Ratel

[siiirhd88]

The 1968 model year on English vehicles imported to the U.S. (and some U.S. vehicles) was when the big safety changes started. These changes included the padded dash or dash surrounds, dual circuit brakes with warning lamp, breakaway window winders and rear view mirrors, rocker switches instead of toggles, hazard flashers and reverse lamps. The wiring diagrams for English cars from 1968 up can be a good reference.

Bob

[S3ute]

Neil I broke out the MGB manual and did some research for you. The test switch has 3 wires. The black/white wire goes to the brake pressure warning switch. The purple wire to the fuse box. And the black wire to a common ground with the instrument lights. Looking at more diagrams some test switches have two black/white wires, one for the switch and one for the bulb. Both these wires run directly to the brake pressure switch. I hope this helps.

Hello again - thanks for going to that trouble for me. Genuinely appreciated.

I think the first set up is for the simple type that just has a failure light but no push arrangement to test that the bulb is working. The second type - which is likely what I’m working with - is probably set up for the push test and temporary check on turning the ignition.

This latter type apparently uses the 6RA relay in the circuit for the ignition test. The anecdotal advice that I picked up off a UK forum is that power goes to the relay when the ignition is turned to the starter phase and this links an earth to the bulb circuit turning it on. Once the ignition is released to the on position that circuit is broken leaving the normal loop circuit to operate only if the switch on the brake unit activates to provide the earth. I’m not quite sure how the push test part of the switch works yet but assume that it also must trigger the relay circuit to earth the bulb. That or it requires a second 6RA relay which is more overkill.

It would probably be more accurate to call it a bulb test switch rather than brake fail test switch since that’s what is being tested rather than the brake pressure unit.

Cheers,

Neil

[S3ute]

The 1968 model year on English vehicles imported to the U.S. (and some U.S. vehicles) was when the big safety changes started. These changes included the padded dash or dash surrounds, dual circuit brakes with warning lamp, breakaway window winders and rear view mirrors, rocker switches instead of toggles, hazard flashers and reverse lamps. The wiring diagrams for English cars from 1968 up can be a good reference.

Bob

Thanks for that update on the changeover in equipment and styling.

I think some of those changes came a bit later on the MGBs sold to our market - perhaps the early 70’s. From memory the main visible changes when the MK 2 came on the local market were replacing the knock off hub spinners with a hexagon hub nut, straight gear lever and overdrive standard along with a heater-demister unit and the flat interior door catches. Pretty much everything else on the late MK 1’s came across - but I could be wrong as I never personally drove or rode in one. Unlike the Triumph TR6s and MGBs sold in the American market ours didn’t have the heavier rubber bumpers and updated dashboards for several years - by which time MGB and MGC sales had fallen away considerably in face of growing Japanese competition.

Anyway, I’ll see if I can find a circuit diagram for that later model with the brake test lamp.

Cheers,

Neil

[S3ute]

Hello again.

I Googled brake test light in MGB and a few useful things were identified.

This one explains how the system basically works on models without the ignition on test circuit.

https://youtu.be/v8ijl6k_1a8?si=yVpkalWhhDHrCKh0

It’s along the lines of what I was thinking which is a start.

Cheers,

Neil

[siiirhd88]

Yep, that video explains the early type of valve. For me the shuttle valve is just a few more connections that can leak, so I usually remove them when I replumb the brake lines.

Bob

[S3ute]

Yep, that video explains the early type of valve. For me the shuttle valve is just a few more connections that can leak, so I usually remove them when I replumb the brake lines.

Bob

Bob,

Thanks and point taken.

Having got thus far, I’ll persevere with it for a while longer but keeping in mind that your advice will be grounded in experience.

I may well look for a blocking bolt at some future stage if it all goes pear-shaped.

Cheers,

Neil

[S3ute]

Hello again,

Making a little bit of progress on the monkey puzzle today.

I pulled the sub-loom out of the packet and tried to reconcile the various wires in the tangle with the nice straight lines and logical order of the various circuit diagrams that I did find on the web. Once figuring I was getting almost on top of it I moved to Stage 2 which involved hooking up the various wires to a 12 volt battery and running current through each wire to see what it did and how close that came to the mental map that I’d run up in Stage 1.



