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roknjohn · 09-13-2022, 04:57 PM

I'm having intermittent issues with building air pressure. Currently, air will only build when RPM is above 2000. (Normally, air would build even at idle). Even at 2000rpm, the pressure starts to slow down above 100psi. In fact, I just returned home from a 250-mile trip and the system never reached the max cutoff pressure (125psi).

Here are a few facts:
1. There is no significant leak on the tank side, my gauges still showed 68psi after sitting all night.
2. I disconnected the line from the D-2 governor to the unloader to rule out the governor. But, there was still no building at all below 2000rpm.
3. I disconnected the supply line at the air dryer to verify air was being delivered to the dryer from the compressor (Bendix BA-921). I could feel air being pumped out at idle.
4. I hooked everything back up, and reved the engine to 2000rpm; pressure began building as normal from 60psi until about 100psi, then slowed. After about 2 mins at high RPM, it finally reached 125psi and the air dryer purged.
5. With the tanks at 125psi and the engine off, I disconnected the unloader line from the governor again. To my surprise, there was no air pressure in the unloader line. I found a leak in the braided hose and replaced, but it didn't solve my issue. Still builds much slower than normal.
6. I bench tested my governor. Works as expected with shop air and gauges. Cut-Out at 125psi, Cut-In around 100psi.

Could the desiccant filter on my air dryer (HALDEX PurEST - DA34160) be clogged and causing these symptoms?

My next step is to temporarily bypass the air dryer to see if that helps. Otherwise, I suspect a sticking unloader valve or compressor head gasket. (No signs or oil leaks from compressor.) BTW, the compressor is not easily accessible on my rig. I dread having to service it.

Any thoughts?

twinboat · 09-13-2022, 05:22 PM

Check the large line between compressor and dryer.

A leak in it is hard to hear because it only leaks when the engine is running. Once shut down, check valves keep air from backfeeding.

roknjohn · 09-14-2022, 11:17 AM

Hey Twinboat!

I just now checked a few things...

I disconnected the air line between the dryer and compressor at the dryer end. I put a gauge and shop air ~130psi to the line going back to the compressor. I heard no leaks and the pressure held fairly well. Conclusion: The supply line looks good.

Then I disconnected the shop air and capped it with a safety valve. I started the engine and the gauge just bounced back and forth really fast around 0-25psi. I expected the pressure to build to the safety limit. The air reservoir pressure was 68psi, well below the governor cut-in. Conclusion: malfunctioning air compressor.

Earlier, I soaped down the compressor pretty well looking for leaks around the head gasket and connections, but no bubbles. I suspect it is a stuck, or intermittently sticking, unloader piston. I ordered a kit for overnight delivery. Do you agree?

I am dreading this repair as that compressor is not easily accessible, and it's a bit rusty...

96 Wideglide · 09-14-2022, 11:43 AM

It would have to be intermittent. You wouldn't have made it 250mi with no air reaching your tanks!

RichieO · 09-14-2022, 12:38 PM

I had a very similar problem, and after paying Freightliner dealer $2100 to diagnose they decided it was the air dryer. They replaced the dryer and I have not had a problem since. If the dryer hasn’t been serviced or replaced in the last five years, that is where I would start.

roknjohn · 09-14-2022, 01:27 PM

Ok, I manned up and made an attempt to get at the unloader valve. It was a bit easier (so far) than I first feared. I was able to remove the unloader cover and the balance piston and spring. I was expecting more pieces.

Here's what came out:

Here's the top view of the unloader hole:

I think the rest is stuck inside:

My rebuild kit won't arrive until tomorrow, so I don't know exactly what these parts look like. I'm trying to proceed with caution.

roknjohn · 09-14-2022, 02:03 PM

Well, with a little effort, I found the Easter Egg.

It didn't move very freely, I could only move it about 1/8" up and down with my finger (felt very stiff). With the help of a pick though, I was able to pry that thing out.

Now, once the new parts arrive, what should I do to prep the compressor? I imagine the inside of the unloader orifice must look similar to this.

twinboat · 09-14-2022, 02:10 PM

From the looks of that unloader, you may want to check the compressor intake plumbing.

Make sure it's all connected and not pulling in outside air. Should be drawing filtered air from the intake manifold.

roknjohn · 09-15-2022, 07:52 PM

Argh! This thing is killing me.

I replaced the unloader piston with new one. Afterwards, the rig aired up just fine. I went through a few duty cycles then took on a 15 mile test run, to fuel up, dump tanks, etc. But on my last stop, air would not build.

I had to rev up to 2500 RPM just to get it to start building air. Once building, though, it seemed to build OK up to the cut-out. Back at home, I bled off the air and again it wouldn't build until I held 2500 rpm for about 10 seconds, then it would build at a lower RPM. I got it up to purge a couple of times, cycling through.

I swear there are no leaks anywhere. The compressor just does not exit out of the unloaded state very well sometimes, and then, it takes a high RPM to re-seat.

Changing out this compressor seems almost like an impossible task. Probably not something I would want to do myself. There's a lot of rust and I don't know what some of the components are. I'm hoping that I can find an easier (and cheaper) resolution.

Tomorrow morning, I'm going to take the unloader valve back out and send a endoscope into the bore to see what I can see. I'm running out of things to try.

Ray,IN · 09-17-2022, 01:56 PM

I suspect the intake air contains a lot of moisture by the rusted unloader valve. Is the bore clean and smooth?
Yes it might be time to have the compressor overhauled/rebuilt if that bore is rusted and pitted.