TDC sensor

tomsheppard · Post by **tomsheppard** » 28 Jan 2003, 14:10

BX Diesel has a TDC sensor which signals to my currently inert tachometer. In order to make it ert again, I need to do some detective work. Does anybody know the nominal resistance of the sensor please? Has anyone an idea of the signal level coming out? This will tell me if the Sensor is duff or if I am going to have to strip out the dash again. If the latter, has anybody ever stripped one of these down and reverse engineered it to get a circuit diagram?
(I just hate having something that I could fix if had the right information!)

DLM · Post by **DLM** » 28 Jan 2003, 16:11

This isn't much help, but you're not the first to try and get this information. There's a section on the BX DIY site about retrofitting tachometers to petrol engines with an ominous space where they said they'd fit in the section about diesels "when the information is available".
http://www.tramontana.co.hu/citroen/ignite/ignite.html
I've had a rev counter on a TGD suddenly stop working (in cold weather) then equally suddenly spring to life again. Have you located the sensor easily enough? I've seen reference to it being on the flywheel end of the engine, but haven't yet managed to locate it.

tomsheppard · Post by **tomsheppard** » 28 Jan 2003, 16:29

I have found the sensor on the bellhousing at the top which is canted back on the BX of course. There is the usual plumbing in the way which is why I haven't stripped it out yet. A cable runs from it with three conductors, one of which is a screening braid. This is certainly the correct sensor, and it connects into a pair of brown wires. If I had a scope trace of the required signal, I would then be able to either (a) see if the sensor is active or (b) by applying the required signal to the brown pair, determine whether the signal was getting to the tach. I suspect that I will need to source an old instrument panel to see if I can find out what goes on within and then post my findings in the common fixes section.

wheeler · Post by **wheeler** » 29 Jan 2003, 03:28

i think the resistance is 380 ohms but i cant remember which pins you measure it across,theres only a few combinations to try !

jeremy · Post by **jeremy** » 29 Jan 2003, 12:16

If you need to test the sensor and don't have an oscilloscope why not listen to it.
If you have a suitable radio or tape player with a microphone input simply connect the sensor to a jack or din plug as required, connect it up, start the engine and slowly increase the microphone gain. You should be able to hear the pulses. I have used this technique to trace faults in ABS sensors, and those sensors produce a hum which increases in frequency and volume with speed.
The sensor is essentially the same as an ABS sensor and produces small pulses of electricity which are then 'processed' by the rev counter to give a deflection on the needle. The generator system is essentially the same as some forms of microphone.
Jeremy

tomsheppard · Post by **tomsheppard** » 29 Jan 2003, 13:19

Thank you all, The audio signal route is a nice simple test and, I suppose that I could measure the clicks at the tacho connection which is the preferred place. I suspect that the two different units for petrol and diesel have only a different calibration resistor in the meter circuit in order to vary the full scale deflection. I can see myself pulling some apart to find out more.Thanks Jeremy and Wheeler for some useful raw data.