This was generally successful in so far as I did work it out and managed not to fritz anything in the process. The coloured cable ties are there to avoid not having to redo it all again once I put it down for more than two minutes…..

It’s set up to connect to a relay from the ignition that creates an alternative earth return on start and hence complete the test light sequence - so much was known before.

What I’m still a little hazy on is how the test switch works within this arrangement. It has to link into that alternative earth return but by-pass the relay activated part of the circuit.



The two central spades of the switch are the positive and negative terminals of the light circuit. The positive takes its feed from a white wire hooked to the fuse box. The negative terminal connects to one of two black and white wires that run down to the switch at the brake pipes and makes the circuit if the switch activates to earth. The second black and white wire to that switch is the one mentioned before as part of the ignition activated by-pass. The two wires are linked at the switch on the pipe so the current loops back up to light panel to complete the circuit.

Now the test part of the failure circuit would normally use that same loop if the relay hadn’t got there first. So, I’m assuming that the remaining two terminals - outer right and left - must create an alternative earth circuit that is connected to the light but am a bit unclear on how it’s to do it. Running a current between the outer terminals gives a circuit but they appear to have no direct connection to the light. It must have a wire run from one of the terminals to an earth point near where the switch is mounted - another wire then from the second terminal to the input side of the relay in order to complete the earthed loop of that second black and white wire going down to the pipe switch.

That’s my thinking for now - I’ll put it to the test tomorrow unless an alternative comes to mind overnight.

Cheers,

Neil

[series guy]

Something that I just remembered is that later on (I’m not sure of the exact year) the dash switch looked the same but there was no press to test function. At first we thought it was a defective switch but it was not. That feature was eliminated.

[siiirhd88]

The brake failure circuit was a lot more complicated on later trucks (and MGBs) where the brake failure warning lamp was automatically tested while the starter was cranking during vehicle start. On the MGB a diode was used on the white with red tracer wire (starter crank) input to the circuit, while on the Series truck a Lucas 6RA relay was used. It looks like the trucks that had the auto warning lamp check used a different shuttle valve switch that grounded at the switch on a failure. Below is a section of the wiring diagram showing the relay and a section from the workshop manual for a truck with additional switches.
I'll keep hunting for more info.

Bob

[S3ute]

Something that I just remembered is that later on (I’m not sure of the exact year) the dash switch looked the same but there was no press to test function. At first we thought it was a defective switch but it was not. That feature was eliminated.

Thanks.

Yes, from digging through some MG forum threads and YouTube clips yesterday that was my conclusion too. The MGBs didn’t have the same Lucas panel switch with the test function. The switch I have is part 589189.

I’m now reasonably sure that I have nutted out how it all works.

Cheers,

Neil

[S3ute]

The brake failure circuit was a lot more complicated on later trucks (and MGBs) where the brake failure warning lamp was automatically tested while the starter was cranking during vehicle start. On the MGB a diode was used on the white with red tracer wire (starter crank) input to the circuit, while on the Series truck a Lucas 6RA relay was used. It looks like the trucks that had the auto warning lamp check used a different shuttle valve switch that grounded at the switch on a failure. Below is a section of the wiring diagram showing the relay and a section from the workshop manual for a truck with additional switches.
I'll keep hunting for more info.

Bob

Bob,

Thanks again for your help.

I did find that diagram yesterday on one of the UK Rover forums. On looking at the Autosparks sub-loom I recognised that that’s how it is configured to work with a 6RA relay which I now have. It’s also compatible with the failure switch that I have on the brake pipes.

The difference is that the sub-loom is designed to use the simpler panel switch that only has the “brake” warning lamp without the press to test function - Lucas part 589189. I have seen another diagram in an MG forum where the test switch fits at the end of the loop to provide the earth to complete the circuit to power the light.

My thinking to make it work with the 6RA relay in place is to put a double adaptor on the relay spade shown as C1 and run an earth from there through the two outer spades on the test switch. That way pressing the test will complete the circuit and activate the light - at the same time if the ignition is turned to start the earth will go through the relay and activate the light.

That should work.

Cheers,

Neil

[S3ute]

Hello again.

Wiring diagrams are a bit like subway maps that give a general but fairly abstract picture of how something more complicated theoretically works. The diagram in the brake failure section of the green bible for both earlier and later trucks is a case in point. It is useful for generally understanding how the circuit works but not quite so in terms of how the actual componentry fits together and works on the truck itself.

