Brake Warning Light Testing

[mattyv87]

Based on all of this is there a good step by step method to troubleshoot a non working brake warning light. I understand that there could be a few different reasons the warning light would not light up but I am looking for some guidance on troubleshooting.

I started with the easiest thing and replace the brake warning light bulb. I have a newer brake warning connector at the H block. I plan to install a new ignition switch next week.

When I directly ground the warning light housing the bulb lights up.

Thoughts?


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If the two wires going into the brake warning switch connector are both grounded with the ignition turned to on, it should have the brake warning light illuminated as I understand it. Therefore, with the switch connected but removed from the H block depressed as well as grounded, it should also illuminate. A long jumper to test grounds at multiple points could help pinpoint a ground issue. This is basically my next step as well.

Check to see if you have good ground to the chassis, firewall, dash and engine. This is probably majority of this circuits problem.

 

[Oldtimer]

When I directly ground the warning light housing the bulb lights

The stock brake warning lamp housing has a floating ground.
The bulb does not ground thru the housing.
It has 2 wires, R/Y, and Violet (purple).
R/Y wire, supplies 12v from ignition switch in RUN position.
V wire, is ground thru pressure differential switch or thru ignition switch in START position only.

If you ground the warning lamp housing, with the key on, and the bulb illuminates
Look for:

1. A short between the housing and V wire.
2. A short between lamp housing and outer shell of bulb socket (maybe the spring when installed in dash).

 

[msommer001]

Good morning,
I was able to do a bit more on this today. With the in the “ON” position, I grounded the brake warning housing, bulb did not illuminate. Then I grounded the point that the violet with white strip wire connect to the bulb socket, the bulb illuminated.

I previously checked the back side of the ignition switch for ground to and when I checked it with my circuit tester, it illuminated, indicating that I had a good ground. Is there another way to check ground at the ignition switch?

At this point, I am going to replace the ignition switch to see if that corrects my issue.

Thanks again for the continued input.


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[DirtDonk]

Easy to check the ground with a test lamp, or an ohm-meter.
Find the little side tab that was pointed out previously in the thread, remove the wire connector from it, and use your ohm-meter probes between that connector and a known good ground. Or the test lamp from battery power to the tab.
If the meter goes to zero, or the lamp lights up, when you turn the key to START, your ground is sufficient for lighting the brake warning lamp.

 

[Slowleak]

Test your new switch before you install it. You don’t want to find out that it is bad after you install it like I did. Those things are no fun to remove.

 

[jerry]

Test your new switch before you install it. You don’t want to find out that it is bad after you install it like I did. Those things are no fun to remove.

This... Is sage advice!

 

[msommer001]

Ignition switch arrived today. Install tonight. I will report back.


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[msommer001]

I was able to get into the garage. Replaced the ignition switch and tested before I reinstalled. Brake warning light lit when I bumped the key into the start position. I put it all back together and for some reason the warning light would not light up.

I did a bunch of testing and it seems like the ignition switch failed/broke. Is it common for a new ignition switch to work one moment and not the next. Going to get another switch tomorrow and try it all over again.


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[DirtDonk]

Is it common for a new ignition switch to work one moment and not the next.

Far, far too often.
In fact, if you buy it from the same source, or get a replacement, or warranty part from the same source, it’s not uncommon for multiples to fail in one way or another.
Might be dead right out of the box, might get a couple of good starts on it, might work for six months.
But if it can fail, a modern part is built to do just that!

Welcome to the modern world of electro, mechanical replacement auto parts. Sourced from multiple overseas locations and built to spec.
Unfortunately the only “spec” followed anymore seems to fall into one category. Lowest possible price.

 

[Oldtimer]

Verify you have ground between switch housing (installed) and battery.
Can be rust or paint between (deep breath) switch, dash, cowl, inner fender, ground wire under starter relay mount, block, battery post.

 

[msommer001]

Verify you have ground between switch housing (installed) and battery.
Can be rust or paint between (deep breath) switch, dash, cowl, inner fender, ground wire under starter relay mount, block, battery post.

When the switch is installed I have good ground on the back housing of the ignition switch.


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[msommer001]

I got a new switch, tested it before I fully installed and then installed if fully each time the brake warning light lit up when key was in the start position. For now this is done…now on to the next project.

Thanks again for all the input and guidance along the way.


