Introduction and horn shorting problem
Posted: Sun Mar 24, 2013 5:24 am
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.
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.