Late BN2 Brake Lights

[Twstnshout]

How does this brake light system function? Where are the electrical connections? I have installed a new sensor at the front right junction. Admittedly I reconnected the existing connections crossed my fingers and was hoping it would function. Not so lucky. I can’t find any troubleshooting help in the service manual or even system diagrams except for the general overall wiring diagram. Any help?

 

[roscoe]

The shop manual diagram is all you have in addition to various threads on this and other forums about this topic. The wiring diagram may seem daunting but if you spend time with it, it will still be daunting but you may see your way to understanding.

Do your turn signals work in the rear? There are many potential failure points for the brake lights and turn signals. The Lucas bullet type connectors are notorious for coming loose, or getting corroded. Often times they are bundled poorly by owners or their mechanics and a bad connection can be a PITA to find and correct. You might first open the trunk and lift the liner to find the wiring harness and locate all the connectors, particularly on the left side if both stop lamps are in op. Be sure the bullet ends are pushed all the way into the connectors. The wires should not come out of the connector sleeve with a gentle pull. My point is be sure they are all pushed in firmly and no wires have broken at the connector. The only other connector for this system is at the flasher relay box I speak of below. It is likley bundled with other connectors right near the relay box.

I'm hoping you are comfortable using a multi meter to at least check for voltage at various points. Since you know where the brake light pressure switch is, first check to see if there is power at one side of the switch. One of the two wires should have 12V when the ignition is on. If you have power at the brake switch, disconnect both wires and jumper them together. If the lights light up, you have a bad switch. If they do not you can probably assume the switch works and your issue is elsewhere. Power for the brake lights comes from the 35 amp fuse on the firewall in the engine bay on the left side. That fuse also powers your wipers, fuel gage, and turn signals. If they work, your fuse is good. If not, check continuity of the fuse and replace if needed.

The ground for the brake lights is through the flasher relay box which is the retangular box, probably with an un painted aluminum cover on the left side of the fender area in the engine bay. They are ugly inside with coils , contacts and what not. Not really repairable but they are replaceable with new solid state units. It has 8 terminals around the periphery which are numbered 1 through 8. Two of them provide a path to ground for the brake lights and the turn signals so that you always have a ground available if a turn signal is being used. Terminal 5 should always be ground for the brake switch. If 5 is not a ground, you likley have a bad relay box. If you can read the wiring diagram you will see the connectors and how the system works.

If you just replaced the brake switch, and your turn signals work, I'd look first at the connectors in the boot. That should be easy. Sometimes all it takes is wiggling some wires with the brake depressed, and press it firmly. These pressure switches aren't like new cars where a slight, light press on the pedal turns on the light. It's a two person job unless you can depress the prake pedal and wedge it to stay on. Three people if one is pushing the pedal, one is wiggling wires at the flasher relay box or at the boot and one is watching the brake lights. If you don't have a ground at terminal 5 on the relay box, a new solid state relay box may be your fix. It is well worth upgrading to the solid state box for reliability. Hope this helps. Let us know what you find.

 

[red57]

Never mind, Jon covered it.

 

[roscoe]

There is one thing, as unlikley as it is, I didn't cover. Ya could have 2 brake lamp filaments burned while the flasher filament still works. That would be pretty strange but strange is an often applied term to electrical systems in these vehicles. I'm casting my vote for a connector in the boot. 2nd would be the relay box or a connector near it.

 

[Twstnshout]

Then shop manual diagram is all you have in addition to various threads on this and other forums about this topic. The wiring diagram may seem daunting but if you spend time with it, it will still be daunting but you may see your way to understanding.

Do your turn signals work in the rear? There are many potential failure points for the brake lights and turn signals. The Lucas bullet type connectors are notorious for coming loose, or getting corroded. Often times they are bundled poorly by owners or their mechanics and a bad connection can be a PITA to find and correct. You might first open the trunk and lift the liner to find the wiring harness and locate all the connectors, particularly on the left side if both stop lamps are in op. Be sure the bullet ends are pushed all the way into the connectors. The wires should not come out of the connector sleeve with a gentle pull. My point is be sure they are all pushed in firmly and no wires have broken at the connector. The only other connector for this system is at the flasher relay box I speak of below. It is likley bundled with other connectors right near the relay box.

I'm hoping you are comfortable using a multi meter to at least check for voltage at various points. Since you know where the brake light pressure switch is, first check to see if there is power at one side of the switch. One of the two wires should have 12V when the ignition is on. If you have power at the brake switch, disconnect both wires and jumper them together. If the lights light up, you have a bad switch. If they do not you can probably assume the switch works and your issue is elsewhere. Power for the brake lights comes from the 35 amp fuse on the firewall in the engine bay on the left side. That fuse also powers your wipers, fuel gage, and turn signals. If they work, your fuse is good. If not, check continuity of the fuse and replace if needed.

The ground for the brake lights is through the flasher relay box which is the retangular box, probably with an un painted aluminum cover on the left side of the fender area in the engine bay. They are ugly inside with coils , contacts and what not. Not really repairable but they are replaceable with new solid state units. It has 8 terminals around the periphery which are numbered 1 through 8. Two of them provide a path to ground for the brake lights and the turn signals so that you always have a ground available if a turn signal is being used. Terminal 5 should always be ground for the brake switch. If 5 is not a ground, you likley have a bad relay box. If you can read the wiring diagram you will see the connectors and how the system works.

If you just replaced the brake switch, and your turn signals work, I'd look first at the connectors in the boot. That should be easy. Sometimes all it takes is wiggling some wires with the brake depressed, and press it firmly. These pressure switches aren't like new cars where a slight, light press on the pedal turns on the light. It's a two person job unless you can depress the prake pedal and wedge it to stay on. Three people if one is pushing the pedal, one is wiggling wires at the flasher relay box or at the boot and one is watching the brake lights. If you don't have a ground at terminal 5 on the relay box, a new solid state relay box may be your fix. It is well worth upgrading to the solid state box for reliability. Hope this helps. Let us know what you find.

Direction and parking lights all work. Horn doesn’t but I haven’t spent any time on them yet. I will do the other tests this weekend. Thanks for the help

 

[BobHaskell]

Jon covered it very well. The brake light switch ground is actually provided by the rear light bulbs. With the brake pedal depressed, you should have battery voltage at both posts of the switch, at terminals 3, 5 and 7 of the DB10 and the rear bulb sockets.

This image is from Norman Nock's Austin-Healey Tech Talk book - inside front cover. Available from British Car Specialists.

Power for the horn is provided by the 50 amp fuse.

 

[John Turney]

Since the direction and parking lights work, you have eliminated most of the troublesome bits. Either you are not getting power to the brake switch, the brake switch is bad (too common), or the wiring from the brake switch to the #5 terminal on the relay box is bad.

 

[Twstnshout]

I checked the power to switch none there. There was a jumper from the old switch to the 35 fuse. I had forgot about it 2 years later . Looking at some of the zillion photos I took i remembered it when I reset the jumper the brake lights work. There must be an issue with the # 17 wire as shown on the electrical diagram. Probably the union connector. I think I’ll formalize that jumper I really don’t like poking around on the 70 year old wiring.

 

[jimmyr]

One item that always seems to trouble old vehicles is their ground connections. Looking at the Nock diagram listed by Bob, the case of the relay box provides the ground path for the relays inside. Also poor grounds in the actual bulb sockets. Just a thought.