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> Brain Trust Help needed on Parking Brake Light, Parking Brake Light Always on......
Gatornapper
post Jul 4 2025, 07:40 PM
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Once again need the help of the great 914 Brain Trust here........

My Parking Brake Light stays on all the time.

The switch is fairly new and it is functioning properly.

Disconnecting the brown (ground) wire to the switch, it has no continuity or resistance to ground. So it is not "grounded out" somewhere.

I see in my great 914 electrical schematic of the ground circuit for the switch and light that there are two diodes in the circuit.

I am guessing that one of them failed.

Can anyone tell me where these two diodes are located?

Or where else the problem might be?

As always,

TIA,

GN
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jim_hoyland
post Jul 4 2025, 08:41 PM
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MC ?
I just went through this… (IMG:style_emoticons/default/smile.gif)
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Jamie
post Jul 5 2025, 09:03 AM
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QUOTE(jim_hoyland @ Jul 4 2025, 06:41 PM) *

MC ?
I just went through this… (IMG:style_emoticons/default/smile.gif)

I'm also trying to troubleshoot the flashing red instrument light. Didn't locate the brake switch illustrated in the shop manuals, and have no good access down beside the seat. Have pushed the MC pin in several times with no effect, no leaks, and brakes are working just fine. Don't know what to try next? (IMG:style_emoticons/default/blink.gif)
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Literati914
post Jul 5 2025, 10:01 AM
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@Gatornapper - if you pull the combo gauge, you'll see the two diodes... (your model may be different, but) on my '72 there are two diodes going to the e-brake/MC flashing bulb - one diode is looped over from the generator bulb and the other diode is where two Brown/White wires are combined (these are e-brake and 'fasten seat belt dash light').

But before looking in to those - pull the other single Brown/White wire from the bulb and see if the flashing stops.. that's the wire from the MC, and you can isolate the problem that way.
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Gatornapper
post Jul 5 2025, 10:47 AM
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Ah yes! On my schematic it does look like one diode is next to bulb. Other one seems to be near the switch but I don't see one there......

Great suggestion, I will do this.

THANK YOU!

Will let you know.

What is "MC"?

GN


QUOTE(Literati914 @ Jul 5 2025, 10:01 AM) *

@Gatornapper - if you pull the combo gauge, you'll see the two diodes... (your model may be different, but) on my '72 there are two diodes going to the e-brake/MC flashing bulb - one is looped over from the generator bulb and the other is where two Brown/White wires are combined (these are e-brake and 'fasten seat belt dash light').

But before looking in to those - pull the other single Brown/White wire from the bulb and see if the flashing stops.. that's the wire from the MC, and you can isolate the problem that way.
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BillC
post Jul 5 2025, 11:05 AM
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QUOTE(Gatornapper @ Jul 5 2025, 12:47 PM) *

What is "MC"?

"MC" is short for Master Cylinder.
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Literati914
post Jul 5 2025, 12:10 PM
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QUOTE(BillC @ Jul 5 2025, 12:05 PM) *

QUOTE(Gatornapper @ Jul 5 2025, 12:47 PM) *

What is "MC"?

"MC" is short for Master Cylinder.


Yea that flashing bulb is not just the E-brake warning light - it's also the Master Cylinder warning light. So the non-stop flashing could be from either circuits going to the bulb. You can pull one at a time and see if the light stops blinking. Alternately you could pull the wires off the MC itself.
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Superhawk996
post Jul 5 2025, 12:23 PM
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Simplified schematic from Spoke Attached Image
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Superhawk996
post Jul 5 2025, 12:25 PM
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QUOTE(Jamie @ Jul 5 2025, 11:03 AM) *


I'm also trying to troubleshoot the flashing red instrument light. Didn't locate the brake switch illustrated in the shop manuals, and have no good access down beside the seat. Have pushed the MC pin in several times with no effect, no leaks, and brakes are working just fine. Don't know what to try next? (IMG:style_emoticons/default/blink.gif)

Troubleshoot with Multimeter using Spokes schematic above
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Gatornapper
post Jul 9 2025, 05:44 AM
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Ah, yes.

Thank you SH for the schematic.

NO diodes at light in left gauge - none in sight anywhere. See none in schematic above either. My 914 is '76.

But I will check the Brake Fluid. I just bled the brakes a week ago with 15psi on fluid reservoir system. So reservoir is full.

GN
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Spoke
post Jul 9 2025, 06:36 AM
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For the MC warning switch, you can disconnect the wires to see if the warning light goes out. Same with the ebrake switch. The flasher will flash with relatively little current through the switches or other leakage.
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Gatornapper
post Jul 11 2025, 06:41 AM
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Thanks Spoke.

Light still flashes with brake wire disconnected.

Gotta get under car to remove MC wire I guess.........yes, light flashes.....

Is wire to MC looking for fluid in MC?

Just bled my brakes and light still flashes. My 914 has 911 MC. Brakes work great, reservoir is full.

GN

QUOTE(Spoke @ Jul 9 2025, 06:36 AM) *

For the MC warning switch, you can disconnect the wires to see if the warning light goes out. Same with the ebrake switch. The flasher will flash with relatively little current through the switches or other leakage.
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914
post Jul 11 2025, 07:23 AM
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Is there a reset button/Switch on the Master Cyl?
914/Alan
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Cairo94507
post Jul 11 2025, 08:06 AM
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On my MC there is- ATE. (IMG:style_emoticons/default/beerchug.gif)
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Jamie
post Jul 11 2025, 08:55 AM
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QUOTE(914 @ Jul 11 2025, 05:23 AM) *

Is there a reset button/Switch on the Master Cyl?
914/Alan

I've tried that reset switch multiple times without resulting in ending the flashing. Will next remove the wire from the MC when I can next get the car up on the lift.
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Gatornapper
post Jul 14 2025, 03:14 PM
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I'll do the same. Lift presently occupied preparing my sweet 944 S for buyer next week. After acquiring my Concours 951 last September, my beloved wife thinks no man needs two nice 944's, and I actually agree.

What in the world does this wire to the MC do? Make sure there is fluid in MC? Why is there a reset on it?

GN

QUOTE(Jamie @ Jul 11 2025, 08:55 AM) *

QUOTE(914 @ Jul 11 2025, 05:23 AM) *

Is there a reset button/Switch on the Master Cyl?
914/Alan

I've tried that reset switch multiple times without resulting in ending the flashing. Will next remove the wire from the MC when I can next get the car up on the lift.
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Superhawk996
post Jul 14 2025, 05:47 PM
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QUOTE(Gatornapper @ Jul 14 2025, 05:14 PM) *



What in the world does this wire to the MC do? Make sure there is fluid in MC? Why is there a reset on it?


The master cylinder switch senses a loss of pressure in either of the two braking circuits.

If there is a pressure differential across the sensing piston in the master cylinder, it trips the latching switch.

This will occur during brake bleeding - thus the reset switch.

If you haven’t bled the brakes and this circuit is being tripped, it is a serious issue and shouldn’t be ignored.
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