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[serial car restorer]

Bringing this back up for more discussion, as I'm fighting it now. Wiring is all new, as is the prop valve switch. I have 2 ignition switches, one new aftermarket, and one FoMoCo original (probably dates from '71 or so).

All testing shows good:

• With ignition on, grounding either purple wire (at ignition or prop valve) causes bulb to light.
• Both switches test good on the bench, with continuity between the spade terminal and the switch body when switched to 'start' position, but not 'run', 'off' or 'acc'.
• When installed in the dash, the ignition switch body meters good continuity to ground.

But when installed in the dash, neither switch closes the ground while cranking, either with the key and tumbler in place, or with just a screwdriver.

What am I missing?

 

[Oldtimer]

Is harness conector on purple wire making good ELECTRICAL contact with the spade terminal on ignition switch?

 

[serial car restorer]

Is harness conector on purple wire making good ELECTRICAL contact with the spade terminal on ignition switch?

Yes, new connector on the harness, very tight fit.

 

[DirtDonk]

Sounds like a typical matter of the ignition switch body not grounding to the dash, and/or the dash not being grounded to the rest of the body.

Is the dash painted? Front and back? Next time you have the ignition switch out, buff little paint off of the contact surface area. Especially if you have paint or powder coat.
And if that’s the case, you should create a dedicated ground wire between the dash panel itself, and the body. Scraped clean sanded clean of paint in that specific area.
Once you attach the ground, you can give it a spritz of paint to protect it from rust. Or use grease. I like to paint over grounds once they are attached and finished.

Do you have the two wire prop valve switch, or the one wire?

 

[jamesroney]

Sounds like a typical matter of the ignition switch body not grounding to the dash, and/or the dash not being grounded to the rest of the body.

Is the dash painted? Front and back? Next time you have the ignition switch out, buff little paint off of the contact surface area. Especially if you have paint or powder coat.
And if that’s the case, you should create a dedicated ground wire between the dash panel itself, and the body. Scraped clean sanded clean of paint in that specific area.
Once you attach the ground, you can give it a spritz of paint to protect it from rust. Or use grease. I like to paint over grounds once they are attached and finished.

Do you have the two wire prop valve switch, or the one wire?

There is one other possibility, and it's easy to test: You need to disconnect the "S" terminal wire at the starter relay to prevent the starter from cranking. Then see if the ignition switch grounds the warning light.

The brake test circuit in the ignition switch works on a different pole. You need to make note of where the key location is when the switch contact is made, and make sure that you get there.

On many newer switches, (and some worn out old ones...) the "Start" circuit will energize before the "Test" circuit closes. Meaning that it is possible to crank, and start the Bronco before you turn the key far enough to "light the lamp."

So the switch will work perfectly on the bench, or with an ohm meter. But as soon as you put it in the dash, and you actually turn the key...you stop turning. Disconnecting the starter allows you to turn the key all the way to the end of travel.

 

[serial car restorer]

Thanks, guys. The switch body appears to be grounded to the dash, according to the ohmmeter. And since I didn't want to be cranking the engine repeatedly while testing, I pulled the S wire at the solenoid before I started the process. So I'm pretty sure the switch engaging the starter prior to the Test circuit isn't the case.

But I may not have a good enough ground from the switch body to the dash. I'll look into that part again.

 

[serial car restorer]

Thought I'd report the end result. Today I tested the quality of the switch body to dash ground by turning on the ignition and then grounding the wire directly to the switch body. Light came on as it should. So I was thinking I must somehow have 2 bad ignition switches.

In the process of testing that theory, I had the ignition switch hanging below the dash so I could switch back and forth between them. While doing this, I did not connect the ACC wire to the threaded stud on the back so I could plug and unplug quickly. And I found that with the ACC lead disconnected, the warning light never came on at all, even when manually grounding either of the wires.

Turns out that in the total wiring replacement process, I powered the warning light circuit from an ACC switched source. And the ignition switch is designed to not only ground that spade on the side during cranking, it also cuts power to the ACC circuits (radio, etc.) during cranking.

So when I switched to START, it grounded the spade just fine, but it also killed the power to the light as well. This even explains why I once or twice thought I saw a quick flicker at the warning light. Sometimes the ground might complete the circuit a fraction of a second before it killed the power.

Moving the warning circuit to an IGN switched source fixed it.

 

[DirtDonk]

Good find! Thanks for following up.
Glad you got it fixed.