Tail light warning problem

Hi all. This problem started right after I rebuilt the front calipers. Before rebuilding the taillights worked and are fit with proper bulbs. After rebuilding, whenever I activated the rear break, the red warning light came would come on and stay on until I cleared it. I rarely use my rear brake anyway so I just disconnected the rear brake taillight switch. Now if I'm sitting at a stoplight and squeeze the front brake hard, the warning panel will signal "taillight" and the red warning light will flash on, but both will go out as soon as I let go of the brake. Unless I squeeze the brake hard, the warning won't come on. What to do?

Oh... my bike is an 82 XJ750 Seca.

 

First confirm that the brake lights are coming on.
Then use an ohm meter to confirm that the rear brake switch is functioning properly.
Both brake switches must be hooked up for the system to not see a fault.

Also check the brake fluid level. There is a float in the master cylinder that will trigger the brake warning lamp if it drops too low. Since you say that it only trips when you brake hard, I suspect that the brake fluid level is a tad low. Be careful to not overfill (unfortunately an easy thing to do), or that will cause the brakes to not function.

 

The "computer" on your bike passes a small amount of current constantly thru the rear tail light/brake lights, and expects to see a minimum current returning.....this is the method which the TAIL (light) warning circuit operates. It detects a reduction in current (below a specified level, as an indication of a broken bulb filament, corrosion, etc. ). Also, the use of incorrect resistance bulbs can cause a malfunction.

The BRK (Brake) warning light circuit senses the level of brake fluid in the master cylinder reservoir via a sliding magnetic float-on-a-shaft system.

This could be the problem. The current can't get back to the computer, so it thinks the rear bulbs are burned out, and lights the warning.

 

Thanks for the response Kmoe and XJ. I just checked all that. Fluid is up, reconnected rear switch. Same thing happens. If I push on the rear brake warning light comes on and stays flashing until cancelled. If I pull hard on the front and hold it, the taillight warning comes on and goes out as soon as I let go.

 

There are either some electrical wiring issues or a malfunctioning computer (or, as stated before, bulb problems). You might also try disconnecting the battery to try to reset the computer.

 

As Len explained "BRK" indicates a fault with the master fluid level and sensor switch. Not sure you are getting this fault?

Also, as he explained "TAIL" is a problem with the taillight or brake light. Since you only get this when applying the brakes it suggests a problem with the brake lights. The brake light circuit can only be monitored when either the front or rear brake is depressed. I suspect the front brake fault you are seeing is when you squeeze hard enough to close the switch and activate the brake light. As for the rear make sure the switch is adjusted correctly and not staying on after depressing the rear brake.

Also note there are two bulbs and both must be in working order to provide sufficient current to prevent a fault from being set.

 

I'm replying in 2019 in case this helps anyone else. This thread suggested a solution for me. I had the TAIL warning light come on while warming up. The tail lights shone brightly without applying the brakes and then went dim - but still on, indicating the bulbs are ok. I replaced (or bypassed) a corroded and apparently charred connector clip that joins three wires by splicing them each separately. A few hours later, the TAIL light warning returned. I imagine from this these replies that the resistance in my splicing wire could be too high. I'm going to try that. Anyone see anything else? Thanks for these ideas.

 

First we need to know if the "TAIL" warning is occurring when the brakes are applied, or if it occurs as stated above - or maybe it is both. From what you described above the tail lights illuminated and then went dim, which indicates a problem in the tail circuit, but not necessarily the brake circuit. Yamaha designed the CMS (Computer Monitoring System) to detect if both bulbs are present when they are illuminated, so if one bulb burns out or draws insufficient current the warning will set.

One easy check you can do is to actually measure the current in the tail and brake circuit to see how it is behaving, you can do this at the signal fuse by:

1. Remove signal fuse and insert DMM in current mode to measure current.
2. Disconnect tail, brake, license connector (4 pin connector}
3. Turn key on and note current reading
4. Reconnect 4 pin connector and note current reading - tail current is current reading minus reading in step 3 - should be about 1.2 amps.
5. Depress front or rear brake and note current reading - brake current is current reading minus reading in step 4 - should be about 4 amps. Note try both switches separately and together - the current difference should be minimal.