"Electricity has mysterious ways". Otto's Gospel 18.4

General Electricity Info

Moss Motors have excellent pages on the tricks of electrics on english cars. For wiring accessories through electro-magnetic switches (relays), check  this site. Also check links-pages.

Wipers Slowing Down

Symptom: Using intermittent switch on front wipers, they sometimes stop - like the job was too heavy. Running full time reveal no problem, only when going intermittent.

Remedy: Oiling linkages solve problem.

Flickering Alternator Warning Lights

Symptom: "Wading" on public roads, i.e. driving in heavy rain, led to the alternator warning light to flicker. Suddenly the steering also went quite stiff. And driving more slowly, I heard the drivebelts whining sound.

Remedy: The symptoms clear - drivebelts worn to a point where they slip. Renewed. For size, check Specs page.

Driving a small trip, I realized that two-or-three warning lights illuminated rather diffusely - sometimes lighting up, sometimes dark, sometimes only glowing. So, this ought to be some kind of electrical current "creeping" through water, bad contacting on clamps or something.

Trying to start the beast again after 10 minutes parking, the Lucas Gospel appear: Total Darkness. Everywhere. No current whatsoever anywhere. Not even in the interior lamp!

Suspected some kind of main fuse blown, but taking a few calls (Haynes manual left at home half an hour earlier), it seemed that an old diesel Disco do not have any main fuses (newer seems to have some kind of catastrophe-fuse under the driver seat). Must have been some kind of a short circuit to drain the battery in 10 minutes???

Pulling every cable in the engine compartment, the problem was easily detected. A nice little rubber-cover concealed the fact that the main cable running (from starter and to the bolt running through firewall to relays and fuses beneath the facia) was loose. The cable had slipped from the clamp, and was neatly hold in place by the rubber cover: looking pretty nice, but not giving any current whatsoever... Rectified on the spot.

Headlights Voltage Drops

The Disco has a wiring setup with 2 headlamp relays in cockpit. This setup is only to save switch from high Amps, as it uses puny wiring for both high and low Amps: 1,5mm².

With such a setup, there is a substantial voltage drop from batteri, through relay and the lots-of-meters wiring - up to nearly 1 volt drop. See table for details. This gives a yellowish, rather weak light - it is alway the last volt that gives the light its characteristics. A 1 volt drop reduce light with 50%, so the drop was substantial on Otto.

So, there is a need for thicker wiring to transport the current with lower loss of voltage - the thicker the better.

There are two options: Either rewire the whole circuits wither better/thicker cables. Or use the existing setup only as control-current for new relays. Last option chosen for its simplicity when doing it - but makes the whole circuit more complex, with lots of wires - especially in the cramped vicinity of the headlights. Check this site for wiring diagram.

To make the routing of wires as simple as possible, one double relay was chosen for each headlamp (one for low- and one for high beam). The old wiring going into the bulb, was rewired as input to relay. This gives an awful lot of wires in the vicinity of the headlamps, so careful colorcoding, strapping of wiring etc is paramount.

With the now-rebuilt circuit, there are 6 relays for the two headlights...:

The 2 original ones:  1 for low beam - now used to control the low beam circuit,1  for high beam - now used to control the high beam circuit.

The 4 new ones: 1 for low beam RH, 1 for low beam LH. 1 for high beam RH, 1 for high beam LH

A combination of 4 and 6 mm² cable used from battery to relay, and from relay to bulb - used the wires found in a plastic bag under-behind-beneath-between-something in the garage.

The result was this - somewhat dissappointing, but definetly giving more light than stock:

BEWARE:

On later Discoverys, there is a Multi Function Unit that can mock up what you're trying to do. Roman experienced a strange thing: The sidelight/parkinglight would only work on their own when engine is shut off. With engine running, it will not light up as it should - only run together with the dipped beam. This seems to happen due to the MFU... but still remains to be sorted out.

What Gauge - How Thick a Wire?

