Volvo 780 Front air tank keeps draining!

I'm posting from my phone so I might be off point for lack of rereading everything. But no, I don't have the I shift. This morning I've checked out the air diagrams and was surprised to realize that the front tank is the wet tank, which we all knew, and the middle tank is the third or secondary tank! The rear tank is the primary tank. So this suggests to me, if I haven't misinterpreted something, which is possible since I haven't had my coffee, that when the forward tank drops on the gauge on the dash, it's the rear tank losing that pressure, not the middle one as I would have guessed. Which means trying to hunt down leaks from that tank to the governor is a waste of time. Also I see that the rear tank supplies the ABS and traction-control system. Funnily enough I have the ABS light on and I think the TCS light too. So that squirt bottle full of soap is going to go from spraying tank fittings to spraying that valve and possibly even spraying chambers because a leak from an ABS valve into the service side of a chamber could possibly be inaudible! The diagram also indicates the ABS and traction control system are perhaps the same module, so perhaps we're getting somewhere.

 

Note: well, now the original reply has posted. This one can be considered redundant. (Can't find a delete button on these posts?!)

I just wrote a reply and the thing got absorbed into oblivion because I wasn't logged in, it seems. Long story short, I found a diagram and to my surprise, if I'm reading it correctly having had no coffee, the wet tank is the front (we knew that), the middle tank is the REAR tank, and REAR tank is the primary tank! Now who using common sense would've guessed that? So I'm totally redirecting my investigation. The idea of the middle tank being the drain now seems wrong. The primary tank seems to feed the rear ABS system, which is also the TCS (Traction Control). Guess what -- I have the ABS and TCS lights on. I don't have I-Shift, but perhaps you have one of those lights on?! Anyway, if I'm reading things right (without coffee!), the ABS valve could be cracked and sending air to the service side of the drive-axle chambers, which can be an inaudible leak. I should've been paying more attention to the ABS codes in the Brake ECU (via diagnostics on your stalk stick on the steering column). Maybe this will lead to the problem finally.

 

So, what your saying is the "rear" tank is actually the "front" gage? Funny you bring up the ABS - and NO I have no lights but I've had to replace both of my front ABS modules for leaking. I need to check the rear (I believe it's a single) but I have spent so much time under the truck with a spray bottle and just can't find anymore leaks...I know from my investigating with the engine off, that when I do a chassis dump and then turn the switch off I can watch the "front" gage drop to replenish the suspension so I have really been looking hard at the suspension but with no luck. I also have checked my brake cans and have not noted any air leakage around the lines or the cans themselves, heard or seen with soapy water.

 

UPDATE.

Found the problem causing the dryer to purge every minute.

But to address your last post first:

> So, what your saying is the "rear" tank is actually the "front" gage?

I looked at the "F" and "R" gauges on the dash today and they correspond in order to the tanks below. F is the middle and R is the rear. (The wet tank is up front and appears to share the tank body with a separate purge tank for Wabco's System Saver 1200 UP.) But the diagrams are calling the rear tank the primary tank, so whatever!

I started the day off prepared to squirt a bunch of ABS relay valves and brake chambers with my quart-sized Zep bottle full of Tide and water looking for leaks. But I remembered one thing that was on the troubleshooting list from yesterday and got to it first. Well, that is after I wasted 2 hours going to a dealer to get a metric pipe fitting (M16 x 1.5 to 3/8 DOT tube) for the wet tank. (For newcomers, wet tank is a term from the days before air dryers; moisture would get trapped into a wet tank (the first tank air travels to from the compressor) and get drained at the end of the day.) Turns out I had a spare fitting already! With this fitting installed into the wet tank (and a new unintended spare on hand), I was able to reconnect the yellow line to the governor. That 3/8" line communicates to the governor the tank pressure. (Via the "reservoir" ports on the governor.) This yellow line shares a 90-degree elbow with another line that has nothing to do with the governor: The fan clutch.

The fan clutch taps into that line at the governor via that elbow and steals air from it to shoot to the fan-clutch solenoid (about a foot away), which then shoots air to (in my case, anyway) a Horton 99A9644, which locks without air. In other words, air must be supplied to the clutch to unlock it; when you hit a hill or your AC condenser needs air, the air is cut off by the solenoid and the fan clutch engages and begins cooling. I built pressure and with the engine off and the key on, I used my 5/8" hose to listen for leaks at the fan clutch. Bingo. It was leaking. But I think if you were to turn the key off, the leak would stop. In any case, the next step was to completely remove the fan clutch from the system. That was easy. Since the clutch runs from that 1/4" line that shares wet-tank pressure with that elbow on the governor, it took 30 seconds to unplug the line (again, which travels a foot to the clutch solenoid) from the governor and plug it with a 1/4" NPT plug.


