Series III FFR brake light wiring

[gromit]

I'm having a problem working out the wiring for my FFR brake lights which don't work.
It appears that there is both a pressure switch and also a mechanical switch on the pedal connected in series. Just to add to the confusion the supply to the mechanical switch seems to come from the blackout switch.
If I supply power directly to the mechanical switch the brake lights come on so the wiring to the rear of the vehicle is OK.

The pressure switch in the brake line appears to be faulty but.....it doesn't seem to be a Land Rover part. Was this a locally sourced component ? Does anyone know what it came from ?
The switch has two round pins and a plug in the harness that latches onto the switch. I can post a picture if it helps.
I guess I could replace the switch but it would be handy to know the thread size so I can work out if a standard LR switch can be used in it's place and I cut the plug off the harness ?

I started looking into this problem at the weekend but oil running from the weep hole at the back of the transmission brake meant I had a new problem to deal with first !

Colin

[Mick_Marsh]

Have you determined the switches are the problem?
I'd be looking for voltage at the tail light wiring first.

[GGG]

Gromit, I don't think that the switch in the hydraulic line involves the stop lights. It would appear that the stop light switch (41) is fed directly from the fuse box and switches the stop lights (64 and 65.) The hydraulic switch I think you will find is to operate the brake warning light when there is a loss of fluid in one circuit. Always open to correction of course but that is the way it works on my S3 GS.
Geoff O.

[gromit]

Have you determined the switches are the problem?
I'd be looking for voltage at the tail light wiring first.

Power supplied to the mechanical switch operates the brake lights so it's the supply to the switch which seems to come from the blackout switch that's 'missing'.

Colin

[gromit]

Gromit, I don't think that the switch in the hydraulic line involves the stop lights. It would appear that the stop light switch (41) is fed directly from the fuse box and switches the stop lights (64 and 65.) The hydraulic switch I think you will find is to operate the brake warning light when there is a loss of fluid in one circuit. Always open to correction of course but that is the way it works on my S3 GS.
Geoff O.

Hi Geoff,
I double checked the electrical drawings on here and the pressure switch seems to be in the reverse light circuit rather than the brake circuit (my mistake) but the reverse lights work although I couldn't seem to get the pressure switch to work ?!.
I followed the supply that goes to the mechanical switch back to the blackout light (not via the fuse panel).
Maybe there is another circuit layout or maybe the wiring has been tampered with in the past

Short term fix will be to wire power direct to the mechanical switch via a switched supply.

If the pressure switch is in the reverse light circuit is it Normally Closed and opens under pressure ? That way the reverse lights would go off when you brake ??

Colin

[GGG]

Colin, I noticed that about the reverse lights myself but cannot explain it as I see no reason for it. The hydraulic switch senses a differential between the two brake circuits and turns the warning light on if you lose pressure in one. I cannot see what that has to do with the reversing lights but hey, I am just an electrician. I have lived without it now for the nigh on 30 years that I have owned my (GS) series 3. One day when I am really bored I may investigate it. My blackout switch failed years ago and I bridged it out, yet another job for when I am really bored which doesn't look like happening in the near future.
If you go looking you can find a number of circuit diagrams for the s3 both civie and military which all seem to have subtle differences. All part of the fun I guess.
Geoff O.

[gromit]

Colin, I noticed that about the reverse lights myself but cannot explain it as I see no reason for it. The hydraulic switch senses a differential between the two brake circuits and turns the warning light on if you lose pressure in one. I cannot see what that has to do with the reversing lights but hey, I am just an electrician. I have lived without it now for the nigh on 30 years that I have owned my (GS) series 3. One day when I am really bored I may investigate it. My blackout switch failed years ago and I bridged it out, yet another job for when I am really bored which doesn't look like happening in the near future.
If you go looking you can find a number of circuit diagrams for the s3 both civie and military which all seem to have subtle differences. All part of the fun I guess.
Geoff O.

Hi Geoff,
I think my mistake was trying to follow the wiring diagrams and assuming they were 'correct' and also correct for my vehicle !

Thanks,

Colin