Constant 12v supply

I have installed an immobiliser in my car but some times it dies just after starting the engine. The immobiliser will kill the engine because of the voltage drop while cranking.

I was thinking of using a capacitor to feed a constant voltage to the immobiliser during cranking but I am unsure about capacitor sizes and wether this would work.

Any ideas would be greatly appreciated.

To find the size of the capacitor you first need to know how much current the immobiliser pulls.
At what voltage does the immobiliser quit working?
From my experience the capacitor does not have to hold up the load for the entire cranking time. The battery voltage drops during the time when a piston is compressing. After the piston breaks over the top the voltage increases bake to normal for a short time. So the capacitor may only have to hold for a small fraction of a second.

It sounds like you require a little more storage capacity in your battery. Either your battery has deteriorated, and will not hold a sufficient number of ampere-hours, or possibly, you have not installed the manufacturer's recommended battery size. A larger, or newer, battery will not exhibit the voltage decrease which you presently observe.

Do you know how long you have had your present battery?

Thanks for the reply's guys.

Battery is only 12 months old and voltage drops to around 10+ volts while cranking and not once during the 15+ starts of the car to check for voltage drop did the immobiliser kill the engine. That is the worst thing, its sort of random, so I'm guessing it may drop below 10 volts occasionally which kills the engine.

I thinks it only the + sense wire to tell the immobiliser the engine is running that gives the problem which only draws .11 mA.

I'd be for returning it to the manufacturer for a full refund! You would think that someone designing a product to be used in an automobile would design it to cope with the brownout that happens everytime the car is started...

I'm with user88 and mike on this one, your immob should be designed to cope with cranking volts drop.
I think something else is up, 10v should be more than enough to keep it alive, I think you have a wiring fault somewhere, did you use rubber grommets where the wires pass through the bulkhead?