Lights on nobody's home

[bopplumb]

I have a small but to date unrepaired problem. When idling the brake pedal becomes hard and the ABS and Brake light come on simutaniously. They remain on until the engine RPM's are increased, or the brake pedal is released. I have replaced the master cylinder. and bled all the brakes. I even replaced the front calipers and hoses. Any advise?

 

[OKish]

hmm, maybe the vaccum pump diaphram or the brake booster diaphram is going south.

 

[bopplumb]

Could I have a ABS (almost braking system) problem? P. S. it is a 93 4X4 CC LE.

 

[Philip]

Check the vacuum line going to the booster. The ABS and brake lights coming on is a sign of low vacuum. same with the hard pedal.



Check the line over good to see if it has rubbed threw.



Check the vac fitting on the booster. It might be cracked.



Check the small vac line going to the low vac switch by the left hood hinge.



If all lines check good. The do a vacuum test of the pump to see if it is pulling a good vacuum. The vac switch if I remember right trips offat 6 pounds of vac.

 

[bopplumb]

Thanks will check them out. What vacume preasure should I have. I have purchased and installed a rebuild of the vacume pump. Followed the video and had no problems doing what was required. But I know pump may be bad.

 

[OKish]

Thanks will check them out. What vacume preasure should I have. I have purchased and installed a rebuild of the vacume pump. Followed the video and had no problems doing what was required. But I know pump may be bad.

vacuum is not measured by pressure. heh, its measured in inches of water or murcury. i would say if its pulling 10 inches at idle its doing good. yea, i would check all of those vacuum fittings too. could be a bad hose or fitting.

 

[bopplumb]

I stand corrected. It was just a incorrect term I used referring to the gauge reading.

 

[Philip]

Not knowing the section of country you live in makes it hard to tell about how much you should have.



Here in IN a reading of 14 to 18 would be a normal reading. If your in high country it would be less. A lower alt would be a higher reading. Thats why people should have the state listed where they live. Somethimes we need to know the region.

 

[bopplumb]

central Pa.

 

[bopplumb]

I'm taking from the responses that nobody feels this problem is related to the ABS system.

 

[blacksheepdiesel]

Idle speed correct?



Trent

 

[bmoeller]

I'm taking from the responses that nobody feels this problem is related to the ABS system.

I'd say so. On my Ford diesel, I had a blown diaphram in the Vac brake booster. Hard pedal and only 6" vac. After that was replaced, back up to 26"



Not sure what is normal on the Dodge, though. My Ram pulls 11". Since I have the hydraboost, no problems with the bakes that way.

 

[cerberusiam]

You have a vacuum leak or a bad vacuum pump. The ABS lights also come on when the system doesn't have enough vacuum.

 

[Moab4x4]

I agree as to the low vacuum issue and not the ABS. The vacuum sensor under the hood tells the ABS system when the vacuum is low and tells it to turn on the brake lights to alert you to a problem.

The fact that you have boost to your brakes at RPM but not at idle says that if your vacuum pump is spinning fast enough, then you have sufficient vacuum.

1, big vacuum leak.

2. vacuum pump rebuild didn't take.



Test the vacuum at the pump with only a guage attached. If you have a set of AC guages you could probably McGyver something up there. Otherwise find a vacuum guage and plumb it directly to the vacuum pump. Then test at the brake booster with everything else hooked up.

Minimal difference = bad pump

big difference = big leak, maybe in the brake vacuum booster itself.



For a rough test, you could use that vacuum sensor on the fender. get the light to come on at idle and disconnect and plug one thing at a time until the light goes off. If it doesn't go off with the sensor hooked directly to the pump its either the pump or the sensor. You will need to adapt hose sizes to do that test though.



BTW how involved was that rebuild? I have seen the kits on ebay and wondered.

 

[dpuckett]

Moab4x4- get the kit from Cummins- $7. 50 a kit. It's just a few seals is all.



mweist- My 92 with 250K pulls about 22in vacuum. My bet is the valve on top of the booster or the booster. But, start with the simple stuff and work your way up.



Daniel

 

[ZedMikey]

vacuum is not measured by pressure. heh, its measured in inches of water or murcury. i would say if its pulling 10 inches at idle its doing good. yea, i would check all of those vacuum fittings too. could be a bad hose or fitting.

Not to be a stickler, but in-Hg IS a measure of pressure. You will notice barometric pressure is generally reported in in-Hg. 1 psi = 2. 036 in-Hg. So, measuring vacuum in psi is just fine.

 

[ZedMikey]

Oh, and 1 psi = 27. 68 in-H2O.

 

[Moab4x4]

Not to be a stickler, but in-Hg IS a measure of pressure. You will notice barometric pressure is generally reported in in-Hg. 1 psi = 2. 036 in-Hg. So, measuring vacuum in psi is just fine.

There ya go sounding like my old physics professor

Absolute pressure vs guage pressure.....

Working in the High Vacuum, High Energy physics world for a while we used microns, or Torr.



In my shop on my vehicles, I will stick with mmHG.

I just wish I had my old roots blower / mechanical pump combo from the labs. My cheap JC venturi pump isn't in the same league.

 

[ZedMikey]

mm-Hg or in-Hg... the units just don't make sense. They aren't derived from the basic units of the system. Psi and pascals are much better, IMO. Pounds/in^2... units make sense and when multiplied by an area, units cancel and you get a force... in-Hg * in^2... that should give (in-Hg)in^2... well, nobody uses (in-Hg)in^2, which turns out to be a unit of force. Go figure. Pascals... N/m^2... yup, makes sense, too. Weee!!

 

[Greenleaf]

I hear a loud sucking sound.



All seriousness aside... ... ... . shut off the truck and remove the big hose from the vacuum booster thing Then listen for a loud sucking sound. If you don't hear much, then your hard of hearing or you have low vacuum which I measure in decibles