engine immobilisers

[parrot]

Can anyone tell me, the plethora of self arming immobilsers on the market; do they work by simply earthing out circuits before whatever function is being immobilised. Presumably at a relay point. If so, would the idea be to earth out the circuit at the relay switch point?

[Screamn_Sleeka]

On what car? on an old carby car, basically, u can hot wire it no matter what, unless ther is a fuel tap. But it is worth it. I dont even know you are refering to with these "kits", you jsut make a remote swith/relay to earth/disconnect an ignition area of choice (pre ignitor...)

[slide86]

i used to have a "custom" immobiliser on my ke70. it featured a switch under the carpet.

one side of the switch went to ground and the other side of the switch went to the -ve side of the coil. when the switch is active, the coil is earthed. providing no spark.

no spark = no start.

quite a lot of immobilsiers do this. i have also seen ones that cut the fuel pump power, so there is no fuel to start. there are several different ways to go bout it i guess.

[Screamn_Sleeka]

yea, like slide86 says, it works well with a immobiliser on a fuel pump and carby, cause they fuel in the carby will work for 20secs, so they recon they get away with it, but they it stalls and wont start

I cnat see how, with out wires, u cant start any old car's ignition...

[oldcorollas]

immobilisers use a relay to create an open circuit so the thing doesn't turn on (whatever the thing is)

LOL... it is SOOOO easy to hotwire carby car...
scarily so...
just alligator clip wire between battery +ve and the coil +ve, then roll start car....

so it's always a good security feature to change to a pump in the boot... sicne then you can cut it's power, and it's usually harder to pop a boot than the bonnet

[parrot]

This is a 'proper' immobiliser that has been installed previously on my EFI car. So I gather oldcorollas that the fancy circuit board keeps the circuit open till the correct matching key tells it to close, enabling the circuit to function normally rather than being earthed. So when reinstalling it, presumably I can take my pick of whatever crucial circuit, safely assuming the unit will earth it out?

[oldcorollas]

no no, it doesn't earth the circuits... otherwise you would have short circuits and fuses would blow...
the circuits are opened (ie broken) and turning off the immobiliser engages the relas, allowing current thru... perhaps on old cars ppl would earth out the ignition coil.. but if someone tried to start car, it would blow the fuse...

but yeah, pick whichever circuits you want to disable and run then tru the immobiliser relays...

i'm not sure on the uality of the immobiliser relays tho.... in mine i will probably changte them to something a little more heavy duty....

[parrot]

Thanks for the feedback guys. Reassuring to bounce things around.

[Mos]

perhaps on old cars ppl would earth out the ignition coil.. but if someone tried to start car, it would blow the fuse...

The fuses don't blow
By earthing out the negative of the ignition coil (which is normally either shorted to earth through the points, or open circuit) you effectively bypass the points, so the earth of the ignition coil is never interrupted, so the ignition coil never generates a spark supply.
Another way of thinking about it - replace the points with points that never open - it will never give you a spark.

Most immobiliser relays are set up in such a way that they are only activated when you switch on the ignition *and* the immobiliser is armed. In other words, when the ignition switch is off, the immobiliser relay switches off and whatever circuit was disconnected becomes reconnected (but because the ignition switch is off, nothing will run anyway).
It is done that way so that you don't waste battery power by switching on a relay whenever the immobiliser is armed.

Mos.

[oldcorollas]

ahh sorry, my bad, i was thinking of the power being earthed...
having negative side earther will just continually run current thru the coil... leading to damage if they run off and leave it connected? i spose dead coil is better than no car