Citroen XM Forum

Many thanks again John although I am fairly confident that this is not the cause of my poor running. With regards, Matthew T.

Just a brief update to report that the tacho is now barely working at all! I have not had much time to carry out further checks, but hopefully this weekend may bear fruit. The tacho issue can only be the crank sensor, gauge or associated wiring and if the sensor was completely duff the engine would not run surely? My theory is that the gauge has been upset or damaged by the rev limiting effect of the fuelling issue. If the problem is insufficient fuel a faulty sensor would not be responsible would it, unless it is about timing? I have tried other sensors and checked the wiring and can obviously do so again. Regards, Matthew T.

It might be worth disconnecting the Tacho in case it is that causing the erratic signal, hence poor running.

I doubt poor running would harm the tacho.

Could someone post some circuits, covering
- crank sensor to tacho
- crank sensor to engine ECU
- crank sensor to anything else it drives (eg Ignition).
Thanks.

It might be worth disconnecting the Tacho in case it is that causing the erratic signal, hence poor running.

Do you mean the tacho might be upsetting the signal Will as the sensor is providing the signal surely?

I attach copies of my earlier annotated diagrams Chris and as you will see two crank sensor connections go to the ecu and the third to ground (sensor located on diagram 1 to bottom left. Since the tacho is fed from the ecu it may be tricky to identify the other wires and I am not sure quite where the instruments are on the diagram!

With regards,

Matthew T.

My apologies Chris I have just discovered that the instrument clusters are on diagram 2 top right at 40 so fed from ecu pin 12! Regards, Matthew T.

Thanks for the diags, Matt. Just printed them out and pored over over breakfast.

Trying to understand several things:

- Tacho (#40) only seems to have two connections: IGN+ (F23), and one other (going to ECU pin 12). The latter is presumably the pulsed signal input to make the tacho read.
But this is unusual wiring for a tacho, which normally would have at least three connections (+12 supply, Gnd, and pulse-input).
Can only presume the tacho is Gnd'd at the Instrument Cluster. Perhaps you could check on the Cluster diagram.

- I looked everywhere for a #160 (TDC sensor, or crank angle sensor), thinking such would be essential for ignition/injection timing, and tacho too. Can't find one of these anywhere - just #152 (Engine Speed Sensor).

- Dean's and your posts of 30-31 January suggest that the (presumably #152) sensor is expecting 3 pulses per engine rev. Your photos of 16 Nov (sheared flex plate) show the perforated periphery of the flex plate. Is it these perforations that trigger the sensor? Lots of close holes, plus ? 3 plain sections ? If this is the case, then the waveform the sensor is collecting is complex: a series of very high frequency pulses, with a longer pulse every 120deg.
Like this . . . . . . . . . . . . ::: . . . . . . . . . . . . ::: . . . . . . . . . . . . ::: . . . . .
Not sure what the ECU does with this; could well be that it simply filters out the high-frequency part, and reads the lower frequency pulses. ???

From this, two things:

- The circuit diagram shows that the cabling from #152 to the ECU is screened and grounded. Might be worth checking this. The screening will be to protect the signal from external interference, which could corrupt the waveform.

- Flex plate mounting:
Seven large bolt holes fix it to crank. Cannot see a positioning peg. How is the plate (which seems to trigger the sensor, at its periphery - correct?) known to be in the right rotational position? This would affect the timing.
Thought that 6 mounting holes could correspond angularly to 3 plain bits at the periphery . . . but with 7 mounting holes, it's a different ball game.

A few QQ, again.

Chris

Chris, while it is possible they could have changed the trigger pattern on the flywheel its unlikely, it is impossible to fit the flywheel out of sync with TDC on pot 1 aligning with one of the blanks on the flywheel at the crank sensor window as the bolt patterns are unevenly spaced.

The cable for the crank and map sensor are shielded with foil inside the rubber sheath of the cable, the ground wire is unsheathed and in contact with the foil.

Many Thanks
Dean Ward
Corralls

OK, I didn't spot the uneven 7-bolt spacing.

Many thanks for your further comments Chris and Dean I would guess that the instrument cluster is grounded elsewhere as suggested. The engine speed sensor is indeed the crank sensor and I can also confirm that the sensor wire is screened and the starter ring only fits in one position due to the location of the fixing bolt holes. My plan is to recheck the wiring and try other sensors and instruments on the weekend and I shall keep you all posted! Regards, Matthew T.