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brake fail light
https://ausmini.com/forums/viewtopic.php?f=2&t=87038		 Page 1 of 1
Author:  1970cooper [ Thu Aug 28, 2014 7:35 pm ]
Post subject:  brake fail light
My 1978 ls brake fail light is on all the time, l am hoping when I bleed the brakes this light will go out.
Does anyone know what puts this light on.i cannot see any wiring around the master cylinder area and my manual does not mention it.
Author:  phillb [ Thu Aug 28, 2014 7:51 pm ]
Post subject:  Re: brake fail light
I seem to remember it having something to do with a split brake system, so there must be a sensor somewhere further in the system.
Author:  boomini [ Thu Aug 28, 2014 8:12 pm ]
Post subject:  Re: brake fail light
Should be a sensor on the master. The light will come on with ign but as soon as the car starts will go out as its wired through the alternator light on the dash.
Author:  boomini [ Thu Aug 28, 2014 8:13 pm ]
Post subject:  Re: brake fail light
Earlier cars had a sensor on the brake t piece on the rh side of the subframe maybe check there.
Author:  slacko [ Thu Aug 28, 2014 11:42 pm ]
Post subject:  Re: brake fail light
GMC 227 master cylinder has a fluid level float switch

Where a pressure failure switch was fitted
cut off connector plug , twist wires together and crimp a lucar connector to the wires
connect to one side of the floatswitch cap
connect a wire with lucar connector to the over side of the cap and run this to a earth
Author:  phillb [ Fri Aug 29, 2014 9:14 am ]
Post subject:  Re: brake fail light
boomini wrote:
Should be a sensor on the master. The light will come on with ign but as soon as the car starts will go out as its wired through the alternator light on the dash.


Yes good point. I had an Ls with this light and remember now it goes out once the car starts. As suggested I seem to remember sensor at t piece or elsewhere to master as mentioned.
Author:  Morris 1100 [ Fri Aug 29, 2014 9:42 am ]
Post subject:  Re: brake fail light
I think it is a differential pressure switch that turns on the light, when the pressure in one side exceeds the other side it closes the low side and turns on the light.
From memory it was built into the master cylinder. I recall the plug being positioned between the two outlet pipes.
Author:  1970cooper [ Fri Aug 29, 2014 10:45 am ]
Post subject:  Re: brake fail light
My original master cylinder had no float switch,but the GMC227 M/cylinder does.

I could wire the float switch up to this light,but there must be another switch somewhere that I have missed.
I suspect it's near the brake pressure valve on bulkhead.
Author:  slacko [ Fri Aug 29, 2014 5:00 pm ]
Post subject:  Re: brake fail light
As morris has said
on the old master cylinder the pressure switch was between the two out lets
GMC227 does not have this , it has a fluid level switch , which replaces the pressure switch
hence the wiring mod
there is no pressure switch any where else that I know of
Author:  Mick [ Fri Aug 29, 2014 5:30 pm ]
Post subject:  Re: brake fail light
Its just another warning light to look at as you spear off the road into the trees without any brakes :)
Author:  MiniBrad [ Fri Aug 29, 2014 6:21 pm ]
Post subject:  Re: brake fail light
Mick wrote:
Its just another warning light to look at as you spear off the road into the trees without any brakes :)



Who needs brakes....

Cheers Brad
Author:  1970cooper [ Fri Aug 29, 2014 6:34 pm ]
Post subject:  Re: brake fail light
Yeah Mick your not wrong more distractions.

Hi slacko,I'll have another look around the engine bay tomorrow for the old wiring.
I think the previous owner may have removed this wiring.
The brake fail light wire must be grounded somewhere as the light is always on.
If I cannot find the old wire I will follow the wire back from the light and connect the ground to the float switch.
Author:  Morris 1100 [ Fri Aug 29, 2014 6:47 pm ]
Post subject:  Re: brake fail light
From memory the wiring for the brake fail switch is a separate pair of wires that don't connect to the main harness.
Power from the fusebox to the dash light, earth from the switch on the master cylinder to the light.
Look at the fusebox and see if you can spot a wire running towards the firewall that joins another wire near the master cylinder area.

Really dim memory time... I think they may have been white and white with a trace?[/clutchingatstraws]
Author:  1970cooper [ Sat Aug 30, 2014 4:24 pm ]
Post subject:  Re: brake fail light
The earth wire for brake fail light was permanently grounded,that is why the light was on all the time.
The earth wire for this light was black with white tracer and yes it was separate from the main loom.

I have connected the float switch in the new master cylinder via this earth and light works as it should.

Thanks for all the info.
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