[Lumps!] Brake lamp failure warning unit

My 79 XJ-S recently had a mystifying, extremely aggravating problem. At
random, the fuse for the brake lights (15A, #5 in fuse panel) & turn signals
would blow. We isolated it down to the brake light circuit by deliberately
not using the turn signals and waiting for the fuse to blow…

At no regular interval, applying the brakes would cause the fuse to blow.
After examining all accessible wiring and the bulb sockets, my husband
concluded the problem was likely in the brake warning relay, located in the
center console under the rear portion of the ski slope. He removed the
‘relay’ and opened it up. Sure enough, a blob of solder on the brake light
input wire (14 ga green/purple) terminal was touching against the steel case
of the ‘relay’. Closer examination revealed the rivet holding the outer
spade terminal to the inner solder terminal had loosened, allowing poor
contact leading to high resistance producing heat, increasing resistance,
leading to even more heat… This apparently got bad enough that the solder
connecting the coils of wire wrapped around two reed switches (one for each
brake light output) got hot enough to melt somewhat. In addition, a diode
in the circuit was cooked by the heat.

We have no idea what a replacement unit costs, my husband (electronics tech
by profession) decided to repair this simple circuit. He cleaned up the
dirty, overheated terminals, tightened (repeened) the rivet and soldered
both terminals to the rivet. Replacing the diode was also very simple,
nothing special about that component he said, any small rectifier diode
would work there. Reinstalled the ‘relay’, everything works great. Brake
lights are brighter than before (wonder why?), and the IP warning lamp comes
on when it should (either or both brake lights disabled and brakes applied).

For those interested, the XJ-S Parts Catalogue refers to this part as “Stop
Lamp Failure Transmitter” P/N C.42291 (actually this is a better name than
‘brake warning relay’, which is how the ROM puts it). For anyone who knows
which end of a soldering iron to grab, this would be a simple thing to
repair, aside from the aforementioned diode, there is a transistor, a
resistor, and two wire wrapped reed switches inside the can. All components
could be easily replaced with like/similar items from any electronics store.
In case of on the road failures, we made up a simple “Y” jumper with male
1/4" spade terminals at the three ends to connect the three green/pink wires
together. This would bypass the transmitter and continue to allow the brake
lights to work.


79 XJ-S

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