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Lambda Sensor Problem


alanrichey

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Seemed reasonable that it has a temperature window where it operates. The heater is to get it there initially and keep it there in stop/start driving when there may not be enough exhaust heat to keep it at 700C. During a fast blast there is plenty heat to maintain its temperature and having the heater working as well may make it too hot and destroy the cell. (850C ish). Felt it may be an error to wire the heater on constant was my thinking.

 

Nigel

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Looks like we found the problem and can close off this thread. Turns out the lambda sensor was incorrectly wired up on the initial build 6 years ago,. This meant the sensor heater has never worked, hence the sensor itself was so slow in starting to work that the ECU never allowed it to go online. Probably the only time it worked was when the car was driven to the garage to fill up with petrol. On the way back the lambda sensor would have still been hot from the trip there and would have starting working straight away for the return journey.

 

Thanks to all the posters for their inputs.

 

And I don't feel so bad now about being stumped as I would never have thought of that.

 

And I can only emphasise to other Superspec owners that they need to use my program to analyse the ECU as that is the only way that we knew there was a problem with both our cars.

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Wish I could take the credit :) It was an owner of a Rover 100, which also uses the same ECU, that realised the 2V we were seeing on the heater circuit was the bias voltage produced by the ECU to the sensor.

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  • 2 weeks later...

As this was my SuperSpec we're talking about, I thought I'd just close off the discussions with summary details of the problem in case anyone needs the info in the future.

 

As Al says we eventually discovered the lambda sensor had never been wired up properly in the first place (apparently done by an auto-electrician!). The lambda sensor on the Rover engine has 4 wires - 2 white for the heater, a grey for sensor ground and black for sensor signal. After finding I only had 2v across the heater wires, which meant it never warmed up enough to allow the ECU to monitor the sensor, I traced the wiring back into the engine loom. Discovered that the sensor ground and heater ground had both been connected to the chassis and the other white heater wire was connected to the sensor return in the loom. The real wires for the heater were tucked up unused in the chassis. I've no idea how the ECU managed to get any signals from the sensor at all!

 

4 wires come from the Rover T16 engine loom near the alternator. The 2 sensor wires need to be in a screened outer sheath to protect from stray noise, and the sensor shouldn't be earthed separately outside the ECU. Wires need to be connected as follows:

screened loom grey (from MEMS pin 18) - to lambda sensor black

screened loom green (from MEMS pin 7) - to lambda sensor grey

loom brown/black (from lambda heater power relay) - to lambda heater white

loom black (from loom earth) - to other lambda heater white (it doesn't matter which way round the 2 white heater wires are connected)

I extended the sheathed screen down from the loom to the sensor with a braided outer hose, and believe the screen itself is earthed through the loom to MEMS pin 29.

 

After rewiring from the loom down to the sensor, running Al's MEMSAnalyser program showed that we now had a proper functioning lambda heater and signal again. Sorted eventually and just shows the sort of issues you can come across when you take on a car built by someone else.

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  • 1 year later...

Back to the Superspec lambda sensor and emissions again :)

 

Although my lambda sensor was working for last year's MOT and it sailed through the emissions, as this year's MOT approaches it started to show some severe dropouts, making the car a pig to drive and trashing the emissions, as the ECU did not recognise the lambda sensor was sending it rubbish and made a right mess of the fuel/air ratio,. I tried with two different sensors and saw the same problem, so it was either the ECU or the wiring.

 

So I stripped out the wiring from the loom to the sensor and was duly horrified. The original builder has simply connected 4 wires (all red <_< ) to the output from the loom and sent them through the same rubber cover to the sensor. That goes against everything you read about the lambda sensor wiring. The 2 heater wires don't matter, they can be any wire and just go to the sensor, but the signal wires should be completely separate and should be a shielded 2-core cable, with the sheath earthed at one end, to avoid interference from the alternator, ignition and the heater.

 

So cue a complete rewire from the loom to the sensor. Obviously early days yet, but both sensors appear to be working perfectly, with no dropouts, and the car is driving so much better than before.

 

So the message to all Rover-engined Superspec owners is to check the wiring of the lambda sensor from the loom (it comes out just by the alternator) and make sure it has been done properly. Remembering, of course, that we have yet to find a Superspec (apart from mine and Andy's that have been fixed) that has a working lambda sensor. :)

 

Al

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