This is a Norgren 1/4" plug. You can get 3/8", 1/2", 5/8" and so on. Excellent for when you have a blown air bag; using a push-to-connect union or two and a few inches of 3/8" tubing, you can plug the air to a particular bag and bypass it until you drive somewhere to change it. https://www.mscdirect.com/product/details/48794002

With the fan clutch completely disabled and the governor port feeding it plugged, I started the engine and built pressure to cut-out at 125 psi. Then I timed the wait till the next purge. It was about 15 minutes! With the fan clutch connected, it's only one minute! So clearly the problem is solved. A leaky fan clutch (and another minor leak or two to hunt down to bring that 15-minute purge interval up). There's a rebuild kit for about $500. I'm going to just buy a remanufactured clutch for $650ish at Fleetpride. So that solves that -- after 2 or 3 years! In July of 2020 I even changed the compressor, thinking it might've been faulty. And I've been hunting around changing parts ever since. Now one reason that I didn't suspect the fan clutch was that I'd installed it a year before in 2019! It was a brand-new Horton. How could it leak only a year later?! BUT. There may have been reason. The line out from the fan-clutch solenoid was kinked. Furthermore, an in-line push-to-connect fitting was leaking closer to the clutch. Both of these conditions reduced the air pressure to the clutch. You are in effect riding the clutch when you don't supply all the required air. How might this cause it to leak instead of just wear down and become useless? Not sure. In any case, a brand-new -- not remanufactured -- Horton clutch failed me within a year.

So, with the fan clutch stealing air from that elbow, it was fooling the governor into thinking the wet tank had 95 psi even though the tank was sending up a higher pressure from that 3/8" yellow line. That's why the compressor would cut in once a minute at around 105 PSI instead of the correct cut-in pressure of around 95. The fan leak skewed the governor's measurement. The other thing that was strange was how if you went from 700 RPMs to 1000, the dryer purging would speed up. So it would appear the fan clutch loses more air when you're on the road and it's spinning and vibrating.

Here's an image of that elbow at the front of the governor with the disconnected black line (it's usually green but I replaced this line too 2 years ago in my quest to find this problem).


Disconnecting the 1/4" line from the governor to the fan-clutch solenoid.


The line to the fan clutch plugged with a 1/4" push-to-connect plug. Should have used a Norgren because this brand was stubbier and harder to remove.



The kinked line from the fan-clutch solenoid to the fan clutch.

Well, @COFLTravlr, I hope this helps you or someone!

 

Wow - great job in pinpointing the issue and great writeup explaining it with pics! Congratulations on finally finding the problem.
Unfortunately for me I have an electric clutch - when you have time can you spend some time on my truck and find my problem.....
I will continue to hunt this down and some day like you I will find it and let others know....

Again great job!

 

Thanks, @COFLTravlr. Still pondering what your issue is. You said you checked the brake cans, and changed front ABS modulators. So there shouldn't be a leak at the front chambers. I feel like something would be coming out of the highlighted hole below if there were a leak. And same for the rear chambers. Also assume your leak is constant while on the highway? It happens without braking? Are you purging once a minute also?




Other things that would be going through my mind:

-- The fact that the leak is inaudible but substantial suggests it's happening in some component, not just at some barely hissing fitting. Chambers, fan clutches, transmissions, rear ends . . . ?
-- This might not apply because the fifth-wheel cylinder should only get air when you're unlocking it, but no leak at the fifth-wheel cylinder?
-- WHAT ELSE is piped to your governor besides the 1/4" line to the compressor, the 3/8" yellow line from the wet tank, and a 1/4" line going to port 4 of the dryer?
-- For serious troubleshooting, figure out exactly what receives air from the tank that is dropping (your middle tank). Now in my case, the wet-tank pressure was dropping because the fan was siphoning air from it through the 3/8" yellow line. The gauge dropped too, so this would show that a wet-tank pressure drop can affect forward-tank pressure, which suggests no check valve between the 2 tanks or a bad check valve in my case (the diagram seems to show a check valve). . . POINT BEING that your faulty part could be getting air from either of the front 2 tanks. You zeroed in on suspension for example.
-- Even an air bag could leak quietly -- say, by having a leak above where the rubber folds on itself, forcing the air to quietly distribute around the rest of the bag. I suppose if it were the leveling valve, you would already have heard that.
-- In the most nightmarish last-ditch attempt to trace the problem, I'd remove lines at the tank and plug them and see which line, when removed, stops the leak, and figure out what that line feeds.
-- I think your air bags get air from a little flying-saucer-like valve that's mounted on your secondary (middle) tank. When the pressure hits 100 (or whatever that particular valve is set to open at), it opens and feeds the bags.

Lots of hot air here (no pun intended) but I hope there's some food for thought! Attaching some diagrams that may help, or maybe you've seen them already. The diagrams at least have numbers (like 57s06, for instance) that can be found next to a description in the document (57s06 -- Tag Axle Supply).