Low front air pressure

My 99 Dutch star air gauges do the same thing. and as Fireup described my oil pressure gauge problem it goes from 40 to 100 then back. These are bothersome but I asked the cummins shop about those and he said to contact the manufacturer about the gauges. He then said that if there was a real problem the engine computer system would let me know as all those systems are cummins then converted in a demox box (I think I called it correctly) to the dash gauges. I will try that series of checks too.

Stim wrote:
I would check owners manual or contact the manufacturer.
There is a chance that there is a regulator to air tank for the front brakes to keep from applying too much front pressure causing a dangerous condition during heavy braking.
Might be part of an anti-lock system.
Just a wild guess after years of driving/working on semi's.

Stim,
While your line of thought might make some logical sense, the air systems in not only diesel motorhomes but, also every semi on the planet, provide the same exact pressure in the front or "Primary" system as they do for the rear "Secondary" system. There's normally a "proportioning valve" in certain air systems that directs the correct amount of air to each set of brakes for a given situation. Take a look at this:

Proportioning valve(s)

Scott

I would check owners manual or contact the manufacturer.
There is a chance that there is a regulator to air tank for the front brakes to keep from applying too much front pressure causing a dangerous condition during heavy braking.
Might be part of an anti-lock system.
Just a wild guess after years of driving/working on semi's.

Hi Scott,
Thank you so much for your help. Right now I'm in a campground and I don't want to annoy my neighbors, so tomorrow when people are not around, I will try this. Thanks again

Well Ro646a,
Just for some preliminary tests which, may or may not reveal anything, here's what I might start with. First, I'd find a flat and level spot to park the coach. Then, start it up and let it run, WITHOUT THE PARKING BRAKE ON 'till you hear the governor cut-out pressure reached and the spitter valve on the air dryer release. Now, according to your report, you should be sitting at 110 psi. in the rear and around 70 psi in the front, correct?

Now, again, with the parking brake off, and engine OFF, but, the ignition ON, step on the brake pedal on and off, at a mild pace until your pressure reads on the front gage, around 65 psi. That is the pressure that by DOT specs, is supposed to sound your LOW AIR alarm buzzer. If your buzzer does sound at or very near (above some or below the 65 psi mark), than at least your gauge is accurate up to, the 65 + or - for that test.

Then, keeping with the same parameters, that is, engine off, parking brake off, ignition ON, keep stabbing the brake pedal, after the low air has sounded, until you reach close to 30 psi. At that pressure, or very near it, plus or minus a pound or two, THAT PSI is when your parking brake knob SHOULD pop out and therefore, your parking brakes have AUTOMATICALLY been set.

Again, this is presuming that at least one of your gauges are reading the correct pressure for the last two tests. Now, for the next test, this, based again on your report, is for the rear gauge since the front can't provide accuracy. In the previous tests, you've ran the air pressure all the way down to lower than 30 psi, correct? So, now, fire up the engine and, if you know how to do it, set the idle to about 1000 rpm. If you don't know how, about 90+% of the diesels out there can achieve high idle from simply turning on the cruise control and then hit "SET", then, repeatedly keep touching the RESUME button to increase engine RPM until it reaches a desired RPM.

But, for this test, you need to do this quickly. In other words, when you fire up the engine, IMMEDIATELY to the procedures for upping the idle. What we're testing here is, the compressors rate of pump-up speed to achieve pop-off on the governor. Using your watch or a if you like a stop watch, observe the amount of time it takes to achieve pop-off, from the time you start your engine.

There are DOT specs on just how long it should take to achieve this but, I've forgotten them. But, at this point, you're not too interested in the time frame but, more or less, that your rear gauge is somewhat accurate with the governor cut-out vs what the gauge reads. If your rear gauge reads 110+ when you hear the spitter spit, that gauge is fairly accurate.

Now, also, when you start pumping up pressure from the low of around 30 psi, watch BOTH gauges to see if they rise equally from the low starting point. If they do rise equally from the low starting point but, your front simply slows or stops at the 70 psi mark, well, I'd guess one or two things here. First, your front gauge is defective. Since the "SYSTEM" has performed somewhat accurately up to this point, it may be presumed that the front air system is also behaving "normally" but, the front gauge is reading inaccurately.

A tech that's familiar with air systems could quite possibly know just where to tap into, with a hand held gauge to see if your front air system is in fact, stopping at 70 psi or, is continuing on to the 110 psi your rear is stopping at. I'm not an expert on any of this but, in many, many cases, in diesel powered coaches, with Freightliner or maybe even Spartan Chassis's, the air gauges are not reading directly from the air system.

They are getting a "converted" reading from what's called the VDC or, in later years (as in '04 and later) the MMDC modules. The VDC is the Vehicle Data Computer. The MMDC is a Multi-Module-Data-Computer. Both of those have what's called transducers built into the circuit boards that also serve as interpreters for many other functions of the engine, trans, ABS, and the air systems.

In the VDC era, quite a few of them developed what's called "cold solder joints". That is, where the terminals of the transducers were soldered to the circuit boards, those solder joints, over time, became faulty. But, if and when that would happen, about 98% of the time, to the ones it happened to, the gauges would drop to ZERO psi and then bounce back up etc. repeatedly. So, is this your problem with your front gauge, not a clue.

Did any of this info help, also not a clue. Without being a tech, and analyzing your coach as it sits in front of me, it's very tough to call this problem from a distance, without seeing it before and, knowing of a remedy. Good luck.
Scott

P.S. As for your tach, I can't even begin to help you with that.
Scott

This is the air for the front brakes, etc. I don't know where else I can get a reading

Ro646a wrote:
I guess my real question is whether there is an easy way to determine if my air gauge is inaccurate or my pressure is too low

Use an handheld air gauge.:S

Personally, I don't have faith in the on board ones anyway.

I guess my real question is whether there is an easy way to determine if my air gauge is inaccurate or my pressure is too low

The tach constantly reads 1/2 the RPMs displayed on my Scangauge, and the front air gauge never goes above 70. The rear air runs at around 110.

Did they both act up at the same time?