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rodekyll · 05-02-2021, 07:52 PM

2002 Newmar mountain Aire on a freightliner chassis. The situation is that the low air alarm keeps going off and the rear air gauge fluctuates erratically. The alarm reacts to the gauge position. The low-dollar test -- blowing the tanks -- suggests I have plenty of air, and that it's the electronics that's giving the problem, but I thought a bit of troubleshooting was in order. I did some reading here and decided that I need to isolate what is actually wrong here: a mechanical air pressure problem in the rear tank or something related to the the Vehicle Dynamics Control (VDC), -- the backbone, the gauge wiring, the gauge, or the VDC unit.

A digression -- One technical point I don't understand is the relationship between the VDC and the diagnostics data port. Does the data port read the VDC or directly from the sensors that it reports? In other words, if the VDC is bad, do diags still work?

Back to the troubleshooting
I started with the dash end elimination, since it's easy to get to, and I'm lazy. Since the plugs all deliver the same set of signals, it doesn't matter which gauge I have which wire plugged in to. So I swapped the front and rear air plugs, since they are adjacent, and I'm lazy. If the wiring to the gauge is faulty, the problem will follow the wire. But the problem stayed with the gauge. Once it hit about 65#, it got twitchy, and the alarm followed the gauge. Some time it would act like a leakoff, and sometimes it would do mindless, full sweeps. Since the gauge and the alarm take the same signal, the backbone is intact, and the gauge is good. So it's either the signal coming down the wire that's bad, or the air at the rear tank has a problem.

Next I cleaned up the connections on the VDC, since it's easy to do, and again, I'm lazy. No joy.

I decided that an easy way to eliminate the tanks as the problem is to swap the red and green tubes from the VDC box, which is located on a piece of frame, under the bedroom hatch in my Mountain Aire. I decided to blow the tanks first to remove pressure from the tubes. Then I disconnected the push-to-release couplings and swapped them. If there was a problem with the air at the tanks, the problem, again, would follow the gauges. But once again, the rear gauge acted erratically, just like before. So the tank air is not the problem (don't know yet if it's good, because the reading is bad). That means the problem is internal to the VDC --the rear air pressure sensor or it's circuitry.

My reading suggested that the cold solder issue could be the problem. That's not lazy work. I decided to check the connections first. Each sensor has four leads soldered to the circuit board. The traces go to resistors. I checked continuity from the legs of the sensors to the traces, and then through the resistors and such. I had good continuity to the traces, and when compared to each other, nearly identical readings though the various resistors on both front and rear sensors. I desoldered/soldered the legs anyway, but still no joy. My conclusion is that it's not the easy cold solder problem, and it's not the sensor connections. The problem is either the rear tank sensor or deeper in the electronics.

At this point, the lazy solution should be to replace the VDC. I could chase solder joints for a week only to discover that it's the chip. But apparently the mfgr has no interest in providing either stock or support for the product. So a new one with my exact part number is not going to happen, and the used market is going to be a crap-shoot. I notice though that the different part numbers have a very similar appearance and connectors.

More research however isn't giving me an interchange list. Does one exist? Are there replacement options? Are there 3rd party substitutions out there? Is there a way to ditch the VDC altogether?

JNSNWT · 05-02-2021, 08:48 PM

If you’ve done all this you’re not lazy .... just efficient ��

Licking over the cold solder joints has kept mine working for 5+ years so far.

Could not hurt to touch them up.

rodekyll · 05-03-2021, 05:12 PM

I went over all the solder joints that I could safely redo with no joy. Symptoms remain the same.

JNSNWT · 05-04-2021, 06:13 AM

Well CRUD! Sorry to hear that.
Since new is NLA I guess your last hope is a salvage yard.

rodekyll · 05-07-2021, 03:17 PM

For those wondering if my lazy troubleshooting was effective and how this turned out, I found the exact part#, one month older, used. It was about $250, shipped. I installed it and the gauges are normal again.

I spent the time finding the exact part# and rev because reading up on very similar numbers, I was seeing Spartan and other chassis indicated, as well as Cummins engines ( have a 3126 CAT). I never got comfortable with the compatibility risk.

That also verifies that I have a toasted fuel gauge sender, since the light is on and the gauge reads empty. It did that before the swap, but being lazy, I thought I'd start my troubleshooting there. Now I have to figure out where that sender is. . . .

A word of caution as you search for the VDC part -- check the website address in the search results before you click on it. Some are misdirects to goddess-knows-what. I didn't stay there long enough to find out. Odd-looking addresses and weirdly similar/specific descriptions in the search engine summary, but coming from different places, are the only giveaways I found.

bizzerbiz · 05-07-2021, 10:45 PM

You came out pretty slick on this deal. I think for many with this same problem such as myself, if the connectors after being re-soldered didn't fix the air fluctuations and alarm. Would have been replaced at Hello. That's Lazy!