TF immobiliser

Hi all
I've had a problem with the immobiliser on my TF for about 2 months. Up until then i could leave it unlocked and when i came to start it the immobiliser would be disarmed by the fob just being next to the transponder coil around the ignition switch. Now i have to manually cancel the immobiliser alarm via the fob key each time. The fob batteries have been changed twice. Paul from Technozen suggested that this coil may be at fault. 
On a meter it reads very low resistance, I had expected it to be open circuit. Is it only possible to test by substitution? 
If I lock and unlock the car and then go to start it there doesn't seem to be a problem.

Thanks

If the coil is OK I would expect to see a low resistance but not zero indicating a short circuit. I assume you are seeing some resistance?
Do you have two fobs and this happening on both? I assume you would have checked this if you have.
substitution would be a good way to go but you could check what you see from the wires to the coil from the SCU. The SCU should output a voltage on one wire when the ignition is switched on with the immobiliser active and should switch it off once it is deactivated.
if you do not see this it could be a fault with the wiring or more likely a faulty SCU.
As you may know, if this is the Lucas system swapping the SCU will then require the engine controller be put into learn mode to learn the code of the new SCU and will require the fobs to be programmed to the controller. Both of these can be done with Pscan or a T4. If it’s the Pektron one you have to get new fobs too, or speak with Teknozen who should be able to help you
 

I have an early (ish) f, this was supplied without the transponder coil. Having to use the fob to re mobilise the car is indicative of either the fob not talking to the coil or the coil not listening. This irritated me to the point where I retrofitted a coil, once fitted it has worked faultlessly.
Have you changed the fob body recently? If you have you might have discarded the fob mounted chip with the old unit.
Our friend (sorry I have forgotten your name) recommend trying a second fob, this is an excellent idea as you would prove that network.
Unless you have two duff chip fobs & what are the chances of that?
M

Chip is the thing of Rover 75/MG ZT but not MGF/TF.

This fault also happens with my 2nd fob.
I'm seeing a resistance of 11 ohms across the coil. Of the 2 wires connected to the coil, with car immobilized I'm seeing 0v on the yellow/brown wire and 11.5v on the red/green wire. This situation doesn't change when I disarm the immobiliser with the fob. 've not done anything to either fob apart from changing the batteries. It's beginning to look like a fault with the SCU? It's a Pectron unit.

regards
Jon 

Thank you Roverlike, that is good information.
I was prompted to comment by a recent incident with my daughter’s car, it refused to start & so a bag of tools was compiled & of on a mercy mission. I was used to handling the original fob & so when I was handed a pristine example to try my luck I started to wonder.
After five minutes in the rubbish bin searching for the husk of the original we were back on the road & a little wiser. The transponder is not much bigger than a couple of match heads & was tucked up away from the transmitter & so easily overlooked, even if you were looking for it.
M