Hi All-

I've just purchased a 2001 E350 7.3. About 20 mins into the drive home the brake warning light illuminated and has continued to illuminate intermittently. Here is what has been done...

E-brake switch: Normal
Brake position switch: Normal
Fluid level: Normal
Vacuum pressure: 24in
Replaced all exterior lighting
Removed pigtail (tailer wiring)
Removed cluster replaced bulbs and cleaned contacts
No leaks in the system when parked or during operation
Vacuum pump is visibly new
Master cylinder is visibly new
Brake fluid reservoir is new

Symptoms: About 7mi into the ride the brake light will come on and kill the cruise-control. The light is intermittent and will fade in and out as well as pop on and off. The problem (sometimes) seems to be effected by bumpy roads. (Again the e-brake switch is functioning perfectly.)

Symptom summation: The brakes at the pedal show no difference in operation regardless of warning light status. If it kills the cruise it must be hydraulic. I have removed the light from the dash cluster to see if the operation of the CC is still affected after the 7mi mark when the now theoretical brake light would have turned on and the problem persists. Also, it will not chime. When the light comes on the van makes no noise what-so-ever, I thought that interesting...

An important note on vacuum: All of the sources that I can find say that this light is activated by A) Hydraulic problem B) Emergency brake deployment C) Vacuum leak. SO...I pulled the vacuum pump supply line and attached it to the vacuum meter (24in). When I started the truck I pumped the brakes and had no (BRAKE) warning light. Question: How does the truck know I have low vacuum? There are no harness connectors to the vacuum pump or brake booster. Furthermore if I remove vacuum from the system shouldn't that sensor fire? If it doesn't that would mean it is the problem. Can anyone confirm this???

Thank you for your help,

-Mitch

 

Hi Mitch, I have zero experience with your setup. The new parts on the van suggest to me that the previous owner might have had the same problem. Can you contact him for information?

The only thing that comes to mind is a vacuum reservoir? If it was leaking would that toss the light? Can you connect the vacuum gauge to monitor vacuum reading while driving? Sorry, all I have is questions. jim

 

Hey Jim-

Thanks for jumping in. Previous owner has held the position that he never saw it before and doesn't understand why it happened to me the moment I drove away. Which to be honest seems really hard to believe... However he has stuck by that since my first inquiry but has not done anything to help me.

Onto your next idea: "Vacuum Reservoir"? Where might that be? (Keep in mind it is diesel it has a vacuum pump, it does not pull vacuum from the manifold like a gas motor.)

You have an interesting idea, to connect the meter while driving. Maybe there is another system that uses vacuum but is only having an issue cruising...?

-Mitch

 

Bleed the MC and is there a switch for the cruise on the brake pedal arm?
Those are the first two things I'd look at.

 

The only thing I do know about diesels is the separate vacuum pump. That's where my knowledge ends!

I am assuming there is a reservoir but with a pump running, maybe not. Gas motors are all over the place for vacuum depending on throttle setting, load, phase of the moon, etc. Reading the vacuum while running should be easy to rig up and at least eliminate that as an issue. jim

 

Yes the master cylinder has a cable switch coming out of the bottom. When thats unplugged nothing seems to happen other than the CC doesn't work.

Do you think there is air inside the MC... Or that the brakes haven't been properly bled?

Thanks,

-Mitch

 

It is possible
I always look at the obvious cheap stuff first

 

That is the correct place to start. Only take a few mins and some brake fluid to try. i'll give it a shot. I just thought that if there was enough of a problem to set off the warning light that the problem would be really obvious... or that if there was something wrong hydraulically (enough to set off the warning light) that I would certainly feel it.

I'll bleed the system and give you guys a shout back.

Thanks,

-Mitch

 

Not to nit-pick, but it's a parking brake, not an 'e-brake'...

I have no idea where the kiddos learned this....but I hear the term quite often lately.

Arrrgg....

 

Hmmm, I remember a TSB on Explorers for this type switch leaking brake fluid and causing vehicle fires. I have worked on cars that used this switch for brake lights and the fluid leakage caused a current drain enough to discharge the battery after days of sitting but not enough to light the brake lights.

Disconnect the wires and ohm the contacts to ground to check for leakage. Easy test to eliminate one more thing. Plus it has my 2 favorite things: cheap and fast!
jim

 

PS, found this
https://www.ford-trucks.com/forums/1...-cylinder.html

 

HOLY CRAP!

Ok... So this evening I started seeing all of the symptoms, just so happened to have it sitting in the driveway at the time. So I grabbed my vacuum tester and started isolating pieces of the system while the anomaly was occurring...I isolated it to a long vacuum line headed under my fuse block. It connected to an object down there with a harness connector. Thats my problem, I just don't know what it is. Anyone have any insight on what that object is or does???

Thank you,

-Mitch

 

Ok So everything I've found calls it a Low Vacuum Warning Switch.

I CANT FIND ONE. Everyone wants to sell me the connector and pigtail... wtf.

Its a silver thing with a harness cable, vacuum connector and its the size of a tuna can. I need a new one.

Does anyone know where to find one of these??

I'm going to bed.

-M

 

Anything like this, Mitch?
Stewart Warner Performance Vacuum Switches 77342-04 - Free Shipping on Orders Over $99 at Summit Racing

Also found some old blog comments but little help.
LOW VACUUM WARNING SWITCH ???
jim

 

Or,
https://presair.com/product-category...vacuum-switch/