Temp mismatch

[Oli77]

This morning while going to work:

http://i900.photobucket.com/albums/ac208/othibau/SSPX0422.jpg

Clearly I trust the old OBD 19.7.0 ;)

But what's with the analog temp display? I tried the dashboard test function (OBD 1.2.0) and it moved the needle to the left a bit after several trials.

Ideas?

[danewilson77]

I don't understand. I want to say your car was overheating.

[Oli77]

No it wasn't the trusted display shows you my engine temp is dead on at 95.

[danewilson77]

I have no idea. For whatever reason.....I would want to trust that analog over the digital. Did it get any higher than that?

Maybe reset the computer?

[Oli77]

Nop stayed there and when I turn engine off, the needle did not come back all the way to the left. As if the analog needle was ofset 10-15 degrees to the right.

I did try the test function where all the lights turn on and the needles go through a full range motion. I thought this would reset the range on the analog.

This did change the display on the analog and moved the whole range to the left a little.

I really want to trust the 19.7.0 OBD reading at 95 celsius, I mean, its been that value for 6 months now.

If it is still not right, I will unplug battery for a few hours tonight.

[danewilson77]

Copy. I like the plan.

[Oli77]

Just went for a drive. All back to normal. Digital temp at 95 Celsius and needle dead-on-center.

I am convinced doing that diagnostic range test reset the needle. I had to do it 3-4 times though.

All good for now.

[danewilson77]

Good deal.

Sent from SIGFest-2012, USA on HTC via TTv2

[kayger12]

Might be the sending unit going.

[Meric]

Did you recently sharpied your needles? Maybe you messed up the alignment of the needles.

[Oli77]

Not sure what boss is saying about the sending unit? What is it?

Meric, yes I did but that was summer last and all has been perfect since then (still could be). I thought about removing the console again and realigning the needle up by hand. I am glad the software way worked.

[danewilson77]

Not sure what boss is saying about the sending unit? What is it?

Meric, yes I did but that was summer last and all has been perfect since then (still could be). I thought about removing the console again and realigning the needle up by hand. I am glad the software way worked.

He may have seen first post, and thought that was fuel gage?

[kayger12]

Not sure what boss is saying about the sending unit? What is it?



Sending unit is the temp sensor in the lower coolant hose. If I understand correctly-- the temp sensor doubles as both the sensor for the ECM (which is the temp the OBD shows) as well as the sending unit for the analog gauge.

My understanding is that these are two different signals sent via two different wires, hence, the OBD reading could be accurate while the dash gauge is inaccurate.

Can someone verify this? Or set me straight if I have that wrong?

[Oli77]

Alright, but if as you describe there is only one sensor, how could we have had that situation (reading 2 different temps)?

I just think my needle had a temporary lapse of reason.

Post 101 in this thread (http://www.zhpmafia.com/forums/showthread.php?6575-OBC-19.7.00-Watcha-go/page4&highlight=watcha%27) mentions 2 sensors, a coolant temp (at T-stat?) and one in the lower hose.

[kayger12]

This pertains to Audi/VW, but is a good explanation:

"The engine coolant temperature gauge uses a heat sensitive sending unit to transmit an electrical signal to the gauge. The sending unit is a heat sensitive variable resistor that is located on or near to the cylinder head and threads into an engine coolant passage. The sensors are a Negative Temperature Coefficient (NTC) type. As the temperature increases, the electrical resistance of the sensor decreases. As the coolant temperature changes, so does the resistance of the sensor. The gauge is calibrated within the operating range of the sensor and interprets the resistance value to display the coolant temperature.

On models produced prior to 1994, the engine coolant gauge temperature sensor is a single wire terminal sensor.

Beginning with model year 1994, the engine coolant gauge and the Engine Control Module (ECM) temperature sensors were combined into one sensor with 4 terminals. The basic operation remains the same in that their resistance decreases as the coolant temperature increases, however the actual resistance values of the 2 sensor circuits are different. The electrical connector of the 4-wire terminal sensor ( 1 and 2 ) is keyed to prevent improper connection of the sensor's electrical circuit."

[Oli77]

Unfortunately, the symptoms I experienced could easily come from a bad sensor (or sending unit), or a bad gauge. It doesn't necessarily tell me that I have 2 sensors (but I think we do).

Could have been gremlins too.

[kayger12]

Yeah, you'll know if the sensor is going bad because you'll start getting more erratic readings.

[MrMaico]

I'm seeing two sensors. Besides the one in the lower radiator hose there is one back by the firewall in the block or head. From looking at the Bentley it looks like the one in the hose controls the electric fan and I'd think the other sender is the one your gauges get their info from.

I'm wondering if it wasn't just a glitch in the gauge. I guess you'll find out if it starts doing it more frequently. Then you'll have to try and figure out what's up.

http://www.realoem.com/bmw/showparts.do?model=BD53&mospid=47713&btnr=12_1079&hg=12&fg=05

part #13......

http://www.realoem.com/bmw/diagrams/v/u/138.png

[kayger12]

^Ah- good info.

[Oli77]

Thanks MrMaico. Bentley says one engine coolant temperature sensor feeds into to the ECM.

Still thinking gauge brain fart.