Electrical Help with dash brake light

Rick88

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Mar 17, 2021
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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?
 
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KRUISR

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Apr 16, 2015
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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.

img_0400-jpg.576141
 
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Rick88

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Mar 17, 2021
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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.

img_0400-jpg.576141
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.
 
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Rick88

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Mar 17, 2021
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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

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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?
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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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.

556951FB-B9E5-4C10-99F9-C176788E5FA8.jpeg
 
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Rick88

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Mar 17, 2021
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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

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Mar 17, 2021
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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.

556951FB-B9E5-4C10-99F9-C176788E5FA8.jpeg
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

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Sep 26, 2017
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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

Member
Mar 17, 2021
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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.
 
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Mustang5L5

Put lubricant all over the balls
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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.

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.
 
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Mustang5L5

Put lubricant all over the balls
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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.

0D1304A4-AB47-48DB-B8DC-9C95ED0B4F05.jpeg


EAEEBC58-911D-45D3-B3CF-08868147DA6C.jpeg


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.
 
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manicmechanic007

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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.

0D1304A4-AB47-48DB-B8DC-9C95ED0B4F05.jpeg


EAEEBC58-911D-45D3-B3CF-08868147DA6C.jpeg


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