Brake Warning Light Testing

[msommer001]

I need some help on the ways I can test my brake warning light. I know that these lights have been problematic but want to do some testing to see if I can get it functional.

I did read this posting but I need a bit more detail.

Brake warning light

Brake warning light

I've been trying to get my brake warning light to work. It wasn't wired when I first got it years ago, and never got it working when I rewired it,but I did wire it The warning light isn't coming on when I turn the ign to start. From the electrical diagrams I have, the ign switch is supposed to...

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[Slowleak]

I need some help on the ways I can test my brake warning light. I know that these lights have been problematic but want to do some testing to see if I can get it functional.

I did read this posting but I need a bit more detail.

Brake warning light

Brake warning light

I've been trying to get my brake warning light to work. It wasn't wired when I first got it years ago, and never got it working when I rewired it,but I did wire it The warning light isn't coming on when I turn the ign to start. From the electrical diagrams I have, the ign switch is supposed to...

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When the ignition is turned to start the purple and white wire on the back of the ignition switch is temporarily grounded causing the bulb to light. Pull the purple/white wire off and ground it. Then the brake light should come on when the ignition is on. If it does, then you have a bad switch or the switch is not grounded to the dash.
You can also remove the plug from your brake warning switch, and ground the wires in the plug. Then turn on the ignition switch. The light should come on when the wire to the bulb is grounded.
The problem I had was a bad ignition switch. That seems to be fairly common.

 

[msommer001]

Okay. I will try that.

I did run a light tester from the positive side of the battery to the brake warning light bezel and it lit up so I believe I have a good ground to the brake warning light.

I probably should change the bulb, just to rule out the easy stuff first.

Thanks again for the information, it is always very helpful.


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[Slowleak]

Okay. I will try that.

I did run a light tester from the positive side of the battery to the brake warning light bezel and it lit up so I believe I have a good ground to the brake warning light.

I probably should change the bulb, just to rule out the easy stuff first.

Thanks again for the information, it is always very helpful.


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Testing a ground to the bezel is not a valid ftest in this case. The bulb does not ground to the housing or bezel, only to the purple white wire. It will light up even when removed from the dash.
You could try that same test to the ignition switch housing. That would verify that the switch is grounded.

 

[msommer001]

Testing a ground to the bezel is not a valid ftest in this case. The bulb does not ground to the housing or bezel, only to the purple white wire. It will light up even when removed from the dash.
You could try that same test to the ignition switch housing. That would verify that the switch is grounded.

Do this with the ignition in the “on” position?


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[Slowleak]

Doesn’t have to be in the on position unless you are testing to see the bulb light up. You want to make sure the switch is grounded to the dash so that it can ground the wire to the bulb.

 

[Oldtimer]

Note:
Ford design has prove out ground path from ignition switch passing thru plug at dual brake warning switch before it grounds the bulb in dash.
If the plug is not connected to dual brake warning switch, the circuit is not complete. This to let you know plug has been disconneted.
Aftermarket brake combination valves (PV4) have a single pin on the brake pressure differential valve, and require splicing both purple/white wires to the new connector, defeating the saftey asspect of notification if plug is disconnected.

Also verify you have 12 volts on red/yellow wire at bulb when ke is in Run position.

 

[msommer001]

I was able to do a bit more testing today.

I checked in back of the ignition switch where the purple/white wire connects for ground. Ran a circuit testers from the positive side of the battery and it did not light up when the key was “on” or when I bumped the starter. I did have ground at the back side of the ignition housing.

I moved the circuit tested to the negative side and with the key on, touched the purple/white wire at the ignition switch and at the bulb housing and both times the bulb lit up.

Thoughts? Time for a new ignition switch?


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[mattyv87]

I'm having an issue where when I turn the ignition on the light turns on as it should but when I push the switch (not installed in the H-block and ground it on the frame the dash light doesn't come on. It's currently dangling below the dash. I bought a grounding kit from North East so I'm going to give that a try first along with reinstalling it in the dash. I'm thinking it's a bad ground.

 

[msommer001]

I was able to do a bit more testing today.

I checked in back of the ignition switch where the purple/white wire connects for ground. Ran a circuit testers from the positive side of the battery and it did not light up when the key was “on” or when I bumped the starter. I did have ground at the back side of the ignition housing.

I moved the circuit tested to the negative side and with the key on, touched the purple/white wire at the ignition switch and at the bulb housing and both times the bulb lit up.

Thoughts? Time for a new ignition switch?


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I will replace the ignition switch to see if that corrects my issue.

Ultimately, I want to use this light for the emergency brake. Since grounding the purple/white wire turns on the bulb. I should be able to use a ground switch at the emergency brake bracket to ground the wire and light the bulb.

Sounds right?


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[Oldtimer]

I was able to do a bit more testing today.

