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Old 04-17-2019, 09:40 PM   #1
lutronjim
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Brake warning light ?

I just had to put a new cylinder and switch in my 70 ss. When I go to crank on switch, the brake warning light comes on - didn't with old switch. I assumed this is like a bulb check.

I thought the proportioning value grounded one side of the bulb turning on the light with 12+ on the other side coming from the switch when the front or rear braked lost pressure.

How is the bulb coming on when switch is in the crank position? I couldn't test switch as I had to drill cylinder down past the 'pegs' because the key wouldn't turn left and when i removed switch it blew apart.

I am also seeing oil pressure light when I turn switch to accessory position - old switch wouldn't turn there. I thought accessory would just power things like radio.

Thanks
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Old 04-18-2019, 08:41 AM   #2
rpmerf
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Re: Brake warning light ?

The brake warning light is connected to the start position on the electrical switch. I think the coolant light too if you have a light instead of a gauge. There are a couple other threads with this topic.

Gauges and HVAC might be on the accessory circuit, so that they are not causing a load while cranking.

There are 4 circuits in the car - always on, accessory, run (coil), and start (starter solenoid.

Accessory on the ign switch should be connected to the accessory circuit.
ON on the ign switch should be connected to the accessory and run circuits.
START on the ign switch should be connected to the run and start circuits.
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Old 04-18-2019, 08:54 AM   #3
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Re: Brake warning light ?

http://67-72chevytrucks.com/vboard/s...d.php?t=741245

Some good info here. Looks like gauges and warning lights should be on the run circuit. Makes me wonder what might had happened with the fuse panel, ignition switch, and wiring. Might have to hit up some things with a test light.
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Old 04-18-2019, 10:16 PM   #4
lutronjim
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Re: Brake warning light ?

Sorry I still don't understand. For the check brake light to come on it needs power and ground. Normally the ground is supplied by the proportion value if brakes are not 'balanced'.

I am seeing brake light coming on when switch is in start position and not when in run position. Brakes are fine - so how is warning light getting a ground when swicth is is start position?
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Old 04-18-2019, 11:10 PM   #5
mongocanfly
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Re: Brake warning light ?

maybe this will help
http://67-72chevytrucks.com/vboard/s...d.php?t=425922

http://67-72chevytrucks.com/vboard/s...d.php?t=462618
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Old 04-20-2019, 10:41 AM   #6
kipps
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Re: Brake warning light ?

I assume your truck is similar to a squarebody in this aspect. On a squarebody, the ignition switch has two pins that are isolated from all others. These two pins are connected to each other in the 'crank' position. One connects to ground, and the other connects to the brake light on the dash.

So basically, the ignition switch is switching the hot for all operations except for this one. In this case, the ignition switch is switching the ground.

The brake light is also grounded through the switch in the proportioning valve. Either the prop valve or the crank position of the key will ground the light.
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