Russ's 2.1 SED Auto now in Mandarin

[russ92xmsed]

Thanks Chris for your help on this... Any ideas are useful at this stage!
Mine has digital Ac and climate control. I'm sure it's typical fan blower issue. Although the engine fuse box is fine and all fuses clean and intact. I hope to get to the blower this afternoon if the weather stays clear.

Paul... I have seen what lurks behind the dash when stripping my old car. Yeah. It's scary. I found some pics from Shane in Australia stripping his dash for a heater issue At least it shows the route of the cables.

Dean... I had the binnacle off yesterday checking the plugs ect. I'm going to do as you suggest, get the old speedo clocks down and see what happens when hooked up. At least it would eliminate if it's dry joints on the original tacho or not.

What confuses me is the hydractive auto suspension message popping up. It clearly piggybacks the main feed to the hydractive light. But that doesn't light up when the message pops up mid drive.
Nothing else on the clocks or dash is being effected.

And I checked out the lights whilst I moved the car last night and they don't flicker or pulse at all. All this with the tacho flicking.

[russ92xmsed]

Just tried the spare clocks, and the tacho flicked about. So it's not the dash. Oh joy!

Still the blower is back to blowing on all settings!

[White Exec]

Russ,
Thanks for confirming the digital AC and climate control. Narrows down the circuits I have to one - big help.
Do you have access to Lexia, to check the TDC/tacho sensor?

John,
Any idea where the tacho gets its supply from?
Not shown on any of my diags.

[xmexclusive]

Hi Chris

Got lucky with the search.

XM21tacho001.jpg

XM21tacho002.jpg

The difference between the above diagrams is manual/auto with the auto having cruise added.
Both have identical self powered TDC sensors and direct wire connections to the tacho.
So no signal processing en route.
The earth (third wire) is simply a signal screen needed for the very low power of the TDC signal.
So that has no connections other than the earth one.
It also means the other problems are due to separate causes.

Hi Russ

Do not go searching in the dash.
Try a new (not recovered) TDC sensor or make up a screened jumper lead to go from sensor direct to the Tacho.

John

[russ92xmsed]

Hi Chris, I don't have a lexia to hand I'm afraid.

Hi John, thanks very much for finding the diagrams. I'll have a good study of those.
Don't worry I'm not about to rip apart the dash! You can get to most of it by taking out the glovebox or getting under it in the footwells anyway.

I think that's an idea, a new lead to the tacho. I did think of that. Maybe a last resort.
Will mentioned that if the TDC sensor was duff, it wouldn't send a signal to the tacho at all. But I do get boughts of it working brilliantly. Almost suggests an earth issue? maybe the shielding has failed somehow.... Bizarre!

Many thanks for the help.

[White Exec]

Hi John,

Tacho sensor clear enough, and advice to Russ for checking/substituting it also.

Puzzle still remains: where does the tacho obtain its power?
Circuits above don't show that, neither do any of the 520-x/xx (Instruments) circuits I have.
F19 would seem logical (from later circuits), but this isn't shown anywhere.


Hi Russ,

Tacho/ABS/speed/distance and like sensors are all similar. Basically consist of a small magnet, around which is a lengthy coil of very fine wire.
When a piece of ferrous metal (eg a peg on a flywheel, or a tooth on an ABS ring) passes close by the tip of the sensor, the magnetic field around the coil is disturbed, and a tiny electrical pulse is generated in the coil. This is then connected to an amplifier of some sort, and can be made use of. In the case of a tachometer, the varying frequency of the pulses (as speed increases) is converted into a varying voltage (by an F-to-V converter), and the voltage is simply displayed either as a moving needle, or a digital display. Essentially, tacho's are voltmeters.

The delicate coil of the sensor can get damaged (heat, vibration, corrosion...) and become intermittent.
Citroen speed/distance sensors (for the speedo/cruise) were famous for this. Our 2.5 ate three under warranty.

The current generated by these "proximity sensors" is tiny, and easily lost down dodgy wiring/connectors.
The lead from the sensor is typically screened, and must be properly grounded, to protect the signal wires from external interference (just like low signal audio devices like mics and magnetic cartridges).

Delicate little beasts.

When/if you get hold of a new one, you could compare it with the original by measuring the resistance of each in ohms (meter across the signal wires; ignore the screen). Often, they are around 1000ohms, but do vary. Would be interesting to know what you get.

[xmexclusive]

Hi Chris

A Mk2 electronic speedo has 3 wire connections from the main pcb.
So I assume that is one signal wire and the two power leads.
The Mk1 tacho has 5 wire connections from the main pcb.
So I assume that is the three signal wires and the two power leads.

It is not Citroen practice to publish any details of the internal circuitry of ECU's and similar modules.
Their definition of and ECU appears to include quite small and simple modules.
For example while simple relays are detailed the glow plug relay contents are not.

I will see if I can find a dash main pcb to gleen a bit more information.

John

[White Exec]

Hi John

Yes, could be the explanation for lack of wiring detail, although section 520- (Instruments) does show supply and details for the speedo and other instruments. Who knows.
Could be that the tacho diagram is a bit more schematic than actual.
Russ at least does have a reasonable way forward with jumping/replacing the sensor.

On the fan speed issue, is Russ's hiccup typical of the way the fan speed controller fails (? transistors), or something else?
Have not managed to come up with anything the two items have in common - except something really basic, like the IGN switch.
Hence the attempt to find out how the tacho was powered.

[xmexclusive]

Hi Chris

Not familiar with the fan set-up on Mk1's as it is years since I had or worked on one.
The F2 problem fuse box is Mk2 only.
There are two types of fan wiring, I think for the earlier XM's.
Not sure why.
I started a bit of research recently because a number of fans and modules have appeared in the heap sorts.
I also needed to sort one of the cars in which the fan was playing up.
I made up a test rig so I could test fans or modules.
You need a support for a fan under test otherwise it overbalances and runs all over the floor.

John

[russ92xmsed]

Back tracking a tiny bit.. So the sensor can go intermittent then and cause a fluctuating tacho. So that could be the problem after all. Right. I'll order a new one. Won't hurt to have a new one anyway. It looks very original!

The fan, ....didn't get round to looking at it today. Tomorrow, if it is dry, I'll fit the spare I have and see if that changes things.
It could be brushes maybe. As sometimes it only works on full power. Anyway, see what happens.

The ignition switch I have changed. Unless there is something after the switch that's playing up, I doubt it's the switch. It worked perfectly in the other car.

But what is causing the automatic suspension message to keep appearing whilst driving?