54 XK120 Turn signal/ brake lights

Obviously, I have something wired incorrectly. My turn signals and brake lights work perfectly until I turn on the lights. Lights work great but no turn signal and brake lights. Thanks for any input. Jeff

Welcome to the fun, Jeff.
Do you have the FHC wiring diagram and understand how to read it?

Do you have this relay unit?

Looks like a earth problem to me.
Probably near the rear lights.

Yes. I have the diagram and I do have the Directional Indicator relay shown in the photo.

Thanks. I’ll take a look at the wiring in the rear area of the car.

1 Like

Everything seems to be connected correctly regarding the indicator relay

Your relay is the opposite way from mine. Is your top cover on the right way?
1 = green / brown
2 = green / red
3 = black / green
4 = green / blue
5 = green / purple
6 = green / white
7 = white / brown
8 = green / yellow

4 & 8 power the coils.

When you say “turn on the lights” do you mean the headlights, sidelights and taillights?
Better go over all connections at the voltage regulator and fuse block with the wiring diagram in hand.


Pardon me for piggybacking on this post. I have had a niggling problem with my splitter box for a little while, specifically the RH signal. I carefully cleaned the connections, paying particular attention to #4 and #8, the two that power the coils, and this appeared to solve the problem for awhile, but then it returned. Looking into it again this afternoon I find that with the cover off and the signal activated the flasher activates as does the dash indicator light, but no RH signal lights either front or rear, however, if I press down on the top of the rear coil the RH signals flash correctly. If shut off and tried again they will not work until I press down again on the top of the coil. I am curious as to what this indicates, lack of adequate juice to the coil, a bad coil, a shot splitter box? Appreciate your thoughts, thanks.


Hello Chris - sounds like the air gap of the coil has increased - usually through age and usage - small adjustment would get the coil to operate as required - could also have dirty relay contact points and/or cold solder joint - without actually seeing the relay, I can not tell you where the gap adjustment is - sent 2/24/2021 1611hrs. EST USA.

I had intermittent problems with right brake/tail…it was in the 8 post relay box…pressing did sometimes correct it. I replaced the box with a look a like…but solid state box. Easy f8iix.

1 Like

Could be the rivets that hold the contact arm to the coil are corroded.

Corrosion or bad points connection somewhere anyway.
The point gap is…a bit more than zero?
Unfortunately Lucas doesn’t tell us.

I would go with about .012-.015"

The box was installed backwards but all the correct wires coincide withe the correct number. Also, I replaced the box and installed it the way I removed it. Although backwards, I’m confident that it’s wired correctly. Also, everything works great, all turn signals, lights, brake lights until I turn on the running lights or headlights. At that point, I loose brake lights and all turn signals front and rear.


Thanks. Due to the relay’s position I think I’ll remove it completely in order to check the rivets etc. How does one adjust the point gap? Also, thank you to the others for their observations.


To both… see if you can borrow a known working 8 leg box and put it in. Like tracking down bad switches it’s important to give this box of miracles a hard look too. The new ones are less WWII Jeep era vintage-y inside, but can still fail.

Bend the contact arms with needle nose pliers.

When you switch off the side lights, do the turn signals and brake lights work again?
The side lights and tail lights are on a red/green wire. I suspect that wire is crossing somewhere with a wire to the flasher or the DB10 relay or the brake light switch. Try removing your fuse #3 to the side lights or disconnecting the red/green wires at the fuse box and see if you still have the problem.


Ha! Didn’t realize it was such a sophisticated process!


Rob, You figured it out! I pulled #3 fuse and everything works correctly. In order to have the fuse in place I will need to track DB10, brake light switch, and flasher, correct?

Sidelights and taillights work with fuse 3 out? They shouldn’t. Trace both ends of every #45 red/green wire. One is touching something that it shouldn’t.

Or it could be in the dash, a red or red/white panel light wire touching the dark green indicator light wire.