Electrical Help with dash brake light

Rick88 · Dec 5, 2021

I've been trying to fix little things with my 88 GT. One I cannot figure out is this stupid brake light on the dash. Mine doesn't even flash when I start the car. I have replaced the bulb with a known good bulb, turned the bulb both ways, checked the e-brake sensor, and the wiring to the e-brake sensor. I get nothing with the sensor connected regardless of the position of the e-brake. I am only getting 1v with the switch disconnected on my multimeter on the purple/white wire everything else is 12v. To me, it seems that the light should come on if I touch both the purple/white wire and the black ground wire together. Does anyone have a wiring diagram for this area, I've searched all day and can't find one? Also shouldn't it be 12v to that wire?

General karthief · Dec 5, 2021

Is the brake fluid level on the same light?
Or maybe there is not a fluid level sensor in our cars, don't remember.

Rick88 · Dec 5, 2021

General karthief said:
Is the brake fluid level on the same light?

From what I've been reading it is. The fluid is fine, the light should still flash when starting the car but it doesn't.

KRUISR · Dec 5, 2021

Did you try the dual brake warning switch? Wiring for the dash light goes through it. This is not the brake pedal switch. It is a 3 wire connector, 2 are P/W, 1 is BK

Found this in another post (diagram is from Mustang5L5). Based on that thread it looks like the dual brake warning switch is on the MC.

Rick88 · Dec 5, 2021

KRUISR said:
Did you try the dual brake warning switch? Wiring for the dash light goes through it. This is not the brake pedal switch. It is a 3 wire connector, 2 are P/W, 1 is BK

Found this in another post (diagram is from Mustang5L5). Based on that thread it looks like the dual brake warning switch is on the MC.

TY! that's what I need! Looks like I traced it back to my ignition switch and got continuity there but not to the cluster. There are 2 P/W wires that go from the ignition switch and from there 1 goes to the cluster. My ignition switch is damaged so I am just replacing it; parts are ordered but won't get here till next week. I will update then.

Rick88 · Dec 15, 2021

Sorry, I'm learning as I go. Where is the dual brake warning switch?

I have continuity from the e-brake to the ign switch, ign to the connector in the driver side kick panel, from any of that to the brake reservoir on the (what looks to be) black/yellow. Then I have connectivity from the green/white on the brake reservoir to the purple/white on the cluster. I'm thinking there is a wiring issue from the cluster to the dual brake warning switch.

Mustang5L5 · Dec 15, 2021

The dual brake switch is the plug on the brake master cylinder.

That plug needs to be plugged in for the warning light to be functional.

If you unplug it, you need to put a small wire jumper across the two terminals for the warning light to function. You can also try this as a way of testing if there is an issue with the MC switch.

The connection to the ignition switch is only for the bulb check function on Start. It’s another ground path that will illuminate the bulb. The two wires connect in the harness plug, so even with the plug disconnected from the ignition the bulb warning light should still function.

You essentially have three positive grounds. The ignition switch (for bulb check only) and at the parking brake switch and at the MC. Only one ground needs to close to illuminate the light, but if there is an issue with the switch at the MC the circuit will be open. That’s why I suggest unplugging and jumpering that plug for testing purposes.

Make sense?

Mustang5L5 · Dec 15, 2021

This is what I mean by jumping out the dual brake warning switch.

Unplug the plug from the MC and put a jumper wire between the two purple/white wires. This will eliminate the switch from the circuit but the jumper is necessary to still allow the warning light to function.

Rick88 · Dec 15, 2021

Mustang5L5 said:
The dual brake switch is the plug on the brake master cylinder.

That plug needs to be plugged in for the warning light to be functional.

If you unplug it, you need to put a small wire jumper across the two terminals for the warning light to function. You can also try this as a way of testing if there is an issue with the MC switch.

The connection to the ignition switch is only for the bulb check function on Start. It’s another ground path that will illuminate the bulb. The two wires connect in the harness plug, so even with the plug disconnected from the ignition the bulb warning light should still function.

You essentially have three positive grounds. The ignition switch (for bulb check only) and at the parking brake switch and at the MC. Only one ground needs to close to illuminate the light, but if there is an issue with the switch at the MC the circuit will be open. That’s why I suggest unplugging and jumpering that plug for testing purposes.

Make sense?

Makes sense, thank you. I’ll test all this out tomorrow as I have ran out of light and patience today lol.

Rick88 · Dec 20, 2021

Mustang5L5 said:
This is what I mean by jumping out the dual brake warning switch.

Unplug the plug from the MC and put a jumper wire between the two purple/white wires. This will eliminate the switch from the circuit but the jumper is necessary to still allow the warning light to function.