This is my take on what’s involved using the Autosparks sub-loom that comes with their wiring kits. This also assumes that the test switch/light is being used instead of a single brake fail warning light.

First off, going to the diagram - 3d is the brake fail warning light and 3a is the shuttle switch on the brake pipe junction unit down on the chassis. Think of 3b as being the test switch part of the test switch/lamp and pretend that 3c doesn’t exist. Move 3b so that it piggy backs to terminal C1 on the relay. If the brake unit fails the shuttle switch at 3a earths and the light comes on. If the test switch is pushed at 3b it earths the circuit and the light comes on. Finally, if the ignition is turned to start the relay circuit W1W2 closes and the earth is completed between C1C2 and the light comes on.

That’s the theoretical setup out of the way.

What comes out of the packet is a bit more complicated but essentially follows those rules.

The sub-loom has a main set of wires running top to bottom for about two feet. About half way up there is a third end with three wires that are inputs and a single wire coming back out that runs to earth via a ring terminal.

At the bottom end of the main set of wires are two black and white wires with fine female connectors which plug on to the shuttle switch. If the brake system loses pressure this switch earths so the two black and white wires would be earthed. You can’t see it but the two male pins inside the shuttle switch are connected so power flowing down one can flow back up the other - in practice this only happens one way with power down one and up the other but not vice versa because of the way the rest of the circuit is configured.

At the middle there is a white input wire which is connected to a live power source (usually at the fuse box) which runs up to the top to become the positive input to the warning light. One of the two black and white wires from the shuttle valve connects to the negative terminal of that light. This is how the fail circuit operates - earth at the shuttle switch and the light comes on. The bulb holder is removed and the two wires are connected to the two centre blades of the test switch. A white wire with a red trace also enters the middle of the sub-loom - this is powered by the start position of the ignition and attaches to terminal W1 on the 6RA relay which is mounted behind the auxiliary panel where the test switch is also located. The third black input wire (coming from an earth point somewhere convenient) has a female blade connector which attaches to terminal W2 to complete the relay input circuit.

At the top end there are two more wires to connect - the second black and white wire returning from the shuttle switch has a female spade connector which attaches to terminal C1 of the relay. This wire has an inline fuse holder. A black wire with another female spade connector attaches to terminal C2 of the relay. The black wire at the middle of the top to bottom wire set with the ring terminal is the other end of that wire attached to terminal C2 and attached to a convenient earth somewhere near the dash completing the earth circuit - turn the ignition and the light comes on because the power flowing down the first black and white wire from the light returns to the relay via the second black and white wire.

That just leaves the test switch - if your model just has the brake fail light you could leave it there - if the switch 3b that we nominally moved across to terminal C1 on the relay was connected to that terminal with a piggy back connector and earthed then that circuit would be complete. Press the switch and the light circuit is earthed and illuminated.

So, that’s how it works on the truck.

Cheers,

Neil

[siiirhd88]

My '80 109 had the brake warning lamp in the small two piece aux panel in the center below the upper dash pad. IIRC it was rectangular and the same size as the MG used, but was only a lamp and had no switch function. I used that location and modified the opening for a hazard rocker switch from a '68 MG.

Interesting that the Triumph cars didn't have a brake warning lamp test circuit until 1976 or so. I think they used a 5 spade relay in that 'lit when cranking' circuit.

Bob

[series guy]

I have to say that it cracks me up when single mark enthusiasts are surprised when you mention that a part from another make fits their car. Like it’s shocking news that all the British manufacturers shopped at the Joe Lucas store.

[S3ute]

I have to say that it cracks me up when single mark enthusiasts are surprised when you mention that a part from another make fits their car. Like it’s shocking news that all the British manufacturers shopped at the Joe Lucas store.

Yes, back in the 70’s when I was looking for parts for my old Series 1 and/or MGB and later a Moke I’d always try to nominate some less popular British or Australian vehicle that had the same part because the prices would invariably be much lower. In many cases less than half for the same part.

Sort of done the same with the present truck for slightly different reasons. Genuine Lucas parts for Rovers generally attract a premium price whereas the same parts fitted to Minis, Austins or whatever are still cheaper - plus in some cases the only viable source of supply.

Cheers,

Neil