Rover 25 / MG ZR: Stopping random alarm activations

If you're reading this, the chances are you've had the alarm in your Rover 25 / MG ZR sound at random times when locked - and you're not alone!

Our very early W-reg Rover 25 suffered with this - and it took some digging to get to the bottom of why. The alarm would sound any time between 5 minutes and 2 hours after locking the car - very annoying!
Ultimately there's only a limited number of ways the alarm can be triggered, so it's a case of eliminating each trigger point in turn. 

Door switches
These are located in the door jamb, as per the following photo: 

Removal is as simple as it looks; one cross-head screw and you can pull it free of the car body. Be careful not to let the spade connector at the rear of the switch disconnect while pulling it free!
With replacements being cheap and readily available from ourselves, we recommend replacement rather than attempting to clean the switch contacts - we had limited success doing this, and only replacement cured and intermittent interior light issue we had. 

You can order new switches here: Rover 200 / 25 / MG ZR Door Switch

Boot / Tailgate wiring loom
The wiring from the body through to the tailgate is prone to failure - this is due to the insulation cracking with age, leading to short circuits. 
You can see where the loom passes through to the tailgate on the right-hand side of the boot opening in the following picture: 

Pull the rubber cover back and you can inspect yours - chances are it's already been repaired or replaced before now, but may have failed again. If in doubt, replace the wiring using lengths of correctly-specified cable - making sure all joints are well insulated. 

Volumetric alarm sensor
One alarm input is the volumetric sensor - if fitted, it can fail and give false 'positive' signals to the alarm module, causing it to sound at random intervals even though there's no movement within the vehicle. 

Not all 25s and ZRs have it fitted, but if present it'll be located above the 'B' pillar on the driver's side, as per this picture:

The unit can simply be unplugged if you suspect this is the issue - they may be available new elsewhere, but these aren't a part we carry in stock. 

Bonnet open switch

The switches themselves are usually very reliable, however the rubber boot protecting it from water ingress often fails, as per the picture below.

With the bonnet closed, the switch is held open circuit. 
If water gets into the switch this can cause the alarm module to believe it has been closed, which would usually correlate with the bonnet being opened - this will trigger the alarm. 

This was the cause of random triggering on our Rover 25 - once replaced, it went back to behaving normally. 

You can order this switch here: Rover 25 / MG ZR Bonnet Switch