Excuse the background, but I'm always complaining about posters who
never provide enough detail.

I have a second-hand ten year old ATV (Cub Cadet). It came from the
Surrey/Hampshire border, in the stockbroker/pop-star belt and came with
an (apparently simple) Sterling Touch immobiliser. I've been using a
detachable battery terminal because there's a bit of battery drain, and
sometimes I don't use it for a month. Recently this terminal has been a
bit flaky so I decided to clean up the terminals and put in a "proper"
isolator like this (except that I think that mine is rated 300A, which
should be OK for this 300 cc diesel).

https://www.ebay.co.uk/itm/Battery-I...AOSw8NVaato u

Unfortunately, after doing this at the battery end (the terminals were
very corroded and needed a lot of cleaning up) I found there was no
power getting through to the isolator "touch point" on the dashboard
(which should "flash" when it is disabled even with the ignition off).
Also the glow plugs and the electric fuel pump don't kick in when you
switch on the ignition, as they normally do, provided the immobiliser is
disabled. (There's a glow plug light).

So I expose the back of the dashboard so that I can see if power is
coming through to the ignition switch, etc.

After taking out the radiator to get access, I discover that I have a
full blown Thatcham-rated aftermarket alarm and tracker system suitable
for a Lamborghini, not just a simple immobiliser. This includes a box
with an IMEI (so presumably used to be able to "phone home" on theft),
plus an alarm sounder, plus (by the look of it) an FM transmitter for
the police to track. All professionally fitted to a high standard, as
far as I can see. It's quite difficult to see what might have been the
original wiring, and which the "aftermarket". The "touch sensor" seems
to be connected to a relay which is connected by fairly fat wires to the
back of the fuse box. It's a diesel BTW (Japanese "Caterpillar", fairly
"low tech"), with an electrical fuel pump that supplies the mechanical
fuel pump.

I suppose the immobiliser could disable the starter solenoid, or the
electric fuel pump, or the glow plugs, or enable the engine cut-off. The
Sterling Touch web site suggests they currently provide two lines of
protection, perhaps the starter solenoid and something else.

The vehicle is still worth £5k or more if only I could get it working,
and lives behind locked gates at an unoccupied but not isolated stables,
but really doesn't need all this protection. But of course it only has
scrap value unless I can get it running again.

Do you think I need to go to an immobiliser specialist (there seem to be
a couple withing 20 miles, but I'd need to get it trailered to them) or
might I be able to unravel it myself? If so, where should I start?

It lives outside, so connectors and spade terminals are beginning to get
a bit corroded. I suppose this might actually be the problem.

TIA!