Strange electric circuit

When I picked up my "new" 400i from the previous owner, he informed me that the engine bonnet opening and the fuel lid opening was not working unless the engine was running. Checking the wiring diagram, it was obvious that this functions are active with the ignition turned off.
When I checked the voltage while pushing the button for the engine bonnet opening solenoid I measured 8 Volts only, which was obviously just at the limit to make it work. Next action was checking the fuse No.2 which controls those two functions plus the electric motors for high/low beam. There was already a 25 Amps fuse fitted instead of the specified 16 Amps. Big surprise: it was blown!. Even with the blown or removed fuse there was an 8 Volts supply to the solenoid! A newly fitted 15 Amps fuse blew instantly when pushing the button! First question: How could Ferrari design the electric board in a way which allows power supply to a consumer indirectly through a number of relay coils and relay contacts without passing a fuse? There is nothing wrong in my individual car. I have carefully checked the wiring diagram and found it true and correct! This explains the 8 Volts measured. Now, I suspected that the solenoid was defective (short circuit) and had it tested. Result: it is in perfect condition and it consumes 60 Amps at 12 Volts! This power is needed to make it work safely since it does not move below 8 Volts (equivalent to 40 Amps). Solution: I have branched the power supply for the fuel lid and the bonnet opening solenoid directly to the battery without any fuse in between. The system is working perfectly now. Second question: How could Ferrari design an electric circuit with a 16 Amps fuse for a solenoid requiring 60 Amps?
Of course I do not expect to get serious answers to my two questions above but may be that one of the readers has experienced the same problems with his car and has found his own way to cure it.

Peter

 

Hi 91tr,

The solenoid has been tested by a winding shop with AC. They say that AC testing would reveal any internal short circuit. Further I have estimated the length of the wire by the dimensions of the coil and have measured the diameter of the wire. The calculated resistance comes quite close to the measured value i.e. below 0.3 Ohms.
At reduced Voltage, the coil draws 40 Amps "only". The very short impulse to unlock the bonnet was apparently not damaging the coils of the relays when the power came through the wrong path. Engine running or not running makes a difference of 1 Volt only at the solenoid terminal.

Thanks for your considerations and comments

Peter

 

When i wished to use the rear heated window for the first time, it did not work. The following investigation revealed several problems within the switch in the center console. The soldering of the contacts between the primary and secondary printed circuit boards (see picture) was not properly done and resulted in unsafe contacts. Then i noticed the burnt leads on the main board from the relay to the plug connectors.
I had the leads on the board repaired and the connections re-soldered in an electronics repair shop. Asking the specialist whether this switch was suitable for 12 Amps (which is drawn by the heated window), he said clearly no.
To protect it from future overload and new damage i have installed a relieve relay.
These switches (similar design for bonnet, trunk and fuel lid openers) do have neither manufacturer nor performance specification anywhere inside or outside. But they are obviously not rated for their pupose. For the lid openers, they only work on short impulse which prevents them from burning but for the heated window it must be suitable for continuous operation.

Any similar experience from late (82 - 84) 400i owners?

Regards

Peter
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