[EricK1300GTSE]

HI, hoping the collective wisdom can help me out. So I have brake function and no ABS flashing etc, AND my taillights work. However I have no actual brake light when I press the foot pedal or squeeze the front lever. I know bmw runs a dual voltage to the brake light, one voltage for the rear light and another for the brake light. I have the warning light on the display telling me the light isn’t working. I tried a few different bulbs but no luck.All wiring seems to check out Am I missing something? Is there a brake switch that is faulty?

 

[CKD1]

Have you checked the lamp/bulb socket with a meter? Could just have a socket making poor connection and swapping the lamp doesn't necessarily fix the issue.

 

[Corkboy]

HI Eric, I know you know your stuff, but just a quick few questions.
1. The bulbs you swapped out, were they single filament bulbs? As you said, BMW uses the same bulb filiment, just ramps up the voltage for the brake light. They also use the same wiring, so if you have a back light, as you say, then the wires are ok.
2. I think BMW have a separate standard contact switch on the front and back brake levers, as opposed to some sort of pressure switch on the brake system itself. Can you confirm? (Just something in the back of my memory about this). If they do, then its unlikely both switches have gone bad

Did you change anything else recently?

Very curious about this one.

 

[EricK1300GTSE]

Have you checked the lamp/bulb socket with a meter? Could just have a socket making poor connection and swapping the lamp doesn't necessarily fix the issue.

Yes, checked the sockets with a meter, 11.67v each. When applying the brakes, voltage stays the same, so that least that narrows it down a bit. Interestingly when using the motoscan app and getting live data from the abs module, I can see pressure values change both front and rear, so the pressure sensors are working.

Will keep checking. Thanks for the suggestion.

 

[EricK1300GTSE]

HI Eric, I know you know your stuff, but just a quick few questions.
1. The bulbs you swapped out, were they single filament bulbs? As you said, BMW uses the same bulb filiment, just ramps up the voltage for the brake light. They also use the same wiring, so if you have a back light, as you say, then the wires are ok.
2. I think BMW have a separate standard contact switch on the front and back brake levers, as opposed to some sort of pressure switch on the brake system itself. Can you confirm? (Just something in the back of my memory about this). If they do, then its unlikely both switches have gone bad

Did you change anything else recently?

Very curious about this one.

Thanks Corkboy, kind words,but I'm still a paduan learner. Tried 3 sets of the same 12v21w bayonet bulbs, all work the same. As per my last comment, the sockets measure 11.67 volts each ( engine not running) and no change when applying the brakes, so I think a switch somewhere is faulty. I can't see a switch at the front or rear, so I need to check near the abs, I think...

 

[Mike in Meadow Flat]

Thanks Corkboy, kind words,but I'm still a paduan learner. Tried 3 sets of the same 12v21w bayonet bulbs, all work the same. As per my last comment, the sockets measure 11.67 volts each ( engine not running) and no change when applying the brakes, so I think a switch somewhere is faulty. I can't see a switch at the front or rear, so I need to check near the abs, I think...

Eric, Mate, what’s the voltage at the battery with everything off? If you are reading less than 12.5 volts, you may have a battery problem. What do you have at the battery with the engine running? You should see ~14.5 volts and that figure should stay pretty constant when you raise the rpm’s.

Try checking the voltage at the sockets with the engine running but the brakes not on. Then try again with the brakes on. The voltage should increase to make the tail light brighter. If the voltage stays at 11.67 volts or the voltage doesn’t increase when the brake is pressed you should think about having your alternator checked.

 

[EricK1300GTSE]

Thanks Mate, will check and report back

 

[Mike in Meadow Flat]

Thanks Mate, will check and report back

Good Luck. I hate electrical gremlins. I have three Pontiac Fieros (2-seater,mid-engined). Thy have plastic body panels and occasionally a bad ground will pop up out of nowhere and cause all kinds of issues from gauges going crazy to tail lights and turn signals not working or working erratically.

 

[Meese]

The tail / brake light doesn’t work by changing voltage. Not exactly.

It works by modulating the tail light voltage via square wave (limited duty cycle) to make the light appear dimmer. When the brakes are engaged, the light assembly gets full (non-modulated) voltage and thus full brightness.

A standard DC meter may not be fast enough to catch this modulation, which can make troubleshooting difficult.

 

[EricK1300GTSE]

Thanks, so is this modulating function in the ecu, or controlled by a switch?

My multimeter picks up voltage V to two decimal places, I noted one reading 11.64 to 11.67, but when switching to mV, it went into O.L, clearly too much for that circuit in the multimeter. I don't believe 0.03v (30mV) would be enough to effect a brighter light.

I did not notice the light changing at all, however tests do far are with engine off. Will check battery voltage and alternator output and go from there

 

[EricK1300GTSE]

Eric, Mate, what’s the voltage at the battery with everything off? If you are reading less than 12.5 volts, you may have a battery problem. What do you have at the battery with the engine running? You should see ~14.5 volts and that figure should stay pretty constant when you raise the rpm’s.

Try checking the voltage at the sockets with the engine running but the brakes not on. Then try again with the brakes on. The voltage should increase to make the tail light brighter. If the voltage stays at 11.67 volts or the voltage doesn’t increase when the brake is pressed you should think about having your alternator checked.

Ok, so checked the following: voltage at battery, engine and ignition off; 12.23V. Ignition on 11.87V, engine running 14.3v. Voltage did not drop below 10v at start up. So this tells me battery and alternator are healthy

 

[Mike in Meadow Flat]

Eric, Great news on the alternator & battery. Wish I could help you on the brake light issue. A faulty switch is my next guess.