A3 compressor, air system, relief valve

Having an issue with the air system after the truck sat for awhile. It builds pressure in the tanks until the safety relief valve starts blowing off, but nothing shows on the gauge in the cab. About once every minute the relief valve will blow due to overpressure, but nothing connected to the air system works...steering, brakes, horn, etc.
My first thought was replacing the relief valve. Thought it was faulty, and blowing air before the system could build pressure. So replaced it, and it did the same.
Reading on it, my next thought was a sticking governor. Tried tapping on it with a hammer, but it did nothing. After that, I had no choice but to move it...driving 180 miles, with the relief valve blowing every so often, and nothing reading on the gauge. But about the half way point the safety valve stopped blowing off, and the steering improved. So I tried the horn, and it worked, too!
I figured something stuck due to sitting too long, and all the bouncing and heat from the move rattled it loose. Had no further issue the rest of the drive. But next time it was started, the same thing happened! Was convinced it was a faulty governor, so ordered a new one since it was cheap. New governor in place, same thing happens!
Bought a set of spiral bound TM's just for this issue, but not much help except finding some part numbers. Reading about it some more, found mention of this same issue in the FMTV forum...safety relief valve blowing intermittently. Points to the compressor valves, and convinced me a head rebuild would be a good idea...but that's a different model compressor. Not finding any similar rebuild kit...or any other parts, ftm...for the 4p5435.
At my wit's end with this problem, so putting it out to the community. What am I not understanding? Any help appreciated!

I'll see if I can help and learn something at the same time. I really should get more familiar with the air system on my M35A3.

Are you getting a warning buzzer with zero air pressure at the gauge? Or is the buzzer not sounding at all?

Roger that! Trying to get more familiar with it myself! The bound paper copies of the TM's are a big help. Much easier to see than electronic copies.

The warning buzzer has never worked on this truck. And I've not yet taken the time to figure out why. My assumption is that it must be unplugged.

Forgot to mention something earlier. My memory on the subject is a little fuzzy, because I had another one at the same time for a while, and sometimes misremember what I've done to which one. Anyway, remembered that with this one the air pressure wouldn't build fully until it warmed up. The gauge would show it building about 1/4 of the way, then it would just drop to zero. After a couple minutes it would repeat, back and forth, until finally it built full pressure. But now, after sitting for several months with no attention, it exhibits the behavior previously mentioned.

Don't the A3s have an air dryer? If so, is it popping off at the dryer relief or a tank relief? If it is the dryer, it is time for a service, the cartridge is most likely full or the valve is no good.

WillWagner said:

Don't the A3s have an air dryer? If so, is it popping off at the dryer relief or a tank relief? If it is the dryer, it is time for a service, the cartridge is most likely full or the valve is no good.

Click to expand...

Yes they do. And I have had it off the truck for a full cleaning and service. Pulled the cartridge to double check after it started the odd behavior, but it's clean and dry. On that note, it's popping off at the tank relief. However, once it builds pressure the dryer dischargers intermittently, as is normal.

My A3 shows air on the gauge about 2 or 3 minutes after startup. Once the compressor cuts off, the air dryer discharges. The air dryer discharges every time the compressor cuts off.

I find it odd your gauge isn't working right. It's building air and blowing the safety and you're showing zero PSI. I suggest disconnecting the gauge and hooking up an air pressure gauge to its airline to rule out gauge failure. If you're not getting any air there, chase the airline from the dash to where ever it goes and see if there are damaged lines or potential failure points.

EDIT: Are you sure it's the tank safety valve blowing off and not the one at the tee at the compressor output in the engine bay? It may have received MWO work to add a safety valve there under the hood.

HDN said:

EDIT: Are you sure it's the tank safety valve blowing off and not the one at the tee at the compressor output in the engine bay? It may have received MWO work to add a safety valve there under the hood.

Click to expand...

Yes. I crawled under the truck to locate the odd noise. It's LOUD! Sounds like an air chisel for about a second every time it blows. Had to crawl up between the rear axles to reach it when replacing with a new one. And that was no easy task, let me tell you, because there's no room for a fat boy to move, and no clearance to work! Was very disappointed when the replacement didn't remedy the issue.

