In early 1995 models, anti-theft protection is pretty basic: when the car is locked a switch inside the Alarm Control Unit (ACU) opens, blocking 12V Ignition power to the DME (T15). This is effective, but easy to defeat with a jumper.

Porsche introduced Drive Block in mid-1995 to enhance security and add remote door locking. Drive Block components include the immobilizer (under left seat), radio-frequency key FOB, and starter interlock relay, R61.

Drive Block is standard on all North American 993's and optional in other markets (UK-spec 993s use a different system). Drive Block strenthens security by blocking coil and injector activation unless the DME is given authorization by the immobilizer. Authorization occurs via digital handshake over the OBDII “Diag K” line. Drive Block cannot be defeated with a jumper.

Key points:

System Primed:

• ACU interrupts 12v ignition power to the DME (same as early 95's)
• DME prevented from starting engine
• Cranking disabled
• Alarm system armed
• Immobilizer warning light illuminates for 15 seconds after ignition is turned on.

System Unprimed:

• ACU enables 12v ignition power to the DME
• Immobilizer authorizes DME to start engine
• Cranking enabled
• Alarm system disarmed
• Immobilizer warning light illuminates for 2 seconds after key turned on.
• Existing alarms cleared

More detail.