Dodgy 750 front/rear brake light

TeddyG

TeddyG

Hero member
Location
Portland Oregon
The front and back brake lights on my 750SF have been working fine- until today, and the brake light is now just continually on. I traced the wiring and see that they share the same wires into the fuse box , and also cleaned the terminals on the rear unit, and the plunger is freely moving. The wiring seems intact w/ full seathing.
So how can I test them and are they connected in a way that if one fails, it affects the other? Are generic replacement switches available or only Laverda ($$) specific?
The front switch is mounted on the Brembo junction housing for the brake lines, and just screws in place w/ no moving parts.
Thank you!
 
Take the terminal off the front - if the light goes out, that's the cause of it staying on. If not, remove the terminal from the rear and same deal. Should be easy enough to fix the rear if it's sticking on. A multimeter would be my choice of testing tool.
 
Yep, PB2's recommended test will determine which switch is not working. 12V power goes to the brake light and the switches complete the return to ground. They're wired in parallel so that either switch will complete the circuit. I'd only add that if the brake light doesn't go out when you disconnect either of the switches, then the wire between the light and one of the switches is shorting against the bike's chassis somewhere, although that seems unlikely as you've already checked the condition of the wires. But there could be a short circuit in the light itself where the wires pass through the taillight housing.

It's been my experience that the hydraulic brake light switches seem to fail more often than the mechanical plunger type.

If it's the hydraulic switch at the front that's failed, the replacement doesn't have to be a Laverda or Brembo part. Any generic switch will do so long as it has the correct thread - M10 taper x 1.0mm pitch. I think a lot of Harley models used that thread, so you could try your local Harley dealer. Alternatively, a lot of European cars also had the same M10 x 1 thread. VW for example.

If the rear switch is faulty, check for any connections to ground on the live side of the switch that shouldn't be there.
 
Dellortoman said:
Yep, PB2's recommended test will determine which switch is not working. 12V power goes to the brake light and the switches complete the return to ground. They're wired in parallel so that either switch will complete the circuit. I'd only add that if the brake light doesn't go out when you disconnect either of the switches, then the wire between the light and one of the switches is shorting against the bike's chassis somewhere,
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The switches are on the live side of the globe. Either switch can give the globe power. The globe is earthed to the chassis. If the light is stuck on either a switch is stuck on or power is getting to the globe other than via a switch.
 
Tippie said:
The switches are on the live side of the globe. Either switch can give the globe power. The globe is earthed to the chassis. If the light is stuck on either a switch is stuck on or power is getting to the globe other than via a switch.
Click to expand...
OK, I could have sworn the switches were on the earth side of the globe. Maybe I'm thinking of another bike. It's a problem when you have too many bikes from different manufacturers, and not enough memory space in the brain to hold all their wiring diagrams.
I suppose that could be described as a first world problem.
 
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With ignition turned on and my meter set to 20 Volts, I've connected the red lead to one of the switch spades that's connected to the switch, and the other black meter lead to the motor as ground, and I'm seeing 9-10 volts for both switches. When I disconnect either switch or both, the bright filament of the tungsten bulb stays on, and I know this is a good bulb w/ a working low filament. The dual gray wires of both switches join together near the fuse box, and connect to the underside of the fuse box. The back light wires have one gray wire and one red wire that connects to the first/left side of the fuse box.
So to recap, the rear brake light remains on at it's highest setting, so I'm obviously overlooking something here.
Thank you for anymore advice
 

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TeddyG said:
With ignition turned on and my meter set to 20 Volts, I've connected the red lead to one of the switch spades that's connected to the switch, and the other black meter lead to the motor as ground, and I'm seeing 9-10 volts for both switches. When I disconnect either switch or both, the bright filament of the tungsten bulb stays on, and I know this is a good bulb w/ a working low filament. The dual gray wires of both switches join together near the fuse box, and connect to the underside of the fuse box. The back light wires have one gray wire and one red wire that connects to the first/left side of the fuse box.
So to recap, the rear brake light remains on at it's highest setting, so I'm obviously overlooking something here.
Thank you for anymore advice
Click to expand...
The switches look like they are getting their power from the 3rd fused bottom connection. Makes sense. The back wires that go to the bulb however are also screwed into the top of the fuse box as you say. Your pictures don't show this connection. Sounds like these are directly powering the bulb when the ignition is on. The bulb should only get it's power when activating the front rear or both switches. So somehow the back wires that carry power to the bulb have to be connected to the other terminal of each switch not the fuse box.
That's just my thoughts.
 
