Titania lambda : reason for high output?
Discussion
As far as I understand it, the sensor acts as half of a voltage divider, with the other half being in the ECU. So for the ECU to see too high a voltage, the supply voltage to the divider must be too high. That comes from the heater circuit. If the heater circuit ground is higher than the ECU ground, I think that could cause these symptoms. If the difference is caused by circuit or contact resistance, that could explain why the symptoms vary with heater load.
GreenV8S said:
As far as I understand it, the sensor acts as half of a voltage divider, with the other half being in the ECU. So for the ECU to see too high a voltage, the supply voltage to the divider must be too high. That comes from the heater circuit. If the heater circuit ground is higher than the ECU ground, I think that could cause these symptoms. If the difference is caused by circuit or contact resistance, that could explain why the symptoms vary with heater load.
Hi Pete..About the only thing I haven't done electrically is to run a new heater ground to the same point at the ecu grounds go to. To be honest I didn't realise that a voltage offset with the heater ground could cause a fault. This is where knowing a little bit about electrics, but nothing about more electronic level lets me down. next job!
Cant you let some air in somehow to force a lean mixture to see if it shifts? I don't think it will be supply voltage related as an odd volt or two is only 10% and you are way outside that once its gone through the potential divider. Can you check the output from the probe against it local earth point?
GreenV8S said:
As far as I understand it, the sensor acts as half of a voltage divider, with the other half being in the ECU. So for the ECU to see too high a voltage, the supply voltage to the divider must be too high. That comes from the heater circuit. If the heater circuit ground is higher than the ECU ground, I think that could cause these symptoms. If the difference is caused by circuit or contact resistance, that could explain why the symptoms vary with heater load.
The wiring diagram shows it as going to battery earth, but then with a scribbled note saying bolt behind center pod, but that looks like the same as the other earths on the ECU?http://tvr-cerbera.co.uk/WorkshopWiringDiagrams/Bo...
There's a couple of connectors in this. Dirty pins seems unlikely but based on recent findings of slow window speeds caused by dirty/poor connectors/wiring.
Update time .. have been for an extended run in the car today and the out of range high voltage seems to be fixed ..
It also highlights the importance of testing something even though it is difficult.
I had previously tested the lambda heater supply voltage with the fuel pump running but the engine not running. This is because the lambda plugs are on top of the gearbox on the cerb and all but impossible to access unless you are taking the lambdas out. It's certainly nigh on impossible to get any kind of testing probe down to them with the lambdas still fitted in the manifolds.
However today has been an important lesson in doing the tests properly even if they are awkward or difficult, and don't assume anything.
The voltages at the lambda heater wires with engine off were fine .. battery voltage near as dammit .. but in testing a bit further I realised the fuel pump speed began being changed when I unplug / replug the lambda connectors. On these cars the fuel pump relay also powers the lambda heater wires, so this change in pump speed I'm thinking is liklely because the wiring cannot support correctly the power for the pump and the additional power required for the lambda heaters... except the heaters tested at battery voltage previously. Further testing showed that yes when cold the lambda heaters were indeed seeing close to battery voltage, but after testing for some time on the dyno I could see the voltage gradually getting lower and lower dropping by nearly 2 volts until it was at around 12volts - it may have gone lower still but I stopped there as I had seen enough to realise there was more going on than I originally thought.
I'd not seen this effect before because previously I had foolishly stopped the heater volt test once I had seen close to battery voltage. I assumed it was ok. Had I run the engine and kept the tester on the supply wires for longer I would have seen the volt drop the other day during initial testing.
This got me checking back through the circuit and particularly that the fuel pump ran all the time once the ignition was on and immobiliser switched out .. this is not uncommon if say the ecu has picked up a fault and the quick dirty and more importantly cheap fix is to just alter the wiring to let the pump run all the time the ignition is on.
