Rebody Brake Warning Lamp

[Jezza]

I am trying to understand how to use a single warning lamp in an ETB speedometer to function for 3 possible triggers.


The BMW instrument cluster has individual warning lights for brake fluid level, brake pad wear and handbrake warning light. The ETB speedometer I am using has a single warning lamp for brakes, and therefore you have to choose which of the three warnings is most important. I would like the light to come on if any of the three sensors are activated.

I believe that it is possible to connect the three individual signal inputs from the BMW loom to the one lamp in the speedometer by adding in diodes to the 3 connections.


My understanding of all things electrical is close to zero so does anybody understand how these diodes can be connected to achieve this? I understand they are directional and will restrict the flow current to one direction only. But I don't know which way the current is or should flow in the Z3 cluster!


On my Year 2000 Z3 I have identified the cables into the instrument cluster for the brake lamp warnings.


Connector X16 (White) pin 17 has a blue brown yellow cable which is for the handbrake switch.
Connector X16 (White) pin 18 has a brown violet cable which is for brake fluid level.
Connector X16 (White) pin 19 has a yellow cable which is for brake pad wear sensor


The ETB speedometer has a single black and white cable which is the brake indicator lamp.


I have found a document from somewhere which has a diagram showing the diodes although the wiring colours do not match those on my car (so I am considering the pin numbers to be more accurate). It also shows a connection to the ground supply for indicator and warning lamps (pin 3 on X17, brown cable)

 

[Pond]

Why do you want a single bulb to do three different things? If you can make it work, how will you know what has triggered the light? Also, are you 100% sure you have brake pad wear sensors?

 

[Jezza]

I only have the option of a single brake warning light, so I would prefer to know if something is wrong.
If the light is on when the car is stationary and the handbrake is on, that's good, it tells me the handbrake is still on. When I release the handbrake, the light should go off. If the light stays on when I release the handbrake, or if the light comes on during a journey, that's bad because it means there's either a problem with my brake fluid pressure (low fluid level) or there's a problem with brake pad wear. Either way, the brakes need attention, and that's something I would like to be notified about.

I don't want my 250 Dashboard to be adorned with masses of xmas tree lights. I decided that I would have the few that are already included in the ETB instruments (Oil warning, Brake warning, Full Beam, Indicators, Alternator Charge warning) plus additional lights for Check Engine, ABS, Rear Fog light and Front Fog lights)

Yes I do have brake pad wear sensor on this car, although that is the least important of the brake warning lights since brake pads are checked when I service the car.

 

[Pond]

I have done similar with mains electric. It was a different application (to run a fan from multiple sources) but is essentially the same.
I used relays. One relay for each feed. The feed from the source to power the coil, then a permanent live on one switch, with the other side of the switch going to your bulb.
Not sure if it would work the same on 12v DC, but as long as you have a common earth I can't see any problems if more than one source was live at once.

 

[ruan]

Think of the diode as an arrow allowing the current to flow from positive to negative. Assuming that each warning signal produces a positive voltage to indicate a warning then connect the 3 diodes so that they "point" to the positive terminal of the shared warning lamp and connect the other side of each diode to a separate warning signal. Sorry can't draw it for you at the moment. Make sure the diodes are rated to carry the current needed by the warning lamp. If smaller diodes are used then they should all go to the switch control on a shared relay and the relay used to drive the lamp.

 

[Jezza]

Thanks Ruan. Only problem is I can't assume each warning signal produces a positive voltage.