Citroen XM Forum

And some more!

Hi Matt,

Couple of QQ:

- what setting is the multimeter on in those photos? Can't make it out clearly.

In first four photos, meter is connected between battery -ve and the engine block.
- What is the red meter lead connected to in the last two shots? Can you provide a photo of where it connects, please?

Chris - the red lead is on one of the two earthing points behind the air box. A pair of thin black cables run from the battery -ve terminal to thes points.

Looking at Matthew's latest photos a different result is seen, the large difference in resistance between cars has gone. All very odd.

Thanks, Steve.

Your post of 7 Mar said that the 5-ohms was observed when measuring between batt -ve and earthing point behind o/s/f headlight. Matt's photo seems to show something else ('behind air box').
The engine's ECUs are, if I remember correctly, grounded to the o/s/f inner wing, which is where Matt's photo'd check should have been done.

Matt's first four photos show the meter check being between batt -ve and engine block. If the meter was set to low ohms, then those readings look decent. What that check likely shows is that the engine block is properly connected to batt -ve . . . confirmed by no complaints about cranking/starting.

What we need to know is about batt -ve to o/s/f (presumably inner wing? needs confirming), and the meter's setting.

The chassis integrity is used for the car negative return.
The initiating points for much of the rusting on XM's is the burnt ring around the spot welds.
Those spot welds are the earth return path between panels.
With 25 years old XM's we should not be surprised at a bit of resistance on the negative return line.
The ECU electronics may not like it at all.

John

Are you sure the valve timing hasn't somehow slipped. For fuel to be in the intake tract it can only have come from gas blow back, which as far as I can see, can only be a result of either an olen valve, or the exhaust back pressure being so great the inducted gas is blown back out.

My apologies for the delayed response Chris, mainly because I am now working away in E. Yorks! In answer to your other queries, the readings are between 0-1.5 ohm I believe and the earthing points checked are between the battery and engine. There were no issues with the other inner wing earths including the o/s for the ecu. With regards, Matthew T.

I do not think it would be physically possible for the valve timing to change Will, without a chain tensioner or guide coming loose and I think I would know about that! The addition of petrol through the intake has also confirmed the engine will run faster given the fuel required. I believe Dean's point about the external O2 sensor is a plausible explanation for the overfuelling and some petrol residue does seem to naturally collect in the manifold well. Regards, Matthew T.

Ok, Matt - but what about the 5 ohm found between batt -ve and the o/s/f ? Presumably an earthing point?
Is that resistance still there (with batt+ disconnected) ?

I think Steve meant between the battery negative and the battery n/s wing earth points as shown in my photos Chris and I am sorry for any confusion caused! The apparent 5 ohm reading was also only present with the battery positive connected. With regards, Matthew T.