Except this wasn't just a case of someone earthing the signal wire for the pump relay coil ... Behind the fusebox and well out of sight someone had wired in an additional relay .. and badly soldered the relay power out to the fuel pump and lambda heater wires on the back of the fusebox. Why they had done this I really don't know, because with the wiring re-instated correctly at the fusebox and running through the correct fuel pump relay the voltages all returned good again, hot and cold.
See attached pic below of the wire I cut out.
Now normal power all the time is restored to the lambdas, and the fuel pump speed barely changes (nice side-effect) ..
What is puzzling though is that the high lambda signal was apparently because of a reducing heater voltage whereas I wrongly assumed that a reducing heater supply would mean a reduction in the lambda sensor output. In reality the reverse appears to be the case here. The sensor is just a resistive element which changes with mixture so this bit really baffles me still. It's almost as if the new extra supply voltage provides a damper eliminating the high signal spike. The lambda traces are still violent spikes up and down just as they should be and just as they always have been on the data logger, but the max voltage reached was too great previously, and now never goes over 1.3volts - and even under acceleration fuelling enriched mixtures never goes above 1.4v. I usually see 0-1.2 volts as the signal range so 0-1.3 I will certainly take!
In the end the rising lambda signal voltage with time wasn't anything to do with sensor tip temperatures as I had thought, but more that the supply voltage was reducing, no doubt due to the high resistance when hot of the wiring below.
Anyway .. I wish there was something waaaaay more technical as an answer to this, but this dodgy bit of wire seems to be the culprit ... fingers crossed anyway .. 3 hours of driving and not a single lambda fault flagged in the ecu .. touch wood .. etc
It also highlights the importance of testing something even though it is difficult.
I had previously tested the lambda heater supply voltage with the fuel pump running but the engine not running. This is because the lambda plugs are on top of the gearbox on the cerb and all but impossible to access unless you are taking the lambdas out. It's certainly nigh on impossible to get any kind of testing probe down to them with the lambdas still fitted in the manifolds.
However today has been an important lesson in doing the tests properly even if they are awkward or difficult, and don't assume anything.
The voltages at the lambda heater wires with engine off were fine .. battery voltage near as dammit .. but in testing a bit further I realised the fuel pump speed began being changed when I unplug / replug the lambda connectors. On these cars the fuel pump relay also powers the lambda heater wires, so this change in pump speed I'm thinking is liklely because the wiring cannot support correctly the power for the pump and the additional power required for the lambda heaters... except the heaters tested at battery voltage previously. Further testing showed that yes when cold the lambda heaters were indeed seeing close to battery voltage, but after testing for some time on the dyno I could see the voltage gradually getting lower and lower dropping by nearly 2 volts until it was at around 12volts - it may have gone lower still but I stopped there as I had seen enough to realise there was more going on than I originally thought.
I'd not seen this effect before because previously I had foolishly stopped the heater volt test once I had seen close to battery voltage. I assumed it was ok. Had I run the engine and kept the tester on the supply wires for longer I would have seen the volt drop the other day during initial testing.
This got me checking back through the circuit and particularly that the fuel pump ran all the time once the ignition was on and immobiliser switched out .. this is not uncommon if say the ecu has picked up a fault and the quick dirty and more importantly cheap fix is to just alter the wiring to let the pump run all the time the ignition is on.
Except this wasn't just a case of someone earthing the signal wire for the pump relay coil ... Behind the fusebox and well out of sight someone had wired in an additional relay .. and badly soldered the relay power out to the fuel pump and lambda heater wires on the back of the fusebox. Why they had done this I really don't know, because with the wiring re-instated correctly at the fusebox and running through the correct fuel pump relay the voltages all returned good again, hot and cold.
See attached pic below of the wire I cut out.
Now normal power all the time is restored to the lambdas, and the fuel pump speed barely changes (nice side-effect) ..
