Tim:
I will try to help, but please note that due to the varieties of Plus 2 wiring that are in my book, I can’t be too specific to your car as not sure which variant you have. For reference, my car originally followed the Federal diagram, but I expect yours may be different if in UK. For reference, Federal diagram is labeled March 68 to March
69 in my book. Note just using this to explain the circuits; yours may be different.
Brake warning light in mine is actuated by the PDWV or “Pressure Differential Warning Valve”, a hydraulic valve plumbed to the dual circuit master cylinder which indicates failure of one of the dual brake hydraulic circuits. Your brake fail warning light, if in a single circuit brake system, may be triggered by a low brake fluid warning light connected to the cap of the single circuit master cylinder?
So, confirm first how your brake warning light is triggered.
In the diagrams I examined, the separate brake warning light and the hand brake warning light are triggered in a different manner, and must be changed to consistent electrical configuration prior to combining them.
The Handbrake switch is powered by a green “hot in run, fused”, and the warning light is grounded at the bulb holder with a black. The switch and warning lamp are connected with Green/Purple. So, the current flows through the switch first, then to the dash lamp, and to a ground located on the warning lamp holder.
The brake fail warning lamp is the opposite. The lamp is powered in the original wiring in my car by a Purple?Hot always, fused?. The PDWV or, I am assuming, the low fluid switch, is grounded when closed. The lamp and switch are connected by Brown-Purple. So, the current flows through the lamp first, then to the switch, and to a ground located on the switch.
So, second, confirm where your grounds in the two circuits are located; they will consistently be black even if the other colours vary in your particular car. Easy way is note if you have a black at the existing warning lamp or not, as you have dash out.
Third is to change the wiring such that both switches go to ground. This is the easiest way to make the switching consistent as you have available power at the dash area, and it is easy and safe to run an additional ground as required to the handbrake switch. In the dash, I wired both my handbrake and brake fail lamps to be powered by Green Hot in Run, Fused
, as this way if the brake fail is triggered, the warning bulb will be extinguished when the key is shut off. In your case, it is just a green power wire to the single bulb holder.
It is essential that the single warning lamp is powered by Green, “Hot in Run, Fused” or White, “Hot in Run , Unfused” or else your handbrake warning will be on when the car is parked and drain the battery! I am betting this set-up did not meet the letter of the law in the US as the brake fail is available only when the car is switched on, hence the weird wiring variations.
Both connecting wires to the switches are now connected to the single warning light. This essentially creates a parallel switch circuit to the single warning lamp, so if either the handbrake is on, the brakes fail, or both happen at the same time, the warning bulb will light. The diagram I looked at for the single lamp set-up uses Black-White for wires from the lamp to the switches, but I would just use the existing wiring in your car now.
One further detail. My car has a simple push button switch, almost as an afterthought, located on the driver?s side panel below the dash and above the tunnel area. This switch is provided to test the brake fail warning lamp. It simply grounds the warning lamp, lighting it up. I do not see this in the single bulb circuit, and I don`t know if you have this feature on your car; I doubt it though. I guess it was required by US law. Anyway, I would not bother adding this feature in a single bulb set-up, as you simply pull on the handbrake to test the bulb.
HTH. Sorry for length and lack of specifics, but hopefully explanation will assist in figuring out the details.