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Thanks Black Jota,
The stop lights have been working fine since I bought this bike, but have suddenly stopped, and nothing has been changed to the wire connects going to the fuse box. I think the only solution is running new wires and possibly install new switches. Odd that both switches would crap out at the same time, and I would think they work independently, but perhaps not. All I know is that these 50 yr old wires are showing their age!
 
TeddyG said:
Thanks Black Jota,
The stop lights have been working fine since I bought this bike, but have suddenly stopped, and nothing has been changed to the wire connects going to the fuse box. I think the only solution is running new wires and possibly install new switches. Odd that both switches would crap out at the same time, and I would think they work independently, but perhaps not. All I know is that these 50 yr old wires are showing their age!
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They work independently or together. Power goes to one terminal on each switch and once activated the switched terminal cable/s power the globe which is earthed to the frame. So there should only be 2 wires leaving the fuse box and 2 or 2 into 1 wire/s going to the globe from both switches. You said the "back wires" which I can only assume goes to the globe is also wired into the fuse box which would explain why the globe is always on. Front switch hydraulic Rear switch mechanical. Unlikely to both fail simultaneously, your globe is getting constant power not from the switches but from somewhere else. You also said that you disconnected both switches and the globe still stays on, so it can't be a switch stuck on. Remove the "Back wires" from the fuse box and see if the globe goes out.
 
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Thanks I’ll try that tomorrow, but oddly nothing has changed to the wires going to the fuse box since I’ve owned the bike. The sheathing to the wires from the front and back switches is intact but perhaps there’s a break in one wire
 
It's frustratingly difficult trying to diagnose someone's wiring diagram remotely. You really need to be there, meter in hand.

I agree with Barry (BlackJota) that the switches aren't the problem if taking them out of the circuit completely makes no difference.

Your photos only tell part of the story. It's difficult to tell which is the live feed from the battery to the fuse box, and we can't see the cables that connect to the brake light itself, so it's not clear where the brake light is getting its power from. There's normally a live feed from the battery and a switched feed from the ignition switch that go to the fuse box, but we can't see which is which.

I'd guess that fat red wire connected to the bottom of fuse#1 is from the battery. Or maybe the battery feed is to the top of that fuse. It's unclear, but it may be a reasonable assumption that fuse #1 has a live 12V feed. That would imply that fuse #2 is switched by the ignition key switch.

The wire labelled "red wire from rear light switch" is connected to the same point as the other big red wire, so presumably that's the 12V supply to the live side of the rear switch. We can't see a power supply to the front switch, but I guess it must be somewhere.
The two cables you've labelled "switch wires" that join together at the bottom of fuse #3 are presumably the switched power return from the front and rear switches. What else can they be? The top connection at fuse #3 has a jumper across to fuse #4. That would imply that the wire connected to the bottom of fuse #4 goes to the brake light. If so, then the power to the brake light is going through 2, maybe 3 fuses! That's a bit convoluted but it could still work.

The thing I'm suspicious of is that other black wire connected to the top of fuse#3 that disappears off to the left of the top photo. If it connects to a 12V supply somewhere (fuse #2 that is switched by the ignition switch perhaps?) then it'll power up the brake light independently of the brake switches. If that's indeed the case, then it makes sense of your test result of disconnecting the brake light switches and the light staying on.

Disconnect the left wire from the top of fuse #3 and see what happens. I suspect that wire shouldn't be there.
 
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TeddyG said:
Thanks I’ll try that tomorrow, but oddly nothing has changed to the wires going to the fuse box since I’ve owned the bike. The sheathing to the wires from the front and back switches is intact but perhaps there’s a break in one wire
Click to expand...
If there was a broken wire anywhere in the circuit the globe wouldn't work, it wouldn't come on
at all.
 
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Dellortoman said:
It's frustratingly difficult trying to diagnose someone's wiring diagram remotely. You really need to be there, meter in hand.

I agree with Barry (BlackJota) that the switches aren't the problem if taking them out of the circuit completely makes no difference.

Your photos only tell part of the story. It's difficult to tell which is the live feed from the battery to the fuse box, and we can't see the cables that connect to the brake light itself, so it's not clear where the brake light is getting its power from. There's normally a live feed from the battery and a switched feed from the ignition switch that go to the fuse box, but we can't see which is which.

I'd guess that fat red wire connected to the bottom of fuse#1 is from the battery. Or maybe the battery feed is to the top of that fuse. It's unclear, but it may be a reasonable assumption that fuse #1 has a live 12V feed. That would imply that fuse #2 is switched by the ignition key switch.