What is puzzling though is that the high lambda signal was apparently because of a reducing heater voltage whereas I wrongly assumed that a reducing heater supply would mean a reduction in the lambda sensor output. In reality the reverse appears to be the case here. The sensor is just a resistive element which changes with mixture so this bit really baffles me still. It's almost as if the new extra supply voltage provides a damper eliminating the high signal spike. The lambda traces are still violent spikes up and down just as they should be and just as they always have been on the data logger, but the max voltage reached was too great previously, and now never goes over 1.3volts - and even under acceleration fuelling enriched mixtures never goes above 1.4v. I usually see 0-1.2 volts as the signal range so 0-1.3 I will certainly take!
In the end the rising lambda signal voltage with time wasn't anything to do with sensor tip temperatures as I had thought, but more that the supply voltage was reducing, no doubt due to the high resistance when hot of the wiring below.
Anyway .. I wish there was something waaaaay more technical as an answer to this, but this dodgy bit of wire seems to be the culprit ... fingers crossed anyway .. 3 hours of driving and not a single lambda fault flagged in the ecu .. touch wood .. etc
Can't say I'm surprised. Had something similar on a cerb recently with no lambda voltage , turns out the entire wire was burnt back to the FB and to "fix" the issue a 30a fuse was used in the lambda circuit supply. It made a mess. An expensive mess as all the plugs needed replacement, a whole new wire front to back and 2 new sensors...
People messing about with things they don't know is probably 75% of my work on electrics.
Well done for finding it. Oh , ECU in the post by the way!
People messing about with things they don't know is probably 75% of my work on electrics.
Well done for finding it. Oh , ECU in the post by the way!
Steve_D said:
I feel for you. Hours of work which are very hard to justify when charging the customer.
Job well done in the end though.
Steve
I will probably end up charging out something like a quarter of the time spent .. I've learned something from it all, and the customer has already had a big bill from the previous garage .. and to be fair it's not the customer's fault I didn't find the fault sooner!Job well done in the end though.
Steve
GreenV8S said:
Excellent bit of detective work there. Would insulation piercing probes have helped you see the running voltages in this situation?
Hi Peter, the issue is access to the part of the wiring that separates out to the lambda sensors .. it's right down the back of the bulkhead on top of the gearbox so although tools of that kind would definitely have helped, its hard enough getting just your fingertips down to the wiring never mind trying to pierce the correct wire with a probe.In the end I got two lambda wiring extenders as used on the catted griffs/chims/V8S and used the (then accessible) plugs on those as my points of access.
spitfire4v8 said:
Steve_D said:
I feel for you. Hours of work which are very hard to justify when charging the customer.
Job well done in the end though.
Steve
I will probably end up charging out something like a quarter of the time spent .. I've learned something from it all, and the customer has already had a big bill from the previous garage .. and to be fair it's not the customer's fault I didn't find the fault sooner!Job well done in the end though.
Steve
Idiots do slap all the time on the bill but eventually lose most of their customers like idiots do
Not taking the piss here. Several moons ago, one of the first things I learnt was to never take anything for granted, always check the full circuit out when the fault is present and bill the customer for that well spent time
Still a proper b
h of a fault though when not being able to gain access for easy testingspitfire4v8 said:
What is puzzling though is that the high lambda signal was apparently because of a reducing heater voltage whereas I wrongly assumed that a reducing heater supply would mean a reduction in the lambda sensor output. In reality the reverse appears to be the case here. The sensor is just a resistive element which changes with mixture so this bit really baffles me still. It's almost as if the new extra supply voltage provides a damper eliminating the high signal spike. The lambda traces are still violent spikes up and down just as they should be and just as they always have been on the data logger, but the max voltage reached was too great previously, and now never goes over 1.3volts - and even under acceleration fuelling enriched mixtures never goes above 1.4v. I usually see 0-1.2 volts as the signal range so 0-1.3 I will certainly take!
Conductivity in semiconductors depends on electron/hole mobility, which is temperature sensitive.Low heater voltage = low sensor temperature = lower electron/hole mobility = higher resistance = high voltage drop.
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