Rover Engine Management Security system
If using the original Rover engine computer (MEMS) in a conversion, it may be necessary to address the computer’s immobilser. The MEMS security system searches for a coded signal from the alarm ECU, and if it doesn’t get it (either none at all or the wrong one) it immobilises the engine in two ways: First by blocking the starter relay, and by shutting off the injectors after a few seconds. There will still be a spark at the plugs, and you will still have fuel pressure, but the injectors will be inoperative.
It is easy enough to bypass the starter imobiliser in by wiring up the starter relay to be independent of the ECU. However, it is not so easy to bypass the injector shut-down function as the injectors are run only by the engine computer – which in turn will shut them down if it doesn’t get/recognise the security code it’s looking for within a few seconds of start-up.
Any Rover alarm ECU could give the signal, but would need programming with the engine ECU so that they know each other. It’s fairly sophisticated, and not simple to set up, so getting the alarm ECU from the same vehicle that the engine ECU originally came from saves the (considerable) programming hassle. More detail on the programming is on the MGF general page Security System – explanation, keys and remotes, disarming, reprogramming
The MGF has yellow/red coding on the wire which carries the security link between the two computers. A black/white wire is the immobiliser link to the starter relay.
To use the security system you may also need the remote from the same vehicle – in theory you will need it but in practice we didn’t in an engine transplant situation, as there were no sensors to reset the security system through removal of key from the ignition, or opening and closing a door or any of the other clever bits of logic that Rover built in. It seems that without sensors in these places the default mode is to leave the security link enabled unless the remote breaks the link and resets it to wait for a further signal to come active.