Series 3 brake failure warning light/test switch wiring

[jvsimmons]

My 1983 S3 has a brake failure warning light/test switch. It has never worked as long as I have owned the truck. I am now trying get this circuit to work. I cannot figure out where the wires in the loom go. There is a basic functional diagram on page 233 of the LR S3 repair manual and I have located other more detailed wiring diagrams. The problem is that the wiring loom for this circuit has more wires than what is displayed in the diagrams. There only three pieces to the system. 1) The warning light/test switch unit has 4 terminals. 2) the shuttle valve and switch with two terminals. 3) The wiring loom consisting of two wires white/black, one white wire, one black wire and one white/red wire.
My understanding is that the two white/black wires go from the switch at the shuttle valve to the upper and lower terminals on the warning light unit. The white wire goes from the fuse panel to the second terminal from the top of the warning light unit and provides + power. The black wire goes to the other terminal, provides ground. I cannot figure out where the white/red wire goes. The circuit does not work with the four wires connected to the warning light/test switch terminals.
Since I am the latest owner and know very little about it's history could a previous owner removed this function.....is there a missing component? Some UK based forums that I have found on this topic make reference to a relay --- but I cannot find in any wiring diagrams a relay in this circuit.
Anyone know about this circuit and where the wires go - PLEASE help.

[vlad_d]

Ha! You give the Series 3 too much credit! XD
it doesn’t have any computers, or complicated on-board diagnostics. Even in - what year did you say- 83? I added/upgraded my ‘73 to have power brakes and a dual-line brakes system(from later Series). It’s still basically a tractor with 1960’s tech. And I say that lovingly (I love my square tractor!) Okay, here’s my advice:

I’ve been over the wiring diagrams and the brake system from scratch. You can get into arguments with people on forums if the wire should be green or white, etc. but that doesn’t matter. Know the systems involved, and you can have confidence of a good repair(and you should, it’s your brakes!

The brake warning light is just that…a light. It doesn’t have any relays. Relays are for switching to higher voltage(think: small voltage switch kicks off higher voltage circuit. That’s a relay.) that doesn’t happen on that circuit. The shuttle valve is your safety device - it saves you in the event of one brake line severing. It’s a fully hydronic, sealed setup. No electrical involved. The “dummy light” just tells you it happened…which you probably already know because either a puddle of brake fluid is in your driveway, or you suddenly feel less grab from your brakes. So, the light is not critical. But, I hate broken stuff, so let’s fix it.

The shuttle valve has shuttle that moves back and forth in a cylinder. There is a round detent in it, at the middle. A ball bearing fits there, and a spring keeps it in place. When one side of the line loses pressure, the pressure from the other side pushes the shuttle off center, and the ball and spring click on the switch on the outside. This turns on the dash light. That’s the basics. The two hydronic lines (front and back) are isolated, so you still have at least one in an emergency(hope it’s the front).

I have found the hardest part of fixing car wiring (and especially on Series) is dealing with the rat’s nest of greasy wires that don’t follow the dang wiring diagram. The circuits are dead simple. Staring at greasy black wires in an engine bay sucks. But it’s a simple circuit. You could even just rewire it with fresh wires. It might be easier than splicing into existing, corroded wires.

The fuse box on series 3 are not even a modern fuse box. Depending on your model, it’s probably like mine…more a “bus bar” with 4 fuses. Hot battery power comes into the bottom bar, and you have 4 fuses connecting it with the top bar. At the top bar are 4 circuits, going out to all your things. If you want it “key operated” you find the line going to the ignition ACCessory hot. You can test this with a voltmeter, or look at the colors at the ignition. Or, you can tap a new line directly from the fuse box, just put an eye loop and connect it to one of the four circuits. Or, connect it to the un-used 5th, etc. The line just lights a low wattage warning light, so it won’t fry anything, and it will be fused. Even if you use the extra bus circuit, I doubt a low wattage light bulb would fry that circuit. The bulb is as strong as the fuse (less, actually) so it probably acts as a fuse itself, burning out before damage.

