Tach Wire (Function in Immobilisers) Query

Hi All,

I have only recently become interested in car alarm systems and I am new to this forum so I may be asking something that has already been answered.

What is the function of the Tach wire and how is it used in Alarm / Immobiliser systems.

The tachometer wire not only lets the remote starter know the engine is running, but it provides for another important function---overrev protection. 

 If the engine races really fast, due to something like a frozen throttle cable or a floor mat jammed against the gas pedal, the remote starter sees this and will shut the engine down right away.

The tachometer wire typically has no function as far as security is concerned.  In the old days, there were systems like the Chapman which grounded one side of the ignition coil to prevent the engine from running.......and then when the unit failed or the wiring shorted out to something, the car would stall out in the middle of the highway.........happened to my father once on his brand new car in '83.

Also, on many new cars, grounding the tachometer wire would likely A) have no effect at all, or B) trigger trouble codes in the car's computer.

Nowadays, we typically only interrupt the car's starter wire.  If anything related to the installation fails, the worst that will happen is the car won't start.  Better to have to pay a customer's towing bill, than face a lawsuit when they lose power on the road and have an accident.

There are occasions where we'll still use a relay to interrupt one of the car's main ignition wires, but preferably only with the customer's knowledge and consent.  For example, a manual-transmission car could be push-started, rendering a typical starter kill ineffective----this is a good time to consider an ignition kill instead.

One exception to all this is the pricey Clifford AvantGuard4, which not only includes an ignition kill, but has a separate, wireless hideaway module that can be used to interrupt another circuit in the car like the fuel pump, for example.  I would presume Clifford has taken measures to ensure that these pieces are as reliable as possible.