TESTING BLUE SENSOR WITH OHMMETER-How do you use an ohmmeter

[irish-e30]

Ok,

Im a begginer at this so firsly since there are so many people complaining about a possible blue sensor fault i thought with a bit of help put a good link on here with pictures to later help others.

Firstly my problem is the car is running at 21MPG, ridiculous with the cost of fuel these days but also its just not right.

My thoughts are,

1. air flow meter or air leak - RULED OUT

2. an obvious petrol leak from fuel pipes - RULED OUT

3. oxygen sensor - RULED OUT (none exist on non-cat converter cars)

4. my adjustable fuel regulator may be at too high fuel pressure ie >3BAR

5. Blue coolant temperature sensor

6. temperature coolant switch

7. ECU may be chipped from previous owner - I think ruled out as its a motronic casing.....unless chip was changed inside...too afraid to tamper with this!

My struggle and question is!!!!!

Ok, how do i test these? (POINTS 5 and 6 ABOVE)
I just bought an ohmmeter, do i just put the meter on the Ohm section >1, then place each pointer and the temp sensor, there are two prongs on the sensor, do i just unscrew the sensor from the engine completely, and put each pointer seperately on the two prongs??

Could blue temp sensor affect the fuel consumption? exhaust fumes smell very petrolly!!!! but inside car you can smell it only from the outside at the exhaust. either way its burning rich right!!

[tezk]

To test the sensor properly its easiest to unscrew..
With the sensor cold, you should get a reading of around 2k ohms across the two terminals..
Dunk the sensor in hot water until its thoroughly heated and test again - its should drop to around 300 ohms..
Of course you could do these test with the sensor still in the car, but it means running it until it warms up and removing it is only a 2min job..

If it isn't giving correct readings there is a potential for the ecu to think the engine is still cold and pass more fuel through than necessary..

Terry '87 320i

[irish-e30]

cheers tezk,

I have it removed, its showing 1300 ohm's, at room temperature, not hot!! that means its screwed right?? also can you tell me from the picture if im doing this measurement correct.

Accoarding to bentley manual it should be
2200-2700 at 20 celcius
300-360 at 80 celcius

Will the car be acting weird once i reconnect these again also?? ie will the ECU have to redo itself?
(ill recconnect them just so i can drive in the meantime)

[tezk]

The reading suggests your taking it ok but that the ecu will think the engine is warmer than it actually is when cold. Do you have starting problems?

It's probably worth picking up a replacement.. If its been running alright up to now then she should be ok for a while longer.. I've never met such a bullet proof engine before, so it should cope

Just as a side note, how much did you pay for your multimeter? I made the mistake of paying minimum amounts for mine and crossing the probes shows a 200+ ohm resistance.. (should be 0, or very close) But then I guess you get what you pay for?

Terry '87 320i

[irish-e30]

I have no starting problems at all, car runs great but i just feel it at the petrol lpump when im getting 21MPG consistantly no matter how i drive.

If the Blue temp sensor is fecked, will it result is such a consumption of fuel like in my case??

I didnt pay anything for this im using a friends one, its analog hang on and ill show you the pics, as for when i cross the probes, i get 0measurement at 1ohm selector, x10 ohm selector, and x1k, but about 2.5 when im measuring on the x10k selector.

[irish-e30]

ok so i found the blue temp sensor, (marked yellow)



here is the sensor removed from car



removed it from car, its cold!! I measured it as per picture below, am i doing this correct? im putting the probes on each pin, and i have the ohmmeter on x1K, its giving a reading of 1.2, but bentley manual says it should read 2200-2700 at 20celcius or room temperature.

here is the reading

[irish-e30]

just checked the ohmmeter again, batterys went dead so replaced them its now saying

6500 OHM at 20 celcius

placed in boiling water bath, its reading 3.5 on selector x1K Ohm,

so that should be correct then as bentley says in the range 300 to 360 at 80 celcius,.....

So does this mean its screwed because the 20 celcius reading is incorrect and not in the range??

[E30Mark]

Trouble is, if the meter isn't calibrated (by the looks of it i guess it ain't!!!) then you can't be certain if the results your getting are true or not. 6.5k means the sender is truly knackered. Either borrow another meter to compare, or buy a new temp sender, about £10 from GSF or £15 from the dealer.

[Globulator]

Coil meter: I have an identical one!
1) You need a fresh battery
2) Select the ohm range - X1K is OK I guess
3) Connect probes together, and twiddle the knob until you get 0 ohms
- If you fail to get 0 ohms, you need a new battery/meter.
4) Now measure the item

If you change the range -> Go back to step 3)

If you do not do this, the reading is meaningless. Maplin do digital ones for about £5 that seem to work OK,
much easier to use.

[irish-e30]

Cheers lads i appreciate that very much!!! i think ill head to buy a digital multimeter today, do they need to be calibrated as above also??

After calibration like you said,

At 20 celcius = 2700 Ohm Correct limit 2200-2700

At approx 80 celcius = 300 Ohm Correct limit 300-360

Also, i was just testing that on an unused blue sensor from a scrap car.

when im testing the one from my own car should i test it in the car at cool temperature, and then again after the car had a run,......... or should i remove it from the car and test it cold and again test it in boiling water???

What would you suggest?

ps if they are removed is there any complications, ie car running funny for a while, not starting, also will coolant leak out from the removed sensor hole???

