Rear Sidelights Not Working

[roy666]

Hi All - suddenly my rear side lights, both sides have failed: all other lights (indicators, haz, brake) working, and front sidelights working as well, checked fuses 6 & 7 .. all ok, bulbs looked ok but changed them anyway, checked power getting to the bulb and nothing there (but then I'm not v good on lecky stuff..difficult to get earths on the rear array). There isn't any other relay/fuse associated with these lights.. is there? What would be the next step..cheers Mike

[Tuga2112]

according to the wiring diagram. there are no relays in that circuit to check

the lighting switch itself is the only common point in both those circuits. i would be looking there first.

after that.
what you are after is a continuity check i guess.
red wire with blur trace and red wire with white trace are the ones that lead power from the front to the rear.

i think grounding for all the lights on the rear clusters is done via the same black wire in the harness. (which would stop the whole cluster from working) but i may be wrong there.

"all of the sudden" and 2 distinct (and completely separate) circuits failing at the same time seems too much coincidence.
are you sure there has not been any work done with electrics recently OR near cables ?

also, what about the number plate lights ? are those also not working ?

[roy666]

Tuga - thanks for the info. Yep, the number plate light is working. I lost all lighting to the dash/instrument lights a while ago.... and I found it was a loose connection to the relays under the dash by the foot pedals... tucked up in there. A wire had fallen down and my feet had caught it. So assumed the Lotus position, took a look, pressed lots of relays back in, got the wire out of the way.. and all worked. Something related to this maybe?

[Saltire]

1E19F9E3-FC20-4C5A-A5E4-770B174702EE.jpeg

There aren’t any relays in that circuit.

I’m very puzzled if the number plate lights are working, because they have common power and earth connections with the respective tail lamps. And it seems vanishingly unlikely that both tail lamp connectors have gone bad simultaneously.

If you have number plate lights, the light switch must be fine - and yet you say there’s no power to the tail lamp bulb. How did you check?

Will scratch my head overnight and convey any bright ideas in the morning

[roy666]

Thanks Sal.. I checked with a voltmeter on DC... used the same earth that the Rear Fog lights are on... not a trickle. Week after next the car is up at Chris Foulds for a C service... I'll see if he has any bright ideas.. cheers M

[dapinky]

Mike,

At the stage you are at, I would consider going in this direction (which may not be the best way, but it's what I'd do!).

First, remove F6 and (hopefully) one of the number plate lights will now fail to work - repeat by swapping F7 to check the other side - this will 'prove' that the 2 circuits are indeed separate, as designed.

Now, with one of the fuses out, work just on one side of the car (it is hard to track down a fault in 2 separate circuits at the same time).

Run a length of wire from a known good earth.... not simply something that appears to work, but a definite earth - to me, that would be direct from the battery terminal, you can't get a better earth than that!!! down to the area you are working on. Check that there is perfect continuity between the known good earth, and anything which "should" be earth.... even if it is enough of a connection to work for a different bulb, it may not be a great circuit.

Next connect this wire to the earth connector of the lamp holder, and check continuity between the wire and the metal bits that hold the bulb..... you are simply trying to prove a DEFINITE earth everywhere along the circuit.

Now take a wire from the positive side of the battery (run it through a 3A fuse if you feel better doing so) and touch the 12V to the other side of the bulb - obviously it should now light up, as we have a proven earth. If it doesn't, either you don't have the continuity you think you have, or the bulb is duff..... try a selection of bulbs until you find one that is known to work - it's only to test the wiring at this stage.

Now 'just' test continuity between the wiring at various points from the fuse box to the light units until you find the problem.

Having said all that, the majority of the circuit is working if the number plate lights are working - so it is likely to be a minor issue, and I will put a fair chunk of confidence in suggesting that it is a poor earth which will flow enough current to operate the small lamps, but not good enough to allow the tail lamps to work.

As a very quick check (if you have one to hand) - try replacing the bulb with an LED one - they will often work with a much poorer earth than an incandescent bulb.

So, my best guess is a poor earth somewhere - possibly in the actual bulb holder in the unit.

[Fredjohn]

As a supplement to Dave's excellent (as aways) advice, take a look at the wiring diagram for the side and tail lights. It may help in visualising the circuits and Dave's diagnostics.

See here when logged in

https://d4pinky.wixsite.com/m100-documents

Electrical manual, Sheet 9 towards the end of the manual.

[roy666]

Chaps - just to finish this off; followed Dave's excellent advice and indeed it was the earth on both light holders. In fact, the light holders had also rusted inside (the contacts).. so I changed them out with some spares i had,.. and now all working. V weird that both went at similar times. Thanks again for the input. cheers M

[Tuga2112]

i think grounding for all the lights on the rear clusters is done via the same black wire in the harness. (which would stop the whole cluster from working) but i may be wrong there.

HEY
i deserve credit too