Is the brake warning light supposed to come on when the ignition switch is turned to ON (but not while truck is running)? Just wondering if i have a bad bulb.

 

It depends on your definition of "ON." As you turn the key past RUN toward START, the brake warning light should indeed illuminate. This is called PROVE-OUT to let you know the bulbs can be trusted.

If the brake warning light illuminates when the key is in RUN, as in the position the key rests while the engine would normally run, then the pressure differential switch is turning the light on to tell you there's a problem. It doesn't mean the bulb is bad. A bad bulb can't light up.

This is all independent of the truck running or not.

 

Thanks. During testing I wasn't going past RUN. I have an aftermarket brake proportioning valve and had never hooked up the warning switch. I'll test again.

 

If I'm interpreting this correctly, you're saying that with the key in RUN, but the switch disconnected, the light still illuminates. That means the bulb is shorted ON somehow - most likely the disconnected pigtail is hitting metal under the hood and grounding it out.

If your aftermarket proportioning valve has a single-terminal pressure switch, the factory light can still be connected, if you're interested.

 

The switch had been disconnected till this morning. I wired it up correctly (one wire to the one wire piggyback connector and the other to a ground in the engine bay). Yeah, you do have to clip out the old connector, as these new proportioning valve switches just have one wire. So the light basically hasn't been coming on for over a year. But I'll make sure I go a little past RUN now. I'll check in later...

 

If you wire one of the PURPLE with WHITE stripe wires to ground in the engine bay, the light will stay on permanently. The connectivity you have described is INcorrect. If your new valve has a single-port switch, connect the two PURPLE with WHITE stripe wires coming from the TRUCK together, then tie that common point to the switch. No ground anywhere.

When seeking help with electrical issues (or any issues really), it's best that you list out any related work you've done up front. In this case, the work you did was the problem.

 

10-4. Believe it or not, the instructions that came with this said to use one of the purple/white wires to ground and the other to the single wire connector. I swear it. But I'll take your advice. This valve is likely offshore and written by someone who doesn't know English.

 

Oh I believe it. You never know what you'll end up with when it comes to that stuff.

 

these are likely the same folks that sold dog food laced with melamine.

 

So the light works fine with both purple wires jumped together.
My proportioning valve warning switch has just the single wire. Are you saying I should jumper both purple wires and connect them to the single wire from the switch?

 

By "works fine," I'm assuming you mean the light turns ON as the key is turned all the way toward START, but shuts OFF when the key returns to RUN. With no connection to the proportioning valve, that's the only behavior your should observe.

Bingo. One of the PURPLE with WHITE stripe wires on the TRUCK-side connector is the "ground out" signal to the BRAKE light in the dash. The other PURPLE with WHITE stripe wire is the PROVE-OUT signal from the ignition switch. It's grounded when the key passes the PROVE-OUT position on the way to START. If you tie these two wires together, the ignition switch can turn the bulb ON to PROVE-OUT the bulb.

If you then tie this common point to the single wire coming from the proportioning valve, the proportioning valve is also now capable of grounding out the bulb to turn it ON. With the two wires shorted together and connected to the proportioning valve switch, both the proportioning valve OR the PROVE-OUT position of the ignition switch are capable of turning the bulb ON by grounding it out. This is desired behavior.

The original proportioning valve switch (from the factory) had a matching two-terminal connector, but the two terminals were shorted together INSIDE the switch for the same reasons I've described here. In the factory case, the short was INSIDE the switch as opposed to OUTSIDE like you must do here to prevent the bulb from coming out during PROVE-OUT (making you think the warning system is operational) if the switch was accidentally disconnected. This was a safety feature. In this case, with the proportioning valve switch disconnected and the jumper INSIDE the disconnected switch, there's no way for the PROVE-OUT position of the ignition switch to ground out the bulb and lead the driver to believe the warning system is operational when in fact it is not.

Unfortunately, you lose that safety feature with a single-terminal proportioning valve, but I don't think you have a choice considering what's available. Here, the bulb can still come ON during PROVE-OUT if the switch is disconnected, but as long as you use good strong connections, it's not a problem.

 

Have you not read any of this thread?

 

Good diagnosis FMC400. It is uncanny that at least the instructions that came w/my valve and also several other aftermarket ones on the internet clearly say one lead goes to the switch and the other to ground. So today, I wired the single switch lead to the paired purple/white wires and all worked as it should. Thanks for your great explanations.

 

Glad to hear you got it worked out. Not sure why aftermarket instructions are the way they are; the behavior depends on which of the two wires is grounded in the INcorrect configuration. In your case, the light stays on permanently. Had you grounded the other wire, the light would function as a warning indicator as it should, but the PROVE-OUT mechanism would be missing (I suspect this is how it works out for a handful of folks). Wiring it the way you've done here enables both the warning light and the PROVE-OUT mechanism to let you know the bulb is good. This is how the instructions should be. This all assumes the switch in your proportioning valve is a ground-out style switch, which it should be if there's a single terminal.

For those interested, the mechanism I've described is shown in the left side of the following schematic: http://www.fordification.net/tech/im...aster_3of9.jpg

 

I have found so many informative threads in this forum, it is amazing. I am curious though, what problems in the brake system does the warning light actually indicate? Reason I ask is that the red brake light in the dash of my 73 F250 has been on since I purchased the truck about a year ago. The brakes seem to function well, the brake lights work, and both wires are connected to the switch that is on the junction block for the brake lines, I assume that is the proportioning valve? What would cause this?.....Tom