Erratic Speedometer

[wheeler]

The gear inside the box is made of plastic, i have on the odd occasion on Xantia’s seen these breaking. If you take the sensor out the box you can inspect it with a lamp & mirror.
I have also in the past used a drill with a peice of injector leak off pipe in the chuck then pushed over the square drive in the sensor. With the scope connected you can see if its working ok & rule out the gearbox internals.

[lraxv]

Hello,

just a quick bit of additional info - I plugged an OBD2 reader into the van today and error code P0500 has been logged (Vehicle Speed Sensor "A" Malfunction). What I've read is that "the vehicle's speed as read by the Vehicle Speed Sensor (VSS) is not as expected" - how does the ECU make that decision? I don't suppose this fault implies a specific electronic fault, does it?

Thanks for your help

[GiveMeABreak]

The PSA definition of that code is: 'Vehicle speed signal : Vehicle speed or info returned not valid'

There are certain reference values that the ECUs can work with - and it's saying whatever it is getting doesn't make sense - so either the sensor is faulty or there is an underlying problem as Wheeler mentioned.

[lraxv]

Hi,

if I take the sensor out, how much oil will escape?

Thanks

[lraxv]

Right - I've tried everything electrical I can think of and am going to have to put this problem down to gearbox failure of some sort, although I've not been able to take the sensor out yet to inspect the gears.

Can I get at the VSS drive gears without major strip-down of the gearbox?..I suspect not

Does anybody know where the easiest place to get at one of the ABS sensor signal wires is in the engine bay? I can't see why, with a bit of electronic cleverness, I can't use that as a way to generate a new VSS feed.

Thanks

[GiveMeABreak]

I read on a different forum that vehicle speed is also read from the ABS sensors - this seemed unlikely to me but, when I investigated, my van has ABS sensors and reluctor rings, even though I'm pretty sure it doesn't have ABS. I'm in the process of removing the sensors to clean them and the reluctor rings in case this makes any difference - I don't think it will.

I just noticed this back in your first post and thought I'd just confirm your vehicle DOES have ABS brakes.

Personally, I wouldn't touch the ABS component wiring - the Bosch 5.3 ABS ECU contained within the block has 2 processors inside checking each other and comparing values received - any signal anomaly here will disable the system I suspect and throw a warning up - which you don't want adding to your problems.

A similar issue here I think:
http://www.citroen-owners-club.co.uk/ci ... operratic/

[white exec]

If I take the sensor out, how much oil will escape?

Not sure which gearbox you have, so I can't look for certain at the Citroen parts diagrams.
If the distance sensor is high up on the box, then you'll not be likely to lose oil when removing it.
Your symptoms all point to sensor failure - no output, maybe no rotation.
Readily checked if you remove the sensor; can be spun by hand/drill to check.
These items do fail.

[GiveMeABreak]

ML5C Manual Gearbox Chris, diagram on first page for relevant part:
viewtopic.php?f=3&t=59895#p572859

[lraxv]

I'm absolutely positive it's not the sensor.

This is a new sensor and it gives exactly the same problem as the old sensor - I found the old sensor and ran it with a drill and it worked as expected.

The VSS is right at the bottom of the gearbox at the back, right next to where the drive-shaft comes out for the O/S wheel.

I went and looked at the instruments more carefully and there is an ABS light - doesn't say much for my powers of observation!

Thanks for your help

[white exec]

Loss of oil likely then. Easy enough to replace - Total Transmission 8 75W-80, and importantly GL-4. Citroen stock and use it on most manual boxes.

Are those sensors actually being driven successfully by the box?

[1080]

The plastic drive for the sensor pinion, as mentioned earlier, can be damaged when changing the drive shaft.
It's fairly easy to replace: driveshaft out, remove 3 x bolts (from memory) holding alloy housing. Pinon drive then comes out from the housing.
Ages ago, I damaged a plastic drive whilst changing a clutch. The speedo then worked erratically. I think this will solve your problem if your speedo works when the pinion spun.

[lraxv]

Thanks for your advice 1080 - withdrawing the driveshaft and getting at the sensor drive that way would certainly be less of a job than splitting the entire gearbox, which is what I thought it would entail...

However...I've got my speedo working again and this is how I did it: I traced the wires from the O/S ABS sensor back to a point in the loom behind the air filter box and spliced in a new pair of wires to take a feed off it. Driving around with an oscilloscope on the passengers seat showed that the ABS sensor (which is a variable reluctance type on my van, basically a tiny AC generator) gave off an AC voltage with a frequency about 12x the vehicle speed in MPH.
The VSS (I'd found out the same way) gives off a 12V square-wave with a frequency about 2x the vehicle speed in MPH, so what I needed to do was convert the AC ABS signal to a 12V square-wave and divide the frequency by 6:



This circuit takes the ABS signal, puts it through a comparator to convert it to a square-wave and passes that into a microcontroller to do the frequency conversion - this part all runs at 0V to 5V, supplied by a 7805 regulator. The output from the microcontroller is then passed through a transistor (a 2N2222) to bring it up to 12V and passed onto the VSS feed (to the ECU and the speedo). The comparator input impedance is high enough that it has no effect on the ABS circuit, so that all appears to be working normally.



Here's the complete system; the black and red wires are the 12V feed that originally went to the VSS, the twisted yellow and black are the feed from the ABS sensor and the white wire is the VSS signal to the speedo and ECU. At the moment, it's just perched on the dashboard - I've added a potentiometer in to the circuit to adjust the output frequency, so once I've calibrated the speedo against my satnav, I'll put it in the engine-bay fuse box, as there's plenty of spare space in there...

So far, the speedo's behaved itself with no more dropping off to zero, so it's definitely looking like a gearbox fault rather than a wiring fault - this may not be the "proper" way to fix it, but it certainly does the job

Thanks all for your help

[RichardW]

That's a top job - well done!

I think on the gearbox in your van the output housing is not separate so to get at the speedo drive you would need to remove the diff casing - no idea if this is possible with the 'box in situ.

[lraxv]

I thought I'd better post an update on this - after a week of working fine, my speedo sensor replacement system stopped working and despite the best part of a week trying to figure out why, there seems to be no reason for it at all.

All my electronics still works, the ABS still works (and isn't throwing any errors), the ABS sensor still works, but my electronics doesn't see the speed signal any more - I've absolutely no idea why.

So, maybe this fix would work for somebody else, but I wouldn't guarantee it as a long-term option, given my experience with it.

I'm off to scrap my van now, unfortunately - b*gger

[old-un77]

Hmmm!

I Like the Arduino? touch!