The wire labelled "red wire from rear light switch" is connected to the same wire so presumably that's the 12V supply to the live side of the rear switch. We can't see a power supply to the front switch, but I guess it must be somewhere.
The two cables you've labelled "switch wires" that join together at the bottom of fuse #3 are presumably the switched power return from the front and rear switches. What else can they be? The top connection at fuse #3 has a jumper across to fuse #4. That would imply that the wire connected to the bottom of fuse #4 goes to the brake light. If so, then the power to the brake light is going through 2, maybe 3 fuses! That's a bit convoluted but it could still work.

The thing I'm suspicious of is that other black wire connected to the top of fuse#3 that disappears off to the left of the top photo. If it connects to a 12V supply somewhere (fuse #2 that is switched by the ignition switch perhaps?) then it'll power up the brake light independently of the brake switches. If that's indeed the case, then it makes sense of your test result of disconnecting the brake light switches and the light staying on.

Disconnect the left wire from the top of fuse #3 and see what happens. I suspect that wire shouldn't be there.
Click to expand...
My take on it is the red wire to the rear brake switch jumps to the front switch to power them both. The two switch wires in the bottom holder are the return wires from both switches. Top wire from that fuse should go to the globe. Can't see but main power from the battery is probably in the top left fuse holder and jumped for other circuits.
Somehow the connector/wire coming from the globe is getting power not from the brake circuit. Are the tail light and brake light connectors touching inside the housing? That would only happen when the lights are turned on though.
 
In your picture you have a pair of "switch wires" going from a fuse. These will go to the front, and rear brake switches. From each of those switches is a wire that will join together near the tool tray on your bike, and a single wire from there to the globe. When either switch gives continuity, power will go to the globe. No other fuses or power are supposed to be in the circuit. With both wires removed from both switches there should be no power going to the switch. If there is you have to find where it is connecting and remove it. Take out the globe, or remove the spade terminals from the tail/brake lamp to remove any chance that the problem is there, and test power at the end of the wire.
 
Dellortoman said:
It's frustratingly difficult trying to diagnose someone's wiring diagram remotely. You really need to be there, meter in hand.

I agree with Barry (BlackJota) that the switches aren't the problem if taking them out of the circuit completely makes no difference.

Your photos only tell part of the story. It's difficult to tell which is the live feed from the battery to the fuse box, and we can't see the cables that connect to the brake light itself, so it's not clear where the brake light is getting its power from. There's normally a live feed from the battery and a switched feed from the ignition switch that go to the fuse box, but we can't see which is which.

I'd guess that fat red wire connected to the bottom of fuse#1 is from the battery. Or maybe the battery feed is to the top of that fuse. It's unclear, but it may be a reasonable assumption that fuse #1 has a live 12V feed. That would imply that fuse #2 is switched by the ignition key switch.

The wire labelled "red wire from rear light switch" is connected to the same point as the other big red wire, so presumably that's the 12V supply to the live side of the rear switch. We can't see a power supply to the front switch, but I guess it must be somewhere.
The two cables you've labelled "switch wires" that join together at the bottom of fuse #3 are presumably the switched power return from the front and rear switches. What else can they be? The top connection at fuse #3 has a jumper across to fuse #4. That would imply that the wire connected to the bottom of fuse #4 goes to the brake light. If so, then the power to the brake light is going through 2, maybe 3 fuses! That's a bit convoluted but it could still work.

The thing I'm suspicious of is that other black wire connected to the top of fuse#3 that disappears off to the left of the top photo. If it connects to a 12V supply somewhere (fuse #2 that is switched by the ignition switch perhaps?) then it'll power up the brake light independently of the brake switches. If that's indeed the case, then it makes sense of your test result of disconnecting the brake light switches and the light staying on.

Disconnect the left wire from the top of fuse #3 and see what happens. I suspect that wire shouldn't be there.
Click to expand...
 
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Tippie said:
In your picture you have a pair of "switch wires" going from a fuse. These will go to the front, and rear brake switches. From each of those switches is a wire that will join together near the tool tray on your bike, and a single wire from there to the globe. When either switch gives continuity, power will go to the globe. No other fuses or power are supposed to be in the circuit. With both wires removed from both switches there should be no power going to the switch. If there is you have to find where it is connecting and remove it. Take out the globe, or remove the spade terminals from the tail/brake lamp to remove any chance that the problem is there, and test power at the end of the wire.
Click to expand...
That's how I thought it was in an earlier post of mine, then Cam had a different idea that the red wire to the rear brake switch and possibly the front switch powered them both, which would work also, if those other two wires were return, then one to the bulb via the fuse. The picture doesn't show that configuration. It shows what your suggesting. As Cam said we are only getting part of the story. We don't have enough info yet to know which way it has been wired. PB2 could also be right in his previous post.
 
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