Get a voltmeter, do some double-checking, but it’s not crazy difficult. The worst part is finding old wires. I suggest just running 2 brand new wires to the switch, 2 to the light, and then connect them to the fuse box and a good ground screw. Honestly, that’s probably what the “4 wires” are all about - 2 to the light , one to your power(either keyed ACC or directly off the fuse box) and a ground. You can test this with a voltmeter (aftermarket switches don’t follow wiring diagram colors). And do the next guy a favor and try to use the same color wires as your diagram (I know it sucks because Napa or whatever doesn’t sell custom striped wires, but you can order them online for like $20). Just get the right colors and you’ll have a good repair that’s factory correct.

You can test the setup by leaving the 2 wires to the shuttle valve off until the end. Touch the wires and your light should come on. If that happens, you’ve done it right.

Good luck! Remember, it’s just a light.

[siiirhd88]

There are several different warning lights and test switches used on the brake failure circuit. Since your wiring has the white with red trace wire, your circuit tests the warning light bulb when the engine is cranking during start. This does require a Lucas relay, a 6CR, I think, and mounts behind the central aux dash panel where the warning lamp is located. There are a few different wiring schemes, depending on the warning lamp, as some are just a lamp and others are a 'push to test' switch and lamp that checks the bulb when depressed.

One member on the 'Guns and Rovers' forum restoring a SIII in Australia recently made his circuit operational. There are a number of entries on several Land Rover forums about wiring this circuit if you do a G oo gle search.

I usually eliminate the shuttle valve and associated wiring on our trucks.

Bob

[jvsimmons]

Thanks for the two replies. Yes there were at least a couple of brake failure warning light/test switch configurations used by Land Rover for the Series trucks......tractors if you prefer. One configuration has just a warning light, one wire from the shuttle valve and a relay, the later configuration ( and the one that I have) has a failure light and a test switch ( to test if the bulb works and that there is continuity thru the system) with two wires from the shuttle valve ( there is no relay in this configuration). I believe the military Lightweights had the later configuration. While my late S3 has the later configuration it has the wiring harness(original) from the early configuration. Fortunately, LR was not the only British automotive company to use a brake failure system in the 70s and later. I located a tech sheet on this system for MGs. A link follows. Using this information I was able to get my system working with the correct wiring attachments.
Not only were the wiring attachments corrected at the warning light/test switch but I also discovered that the switch at the shuttle valve did not work. I took it apart, cleaned the two connection pins, relocated the internal connection to the pins ( so that when the plunger in the switch was depressed it made contact with the two pins) - very simple switch and easy to repair. Problem solved! Hope that others wanting to get their dual line brake warning light/test switch to work look at the following tech sheet for MGs.

https://mossmotors.com/media/instructions/141-280.pdf?srsltid=AfmBOorc6UuTrgWD9qQFKga647dRePRuRG-U98UxJzaZoc5jFJH5foRB

Jim

[jimrr]

I did as vlad mentioned, bypassing the old wires and jumped past them for the fuel/temp yesterday, and many before! I have the green factory manuals. bought new at Roland Motors in seattle about 1976 and I have Haynes and something else. You can not always go by the book but as mentioned it is pretty basic stuff and if it is a sunny day you can always manage to make it work. Lucas (aka 'prince of darkness') i've always found to be ok wiring not so deserving of its moniker.

[vlad_d]

Well done! And good advice on the other circuit types. Thinking about it, it makes sense to have a test circuit for the bulb…because a dead bulb would read as “no problem” and that’s not much help. Glad to see LR /S3 improved since 1970’s.

I was also going to suggest you give the guys at AutoSparks (in the UK) a call. They sell some decent reproduction wiring harnesses( I got one in the garage I’m waiting for good weather to finally put in). They have some optional pigtails and accessory looms, too. I thought I saw one for the Shuttle valve and light. Either way, they might be a good resource to keep in mind.

But you solved it! Cheers…

[vlad_d]

You’re going to hate me. I was double checking they had it (didn’t want to post to be wrong) and here it is at about $14:

https://www.autosparks.co.uk/land-rover-series-3-brake-fail-wiring-harness

I’m sure you’ll need some other wiring later - we all do eventually . Not sure our hosts stock many wiring looms/kits, so hopefully they don’t mind me sharing an outside resource.