www.RX7fb.com

Something I've not looked at before, but tucked under the dash, near the "CPU" box on the driver's side is this item (case removed to show innards):


What does it do? Well it illuminates the brake fault light on the dash if one or both of the brake lights aren't working.

There was one of these relays attached to one of two CPU units that gt_james passed to me at Goodwood last Saturday for testing.

By a weird coincidence, Dave Collins, another FB owner who attended on Saturday, mentioned that his "Relay Check Lamp" had gone faulty and he'd bridged it out to stop an intermittently flashing warning light on his dash.

I decided today to check the wiring diagrams and trace the circuitry to see how this device works, using Jame's relay unit, with the aim of repairing Dave's faulty one later (which he will drop to me when I see him again in July).

The circuit is very simple:


When the brakes are applied, the stop light switch (top right in diagram) applies positive 12 volts to the unit. If the correct current (about 4 amps) flows through the reed relay coil, then the reed switch closes. This effectively grounds the base of the D571 transistor, turning it off. With no current flowing through the transistor, the dash warning light will remain off.

If one, or both, brake lights has failed, then insufficient current flows through the reed coil to activate it. With the reed open, current flows from the +12volts supply through the 2 x 1k resistors to the base of the transistor and turns it on, so illuminating the dash warning light.

From Dave's description, it would seem the most likely fault is the transistor allowing current to flow when the brake switch is off. Equivalent transistors are readily available, should this prove correct. Watch this space....

Nice post there aye bro. I had notice this little fulla under my dash a while ago, but never paid too much attention to it

Only just seen this post.........Would LED bulbs trigger the warning light & if so would the CANbus bulbs be OK?

LED's would most certainly trigger the warning light as around 4 amps have to flow to the bulbs to "hold off" the warning light. LED's will consume far less.

I've no knowledge of canbus bulbs, but they would need to draw 3+ amps in total.

There are solutions, such as bridging out the checker, or modifying the circuit to work at a much lower current threshold.

Transistor now replaced on my brake light check unit. Works OK on bench so will now fit back in the car and see if that has fixed the intermittent flashing of the warning light as Malcolm suspects.

The light was flashing on and off during normal running when brake pedal not pushed. Would go off as soon as brakes used.

Dave

I think this should fix it Dave, as it's the most likely cause of the fault. The only way the dash light can illuminate, based on the cct diagram, is if current flows through collector to emitter of the transistor. That can't happen unless it's faulty, or, somehow, a +ve signal is applied to the base of the transistor. Again, based on the wiring diagram that is highly unlikely, unless the brake switch AND the reed relay BOTH have intermittent faults, which is highly improbable.

If it is still faulty, I will have to scratch my head harder, maybe a loom issue, or something even more obscure

Hi, I know this is kind of an old thread but hoping someone can help.

I've swapped all my lights to LEDs so therefore the brake warning light comes on when I press the brake pedal.

Can anyone provide instructions on how to "bridge the checker" as described above to stop this warning light? I assume all I need is two spade terminals and a wire but unsure on which connections need bridging.

Remove the module and bridge the "ST.SW" and "S-L" ("STop SWitch" and "Stop Light"), as shown in diagram above with two spades and a piece of wire in the respective module socket. That will connect the brake switch to the stop lights directly.

The pins on the module that you need to bridge are these two, just carefully work out which socket pins correspond and bridge them out.

Worked a treat, thank you!

Hi Casey,

Had another related question. Have also swapped all my indicators to LED which is causing a hyperflash issue.

I've bought a LED relay and adapted it to the CPU plug but still had the same issue. This brought me to your post on flasher/warnings CPU.

From what I gather the indicator timing is based on the circuit load not the relay. Looking at your very useful wiring diagram if I were to bridge the reed coil switch on the CPU at the top of the diagram would this revert back to normal indicator flash speed?