Land Rover Lightweight

Infra-red Working

The Infra-red setup in a British Military Vehicle was designed so that with the throw of one switch, ALL lights other than the dipped Headlights were disabled. The Dipped Headlights had a cover that could be placed over them to ensure that only Infrared light was displayed. The driver would then don Infrared goggles to enable him to see. This set up was very complicated and you only have to look at the differences in the circuit diagrams between IR and non-IR vehicles to see the complexity added. There were problems with the reliability of the IR Switch and the IR Relay and Wiring had a habit of burning out!

In the OFF position terminals 1 and 5 are connected.  Terminal 1 carries the feed to the Dipped Headlights and terminal 3 has the feed from the 6-way Light switch i.e. the Dipped Headlamps are controlled by the light switch.

 In the ON position the Dipped Headlamp feed (terminal1) is broken away from the Light Switch (terminal 5) and is now fed from the power feed to the IR Switch (terminal 3) This power feed is now also connected to terminal 2 which feeds the IR Relays.

These switches were infamous for their failure rate. If the switch failed you basically could lose all the lights. When I acquired my switch recently, the contact between terminals 1 and 5 in the OFF position was lost. This would mean in the normal working condition the feed to the Dipped Headlights from the 6-way Light Switch would be missing i.e. no Dipped Headlights.
In my late 1983 Series 3 Lightweight the switch was removed and wired out, probably from manufacture.

Click on image for full size Photo

A cautionary Tale!

I used to blame my kids for fiddling with switches and the inhibitor was always a big attraction.

However the (wet) day after I was left in the dark on a wet night in Snowdonia . I had no alternative but to investigate with help of my AVO meter.

I ended up filing the alloy rivet heads of the side to disassemble, the sliding contacts were clean and even had a bit of petroleum jelly on. However I was able to get to work with the resistance scale and found that there was no need to have taken it apart. They don't often burn out - if you disassemble just put a blob of Araldite where the rivet head was before you filed it off.

I have found the same problem a few times , look at the small washers at the copper staked terminals. IIRC the washers may be tin coated but they corrode against the copper and the small residue has a high resistance stopping current flow. To get them working again all you do is turn the washer with a small screwdriver (may require a small hammer tap) , this gets a good electrical contact and then just WD40 spray or smear with Vaseline.

I suspect the problem is worse on a Lightweight because the weather seal between vent panel and bulkhead hardly works and in heavy rain you can get water down the back of the panels (and everywhere else)

'FULLTILT' from the Ex-Military Land Rover Association Forum

The offending washer!
I did as FULLTILT suggested, and lo and behold fixed!
What I hadn't realised was that the internal switch connection in the OFF position was between terminals 5 and 6 and that the linking bar between terminals 6 and 1 carried the circuit, hence the faulty connection at the washer can cause the problem.

Slightly different version of the switch