conclusion here?

Post Mon Jul 21, 2014 9:00 am

Bentley manual says;
-10c =7000-11600ohms
20c = 2100-2900ohms
80c = 270-400ohms

So you need a huge difference between cold and hot.

Your results give this,I conclude that your sensor and it's wiring are OK.

Post Mon Jul 21, 2014 9:14 am

The Wiki states that for cold, the reading should be between 3500-4000 ohms, and once warm should be around 330 ohms.

My readings are considerably different to that.

Post Mon Jul 21, 2014 9:14 am

Yeah, those results look fine. Bare in mind that the Wiki cold value is for an average British day of about 10degrees. It's the amount of change that's important here.

Post Mon Jul 21, 2014 9:22 am

German-Whips wrote:The Wiki states that for cold, the reading should be between 3500-4000 ohms, and once warm should be around 330 ohms.

My readings are considerably different to that.

And you tested yesterday....
Now,unless you keep your car in a cold store

the AIR temperature around your stone cold engine would have been in the region of 10c/20c

As I posted,at 20c,a reading of 2100/2900ohms is bang on the money.

Post Mon Jul 21, 2014 9:40 am

I was kind of hoping to hear that the sensor needs replacing so I could reassure myself that's what causing the low idle issues.

Looks like I now have to check elsewhere to try and sort this low idle!

I suppose that's the good thing about testing things, saves you from blindly replacing parts and not actually knowing what's wrong!

Post Mon Jul 21, 2014 10:04 am

I don't agree! Those Bentley manual figures differ considerably from the data I have, which gives a temp. of about 27 degrees for 2500 ohms and about 89 degrees for 250 0hms.
The hot reading depends on what thermostat is fitted to the engine, and the cold reading depends on the ambient air temp. and/or any residual heat in the engine from the time it was last run, but these readings are far enough out to at least make the sensor suspect.
I'd have it out and compare it with another one at various temperatures.

Post Mon Jul 21, 2014 11:02 am

Brianmoooore wrote:I don't agree! Those Bentley manual figures differ considerably from the data I have, which gives a temp. of about 27 degrees for 2500 ohms and about 89 degrees for 250 0hms.
The hot reading depends on what thermostat is fitted to the engine, and the cold reading depends on the ambient air temp. and/or any residual heat in the engine from the time it was last run, but these readings are far enough out to at least make the sensor suspect.
I'd have it out and compare it with another one at various temperatures.

Those were exactly my thoughts when I was looking at my figures and the figures on the zone which were 3500-4000 & 330, cold and hot respectively.

I thought to myself that the difference between 2500 and 3500 is pretty considerable, and I made sure the engine was stone cold as I wanted to achieve a true figure for cold startup. I had left the car overnight, and checked in the morning. The coolant surely must have been below 27 degrees I'm guessing.

Slightly off topic here, but I've also been wondering why on earth did the guys rebuilding the engine not use proper gaskets around the throttle body region instead of the shite blue silicone sealant crap? I see that the AFM cover also has this sealant, which leads me to believe that the AFM has been opened up and bodged/adjusted?!

Post Mon Jul 21, 2014 2:28 pm

Do not take too much notice of Brian today,it seems that he's working under pressure....

Just been out to the garage and checked the blue sensor on my cabbie... 2500ohms...or slap bang in the middle of Bentley's claim at 20c,my garage interior is about 25/30c ATM....