[Brianmoooore]

Afternoon all. Just been away for a couple of days, picking up Touring number eight.
Was in an auction and hadn't even got a bid with 20 seconds to go, so I felt sorry for it!
Irish-e30; The sensor you tested above is in perfect working order. Am I right in thinking this is not the one on your car?
If so, take the pressure cap off the bottle and put it back on again to release any pressure, and swap the blue sensors over. A very small amount of coolant will leak out, but only slowly.
Test the old one with your meter in the same way as before. Nothing wrong with your meter - it's identical to the one I carry in my car at all times. Just remember to always short the probes and 'set zero' on any ohms range.
For most car work, an analogue meter is probably preferable to a digital.
If the old sensor turns out to be good, direct your attention to the injector loom plug and socket under the inlet manifold.

[irish-e30]

yes brian,

this is not the one on the car, i was just testing it first to see if in fact thats how its done. pity cos i just went and bought a digital one!!

Anyways can i just test the sensor while its on the car? just unplug the plug, and test the exposed sensor wires?

The whole reason im testing all this is not because I have heating issues, its purely and only because I have a high fuel rate consumption, and i was told they are a common cause.

Is the injector loom plug and socket under the inlet manifold another cause of high fuel consumption??

currently im getting 21 MPG i want at least 32-34 MPG am i being unreasonable here???

[Brianmoooore]

32 - 34 MPG on a M20 engine is a bit optimistic, even if you drive like your granny. About 27-28 MPG is realistic. 21MPG is poor, but achievable with a sufficiently heavy right foot.
No problem in testing the sensor on the car. Test it cold, reconnect it, get it up to temp and test it again.
Blue temp sensor has no relevance to how hot the car gets - that's solely down to the thermostat.
The signals from both temp sensors go through the injector loom plug and socket, and a (common) poor connection here simulates a dud sensor.

[irish-e30]

cheers brian, as i said im running the car at idle now til it gets up to its normal temperature, mid-way. then ill switch it off and re-test the blue sensor and as far as i know from that bentley manual it says at normal operating temperature (mid-way) the resistance should be between 300-360 at this temp if normal, if not then the blue temp sensor is faulty i take it???? and also causing an increase in fuel consumption??? as the engine constantly still thinks the car is cold and so adds more fuel??

sound right??? im still doubting myself! "beginners doubt"

[irish-e30]

At normal running temperature (mid gauge):

reading of blue sensor was:
230 OHM
No continuity

Bentley manual again is stating it should be 300-360?

Im lost should i replace it or would that be pointless? I have not took the cold reading yet ive to wait a few hours

[Brianmoooore]

230 ohm is a bit on the low side for normal running temp. Where did the temp gauge get too?
If it went above half, there's a distinct possibility you have a dud viscous fan coupling as well!!
230 ohm hot isn't going to cause any problems in itself, especially fuel consumption related ones.
Wait and see what the cold reading is now, as the sensors can fail with a low cold reading as well, which will give starting problems when cold and flat spots when accelerating until operating temp is reached.

[irish-e30]

hey brian thanks for that.

There are no flat spots, and no problem with acceleration. the only issue is fuel consumption is high.
The temp gauge sat to just barely below 1/2 or mid-way where it always sits when even on long drives!

Damn, what else could be causing the high fuel consump?????
So far tested:

Fuel tank - Not leaking
Fuel pipes - Not leaking
Air pipes and AFM and air filter - All OK
Blue sensor - OK (well ive to test at low temp now but no fuel consumption issues with it)
Oxygen sensor - Not fitted as my car is not a cat
coolant temperature switch - Not fitted on my car as per above


There is still a smell of petrol from the fumes at idle, also the idle is jumpy and sits below 1000 RPM maybe 800?? Idle seems to be fine then rises and falls back again.

The only thing im considering left is the non-standard adjustable fuel pressure regulator thats on the car since i bought it, should i get an original one or just adjust the adjustable non-standard one to 3.0 BAR???? Ive no idea what the fuel pressure is at the minute, ive no fuel pressure gauge!

[Brianmoooore]

If that piece of aftermarket junk is set high, then it will definitely use more fuel!
I would certainly replace it, as a) they give no advantage, and b) they have been known to incinerate cars.
Make sure you replace it with a 325 (3 bar) one, as 320 ones are 2.5 bar.
Still that connector under the manifold to check out - not just the pins and their sockets, but the wires under the lower rubber boot. Water gets in from the top and rots the wires.

[irish-e30]

Cheers Brian,

That piece of boy-racer crap is about to meet its death, ill get rid of it anyways, i want originality.

Brian, I have no clue of the "connector under the manifold" I dont know what it is you want me to check here is there any way you can take a picture of it on ur own car or from a website or point it out on my own picture.

There is a wire burnt along there somewhere

right we're getting places i hope

[Brianmoooore]

Someone posted a pic of part of this connector a few days ago in answer to the same question, but it's quite easy to find.
Under the inlet manifold there is a black painted steel structure that has various pipes and wires clipped to it. At its lowest point, and just about in the middle there is a round plug and socket, similar in construction to the diagnostic connector or the C101 engine loom connector, but smaller (about 30mm).
The connector is mounted in a horizontal hole in the metalwork, with the cable going vertically down into it from the top and vertically out the bottom.
Water can get into the plug/socket part and corrode the pins and it can get trapped in the lower rubber boot, where it completely rots through the wires.
Remove the air filter/AFM assembly for access.

Burnt wire?

[temp]

Irish-e30 here (waiting for my lost password)


Ah so im over in the wrong spot all together, wrong side, is it the wiring that goes into the rubber boot? im useless!let me look for that this evening, MOT tomorrow!!!!

[irish-e30]

The signals from both temp sensors go through the injector loom plug and socket, and a (common) poor connection here simulates a dud sensor.

Is this what your on about Brian....ie just follow the wires back from the path of the sensors?