Aprilia ignition and lighting switch: Marelli MHBA9.

[Tony Stephens]

Anyone got an idea of the function of terminals 2 and 31 on my Aprilia dashboard switch? They do not feature at all on the wiring schematic, and my multimeter does not show them connecting to anything else in any key position.
If I look at the Bosch terminal designations, which appear to coincide with Lancia and Marelli practice, they are respectively a "short-circuit terminal" and "direct return line to battery -ve or ground" -in other words, earth.
This looks promising, as I am trying to find a switched earth path so that I can run the car on a Scintilla Vertex magneto, which I believe was an option. This would (should) mean that the mag. is shorted to earth until livened up by pushing in the key, as you would to turn on a coil ignition system, while the rest of the ignition-controlled electrics are powered up with volts.
All fine except I cannot make these two terminals undertake any function which I understand. I suppose that 30 years of managing just an ERA magneto earth wire have blinded me to any other electrical possibilities.
Any help out there?

[simonandjuliet]

Hi Tony,
Some photos of notes I took when wiring the Cabrio

Let me know if more detail/explanation needed

Please share the Scintilla wiring, I have one that I have considered fitting

[Tony Stephens]

Hi Simon, thanks very much, that could be the key(!) to it. I need to be braver, and take the switch to bits to Investigate.

If I understand your sketches, which are nearly as fine as mine, the top one is with the key out, and T2 and T31 joined. Magneto earth wire shorted to earth, thus engine off.

In the lower sketch, Magneto earth open circuit, i.e. Magneto operative, and T15  (ignition-controlled accessories) supplied with volts from T30/51.Everything, including the engine, should be working.

Lights are controlled by the two 56's, 57 and 58. The mechanical character of the key stops you leaving the car switched off but with the headlights on.

If that is right, wiring up a Scintilla Vertex mag would be a question of connecting the earthing wire to T2 and removing the distributor and coil in their entirety including the coil wire to the no 1 fuse on the fusebox.

All of which sounds fine when sitting watching them splash and spin around in Turkish GP practice, I'll have to get it to work on the bench and confirm.

[Kari]

Tony,
I just have consulted a list of BOSCH terminal designations and it confirms that Nr. 2  connects to the magneto and Nr. 31 to the ground. When key is in: contact open, when key is out: (pulled back a bit) contact closed. I have tested this on a similar Marelli switch. I think that's the only function of those 2 terminals.

I hope that helps  Karl

[simonandjuliet]

Tony, glad it helps, albeit a little bit - these are just my garage sketches. I have a pile of paper in the garage to try and work things out as I go along

A word of caution ! I had a near scrap switch that I dismantled, you do risk snapping the clasps when you open them, so if you want photos of the real thing , I can open it up again

[Tony Stephens]

No, I'll test it with my trusty meter and maybe not even take it to bits.

With your and Kari's help, it tends to confirm my suspicion that the problem was in my brain rather than in the switch.

Incidentally, don't be tempted to use an old unserviced magneto as you are doomed to disappointment. Mine is unused old stock, but I still had it overhauled with a new coil and condenser, not very cheap. The spark would really make you jump now, not at all good for my pacemaker!

Thanks, I'll get back to you with the outcome.

The car is miles away from working, I'm just thinking well ahead.

[Tony Stephens]

Done it! With help from you chaps, many thanks indeed.

I started by using an aerosol switch cleaner, but that wasn't enough, so it was necessary carefully to take the switch apart. It was obvious after cleaning the contacts that the connection between 30/51 and 15 terminals would work ok, but that the one for the magneto kill switch, no 31 and no 2 terminals, was physically not making contact when the ignition switch was in the "off" position, and therefore not providing a path to earth to stop the magneto sparking.

This meant careful bending of the little brass leaf spring to adjust the contact position and correct the fault. Nasty detail work!

Switch back together now and everything apparently in order.

Thanks very much for the help, it would have taken much longer without your analysis, Simon.

[Tony Stephens]

I have been giving this further thought, and although electrically sound, the contact earthing the magneto within the switch is fairly delicate. I think I will incorporate a hidden switch to earth within the wire from the Scintilla to the key switch. That will serve the dual purpose of being a theft deterrent, and also a failsafe to enable you to turn the engine off if a fault intervenes on the key. Belt and braces will do no harm.