C5 Revcounter fault

waterwytch · Post by **waterwytch** » 04 Mar 2009, 17:54

I'm sorry if this has already been posted,but My dad (a card carrying technophobe) has a problem with the revcounter on his C5 2.2HDI,its showing about half (or less)of the actual engine revs IE 2000 rpm nearly flat to the floor,the engines revving no problem,pulling ok,he recently had a new battery fitted after the orignal failed on a cold morning,it a mark 1 exclusive on a 01 plate,I did auto electrics years ago in the 80's for BL so any cures you can talk techy,its got me stumped,though I did tell him to buy another Xantia

KevMayer · Post by **KevMayer** » 05 Mar 2009, 16:54

The info for all the instruments on the mk1 C5 is supplied as digital info on the VAN (Vehicle Area Network) so there's not a cable driving the tacho in this case. The VAN is a 3 wire network which connects many components together.

If the revs are being displayed incorrectly there isn't much you can do without the aid of a Citroen/Peugeot computer which will mean going either to a main dealer or a specialist. I don't think there'll be much difference in cost if you choose either.

One thought I have is that could the revs be limited to 2000 rpm because the C5 is running in limp home mode. This happens if the engine ECU sees a fault. The C5 will resort to limp home mode and I believe the revs are limited to 2000 under these circumstances.

If you feel the car is running well then it won't be limp home mode and i'm sure you'd notice the difference.

p.s. Just had another thought... I believe there is an engine speed sensor which is a proximity probe near the flywheel. This send the speed info to the ECU. The probe detects the passing of a raised piece of the flywheel steel as you probably already know. It may be worth checking this. I'm not sure what you'll find. Somehow the signal may be divided by 2. Could there be 2 raised bits per revolution and one has dropped off ?

waterwytch · Post by **waterwytch** » 05 Mar 2009, 17:08

Thanks Kev,the engine is pulling like a train no problems as I had it over the ton blowing it out seems maybe a dashboard fault,I'll see if theres a mention of a twin "slug" on the flywheel for the sensor in what little documentation I have on the hdi's,the speedo seems to have had "issues" recently too,the needle only moving from zero at about 30,may need a reset on Citroen Diagnostic system,he will moan about the price of that,typical pensioner