I am not familiar with A3s, but I have experience in OTR truck stuff. If when the A3s were built they set them up like modern stuff, there should be a wet tank, the first tank air gets pumped into from the compressor/dryer and then out to the other tank(s). At the outlet of the wet or the inlet of the second and 3rd tank there should be a check valve. If for some reason, those check valve(s) was installed backward, it will cause what you see, no air on the gauge(s) but the dryer popping off. The relief is in the wet tank as is the sensing line for the governor, that is why the dryer is popping off, however, the dryer shouldn't be popping off constantly, it should only pop off when the tank gets to 120, the governor sends a signal to the dryer valve and the valve spits. The relief valve should be a 150 psi pop off.

I re read your post a few times, but I am still confused. Is just the relief going off or the relief AND the dryer? And, no air in the primary, secondary, if it has one, tank, only the wet?

You did install a new governor. Incase you replaced it and just followed what you had in your hand, lets go over the plumbing.

It should be plumbed with 2 lines if it is mounted to the compressor and 3 if remote mounted. There are a total of 7, 1/8 ntp threaded portss in it. The center 3, the ones between the mounting bolt holes are for the governing/sensing lines, 2 ports are used, 1 is for the unloader, so if it is on the compressor, no line or plug, just bolt the open port to the compressor, one of the other 2 ports needs to be plugged and the other should have the line to the dryer, this is the signal line that tells the dryer to purge.

The second set of 3 ports are reservoir pressure ports. These let the governor know what pressure is in the tank so that the governor can send the pressure of the tank to the unloader and stop the compressor from making more air. Only 1 of these are typically used, one line from the tank, that's it. The other two ports should be plugged.

The last port is a vent. no plug! It is at the adjustment end on the bottom. It can have a vent that looks like an axle vent, a line attached to it venting it to atmosphere somewhere out of the engine compartment, but it needs to be open to atmosphere and normally facing downward to prevent junk, dirt, sand, rocks, whatever, from falling into the port.

My gut feeling is that the unloader is messed up. You stated that the compressor on your truck isn't what the TM shows. Take a picture of it and let me see what you have.

WillWagner said:

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I re read your post a few times, but I am still confused. Is just the relief going off or the relief AND the dryer?...

...

My gut feeling is that the unloader is messed up. You stated that the compressor on your truck isn't what the TM shows. Take a picture of it and let me see what you have.

Click to expand...

Apologies for the misunderstanding. And thank you for the thoughtful and detailed reply! It's only the relief valve going off. However, the dryer does work as normal after whatever the problem is "unsticks" itself.
So.... I've not traced the routing of the full air circuit yet. I'm going to have to study your explanation some, and follow along with the diagrams, but it is making more sense. What seems to be happening is the compressor is pumping air into the first tank at least, and it just keeps pumping until the relief valve pops off. But somewhere after that the pressure is being blocked from the rest of the circuit, it's not reaching the gauge in the cab, or the governor that that regulates when to stop.
What I said about the compressor in the beginning was that...in the one forum post I found that addresses a somewhat similar problem, the vehicle in question was an FMTV. I continued along because those vehicles were originally equipped with the same engine as in my A3, the CAT 3116, and so the compressor would have been the same, also. However, it was discovered that they were dealing with a later model, and so the compressor was different.

I had an almost identical problem with my M35A3 years ago. I replaced the governor and it cured the problem. One side note is that the new governor had plugs installed in ports that the original governor had left open. I plumbed the new governor to match the old governor plumbing. I haven't had any issues since. I should also tell you that two different diesel mechanics at two different truck shops swore up and down that my compressor had failed. Thank God I didn't listen to them. The only other problems I've had with the pneumatic system on the A3 was a failed regulator for the steering. $25.00 for a new one fixed that problem. I also replaced the air dryer once. Believe it or not, I found an NOS air dryer for less money than a replacement 24 VDC heater, only $25.00 if I remember right. I also installed the MWO myself that HDN mentioned. Have you studied the pneumatic diagrams in the TM? If you follow them, you can eliminate a lot of the system based on the symptoms.

glcaines said:

I had an almost identical problem with my M35A3 years ago. I replaced the governor and it cured the problem...

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I was very disappointed when the issue continued after replacing the governor. Had the same issue with plugged ports. But I was very careful to switch all the fittings over and make the new one just like what came off. There's even one port that's open to the air, but has a screen in it. The new one didn't have that, so I had to hunt up a small, stainless steel screen to stuff in it.

My next course of action is to crawl around under the truck with the TM in hand, tracing out every air circuit. That probably should have been step one, but I'll eventually get it all figured out. Really appreciate the input from the community!