1993 F-350

Seriously, Ford, no brake light switch adjustment? What were you thinking?

It has happened several times now. I am driving along and tap the brake pedal to disengage the Cruise Control and the engine increases by several hundred RPMs. Last time it was probably 700 RPMs.

When I am close to home I stop to observe the reflection of the brake lights off a stop sign. There is some hydraulic pressure in the system, because the truck will stop rolling forward, before the brake lights turn on.

I think to myself that I'll just adjust the switch so it closes with less brake pedal travel. Surprise! There is no adjustment. The way it is, there has to be many dimensions that have to be just right for the switch to close at the right time which is before the master cylinder develops pressure. Normal wear will make this worse, requiring more pedal travel for the switch to close.

Is this why Ford had that recall where they installed a new harness and pressure switch? How many people got rear ended because of a delay in the brake lights shinning?

I could try to replace the switch but there is no assurances that it will work any better.

Ideas? Suggestions?

Have you experienced this?

 

It's because it's hot all the time & causes fires spontaneously.

 

What is hot all the time? If it is hot I can see why it causes fires.

 

The speed control wiring. And the meaning intended was that a delay in the brake lights coming on has absolutely nothing to do with the recall.

As far as your complaint, that doesn't sound like a brake light switch issue to me. Why would the engine speed up if the brake lights don't come on? I could understand that if you have an automatic and the torque converter clutch was unlocking but the speed control wasn't coming off. But the torque converter clutch is also triggered by the brake lights. So if it was the brake light switch the engine speed shouldn't climb.

 

Oddly enough, I do know what he might be referring to.

I have recently been experiencing a RPM surge when tapping the break pedal to come out of cruise.

As the cruise is "disengaged" the RPMS flare upward a bit before the truck begins to decelerate. (sounds almost like over shifting into neutral)

I have no idea if there is any non-illumination/delay of the brake lights since I ain't back there on the interstate...

My servo is not original & is from a 2000 up something Crown Vic.

 

My memory told me that a adding a pressure switch was a part of the recall. After looking it up I see that I was wrong. Correct me if I am wrong, but isn't the pressure switch in the brake hydraulic system a backup to disengage the CC?

Why would the engine speed up if the brake lights don't come on?

Because when the brake light switch closes it signals the CC to disengage. If the truck slows down with out that signal, the CC will apply throttle to speed up the engine. On my truck there is brake hydraulic pressure, thus slowing the truck, before the closing of the switch sends a signal to the CC controller to disengage.

When this happens I press the brake pedal harder and things work normally, after getting a scare.

 

I have no idea if there is any non-illumination/delay of the brake lights since I ain't back there on the interstate..

Could you do me a favor? Sometime at night, stop somewhere safe where you can see the reflection of the brake light. Slowly press on the brake pedal and note if your truck stops rolling before the lights come on. That is if you have an automatic transmission.

 

The slight increase in RPM when pressing the brake is the lockup torque convertor disengaging. As for the electrical circuit for the lights being energized at all times, I'm fairly sure that every vehicle I have seen had brake lights that function with the key off.

 

I think the brake light switch sensitivity is common on these trucks. I can almost completely depress the brake pedal without turning on the brake lights if I press slowly enough, and I watched another OBS truck's brake lights turn off while at a red light and stopped. That being said; with the engine off, and no vacuum in the brake booster the brake lights turn on with the slightest touch of the pedal.

 

Not in this case because I feel the acceleration. 700 RPMs is not silght.

My test watching the reflection of the brake light show that the brakes are engaged before the lights turn on. It can be inferred from this that the CC is still engaged and is compensating for the reduction of speed by increasing throttle.

Me me too except W123 Mercedes like my 1984 300TD. The cigarette lighter and glove box light are not powdred with the key off.

 

If ANY hydraulic pressure is required to light the brake lights, that means it's the PRESSURE switch on the master cylinder kicking in. That's a SECONDARY switch. Most likely that means that the PEDAL switch on the pedal arm in the cab has failed. You should be able to jam a snow brush or other such object between the seat and the pedal, and induce minimal pedal movement, to light the brake lights. If that's not happening, check that switch (with a meter).

 

The brake light switch is mounted to the brake pedal, and closes when the brake pedal pushes it into the rod coming through the firewall. The problem I saw while playing with it earlier is that the brake pedal can push the rod in before the switch actually closes. Since these trucks have power brakes that small amount of rod travel is enough to engage the brakes without closing the brake light switch. I want to assume that the brake booster is always slightly pulling on the actuating rod, so that brakes are applied as soon as the pedal is pressed. This might also explain why the switch closes immediately when the truck is off without vacuum, because the booster isn't pulling on the rod the switch never has to catch up to it to close.

How this pertains to cruise control: the brake light switch is the primary input to tell the cruise control to cancel. When the switch behaves like described above during light braking the cruise control will try to compensate by increasing engine speed, until pressure is sensed in the backup switch located at the front of the master cylinder.

The easiest solution I can think of is to press the brake pedal harder. There might also be some wear in the pin connecting the rod and the pedal, but I don't have another vehicle to drive while I tear apart my truck.

 

It's converter flash from the clutch disengaging. The cruise motor can not shut the throttle faster than the clutch unlocks so you get a flare in rpms. I have had this in most of my vehicles, excluding the manuals. But, even then if I stab the clutch I get a rise in revs.
Now, if you are able to depress the switch slightly without lighting the lights then it sounds like a dead spot in the switch. My clutch switch went out in the 95 and wouldn't allow it to start, however it would still turn off the cruise because there are two circuits actuated by the same rod. (I bypassed the clutch switch and tied into the horn relay go start because I was working too many hours and didn't have time to do the switch at that time lol) I'm not positive the brake switch is the same but it makes sense because I would think that running your flashers would shut off cruise and on my truck that is not the case.

 

That sounds a lot different from what you described in your first post:

What most people would call "tapping" the brake pedal won't slow down the truck. It should briefly flash the brake lights, unlock the torque converter and disengage the speed control, but it wouldn't be enough to give any hydraulic pressure to the brakes. So my questions and comments were based on the idea that you were seeing this when you just tapped the brake pedal.

If you are actually applying the brakes and the brake lights aren't coming on, that sounds like at least a gummed up brake light switch. I'd pull it off and try to clean it up, or just replace it with a new one.

 

There are two brake switches in the 92 and up cruise control circuit. The brake pedal switch which controls the brake light as well as an output to the cruise control and ECM and the brake pressure switch which also provides an output to the cruise control.

The brake light switch does not sound like it is failing, but it may be possible that the brake pressure switch is?

Just a wild guess at my end.