Question of a complex technical nature

[Mase]

Hi all,

I can't post in Technical for some reason...hence posted here.

Does anyone know exactly why a simple clean of the TDC elect connections at the TDC (CAS) will eliminate the need to turn the key twice to start? It's very common here in Aust but none of us really understand the reasoning behind it as a solution to this common hard start prob.

Regards,

Mase

 

[davebem]

How I understand it the tdc sensor monitors the position/speed of the crankshaft, this info is used by the ecu to control the ignition and timing. If the sensor is faulty the ecu will be getting the wrong signals at startup. Where it is positioned it gets covered in dust and can also corrode.

 

[Mase]

Hi davebem - thanks for that, already have the whole functionality of the TDC sus'd. The question being is why do so many people have to turn the ignition to 'on' then 'off' again then 'on' before the car will start. Once started the car will run fine. Some guys have to do the on/off routine everytime they start the car. There seems to be an 'overide' function within the ECU that says "if no / or poor TDC sensor signal, then run from last state crank position & time spark from maps in ECU". My thoughts are that if volts drop too low on engine crank the TDC outputs poor pulse info to the ECU / or ECU fails to read inductive output. After the second key trun the ECU reverts to a back up mode as described above, volts rise as engine has started and the ECU can more effectively read the pulse from the TDC.

Does anyone know if a voltage is supplied to the TDC coil or is it purely passive?

I am wondering if anyone in the UK actually knows the explanation for this curious problem and why a simple clean fixes this dual start scenario.

Cheers

 

[bmh.01]

As is my understanding the TDC sensor is a hall effect device so the target on the flywheel induces a voltage in the TDC coil as it passes the sensor the ECU interprets this pulse and works out the crank position from it. From this it works out the cam positions so it knows what piston/cylinder is doing what.

The trouble with it is in its positioning (although it can't go anywhere else) as it gets covered in the clutch dust as the clutch wears and this effects it picking up the target on the flywheel. Why it starts on the second try I don't know.

Its solved as you say by cleaning the sensor and ultimatly changing it for a new one.

 

[Mase]

Thanks bmh, yes I have read it's a hall effect sensor. The 2nd turn of the key thing is a real mystery to my inquisitive mind..and will likely stay that way until someone with intimate knowledge of the whole ignition system chimes in which is unlikely. I understand that Renault are on their 3rd rendition of the TDC so it's obviously a known issue although the 'tech notes' that the dealers here in Aust have mentions nothing. It's almost like Renault want it that way.

I have had some dialog with BenR on the but it stopped short of the TDC twin start situation.

I suspect the newer TDC's produce a high voltage fluctuation as the target passes.