Introduction and horn shorting problem

[StewartD]

This is my first post on this forum, so I will briefly introduce myself and my interest in Ducatis:

I live in Melbourne, Australia. I bought a Ducati Mach 1 in 1973, when I was 18 years old. I raced it at a few race meetings and rode it on the road. I have recently been restoring it. It was in a number of boxes between about 1977 and 2004!

I had a 450 Desmo (disc brake model) from 1979 till May 1990. For most of this time it was my daily transport. I estimate I did about 200,000 km (124,000 miles) on it. After this I had a TL600 Pantah which I got rid of about 15 years ago.

My Mach 1 horn has a shorting problem. As soon as power is connected the horn operates.

On a standard Mach 1 wiring setup the positive connection is made to one of the horn’s double terminals. The wire from the other double terminal takes the power to the brake light switch. The single terminal takes a wire to the horn button which simply earths it. I deduced that the internal components connected to the ‘earthing’ single terminal were not correctly insulated from the horn’s casing, and so the horn operated whether or not the horn button was pressed.

The horn was disassembled to investigate. All components are shown in the photo.

The Chrome plated cover and the diaphragm are riveted to the horn body with six 3mm diameter rivets. These are carefully centre punched (from the horn body side), before drilling out. Use a drill slightly larger than 3mm. Be careful not to drill too deeply. Drill a little and then try hitting the rivet with a drift smaller than 3mm to see if the drilling was enough. Repeat this process in small incremental steps so that the horn body is not drilled too much with the larger drill. A little damage is probably inevitable unless you are an ace tradesman!

When I had the cover and diaphragm off I could not see any obvious fault causing the shorting so I assumed it was shorting between the contactor components that are riveted to the chrome plated bracket shown to the right, inside the body, in the photo. To drill these rivets out and the rivets holding the bracket to the body, you can follow the preceding method as they are all 3mm rivets. (In the photo, the rivets holding the bracket to the body, have been replaced temporarily with M3 button head socket screws.)

The photo shows the moving contactor out of position slightly, but still connected to the internal positive wire. It is mostly hidden by an insulator. The moving contactor is pushed down by the central plate connected to the diaphragm. The insulator prevents electrical connection between the moving contactor, which is a thin sheet of spring steel, and the diaphragm plate.

In the photo, at right of body are the other three contactor components laid out in the order of: left to right is from bottom to top.

At left is the copper terminal that sits on the moving contactor. The loose end of earthing wire, (in the body at right), is soldered to the small hole in this copper terminal plate.

In the middle is the fixed contactor which is a triangular piece of steel with an electrical contact point on its underside. This contactor still has its insulating material adhering to its top side

At right is the top plate. It was slightly damaged by drilling the rivets out.

The chrome plated bracket and the top plate have 3mm diameter holes; the other components have 4mm diameter holes. When the drilled out rivets were inspected they had a bit of rust on them and I assume that this rust had been the cause of shorting over the 0.5mm gap between rivets and the fixed contactor holes, or the copper terminal holes.


I plan to reassemble the components using M4 polycarbonate screws (shown in photo). The only alternative to this would be to find a insulating sleeve material of 3mm inside diameter and 4mm outside diameter.

I am worried that the polycarbonate screws may not be strong enough.

I would like the forum member's opinion on the strength of these polycarbonate screws. I bought them at an electronics hobbyist store.

Also any ideas about the alternative of using an insulating sleeve and 3mm socket head screws instead of rivets would be welcome.

Other items shown in photo are:

Adjustment screw and associated spring. This screw is M4 x 25 long, countersunk slotted head. It is damaged and so will be replaced.

New steel rivet for internal bracket to body fixing. It is 3mm diameter x 6 long, the head is R3 spherical. (Home made)

New Aluminium rivet for cover, diaphragm to body fixing. It is 1/8" x 5/16" long. My brother had 6 of these lying around so laziness dictated that these are close enough to the 3mm diameter originals.