Alright the light works with the jumper but not when plugged into the MC. I’m assuming I need to replace my MC then? I will try cleaning the connection first.

manicmechanic007 · Dec 20, 2021

Your bulb is missing?
You can get (buy) the sensor on the bottom of the master cylinder separate from the reservoir
Replaced a ton of them over the years (lamp lit when fluid is full)
Ford dealer part
Its the brake low fluid level sensor
The brake lamp will also illuminate if the proportioning valve piston is stuck to either side
Used to see people pull the bulb out when the proportioning valve stuck
Kind of difficult to get the valves unstuck sometimes by bleeding and then the valve must be replaced

Rick88 · Dec 20, 2021

manicmechanic007 said:
Your bulb is missing?
You can get (buy) the sensor on the bottom of the master cylinder separate from the reservoir
Replaced a ton of them over the years (lamp lit when fluid is full)
Ford dealer part
Its the brake low fluid level sensor
The brake lamp will also illuminate if the proportioning valve piston is stuck to either side
Used to see people pull the bulb out when the proportioning valve stuck
Kind of difficult to get the valves unstuck sometimes by bleeding and then the valve must be replaced

The bulb is there and works only when the dual brake switch is bridged (at least for the hand brake). I'll look into just replacing the sensor if cleaning it doesn't work. Thanks for the info. If that doesn't work I may just replace the proportioning valve if I find that is the issue. Going to have to replace it anyway when I do disk brakes might as well get it out of the way.

Mustang5L5 · Dec 20, 2021

Rick88 said:
The bulb is there and works only when the dual brake switch is bridged (at least for the hand brake). I'll look into just replacing the sensor if cleaning it doesn't work. Thanks for the info. If that doesn't work I may just replace the proportioning valve if I find that is the issue. Going to have to replace it anyway when I do disk brakes might as well get it out of the way.

Has nothing to do with the proportioning valve. It's strictly part of the master cylinder reservoir. If it was bad, you would replace this. My guess is there is a bunch of gunk inside the MC that is blocking the sensor from doing what it needs to do.

I believe the Cardone 1R4005 is the only game in town now for a replacement.

https://www.summitracing.com/parts/aaz-1r4005

I actually think the EVTM is wrong in the sense that the low fluid light is not a dual-brake sensor like the 1986 and earlier cars. It's just a fluid level sensor after 1986.

If you are doing a 5-lug conversion anyway, i would just wait. You'll need to change out the MC anyway and they come with a new reservior. Don't do the job twice.

I'd leave the jumper in place so you can see your parking brake light though.

Mustang5L5 · Dec 20, 2021

Ended up finding my original ‘88 MC.

The sensor is removable. However it’s a PITA to get out. Took me a few mins with the Mc in my hand.

It appears that it functions by detecting the float in the reservoir descending down near it.

I do not see a replacement available without buying a new reservoir.

manicmechanic007 · Dec 20, 2021

Mustang5L5 said:
Ended up finding my original ‘88 MC.

The sensor is removable. However it’s a PITA to get out. Took me a few mins with the Mc in my hand.

It appears that it functions by detecting the float in the reservoir descending down near it.

I do not see a replacement available without buying a new reservoir.

Yes ignorant to get out. The clip is tucked under the reservoir

OfflineHomestead · May 11, 2024

Mustang5L5 said:
The dual brake switch is the plug on the brake master cylinder.

That plug needs to be plugged in for the warning light to be functional.

If you unplug it, you need to put a small wire jumper across the two terminals for the warning light to function. You can also try this as a way of testing if there is an issue with the MC switch.

The connection to the ignition switch is only for the bulb check function on Start. It’s another ground path that will illuminate the bulb. The two wires connect in the harness plug, so even with the plug disconnected from the ignition the bulb warning light should still function.

You essentially have three positive grounds. The ignition switch (for bulb check only) and at the parking brake switch and at the MC. Only one ground needs to close to illuminate the light, but if there is an issue with the switch at the MC the circuit will be open. That’s why I suggest unplugging and jumpering that plug for testing purposes.

Make sense?

Mustang5L5 is the man. Verified wiring at Master Cylinder, repaired terminal pin and now have a brake light.

Stopman33 · Dec 22, 2024

Hello, I am new to the forum and fox body's in general. I am having the same issue with the BRAKE light not coming on in the dash as well. I have tried to jumper the MC plug but no luck, light is still not coming on in dash,

Mustang5L5 · Dec 22, 2024

Take the Pink/White wire on the brake MC low fluid switch, and ground it. Since there are two wires you'll likely want to try this with both of them. Does the light come on with key on? If not, it's a cluster/bulb issue. Most likely cause is a bad ribbon cable if i had to guess.

Brake light bulb gets positive 12V and then it's a matter of completing a ground either with the brake light low fluid sensor, or with the parking brake switch. There's also a momentary ground during key start to perform the bulb test.

Grounding pink/white (wire 977) should turn the bulb on with key on

manicmechanic007 · Dec 29, 2024

Did you check the bulb?
If the brake lamp is on for years, it will burn out

Electrical Help with dash brake light

Rick88

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General karthief

wonder how much it would cost to ship you a pair

Rick88

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KRUISR

Rick88

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Rick88

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Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

Rick88

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Rick88

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manicmechanic007

Rick88

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Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

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OfflineHomestead

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Stopman33

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That is…until I whipped out my Bissell

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Electrical Help with dash brake light

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wonder how much it would cost to ship you a pair

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That is…until I whipped out my Bissell

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That is…until I whipped out my Bissell

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