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Old 24-08-2009   #1
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Question Punto Mk2 Lambda Sensor Resistance

Hi there

Im trying to fault find an issue with my engine, its running as rough as a VW Camper van! I have recently changed my cat, it cracked off at the weld, story for another thread.

After changing the CAT the car seems to have lost power, and sounds rough. I had real trouble getting both sensors off, the pre and post cat sensors. I really had to hammer and bash them off.

So, im trying to work out if I have shagged them in the process.



As you can see from above this model has two sensors, and from eper, it seems they are both the same.

looking at the following electrical circuit:



where K40 is one of the sensors, it seems the sensor is made up of a resistor, which im guessing changes with temperature, and another component, which looks like a heat sensitive capacitor or thermocouple.

So, i've been out to the car and got the following measurements:

COLD
---------------------
Pre-CAT terminals
1 - 2 Open circuit, 0V AC, 0V DC
3 - 4 Continuity, 8.4 Ohms (meter set to 200scale)

Post-Cat terminals
1 - 2 Open circuit, 0V AC, 0V DC
3 - 4 Continuity, 9.3 Ohms (meter set to 200scale)

WARM (5 mins idle)
---------------------
Pre-CAT terminals
1 - 2 Open circuit, 0V AC, 0V DC
3 - 4 Continuity, 12.5 Ohms (meter set to 200scale)

Post-Cat terminals
1 - 2 Open circuit, 0V AC, 0V DC
3 - 4 Continuity, 13.3 Ohms (meter set to 200scale)

HOT (10 mins)
---------------------
Pre-CAT terminals
1 - 2 Open circuit, 0V AC, 0V DC
3 - 4 Continuity, 14.1 Ohms (meter set to 200scale)

Post-Cat terminals
1 - 2 Open circuit, 0V AC, 0V DC
3 - 4 Continuity, 15.3 Ohms (meter set to 200scale)


I cant seem to measure any change between terminals 1 and 2, I suspect i need to scope these terminals, sadly i dont have one avaliable.

Terminals 3 and 4 are increasing in resistance in line with temperature, and both are within 1 degree of each other, i think this is negligable, but i might try swapping them over.

interestingly, having the sensors unplugged versus plugged in made no difference to the revs or noise at idle.

so my question is, is there anyone out there willing to go and measure their sensors. If you have a multimeter to hand and a spare ten mins, it would be a real bonus, not only to me, but i think this could be of interest to others.

thanks in advance.
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Old 24-08-2009   #2
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Red face Re: Punto Mk2 Lambda Sensor Resistance

Ah, google and youtube are our friends!

It seems that these are heated sensors. pins 3&4 are heating elements, resistors that heat up to over 300 degrees believe it or not. Their resistance does change with temp, but this is a by product if you like. they should be around the 7 ohm mark at 20 degrees C or there abouts.

pins 1&2 are joined to a variable power source, a thermocouple that gives of an AC signal when heated, but only a few hundred millivolts, in the region of 500 to 1000mV (0.5 to 1.0V under load)

So it looks like im going to have to try and find either a really good voltmeter or an oscillioscope to check these two sensors out.

Just when i had ruled them out too!
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Old 26-08-2009   #3
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Lightbulb Re: Punto Mk2 Lambda Sensor Resistance

took the car into work today and hooked up both lambda sensors to a work collueges oscilloscope.

im no expert at these things, but with some help from youtube i managed to get both sensors outputting a signal of between 0v AC and 900mV AC.

im fairly happy that these are not causing my problems, but this may be interesting info to someone else out there.
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