Low Vacuum warning light

[Bikersmurf]

‘97 E350 with 7.3 PSD

Been chasing a number of things that have been making the (BRAKE!) idiot light come on.

Fluid level now full
Parking brake off

Now only one thing left that comes and goes... Low Vacuum! Pump the brakes at idle a few times it comes on... rev it up for a minute and it goes away. Unplug the vacuum sensor from Vacuum and it stays on.

Now I believe I the rear brakes need to be adjusted a bit more.

Could the vacuum pump be getting tired? I replaced it about 3 years ago and it only has about 1000 hours on it. Since a new one is about $200 I’d rather not just throw money at it.

Thoughts?

 

[damarshall7]

I have the same problem, and i cant get any consistency with the light coming on or off. Im good on the fluid, parking brake is off, and i can only assume the vacuum pump. Does your vacuum pump make noise when braking? If you figure this out, please let me know. Not having crusie control can be a PIA

 

[NevadaLover]

Disconnect the belt, then disconnect the vacuum line from the pump, spin the pump by hand while holding your finger over the suction port on the pump, it should pull pretty tight if the pump is good!
That is, if you don't have a vacuum gauge available!

Fluid level, parking brake not released (or switch malfunction) and vacuum are the only things that turn on the brake malfunction light.

 

[Bikersmurf]

I have the same problem, and i cant get any consistency with the light coming on or off. Im good on the fluid, parking brake is off, and i can only assume the vacuum pump. Does your vacuum pump make noise when braking? If you figure this out, please let me know. Not having crusie control can be a PIA

The Vacuum pump doesn’t seem to be making any noise... that can be heard over the 7.3. If anything it’s quieter.
Its quite consistent. When I start the rig the light is on. After a bit it goes off. If I pump the brakes 3+ times it comes on. If it idles a while it’ll go off, or if I rev it up a bit it shuts off faster. If I hit the brakes right after it goes off, it comes back on.

After pumping the brakes several times, (especially in reverse) the pedal starts to get harder. I’ve found the brakes getting harder with applications issue happens less when the brakes are adjusted tighter.

It was feeling like they were getting out of adjustment so I crawled under the rig to snug them up a bit... but messed up and loosened them off a ways first. I’ve since snugged them down some, but they’re not snugged up enough yet. Because of the LSD, I need to put it on sxlestands and adjust them... likely bleed them also.

I also should pull the drums, inspect, and ensure everything is in the right position.

A couple years back, I swapped the ‘97 Master with an ‘01 Booster & master. The newer Master has a larger bore and the brakes were more responsive. The booster has seemed to use more vacuum per application than the original booster.

 

[Bikersmurf]

Disconnect the belt, then disconnect the vacuum line from the pump, spin the pump by hand while holding your finger over the suction port on the pump, it should pull pretty tight if the pump is good!
That is, if you don't have a vacuum gauge available!

Fluid level, parking brake not released (or switch malfunction) and vacuum are the only things that turn on the brake malfunction light.

I’ve got a few vacuum gauges around so I’ll try to check how much it is pulling. If it wasn’t such a pain to pull the serpentine belt I’d have already pulled it,,, the Down side of an E350 Ambo with a 7.3, 215 amp alternator, A/C, dual battery banks... NO space

 

[shenrie]

I just had a vacuum pump go out on me last month on my 03 7.3 and there was no idiot lights at all. Scanguage didn’t register anything either.

We were on the road and the noise was noticeable, but sounded more like a relay going bad under the dash. Couldn’t figure out what it was until I was trying to go from 55 mph to 30 for a sharp corner, then it was painfully obvious.

 

[yager]

Also, you can check the vacuum lines.
Use cap/plugs to cap off various lines, and test.
Check the main feed from the motor.
Its not uncommon on a 10+ years old van for a line to not be broken, but have a minor leak.
Check the heater control lines too, they control by vacuum. ( the end piece are rubber)

 

[Bikersmurf]

Also, you can check the vacuum lines.
Use cap/plugs to cap off various lines, and test.
Check the main feed from the motor.
Its not uncommon on a 10+ years old van for a line to not be broken, but have a minor leak.
Check the heater control lines too, they control by vacuum. ( the end piece are rubber)

I’ll check them too. I’ve got lots of spare lines and fittings do it wouldn’t be hard.

 

[djbonsu]

Check your vacuum lines, vacuum pump, vacuum manifold AND brake booster (well maybe there isnt a way to check that. I too had a problem right before our cross country trip and I narrowed it down to a tired brake booster. Swapped in a new one and everything was good to go. Also, where is this low vacuum light?? I have never seen one on the van 7.3s. Is this an ambulance feature?

 

[NevadaLover]

Check your vacuum lines, vacuum pump, vacuum manifold AND brake booster (well maybe there isnt a way to check that.

Check the booster by using a vacuum pump hooked to the line going into it, it should hold vacuum with no loss, if it bleeds off more than a few inHG in 5 minutes it is probably tired and needs to be replaced.

 

[Bikersmurf]

Brake booster holds vacuum overnight, so it’s likely not the culprit.

It’s not an ambulance feature. It is the red (BRAKE !) light that comes on when the parking brake is set, brake fluid is low, vacuum is low, and the third Wednesday of the month after 10 if it’s a full moon.

I’ve got spare vacuum lines, a spare manifold, and many other spare parts... just need the time, energy, & daylight to investigate. Not driving it a ton at the moment, so it’ll wait a bit.

 

[bdog1]

I had a cap come off a small distribution manifold on the firewall. (Like a six way adapter.) But it’s usually the diaphragm in the pump. You can just buy the diaphragm housing separate.


Sent from my iPhone using Tapatalk

 

[Bikersmurf]

I had a cap come off a small distribution manifold on the firewall. (Like a six way adapter.) But it’s usually the diaphragm in the pump. You can just buy the diaphragm housing separate.


Sent from my iPhone using Tapatalk

It has only been about three years since I put on a brand new pump, so I’m hoping that it’s something simple instead. The stores like to gold plate everything before selling it here,

 

[Bikersmurf]

So.... No vacuum loss from any lines. Vacuum holds constant if heater controls played with.

Pump brigs pressure up to about 15 in. So long as the brakes are not pushed. Press the brake and the pressure drips 3-4 in... release and it dips a touch more. After 3-4 pumps the pressure drops down to almost nothing.

It recovers after a bit.. especially if above idle. I’m not sure if the booster is using too much, the pump sucks (to Slow), or a combination of both.

 

[IdaSHO]

You sure its even a true fault?

How about the e-brake pedal not returning all the way, or a bad e-brake pedal switch?

The ebrake pedal on my '96 PSD is a bit lazy, doesnt return all the way, causes the light to come on occasionally, normally after hitting a big bump.