Electrical Brake Light Switch

Shane8996GTLX · Nov 5, 2017

I'll have to take a picture of where it goes and show you.

Shane8996GTLX · Nov 5, 2017

Here is 4 shots that should help.

General karthief · Nov 5, 2017

First you need to soldier those wires together and put heat shrink on them, remove the boo switch and plug in the wires, according to the wire diagram the wire runs to the mfs (turn signal switch) someplace between the boo and mfs it should split to the tbl (third brake light) follow that and make sure it power.
The reason you are removing the boo from the peddle is so power is on, you don't have to hold the peddle down.

Shane8996GTLX · Nov 5, 2017

What wires am I plugging in ? If you mean the pigtail to the boo then I will do that once it's sottered

General karthief · Nov 5, 2017

Shane8996GTLX said:
What wires am I plugging in ? If you mean the pigtail to the boo then I will do that once it's sottered

Yes the boo switch connector

Shane8996GTLX · Nov 5, 2017

Ugh I'll never figure this out ! Lol. How do I check power for the third brake light.

General karthief · Nov 6, 2017

Looking at the wire diagram it shows it splitting off from the wire that leaves the boo, we need to find that connection
This connection will be after the boo switch and before the mfs (turn signals)

wmburns · Nov 6, 2017

Just wondering if you are even using the Volt-Ohm meter correctly. If you are using the setting that "beeps" the VOM when the leads are touched together that is for measuring resistance.

You want to use the DC volt setting. This should read 12 volts when directly connected to battery.

From your set up the bare wires are not causing the problem. IN fact they should be a benefit as touching them together SHOULD cause the brake lights to come on. IF touching the bare wires together does not cause the 3rd high mount brake light to come on THEN there are other things wrong here.

You need to go back an perform the tests to prevent a wild goose chase. Must of the previous advice was given with the understand there is +12 volts present at one of the leads going to the BOO switch.

General karthief · Nov 6, 2017

wmburns said:
Just wondering if you are even using the Volt-Ohm meter correctly. If you are using the setting that "beeps" the VOM when the leads are touched together that is for measuring resistance.

You want to use the DC volt setting. This should read 12 volts when directly connected to battery.

From your set up the bare wires are not causing the problem. IN fact they should be a benefit as touching them together SHOULD cause the brake lights to come on. IF touching the bare wires together does not cause the 3rd high mount brake light to come on THEN there are other things wrong here.

You need to go back an perform the tests to prevent a wild goose chase. Must of the previous advice was given with the understand there is +12 volts present at one of the leads going to the BOO switch.

Thanks for jump'n in here, he and I both need help here, I'm just going by a ruff wire diagram picture with no real experience with this year mustang.

Shane8996GTLX · Nov 18, 2017

I'm back at it again trying to resolve this issue.

Shane8996GTLX · Nov 18, 2017

Just gunna have to buy a test light for better analysis. Clip to battery and recheck

General karthief · Nov 18, 2017

Let us know what you find

Shane8996GTLX · Nov 18, 2017

Bought a new multifunction switch to see didn't help but was wore out. Area you said to look at between boo and mf switch from visual inspection shows no damage

wmburns · Nov 18, 2017

Since the high mount 3rd brake light does not go through the multi-function switch it's almost certain that replacing the multi-function switch would not solve the problem IF the high mount 3rd brake light is also affected.

What were the results of touching the two bare wires together? Does the 3rd high mount brake light come on or not?

Shane8996GTLX · Nov 18, 2017

After sotering the connections and before taping them up I touched them and no lights came on. There is power to one wire. I did do dc volts and touched a ground etc to check for power. Went over all fuses again. All bulbs in tail lights work.

Shane8996GTLX · Nov 19, 2017

After sotering the connections and before taping them up I touched them and no lights came on. There is power to one wire. I did do dc volts and touched a ground etc to check for power. Went over all fuses again. All bulbs in tail lights work.

wmburns · Nov 19, 2017

IF this were my car the next step I would do is to force power to the high mounted 3rd brake light from the rear. Using either an "old school" battery charger or a long jumper wire from the battery. What this will do is:

Confirm that the 3rd brake light is actually capable of working.
Confirm IF there's a short in the circuit.

Soooooooooo there are a couple of possible test results:

3rd brake light does not Work: Assuming the bulbs are good, then the next thing to CONFIRM is the trunk groung.

3rd brake light does work: Likely the problem is a wiring harness fault. More tests will be needed to confirm.

Rather than try to write a trouble shooting novel I'm going to wait until the results of the next test is posted.

Note regarding powering loads via a battery charger. Newer battery chargers have reverse protection. This kind of battery charger can't be used to power external loads.

EDIT: if this were really my car I would short/jump the brake light switch. Then I would confirm power at the 3rd high mount brake light. Next step would depend upon if there is power or not.

I would also test and confirm that the ground is able to carry a real current using a simulated load.

Shane8996GTLX · Nov 19, 2017

I bought the test light confirmed power at fuses 1 and 9. Touched the brake light switch and power to one wire confirmed. Pulled back the carpet and touched the red/gr wire power confirmed by the driver's seat.

Neuron · Nov 19, 2017

I picked up a Home Security System 12 volt battery for testing, they are only $10-$20 and I just use my small charger to keep it charged. The cigarette adapter died on my air pump so I put on longer cables with copper clips so I can pump up tires using the Security Battery and not run down the car battery.

wmburns · Nov 19, 2017

Shane8996GTLX said:
I bought the test light confirmed power at fuses 1 and 9. Touched the brake light switch and power to one wire confirmed. Pulled back the carpet and touched the red/gr wire power confirmed by the driver's seat.

NONE of these tests have confirmed the circuit from the brake on/off switch back to the trunk. Nor has it confirmed the brake light bulb assembly itself.

An alternate way to test is to CONFIRM power at the 3rd high mount brake light R/LG right at the light assembly connector. Remember the objective here is to confirm just how far the power is actually going.

Electrical Brake Light Switch

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wonder how much it would cost to ship you a pair

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Electrical Brake Light Switch

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