[JimF]

Hi Stewart,

I wish I knew what to tell you regarding the poly-carbonate screws. Certainly the screws won't be under much stress save for sheering forces which too should not be large. I am thinking that maybe when you hit a bump in the road there will be some shear forces on the screw.

I know I would try it, but you might put a loop of safety wire through the horn grill and and anything else you can that might tumble down the road if the screws should break at least until you are reasonably sure they will hold. I would hate to see you loose original parts though I think horns are easy to source.


Do you think this going to fix it? It's odd that the horn works but that it's finding it's connection to the frame without having to to through the horn switch. I've read your text a couple times and I can't figure out if you've conclusively identified the fault.


Jim

[StewartD]

Hi Jim,

I intended that the polycarbonate screws would only be used in place of the two rivets that held the contact points to the chrome bracket (that is riveted to the inside of the body). As mentioned I have 6 new aluminium rivets for the horn cover & diaphragm to body fixing.

Since then I have found an electronics circuit board mounting kit that has some insulating washers that are the right inside and outside diameters to act as a sleeve between the 4mm diameter holes and the 3mm fasteners.

Cheers,

Stewart

[DewCatTea-Bob]

This is my first post on this forum,

____ I must welcome you to our w.site ! ... It seems you are a rare-type who pays much attention to finer-details,, so if you similarly care about the rest of your Duke's parts, then your Duke has certainly found a great home for itself !



" On a standard Mach 1 wiring setup the positive connection is made to one of the horn’s double terminals. The wire from the other double terminal takes the power to the brake light switch. The single terminal takes a wire to the horn button which simply earths it. "

____ Your understanding is correct (for ALL battery-equipped Duke-models) !



" I deduced that the internal components connected to the ‘earthing’ single terminal were not correctly insulated from the horn’s casing, and so the horn operated whether or not the horn button was pressed. "

____ I-myself don't happen to find this particular topic to be of much due interest, and-so won't be of much help to you with it,, as I haven't so deeply dug-into any of the horns since before the '80s, and have since let whatever I once knew & understood about them, become forgotten. _ However your presented (& rather strange) issue is something which I've never experienced. _ And-so yet, I have a couple questions...
First though, I assume that one of the first trouble-shooting steps you've tried, was to disconnect the black/grounding wire (that's circuited up-to the horn-button), off-from it's terminal on the horn itself,, and-yet the horn remained in active-state, is that correct ?
__ Anyhow, did you ever try-out swapping-around the horn-connections of that black-wire with that of the power-wire (which is light-blue with black-stripe -[w-c.models]) ? _ Cuz if the body of your horn is ground-mounted, then the polarity of the connected wires (to the horn's wire-terminals) could possibly make an (normally unintended) electrical/functional (and-thus audible) difference.
Also,, if that wire-connection swap change makes no difference, then have you ever tried to find-out whether-if your horn-on issue remains or not, with the horn totally isolated (dismounted & held away) from any contact with ground -(all other metal-parts of the entire bike) ? _ As I'd expect that doing so, would then HAVE-to (temporally) cure the issue.
And if so, I then would've expected it to be easier to merely electrically-isolate the entire horn from ground, (rather than to go-through all that which you've gone-through thus-far).


Hopeful-Cheers,
-Bob

[StewartD]

Bob,

First though, I assume that one of the first trouble-shooting steps you've tried, was to disconnect the black/grounding wire (that's circuited up-to the horn-button), off-from it's terminal on the horn itself,, and-yet the horn remained in active-state, is that correct ?

Correct - Horn sounded when positive wire was connected to the double terminal, no wire connected to single terminal that horn button (earthing) wire connects to.

__ Anyhow, did you ever try-out swapping-around the horn-connections of that black-wire with that of the power-wire (which is light-blue with black-stripe -[w-c.models]) ? _ Cuz if the body of your horn is ground-mounted, then the polarity of the connected wires (to the horn's wire-terminals) could possibly make an (normally unintended) electrical/functional (and-thus audible) difference.

I didn't try this; I think I would have shorted things out if I did this.