Choosing the correct gauge wire is important if you want to avoid a high voltage drop. To do this, you use the nomogram at right (click on picture) - found on a swedish site and reproduced here without permission).  1) Check the effect (Watt) of the item your planning to put on, using left column - go to middle column: Find the correct gauge wire (mm2).  2) Then: Find the length of the wire your planning to put on the vehicle. Multiply this with 2 (current has to go both ways); find your length in the right column.. Go to middle column: Find correct gauge wire to that length of wire (mm2). 3) Choose the thickest gauge in mm2 adviced by effect in Watt (#1) and length in meter (#2), as this will be the restricting factor to determine correct gauge.

Reverse Light Switch - Diff Lock Switch

On MOT-station, the reverse light was no longer operational. Bulbs were OK. Turned out one of the terminals had broken when pressure-hosing the vehicle's undercarriage minutes before going to MOT'ting. Removing MOT-failing oil meant MOT-problem because of faulty reverse switch. MOT officer said "ahhh your bulb has blown...you can renew it and no need to show it again, just renew the bulb". Of course no need to tell him bulbs were fine...

The two gearbox switches are about the same, and are tested same way, so procedure applies to both.

Reverse switch is on top of main gearbox, facing rearwards, right behind the gearlever linkage, directly below handbrake lever - i.e. if you should be able to see it, you will have to look on the left hand side, above the gearboxes - stretching an arm between gearbox and chassis; or you will have to peep over the handbrakedrum. You'll see or feel two electric wires coming into a brass item. It is really cramped and very low visibilty. Best choice is remove centre console and drill out the pop-rivets.

Mind: There is also a similar switch on top of transfer gearbox, close to the transfer lever linkage - follow a line rearwards from front propshaft; here you'll be able to see the wires, and it is relatively easy to get access. This is diff-lock switch.

More or less common faults and problems: 

To check switch (same for diff-lock), do this:

1) Are the wires connected to the switch terminals?

2) Disconnect battery negative earth. Find two pieces of wire with clamps on them; put them on the switch, and a test-meter onto them = better access. You can also make up your own instrument: a bulb, battery and wires. Closing a circuit will give light to the bulb. This is easiest way of adjusting switch seating depth. Tested the switch for continuity (ohm): Open loop while in forward gear (or diff unlocked) = correct .

3) Put gearlever in reverse (or lock diff, check it is really engaged), and test  the switch for continuity (ohm): Open loop while in reverse gear (or diff locked)= faulty switch or mounting of switch: an electric circuit should be closed when reverse is applied, thus giving reverse lights (or warning light for diff). So either the switch itself is faulty or the switch is not seated deep enough into gearbox - which seems quite illogic because it has worked fine for more than 10 years already.

The fix

The switch was renewed and adjusted while centre console was dismantled due to heater cleaning and overhaul. This is by far the easiest way to reach and adjust the switch, but it can be done from beneath as well. From above you'll have to drill out the pop-rivets on the metalframe of the rubber gaiter surrounding the gearlevers. Removed old switch, swapped old locking nut onto new switch, roughly put it into same position as on the old one, and screwed the switch into position The electronic multimeter was connected to the two terminals of the switch via to lengths of wire for ease of access, and switched to audible alarm for continuity. Gearlever in reverse, screw the switch into gearbox until buzzing appears. Put in neutral (no buzzing should sound), put in reverse (should buzz) - when it works correctly, turn the locknut to lock switch into position.

More Help

How to wire a trailer socket.

Installing a rear working lamp on tail door (from www.Discoweb.org)

Wiring diagram (pdf file download) for using the Land Rover Fog Lamp switch - by Mike Dodson  (at www.Discoweb.org)

Wiring diagram for 4 Hella 4000s on the roof rack using factory LR switches - by Paul D. Morgan (at www.Discoweb.org)

Wiring diagram for 4 Hella 4000s and one rear work lamp - by Greg P. (at www.Discoweb.org)