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Ask Greg Szczygiel Your Own Question
Greg Szczygiel
Greg Szczygiel, UK Car Mechanic
Category: Car
Satisfied Customers: 12
Experience:  Vehicle technician and electrician currently working in reputable independent garage in UK. Fully acredited ATA diagnostic technician.
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- What causes an immobiliser to stop arming? I can start

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- What causes an immobiliser to stop arming? I can start the engine without the fob - the red led does not illuminate

Greg Szczygiel :

I'm Greg.

Greg Szczygiel :

Could you tell me how long did you own this car?

Greg Szczygiel :

My apologies, I have just noticed your Already tried message.

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What is the brand and model of aftermarket immobiliser on You defender?
Customer: replied 3 years ago.


Sterling Excel

Ok,It looks like your immobiliser control unit or wiring is at fault.
This type of control unit should automatically arm after 60 secondes after turning ignition off.
Have you changed any optional settings of the unit since it worked ok?
Customer: replied 3 years ago.

Greg - I would not know how to adjust or what those options were.


Ok no worries,leave it with me while I'm checking operating manual of your immobiliser I presume you haven't got one ?
Customer: replied 3 years ago.

The manual I have assumes "normal" operation. No troubleshooting.

Thanks again

Yes, It should include optional or personalisation settings which can be changed by the user.
Customer: replied 3 years ago.

Only to add a new fob or delete an existing fob. No details scenario where the immobiliser is not arming

This unit should automatically arm after ignition off, do you know where is it connected to permanent power supply (positive) and ground ?
Can you tell if the unit itself is powered ?
And finally do you know which circuits does it cut ?
It looks like it thinks ignition is on all the time.
Can you check orange wire when ignition is off there should be 0 volts.
Customer: replied 3 years ago.

Greg - Thanks answers - but they have not answered the question. - Like I said - the unit has been working month - and suddenly stopped working - I asked - what causes this scenario - all has been suggested is I start pulling and prodding wires - which as a non mechanic / car wiring specialist I am less than qualified

So - the unit thinks the ignition is on all the time - what cause this to happen - is it a radio frequency or a disruption of wiring due to perhaps it being cold. Is there any way of programming the unit using software?



The following conditions cause immobiliser systems to stop arming:
1.Immobiliser unit did not receive request to be armed.
The request comes from the remote (fob), ignition switch, time after ignition off which normally is no more than 60 secondes.
On your type of immobiliser even if you don't press the fob button or the fob is away of the car it must still arm automatically.
I presume this is how it was before it failed.
You didn't make any changes of the unit's configuration so this is how it should be anyway.
2.Immobiliser unit has failed internally and it needs replacing.
So in order to start troubleshooting, full wiring check must be done.First and most important thing to check is voltage on the wire connected to ignition switch.
Voltage of this wire tells immobiliser unit to arm because user has switched ignition off, removed key from ignition switch and left the car.From this moment immobiliser unit waits up to 60 secondes and arms automatically to protect against theft by cutting ignition, starter or anything else depending how it has been installed.
My suggestion was to check the voltage on the orange wire.
If it is +12v after ignition off it means that following fault can exists:
Short circuit in wiring, orange wire is shorted to other wire with permanent voltage of +12v.If this is the issue, immobiliser unit will than ignore any signals from the fob.
Water ingress into the unit or wiring of the car.
Internal fault of the hardware or software which you can fix only by replacing the whole thing.
There is no way of programming or reprogramming of this unit.
Frequency from other sources like radios or magnetic fields are possible but only in certain locations where vehicle is parked.
My strong advice to you is to book your vehicle with good, independent garage with the experience on aftermarket immobilisers.
I'm happy to assist you further and I strongly believe this is the answer to your question.
Greg Szczygiel and other Car Specialists are ready to help you
Customer: replied 3 years ago.

Unusually - left couple of days (not started) and the unit has resumed normal operation. Thanks again insightful knowledge

You're welcome, If you need any further assistance let me know.