Master caution panel tail light warning

Last year I changed the hydraulic fluid on my 82 XJ750 Seca. Ever since then I've been getting a tail light warning when I brake moderately... light braking doesn't set it off. I've disconnected the brake light switch for the rear brake, thinking that might have been it, but still happens. Anyone have any idea where to start on this?

 

Your brake fluid level is too low. The warning is triggered by a float in the master cylinder.

 

Just to clarify, if you are getting a "TAIL" warning then the issue is with the tail light or brake light. If you are getting a "BRK" warning, then yes the master cylinder fluid level is likely low and the CMS system might have prevented a disaster of loss of front brake.

 

Level is right up there... so hard to check, the only thing I don't like about this bike. At first I only noticed it when using the back brake, which is why I disconnected the rear switch. The warning I'm getting now is "tail".

 

The "TAIL" warning can be set by four conditions, since yours only occurs with the brake applied then only two are applicable:

1. The bulb filament check is failing. This condition requires only one incandescent bulb to be functional and the fail will occur without applying the brakes.

2. The current is inadequate through the sense circuitry in the CMS when the brakes are applied and the brake light illuminates. Inadequate current will be caused by either one bulb burnt out, incorrect wattage bulbs installed, faulty switch / wiring, or worst case a problem with the CMS internal circuity.

So, be sure both bulbs are illuminating when the brakes are applied and verify that the bulbs are 1157 (27 watt).

But hard or moderate front braking does. The brake light should illuminate with light front braking, so check adjustments and switch operation.

 

Checked the bulbs... both are 1157s, both work properly. Where is that switch and it's adjuster?

 

On the right control assembly. The switch is not adjustable, but the free play in the brake lever is and should have sufficient free play so that light braking will release the front brake switch plunger enough to illuminate the brake lights.



Unfortunately none of this is going to fix the "TAIL" warning that you have. You will likely need to do some current measurements of the brake light circuit to further evaluate. The easiest place to do this would be at the signal fuse.

The basic premise of this would be place the DMM in the 10 amp current mode.
Remove the signal fuse and place the DMM across the terminals. Turn the key to "ON"
Note the current reading (The tail and indicator lights will be on drawing current)
Pull the front brake lever to illuminate the brake light, note the new current reading, release the front brake
Engage the rear brake lever to illuminate the brake light, note the current reading
Engage the front and rear brakes to illuminate the brake light, note the current reading