Failed MoT on rear light issue - where's the earth point?
 

My 1988 360 GLE failed its first MoT in two years - two very minor issues.

With the headlights on, pressing the brake turns off the bulb completely on the right hand side. I'm thinking this is down to a bad earth. Where are the rear lights unearthed to, or am I looking in the wrong place?

Any advice appreciated.

Rather than try finding the original earth, could you take a new one from the offending light, to at least test your theory?

I could attempt to, I will be honest in that electrics are not my strong point at all, I'm more mechanically-minded. But I shall try to see what I can do.

Might it just be a dodgy bulb, Joseph? Worth substituting it with a known good (new) one to eliminate it before delving deeper? I've had bulbs that appeared sound playing silly b's with me in the past ... :regular_smile:

Regards, John.

If memory serves, the 360 rear lights have a crude multiplug to connect the loom to the clusters, very much like the Fords of the time.

The fault sounds very much like a bad earth on the stop/tail bulb, could just be the bayonet of the bulb is corroded and needs cleaning or the bulbholder itself or if my memory is correct about the multiplug, the contacts in that and on the cluster - clean them with a soft wire brush, emery cloth or similar but not sandpaper. Once clean and normal service is resumed, disconnect the multiplug from the cluster, smear silicone grease on the contacts of both and reconnect with a final test. The silicone grease will exclude more moisture from corroding the contacts again. :thumbs_up:

The galvanised strip in those lights is pretty crude as well. As above, a good clean, contact cleaner spray and don't forget to emery the bulb contacts too and it should be fine. They are prone to water ingress too. Our rear lights have a tiny hole drilled in the external surface to allow rainwater to drain out, otherwise the light is filtered through a pool of water.

Well having went from one working brake light with lights on, I've now got no working brake lights with lights switched on!

Tried replacing bulbs but haven't got a spare of each, switched bulbs round and this is what happened. Old cars eh....

"Borrow" a pair of bulbs from the rest of your fleet there - once you've proved the point that the bulbs are defunct, you can get your car MoT'd and then buy a 10-pack of bulbs on Amazon, ebay or similar for a couple of quid delivered.

Are the circuits individually fused, Joseph, and if so, are they intact? If there is a fault with the light unit it is also possible that it/they may have 'blown'. The advice re cleaning bulb holders is sound, but the fault may have caused the fuse to 'blow'.

Do you have a high-level brake light? If so, is that working?

Joe H. is also correct in pointing to water ingress; we had the same problem with a 744 and resolved it in a similar way - a 1.5 mil bit in a hand brace and very light pressure, IIRC.

Regards, John.

Thanks folks for all replies so far -

Replaced both with two good bulbs I managed to get, and we are back to the right hand brake light not working when tail lights are on. 3rd level 'middle' brake light works fine.
I'll check re fuses to see if this is the case, I lightly cleaned bulb holders but to no avail yet.