78-83 928 Brake light failure - See here for advice!

[WitchFinder]

Not sure if this is useful, or has been discovered before, but I thought I'd share anyway. I spent Saturday tracing out why my brake lights weren't working.....skinned knuckles and swearing later, I pass on these tips to save you some trouble and stress 🙂

1.- DO NOT remove the plugs from the master cylinder pressure switches unless you want to spend a lot of time either pulling pieces out to make room, or trying to squeeze your hand in around a tiny space!

You can check most of the functionality of the brake/tail/side lights from the plug for the lamp check controller....as I found out. Please note this is only true for 78-83 models, later models have a slightly different unit and may not function this way. The lamp check controller resides just above the passenger side footwell fuse panel.

There are 12 pins on this plug:

Pin 10 is the 12 volt input, check from here to any ground and if you don't have 12 volts here, you have a bad supply wire or a blown fuse.

Pin 3 and Pin 4 are the pins that feed 12 volts to the common side of the pressure switches. Each pin feeds one switch, on the diagram they are marked L and R, I assume that the left switch is the one closest to the firewall, with the right switch being closer to the radiator end, as all my testing was done on the plug for the switch closer to the radiator, and that was on Pin 3.

Pin 2 is the return 12 volt signal from both pressure switches.

Pins 12 and 9 are the 12 volt feeds to the brake light bulbs.

So, given we have 12 volts on Pin 10, if we jumper Pin 10 to Pin 3, then measure between Pin 2 and ground, we should see no voltage. Now depress the brake pedal with the ignition on, you should see 12 volts on Pin 2 with the brake pedal depressed. Repeat the same with Pin 10 jumpered to Pin 4 to test the other switch. If you don't get any voltage on Pin 2 then the pressure switch has either failed or has bad connections, and you're going to have to do the aforementioned squeezing and removing, you poor sod!

Once the switches are tested and proved OK, you can jumper Pin 10 to Pin 12 and Pin 9, this will feed 12 volts direct to the brake lamps, one pin for each side. If it lights up, all good. If not, you have a bad bulb, or a bad wire. You can pull the bulb and check for voltage at the contacts to determine if its bulb or wire if there is no obvious damage to the bulb filament.

As an aside, Pin 6 and Pin 8 and for the front parking lights, jumper Pin 10 to them for a quick bulb check.

Pin 5 and Pin 7 are for the tail lights, however these will not light up when jumpered unless the headlights are also on....

If you do all this testing, and it all checks out, then you have a faulty bulb check controller......just like me!

As an aside.....anyone have a lead on a good second hand bulb check controller? I could wire it out but I'd prefer to replace it....

 

[tazzieman]

I gave up trying to fix my brake pressure warning light. All components bar the controller were replaced : I broke a finger trying to remove a brake pressure switch , replaced the switches (2 faulty new ones from xxxx) and went through everything else.
In the end I stuck black tape over the flashing lights...

I was led to believe the central light controller "doesn't malfunction" but????

A horse don't need no lights 🤬
 

[WitchFinder]

Hmmmmm could very well be an issue with the controller.

In my research I found that the brake pressure warning is a very simple system. There are two warning loops, one through each pressure switch with each switch monitoring one side of the hydraulic circuit. Both pressure switches are a single pole double throw type, i.e. one common pole, with two contacts, one is normally closed and the other is normally open. In normal operation, the 12 volt feed from the bulb check unit is fed through the normally closed side back to the warning system controller. As long as the brake pedal isn't active, both switches feed 12 volts back to the CWS controller and everything is OK. 

When you press the brake pedal, the switches change state, and the normally open contact closes, feeding 12 volts back through the bulb check controller and onto the brake lights. As long as both switches close, the normally closed contact breaks and 12 volts is removed from both warning channels into the CWS controller and everything is OK.

A warning is triggered when one of the pressure switches fail to close the normally open contact- the CWS controller receives 12 volts on one channel but not the other, and interprets this as a loss of pressure in the Master cylinder on one hydraulic loop.

Due to the dual channel nature of this system, even if a single switch fails, both brake lights will still work.

In your case tazzieman, I'd expect there is a possibly a bridge somewhere in the CWS controller or on the wiring, most probably some corrosion or carbon buildup or a rubbed through wire that is feeding the 12 volts into the warning channel regardless of the pressure switch state. Good luck finding it though 🙂

 

I tell you what, the more I look into the electrical system, the more I'm glad I have a background in electrical control and automation, this old school relay logic is a piece of piss. I'm actually curious to see if I can replace a big chunk of it with a programmable logic controller....... 

 

 

[tazzieman]

1 hour ago, WitchFinder said:

In your case tazzieman, I'd expect there is a possibly a bridge somewhere in the CWS controller or on the wiring, most probably some corrosion or carbon buildup or a rubbed through wire that is feeding the 12 volts into the warning channel regardless of the pressure switch state. Good luck finding it though 🙂

I replaced the corroded wire on the for'ard switch but from memory couldn't get near the rear wires.
Think I need to disconnect the controller!