Dim-dip bypass

WET wintry weather always seems to bring plenty of electrical problems with old Land Rovers. The bullet connectors favoured by Solihull for many years are non-waterproof and prone to corrosion: light units can suffer from corroded contacts and extensive use of lights, wipers and heater will tend to finish off alternators and batteries that are not in the best of health. As a result, when a vehicle comes in for a service and MOT test I can usually reckon on spending a bit of time persuading all the lights, wipers and horn to work properly.

Corroded connectors and earth tags cause most of the problems. I have plenty of the black-cased bullet connector blocks on the shelf: a new connector, cleaning the bullet terminals with fine abrasive paper and a squirt of contact cleaner will fix most lighting problems in short order. The biggest problem is trying to access the electrics in the first place. Defender Station Wagons usually have the rear seat frames bolted so close to the rear light covers that they are impossible to remove without taking the seats out. At the other end of the vehicle the connectors for the front lights are tucked away behind the headlight bowl and not easy to access. You can just about get to them with your hands, but you can’t then actually see anything. Older vehicles have solid-backed headlamp bowls. Taking these out will give good access to the tangle of multicoloured spaghetti behind them, but to do this you have to remove the headlight surrounds, which on most Defenders are held in place by the sidelight and indicator units. The screws for these will have rusted to shapeless blobs, and even if the light units worked before you disturbed them, they will probably have stopped working when you put everything back together. The most likely cause of this will be a corroded connector: to fix this you will need to take the headlight bowl out again.

The old-but-good One Ten pick-up that came into the workshop a couple of weeks ago had more than its share of electrical problems. The main job on the list was a new rear crossmember: the original had done well to last 30 years but was now looking rather moth-eaten along the lower edge and, crucially, around the mounting points for the tow hitch. Rear crossmember replacement on a One Ten is physically hard and time-consuming work but not technically difficult. The fuel tank has to come out to do the job and I always worry about whether an old tank might spring a leak when disturbed. This particular vehicle had a brand-new tank, so no worries on that score.

With the vehicle restored to full structural health I turned my attention to the Mot-related electrics. A quick light check showed that I was missing both front indicators, one sidelight and one tail-light. The headlights did not always immediately extinguish when switched off, which is not a problem I had come across before. And both headlight units were cracked and would need replacing. With the headlights and bowls out of the way I decided to sort out the other problems first.

I assumed that the non-extinguishing headlights must be caused by a faulty switch. Headlights on these vehicles are wired direct through the column switches (lighting and main/dip) rather than via a relay, and the lighting switch often gives trouble in old age. I normally keep one on the shelf. The non-working sidelight turned out to be caused by something I see far too much of – a melted bulb holder. The side and indicator lights on Defenders and earlier models are cheap and nasty pieces of under-engineered junk. The bulb is retained by two short projecting pins that engage in slots moulded into the plastic body of the light unit. The bulb gets hot, melts the plastic and the sprung contact on the end of the bulb pushes it out of the body until it is no longer making good electrical contact. The only solution is to replace the light unit with a new one, which is what I did.

The non-working indicators were both caused by corroded bullet connectors: the tail-light had furry green corrosion on the earth contact inside the bulb holder. All routine stuff and easily sorted. I plugged the bulbs for the new headlights into their holders, quickly checked that they both worked on dip and main beam, fitted the headlight bowls, headlights, surrounds and light units, checked the headlight alignment and then did one final test of sidelights and indicators to ensure I had not disturbed anything. At this point things went weird. With the sidelights on, if I switched on the left or right indicator both headlights started flashing. About the only place where there is any potential interaction between the headlight and indicator circuits is in the combined indicator/dip/horn switch on the steering column. I swapped this for a new one and it made no difference.

Faced with baffling electrical faults I find the best approach is to make a cup of tea and then sit down and study the wiring diagram. I fairly quickly identified the cause of the problem. Back in the 1980s, someone in authority decided that vehicles sold in the United Kingdom should be fitted with a ‘dim-dip’ system, which dimly illuminated the headlights whenever the sidelights were switched on with the engine running. Dipped beam headlights were apparently considered too bright for night driving in built-up areas with streetlights. The dim-dip requirement was eventually binned, but not before a very large number of cars and commercial vehicles had been