Also,, if that wire-connection swap change makes no difference, then have you ever tried to find-out whether-if your horn-on issue remains or not, with the horn totally isolated (dismounted & held away) from any contact with ground -(all other metal-parts of the entire bike) ? _ As I'd expect that doing so, would then HAVE-to (temporally) cure the issue.
And if so, I then would've expected it to be easier to merely electrically-isolate the entire horn from ground, (rather than to go-through all that which you've gone-through thus-far).

Yes to this. The horn operated normally when isolated from the motorcycle frame, with wiring connected as standard. This would have been a much easier fix if insulation could be provided at either end of the bracket, but it would only be a matter of time before the insulation between the moving point terminal and the rivets deteriorated. When this occurred the positive wire would be earthed.

I have now fixed the earthing problem between the horn body and the fixed point terminal. I did not use the polycarbonate screws that I had been thinking of. An electronics shop employee said they lose strength when hot. Instead I used some insulating sleeves that are sold as an insulating kit to mount electronics circuit boards. They are shown in the attached photo I took of all the contact points components. The order of assembly is: starting with the bracket at top left, the items left to right, top row, then left to right, bottom row, are placed in that order on top of the bracket.

At left, top row is the bracket and the rivets I made from mild steel that will reconnect bracket to horn body. Above bracket is the horn adjustment screw and spring. The screw is M4 x 25 long, countersunk slot head.

Next to right of bracket is the insulating material that I will discard and replace with the two insulating sleeves (shown above). Note that the sleeves have a flange and might be described as 'top hat' section. Also note orientation: The flange is flat against the bracket.

Next in photo are the moving point terminal, the moving point and the moving point insulator. The thickness of the copper terminal is 0.3mm and the spring steel sheet of the moving point is 0.4mm. The insulating sleeve is long enough to penetrate this combined thickness of 0.7mm.

Note that the moving point insulator is the same size as the moving point. It needs to be this size as the diaphragm presses down on it, but must be electrically insulated from it. The diaphragm is shown in the previously posted photo. It consists of a disc of spring steel and a central plate that the electro-magnet attracts. The two 'ears' of the central plate bear on the moving point insulator, either side of the moving point, that protrudes through the insulator's hole.

In the bottom row of the photo, at left is the fixed point terminal and the fixed point. Next to right is the insulating material that will be discarded and the insulating sleeves that will replace it. The insulating sleeves are placed with the flange at top. The steel plate shown next to right, is placed above the insulating sleeves and the assembly bolted together.

The second photo shows the assembly held together with M3 x 16 long stainless steel socket head cap screws. Note there is an extra steel plate below the bracket. This plate is 2.5mm thick and is tapped M3. The nuts therefore act as locknuts.

When this subassembly was trial fitted to the horn body it was seen that heads of the socket head screws were above the level of the horn body flange. The diaphragm would touch them and its free movement would be compromised. The washers under the heads of the socket head cap screws were discarded to solve this problem. Also when the earthing, (horn button) internal wire was soldered to the fixed point terminal (at left), the solder was also seen to be very close to the level of the horn body flange. It was bent down slightly. It is equally important that it not touch the bracket or the diaphragm; both will earth it.

I have now reassembled the subassembly to the horn body using the rivets. To set these rivets I rested the spherical head onto a 5mm nail punch. The nail punch has a spherical female cavity and so the rivet head which is to the outside of the body and visible, is not flattened. In hindsight I should have had a couple of extra rivets to practise setting. I set these rivets cold and they ended up a bit of a mess on the inside of the body which fortunately can't be seen. Next time I might try heating them red hot. To do this I think an assistant would be needed to ensure that things happened quick enough.

The diaphragm and horn cover are now assembled to the body and held with button head socket screws. From a distance it is not obvious that the original rivets are not used. The horn is working but I think the screws are a good idea if future maintenance is needed.

Cheers,

Stewart

[StewartD]

Sorry Guys,

I don't think I got the photos attached, here they are.

Stewart