Rev Counter on Diesel

[Stuarted]

The rev counter on my 1.9 diesel estate does not work.

I understand that there is a 'sender' or a sensor / pickup device on the engine to drive this - obviously the rev counter can't work off the "ignition" electrics on diesel! Where is this sensor and what does it look like please?

The only workshop manual I have is for a petrol BX. Can anyone advise please me how I can check if the fault with my inoperative rev counter is with the dashborad instrument, the sensor on the engine, or the wiring?

[bx_pallas]

Behind the Battery you find a brown 3-way connector with two wires going down to the sensor (below the connector the cables are manteled, either black or white). The sender sits on the flywheel, engine side, next to the right hand drive shaft.

[Stuarted]

Excellent - thank you!

Do you happen to know what electrical signals should be present, please, so that I can check if it's the clock, the sensor or the wiring where the fault is?

[tim]

This seems to be a common BX problem, and reminds me that I was going to put a post up about this. When I first got the Red One running, the rev counter wouldn't work for 3 or 4 miles then it would happily chime in for the rest of the journey. Then, after a couple of years, one day it decided to work OK for the first two or three miles, then.... yep, you guessed it. And it's been that way ever since. Temperature made a difference too, in winter it takes longer to die, now it's warm it goes almost immediately. Andy's non-turbo one doesn't work at all, and the wife's wanders about all over the place and never shows more than half the true reading.
So is it the pick-up, which can be unplugged, or what it picks up from - which can't be. If it's some kind of pickup ring inside the engine it would be a sod to get to - would just a clean do it or is there something electronic which needs replacing?

[mat_fenwick]

I think it's a Hall effect sensor - it picks up signal from the flywheel teeth. Usual failure mode is cutting out when hot; my suspicion is that a connection fractures within the resin encapsulation after repeated heating and cooling, and when hot this connection is broken. It's shared with other XUD engined vehicles so should still be easily available.

[Stuarted]

Thanks - helpful.

I can retrieve the sensor off a scrap BX where the rev counter worked OK.

However it would save time if there were a straightforward way of checking for sure if it is a faulty sensor (and not a wiring or an instrument fault) which is preventing the rev counter from working at present.

(Just to clarify, in view of the interesting replies so far: the rev counter does not work at all at present, hot or cold.)

[mat_fenwick]

I'm away from a BX at present so can't check resistances, but I think that one of the 3 wires is for the cable screen. A simple continuity check between the other 2 may help - if it is open circuit then there is definitely a sensor problem. If you have an oscilloscope you will be able to check what output (if any) is present; if not then a normal multimeter set to measure AC voltage may show some kind of output from a good sensor.

[bx_pallas]

The sensor is indeed the most common fault. You should measure a resistance in the 100ohm range (test at the brown connector, disconnected!). Could also be possible that the sensor is not close enough to the flywheel, the gap needs to be as narrow as possible. To adjust its position, loosen the little screw on the plasic mounting.

The connections at the back side of the rev counter instument are: (blue connector)
Pin #1: +12V (over ignition and fuse F1)
Pin #2: Ground
Pin #3&4: The sensor (via the brown connector behind the battery)

The F1 fuse (the very left) is also worth checking (first of all!), blown fuse = no rev counter!

[Stuarted]

Thank you, gentlemen - all good stuff. Very helpful.

[Tim Leech]

They are still avalailble new from Citroen/Peugeot also worth trying eBay.

[Stuarted]

Thanks for all the info, folks.

Replaced sensor with the one off a scrap BX. As for adjustment, the best way seemed to be to slide the sensor in until it just touched the flywheel teeth, and then back a bit to give a bit of clearance. Rev counter now works a treat. Judging by the chewed-up end of the old sensor, it was probably damaged by being adjusted too close to the flywheel teeth.

Wouldn't have relaised the position of the sensor was adjustable if it were not for the info here. Thanks.

[electrokid]

Glad it's fixed

Yes the sensor needs to be as close to the flywheel as possible - 100 ohms sounds like a coil sensor - I know from experiments with a tacho on the test bench that the input signal needs to be quite high - slightly greater than 12 volts.

Where the tacho fails after the engine has been running for a while it could be sensor adjustment - magnetism reduces with increasing temperature (the curie curve) so any magnetic sensor will have an output which reduces as temperature increases.

[themildbunch]

Where the tacho fails after the engine has been running for a while it could be sensor adjustment - magnetism reduces with increasing temperature (the curie curve) so any magnetic sensor will have an output which reduces as temperature increases.

Yes, a perfect description of my issue too after a recent engine swap - rev counter works fine then slowly becomes a little 'sticky' behaving oddly, then stops altogether after about 10 / 15 minutes.

[electrokid]

Yes, a perfect description of my issue too after a recent engine swap - rev counter works fine then slowly becomes a little 'sticky' behaving oddly, then stops altogether after about 10 / 15 minutes.

With the (relatively) small increase in temperature that the sensor will experience, the effect will be quite marginal so you probably only need to move it very slightly closer to the flywheel. (In other words it's probably very close to where it should be anyway.) The story above suggests the need for caution to aviod a destroyed sensor so good luck with that

If the problem is related to temperature then your tacho will take longer to fail on a cold day.

[mat_fenwick]

Hmmm, I wonder if it is that rather than just my theory of a connection being pulled open circuit due to thermal expansion. I must admit I've never tested a failed sensor, just replaced them, which cured the problem. Previous ones have lasted between 100 and 130k miles so the one I have in now is getting on for failure time if these lifespans are typical.