Speedo fault diagnosis

[BrendanM]

My Speedometer has the classic symptoms - sometimes it works, often it does not.

To avoid wasting time trying to fix the fiddly connector under the speedo I want to be sure that the speed sensor on the gearbox is working OK.

I was thinking of making a connection to the wire that goes into the speedo plug (1st pin on the left I assume) and connecting it to a sensitive but high resistance headphone so that I could hear the voltage pulses sent from the speed sensor.
If the sensor output give a continuous tone even when the speedo stops working then I can be fairly sure the fault is on the speedo side.
I have an XM manual on a CD and I believe that pin 6 of the speedo plug is the 'chassis' connection.

Has anyone tried this? Can you think of any snags?

Many thanks,
Brendan

[Peter.N.]

Hi Brendon

Presumably your car is a Mk2 with an electronic speedo, I have had problems with two of these and the fault on both was dry joints on the instrument panel, they were on one of the edge connectors, I think it was the green one, if you carefully inspect the pins where they are soldered to the board you will probably find cracks in the solder, re solder them and away you go.

Having re read your post it seems you are already familiar with this procedure, it doesn't take many minutes to get the panel out - if you know where all the screws are, so I would have thought that would have been the easiest route to go down.

Peter

[Jan-hendrik]

Peter may be right.
Personally I've had the same symptoms on both XMs I've owned and the cure was replacing the speed sensor unit.

[xmexclusive]

The Mk2 speedo wiring has a plug join by the back of the LHM tank.
I think the plug and socket used are the same as on the sender unit.
Breaking in here to look at signals or send them might be the easiest test point.
Perhaps use spare sender run by an electric drill to test the speedo.

John

[BrendanM]

I replaced the sender unit and that stopped the problem.

There was oil in the faulty sender. Could that be the only fault?

[HOWROYD]

OK, but where is the "sensor unit?
Is it a standard replacement part, and if so where can I find one?
Is a 1929 model amateur mechanic devoted owner of a 1997 XM
capable of undertaking such work?

Advise me, please.

[xmexclusive]

Hi HOWROYD

The sender is fixed on the top back of the gearbox.
They usually have a metal heat shield and are fixed with a 10mm bolt.
The electronic drive boxes are generic as it is the push in drive cog that varies.
The drive cogs do not wear and are reusable.

The problem is getting to the sender to replace it.
From below it is difficult to see what you are doing.
From above there is a lot of smaller items to remove for access.

Sender units are available on ebay but with lots of Chinese copies.
No reason to suppose these will not work ok but a pain if they do not.
Genuine Citroen part was the way I went.

John

This post has been edited by xmexclusive on February 10, 2016 11:57 am