Something I noticed about the Mk2 brake lights

[AntonovAN12]

The other day I was behind another Mk2 in a queue of traffic. It was after dark so their red tail lights were on.

When they stopped, one brake light came on as normal. On the other side, the tail light went out, but no brake light came on.

The Mk2 rear lights use the twin filament 380 P21/5W bulbs for the tail/brake lights. I assume that the bulbs must be wired up so that the tail light filament is switched off when the brake pedal is pressed. On that particular car, the brake light filament was likely burned out. The brake filament always fails first on those bulbs. That is not surprising as it is switched on and off so many more times than the other.

I have never seen the tail light switch off when braking on any other vehicle. There must be a reason the designers wired it up in that way. To me it seems like an over complication of the circuit.

[MarkRS9]

I have seen the tail light go out when the brake light is blown on many different makes of cars.

[mjt]

4 hours ago, AntonovAN12 said:

The other day I was behind another Mk2 in a queue of traffic. It was after dark so their red tail lights were on.

When they stopped, one brake light came on as normal. On the other side, the tail light went out, but no brake light came on.

This would happen if the earth for the light unit is broken on that side. Then, with 12V on the tail light and 12V on the brake light there is no voltage difference across the two, which, in the absence of the earth, are effectively in series (see RH diagram).

When the brake light is off there is a path to earth for the tail light via the brake light filament on that side then through the brake light filament on the other side to the earh of that light unit (see LH diagram). Although this means there are 3 filaments in series the brake light filaments are a much lower resistance than the tail light so the slight dimming this produces is probably barely noticeable.

 

[isetta]

I agree it is most likely faulty earth.  You would also see odd behaviour when indicators switched on, making red lights flash.  Many cars over the years have this kind of problems.   Recently fixed a friends bmw 325 which had same problem .  Wired new earth.  It was a very common problem on mk3 escorts where the plug connector on rear lamp seemed to get corroded up.  Sometimes connectors show signs of overheating.  Earth is worst as it carries highest current. Luckily earth is easiest to rewire.  If you want to do it quickly get a length of wire. Strip some insulation off each end. Wedge one end of bare wire down the side of bulb in the bulholder. So it is against metal side of bulb, not touching any terminals at the base.  Fix other end to somewhere where it will make contact with car body.  Might involving filing some paint off so can connect with bare metal of car body.  There are better ways of a permanent fix but this is quick eg you have a mot tomorrow and just found the problem

[isetta]

Ps, make sure wire is at least as thick as the original earth wire. 

[AntonovAN12]

Thanks for the replies. I hadn't considered an earth problem. I have had that happen on another car before. Cleaning the rear light housing pins sorted it.

[Janner7859]

I had a Renault Megane a few years ago that failed it's MOT because the tail lights dimmed when the indicators flashed. That was caused by a bad earth on the rear light cluster.

[StephenFord]

9 hours ago, Janner7859 said:

... That was caused by a bad earth ...

Potentially an increasing issue - St Greta has been warning of a bad Earth now for years, apparently it's an emergency 🤣