Exhaust temperature sensor

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Exhaust temperature sensor

Postby Mjolinor » March 5th, 2017, 4:12 am

I have a diesel smart car that is throwing error codes for the temperature sensor in the exhaust. These cost £140 so not something I will simply buy without knowing why.

I assumed that this sensor would probably be a thermocouple, hopefully type 'K' so made one up and stuck it in. With no sensor connected I get "high temperature" on the diagnostics, with a thermocouple plugged in I get "low temperature" on the diagnostics so this is telling me that it is probably a PTC thermistor.

In my experience there is no way a PTC thermistor will cope with the temperature range needed. The bit I took out is nominally 220 ohm or so in my workshop (18 c or so) and will increase to 340 ohms or so when I hold it (guessing maybe 33 c or so). It has to cope with 700 c or so when in the exhaust so warming it up with a blow torch to the dull red temperature (approx 650c) I have a resistance of around 500 ohms.

Polarity does not affect the readings so probably not a diode, maybe a bridge?

Any suggestions as to what this is that will cope with such a range and such a hostile high end temperature?

It is a two wire device and neither wire is connected to the body of the thing.
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Re: Exhaust temperature sensor

Postby k-ww » March 5th, 2017, 5:11 pm

Doing a bit of google-fu, I came across this video:

https://youtu.be/hXUF6MACayQ

It suggests ways of testing - hope it helps.
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Re: Exhaust temperature sensor

Postby Mjolinor » March 6th, 2017, 2:35 am

Cheers for that.

I will have ignored it as I tend not to like video for information, the musical crap that people seem to think helps really winds me up. :)

Interesting watch but I have already been there, what I need is something to replace it without forking out much money and without too much circuitry. It seems to me to be stupid to not use a thermocouple for this particular task and I am thinking that making a thermocouple amp and associated level shifting to fool the ECU is going to be the best way.

Mercedes have had four versions of this sensor now because it has been known to fail since 2009 and they still haven't sorted it but are happy to let the customer pay for their research and failure while they have fun making new ones that also fail.
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Re: Exhaust temperature sensor

Postby k-ww » March 6th, 2017, 6:58 am

Well, you could always just wire a 1-2k pot in place of the sensor, and dial in the temp....

or visit a junkyard for a replacement.

Are you sure the ECU isn't bad - that was one of the things the video suggested.
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Re: Exhaust temperature sensor

Postby Mjolinor » March 6th, 2017, 1:20 pm

It seems I have fixed it. A screwdriver in the hole and rattling it about resulted in much lumps of carbon existing the hole. I reckon the design fault is that carbon build up round the sensor prevent it from reacting fast enough. The later versions of this sensor have a heat shield around it and are thicker all the way along the probe. I reckon that is to stop carbon build up directly round the sensor.

Fault codes gone now and not come back today which they would normally have done so maybe it is OK, time will tell. :)
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Re: Exhaust temperature sensor

Postby k-ww » March 6th, 2017, 3:39 pm

Ah, the joy of fixing something out of frustration.

The important thing is that you fixed it w/o cash expenditure.

The carbon could also have cause a partial short from the shell of the sensor ['earth ground']
to the high side of the signal output, causing a erroneous reading.
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