I checked in back of the ignition switch where the purple/white wire connects for ground. Ran a circuit testers from the positive side of the battery and it did not light up when the key was “on” or when I bumped the starter. I did have ground at the back side of the ignition housing.

I moved the circuit tested to the negative side and with the key on, touched the purple/white wire at the ignition switch and at the bulb housing and both times the bulb lit up.

Thoughts? Time for a new ignition switch?


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I would suspect ignition switch at this point.

With key ON, and connecting PU/WH wire to ground, bulb illuminates.
Power source, bulb, and wires in harness are all good.

With key in START position, the spade lug at arrow (PU/WH wire is connected to) gets shorted to brass tab in circle.
The brass tab is caught under diecast housing, and housing should be grounded to dash. You said you had ground at back of housing.

Edit: Yes on adding a ground switch on parking brake bracket.

 

[Oldtimer]

I'm having an issue where when I turn the ignition on the light turns on as it should but when I push the switch (not installed in the H-block and ground it on the frame the dash light doesn't come on. It's currently dangling below the dash. I bought a grounding kit from North East so I'm going to give that a try first along with reinstalling it in the dash. I'm thinking it's a bad ground.

The lamp assembly does not need to be installed in dash to work properly.

I would suspect the brake pressure warning light switch.

Unplug connector from switch. Check for continuity between contacts in switch and pin when it is depressed.

 

[msommer001]

I would suspect ignition switch at this point.

With key ON, and connecting PU/WH wire to ground, bulb illuminates.
Power source, bulb, and wires in harness are all good.

With key in START position, the spade lug at arrow (PU/WH wire is connected to) gets shorted to brass tab in circle.
The brass tab is caught under diecast housing, and housing should be grounded to dash. You said you had ground at back of housing.

View attachment 937315

Edit: Yes on adding a ground switch on parking brake bracket.

I appreciate the photo. This helps me understand how this all works. I will report back once I get the ignition switch changed.


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[mattyv87]

The lamp assembly does not need to be installed in dash to work properly.

I would suspect the brake pressure warning light switch.

Unplug connector from switch. Check for continuity between contacts in switch and pin when it is depressed.

View attachment 937317

Sorry msommer001 to hijack slightly.

I've tested the switch and it does in fact work. With the switch connected but not grounded the light comes on with the ignition. I'm thinking frame surface rust and bad grounds are leading to the problem. Maybe I'll make a jumper to go from it to the main ground to verify wiring isn't hacked up under the dash and that I just need to correct some ground issues.

 

[DirtDonk]

(Edited for posterity… Not to mention accuracy!)

Which switch are you talking about? There are two in this discussion.
The little plastic switch that screws into the brake distribution block, and the ignition switch in the dash.
If it’s the ignition switch that’s hanging, then there’s no way for it to ground to the dash.

Maybe you got that already, and if so, sorry. But I wanted to make sure that when you said “switch“ we were all on the same page.

 

[Oldtimer]

The little plastic switch that screws into the brake distribution block . . . .
It doesn’t need to ground at all. When the switch button is pressed, the two wires are connected. It’s simply an open/close switch.

Paul,
The switch does in fact need to ground.
The two connector pins in the switch are connected to each other all the time. This is part of the prove out circuit. If the harness becomes unplugged from the switch the ground path from the ignition switch thru the harness to the lamp will not be completed, and the lamp will not light when starting Bronco.
If there is a pressure differential in the hydraulic system, the shuttle will move sideways pushing the pin against the contacts in the connector. This completes the ground path from the frame, to the H-block, thru the shuttle, to the pin in the switch, to the contacts in the connector, down the purple wire to the lamp assembly at the dash.

Also worth noting on these 50 year old rust buckets, the only ground wire between body and frame is the one between the main gas tank and frame, and that's relying on the tank being in contact with the bottom side of the floor.

EDIT: clarified "connector pins in switch".

 

[Slowleak]

You guys are both kinda right but, for clarification, it’s actually the two pins in the brake SWITCH that are connected at all times.
If you remove the connector from the switch the warning light will not function when the key is switched to start but, with the connector plugged into the switch, the light will function when the key is switched to start, even if the brake switch is not grounded and is just “hanging”. In that case the circuit is still complete and the ignition switch is doing the grounding but, for the switch to activate the warning light in the event of a brake failure, the switch does have to be grounded.

 

[mattyv87]

The way slowleak and oldtimer described it is how I understood that it functions. I think part of my problem is poor grounding. I bought the grounding kit from northeast. I'm sure that'll fix the issue.

Again. Sorry to hijack.

 

[msommer001]

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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[DirtDonk]

Right again, Oldtimer! I was forgetting the basic function, which I have done now and then over the years. Sorry for the confusion. I should’ve been sleeping!