PORSCHE 928 S4.GT/GTS, ENGINE, BONNET/HOOD/LID SWITCH 

With the 1987 introduction of the Series 4 Porsche 928, a redesigned cooling, fan/flap system included for the first time, an electronic engine lid switch. This was introduced as a safety measure to stop the fans switching on and running when the engine was warm and the bonnet open. It has a permanent 12 volt power feed at the blue/green wire terminal and is wired into the original Porsche 928 alarm, the under engine lid light and the interior lights. These switches are no longer available new from Porsche so working, 2^nd hand units must be sought for renewal.  

Many owners seem a little baffled by the switch’s function. It is quite simple in idea/operation though. 

1. With the key in the ignition, engine running and engine lid closed, the switch is in open circuit, allowing power to the 2 cooling fans on S4/GT/GTS models. 
2. With the key out of the ignition, engine off and engine lid open, the switch is in a closed circuit mode, cutting power to the fans. Hence, the fans cannot run if you have your hands down around the fans. 
3. With the key in the ignition, engine off and engine lid open, the switch is also in a closed circuit, cutting power to the fans.  
4. If the switch fails, it automatically creates an open circuit, thereby allowing the fans to run with the bonnet open until you renew the failed switch. 
5. The 928S4 handbook has a short piece of writing about the switch: see paragraph. It states that they key must not be in the ignition to ensure the fans stay off. 
6. You can run a simple test to see if the engine lid switch works.
   • Precondition engine cold and AC off
   • Ignition key removed from ignition lock
   • Remove the 2 wires to the inlet manifold temp switch situated right on-top of the manifold. Jumper these 2 wires together with a piece of wire with 2 spade terminals.
   • The fans should now run at moderate speed to cool down the "hot" engine which we simulated with the bypass
   • Turn the ignition on and the fans should stop running. The system is only for cooling when the engine is very hot and the key is removed from the ignition
   • Close bonnet, turn ignition off and remove key: fans should start running.
   • Open the bonnet. The fans should not run anymore as a safety measure. Press down by hand on the engine lid switch; the fans should start to run.
7. Hope this info is helpful as there is little info on these switches to be found.  

best, Dave.