XM ABS fault

The 'ABS out of use' warning message appears on the dashboard of my 1997 XM (2 litre Turbo Petrol) shortly after the engine starts.
The brakes still work well.
I want to do as much testing/checking as I can before I hand it over to the garage to repair. What are the most common faults?
I plan to measure the resistance of the wheel sensors first. If the Haynes manual schematic is correct I should be able to disconnect the plug from the ABS control unit and connect a multimeter to the plug pins.
I will also check the earth connections and sensor gap.
Is there anything else I can check?
The Haynes manual recommends disconnecting the battery earth lead before removing connections. Will that have any other effects on the cars electrics when I reconnect the battery (i.e. immobilising the radio)?
If the ABS control unit is faulty, am I limited to XM units?
Where is the ABS hydraulic unit mounted? Is it on the left hand side under the battery?
Many Thanks,
Brendan McMahon
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saying:

I'll put money on it being one of two things. - a front wheel sensor - the under-bonnet fusebox

I don't believe the check is quite that simple. It may be a nice clean break in the cable to the front wheel sensor - they get flexed extensively and regularly, with suspension movement and wheel travel - but it may be more difficult. If you've got access to somebody with diagnostic equipment, the ABS ECU will have logged a fault code telling you which it is.
The under-bonnet fusebox is a REGULAR cause of hassle on s2 XMs. There's very poor weather proofing on the underside, so it corrodes badly within. The ONLY thing to do with it is to get a decent auto-electrician to chop it out and replace it properly. Anything else is just delaying the inevitable. Problems will include cooling fans, climate control, ABS and gear/start relay on automatics. I had all four on mine...

It won't be.

Yes.
It won't be.
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gurgled happily, sounding much like they were

Thanks Adrian, you might be onto something.
The 30 amp fuse supplying the air conditioning regularly overheats [melts plastic] and is probably due to a corroded fuse box. I'll check that first.
Many thanks, Brendan
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