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Understanding LR lambda sensors


AncientGeek
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I'm trying to figure out what electrickery a PO has done.

He has retrofitted lambdas to a non-cat system and changed the tune resistor to the UK cat type (3900 I think, I forget the colour).

The lambdas look like the NTK ones and the wiring has been bodged to suit.

As I understand it these are the Titania type which do not generate a voltage.

So to test them, I should be measuring the resistance on them, right? While hot I know.

And on the supply side the ECU should be supplying what? 1 volt I expect. So with the lambda disconnected I should measure 1 volt. and 12 volts across the heater wires. Does the ECU only supply this while the engine is running or do I just turn the ignition on?

I see the shielded blue wires from the lambdas have the shield connected to ground on the ECU side only. I'm not sure if this is a mod or if they were done like that at the factory.

How much does that mega-thingy cost again? :)

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OK, what I read on the net was rubbish.. they do generate their own voltage and the ECU measures it. Averaged at about 0.6 volts at hot idle which seems fine. Generated a vacuum leak and it drops right down.

Now to try and make sense of what he's done to the MAF and ECU loom...

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I thought the hotwire systems all used Titania sensors, although you could wire Zirconia sensors to it they wouldn't do anything as the ECU wouldn't get a meaningful reading.

I think what you've read about the two types is correct (Ti are resistive, ECU supplies 5v to them across a resistance bridge, Zr ones generate 0-1v and the ECU just reads it direct) but now sure how you'd frig a Hotwire to read them... not sure I want to see what's been done to yours :ph34r:

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I wonder....

I don't think that your lambdas are doing anything useful at the moment - I think that the PO had a fundamental misunderstanding of how they work... You said when you introduced an air leak, that the reading went right down? So surely that means that the ECU is NOT compensating and correcting the mixture like it should if it was getting any meaningful information from the lambdas?

As FF said, the two types of lambda are fundamentally different in their operation and to use a zirconia one in a titania circuit would require some pretty radical modifications to the electronics. Since yours are generating a voltage they are zirconia and will not give the 14CUX any useful information and may well generate a fault code if connected.

Or are they really generating a voltage? Did you measure them while disconnected from the ECU? Could it be that you are reading an ECU supplied voltage?

The tune resistor tells the ECU what the setup is and so if yours is the 'I have lambdas' (white) resistor it will be attempting to read the mixture and compensate, as opposed to running open-loop if the 'no lambdas fitted' (green) resistor is there, and so from the fact that you made such a nice 0.6v average - perhaps the PO was some kind of a wiz and he made the system work? Or perhaps it's actually running open-loop anyway (mine is open loop and does that).

Personally I think that you need to do a little more checking:

(a) Find the tune resistor and check what it is.

(b) Disconnect one of the lambdas and (with the engine hot) put a volt-meter across it. You can totally disconnect it or leave just the heater wires connected, it doesn't matter since heaters are not required when the engine is hot. If you get a voltage flicking around between 0-1v then you have titania ones which will not work with a 14CUX. If you see no voltage then you have zirconia which are the correct ones for the 14CUX and should work if the correct tune resistor is fitted.

Finally: There has been some discussion on this forum as to whether or not the 14CUX has a "limp home mode". Although there are several references to this on fairly reputable websites, I don't thing a definitive answer was found. Just in case there is a limp home mode than you might like to disconnect the battery for 15 seconds between testing as this will clear any fault codes.

Hope this helps - sorry if all I have done is muddy the waters...

Roger

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The tune resistor is white. I even measured 3900 Ohms across it. They were never sold like that in this country.

I measured the voltage from the ecu on the sensor wire (with the lambda disconnected)and there was none. But perhaps the engine needs to be running.

I never tried it the other way by measuring a voltage across a disconnected lambda.

There's a piggyback ECU in here but it does not seem to use the lambda circuits. Crikey indeed.

Probably rip it all out and do some business with a well known forum member here. But in the meantime I'm curious.

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If no tune select resistor is fitted the ECU runs in catalyst mode, is that what you mean by limp-home?

With the green resistor fitted the ECU runs a fixed fuel map with no learning or self-tuning. In truth the learning is a self-callibration during initial engine start up and has I'm sure been discussed before.

With lambdas fitted and working the engine starts up using the MAF sensor to get initial fuelling then once the engine has settled revert to running closed loop on the lambdas. With rovacom you can watch the lambdas reading in real-time.

With lambdas fitted you can get the engine running without a MAF fitted (when mine broke that's how I got it home) but they don't run as well.

This website explains it better than I can at this time in the morning.

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I took out a lambda. It has these numbers on it:

JAPAN

5350 03c14

Could not find anything on on google for that.

Wiring is blue, black and black. Seems these are more common colours for a zirc sensor.

I put a lpg torch on it (gently) and I measured up to 1 volt across the sensor wire and chassis ground!

So looks like Zirc.

WTF was he thinking?

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I took out a lambda. It has these numbers on it:

JAPAN

5350 03c14

Could not find anything on on google for that.

Wiring is blue, black and black. Seems these are more common colours for a zirc sensor.

I put a lpg torch on it (gently) and I measured up to 1 volt across the sensor wire and chassis ground!

So looks like Zirc.

WTF was he thinking?

Probably just "Well any lambda sensor will do...these are much cheaper" :rolleyes:

Just because something is mounted in a nice way, doesn´t mean it´s working right...found many goofy, homemade wirings in my RR.

I would replace with the right type lambdas and re-wire back to the ECU. Just to be 110% sure. No need to be burning more fuel than absolutely needed. The V8 is thirsty enough in the original state.

/Bo

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I would replace with the right type lambdas and re-wire back to the ECU.

Definitely the best option - although you may find that the thread is different - I'm not sure but I have a feeling that they are...

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I vote rip it all out and Megasquirt it, then you know for sure what everything's doing.

Already talking to Nige... ;)

About the thread.. I think the original lambdas were a spark plug thread instead of the 18 mm that is common today.

I'm still baffled why someone would have gone all this trouble and then make such a basic mistake. He got got the original round connectors which he must have imported, welded on bungs, and a piggyback ecu that HAS to be dyno-tuned.

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About the thread.. I think the original lambdas were a spark plug thread instead of the 18 mm that is common today.

You can find most the information here: My link

They don´t specify whether it is 12 or 18 mm - both sizes are found on the titania types. I can measure on my 1990 RR if you need the info.

/Bo

OK, just found out, it´s 12 mm... :(

Edited by Roverbo
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