Air system compressor

[RedT]

I have an '04 Meridian on a Freightliner chassis powered by a CAT 3126E engine. Recently upon start the air failed to build-up. Air line from compressor to Haldex Pure Air dryer had pressure, but line from dryer to system did not. Bypassed dryer, air came up and system operated normal.

After arriving home, I contacted Haldex to ask if the DQ 6026 and DQ6020 kit would remedy the problem. They told me to return all lines to original configuration, disconnect the control line from the compressor and operate the engine.
If air came up, that indicated a bad governor.
If air did not come up, that indicated a bad unloader. Haldex indicated the unloader was mounted to the compressor.

Don't want to throw money at this problem (the DQ6026 is only 12 months old) but am willing to listen to the resident experts.
Thanks for any info you can provide.

 

[Gary RV_Wizard]

Have you done the test that Haldex recommended? Result?

You may know more about these components than most anybody here. The DQ6020 is basically just the dessicant package, so that doesn't seem likely to be part of the problem. Based on the description of the parts, it seems the 6020 could be, or the "unloader" (I have no clue what that does, but Haldex seems to think it is a candidate). None of the parts seem expensive (priced on Amazon).

 

[RedT]

Thanks for the reply, Gary. I also posted on the Diesel RV Club forum, but no answer yet. I'm due to change the desiccant and filter anyway, so will probably buy both the DQ6026 and DQ6020 kit hoping that will resolve the problem. I'm reluctant to throw money at the problem hoping something will fix it.

Haldex seemed to think the problem was the compressor unloader, but I've not found a document that explains the function and operation of the unloader.

Which does not explain why the system aired up (after bypassing the air dryer) and maintained 125 PSI in both tanks for the next 4 hours driving home? I'm not convinced there's a problem with the compressor, the governor, or the unloader valve. If the unloader valve functions as the name implies, it is relieving back pressure on the compressor once the pressure builds up.

My understanding of the function of the air dryer is to filter the air and remove moisture from the air delivered by the compressor. If the system appears to operate as designed with the air dryer bypassed, I'm thinking that is where the problem is. 

 

[Jeff]

I had an issue with the Haldex in our coach when we first got it. Bypassing the dryer was the only test Freightliner suggested and it showed a bad dryer. There wa a bulletin on the turbo valve which I had repaired. As the summer went on different issues kept popping up that we're finally resolved by taking the advice of a Freightliner tech and replacing the dryer with a factory rebuilt unit.


I wish I had done that the first time around!
Edit: Fixed/removed tags.

 

[RedT]

Thanks Jeff...... I was hoping to not have to spend the $487.52 plus tax for a dryer.....      :-\

 

[Gary RV_Wizard]

Which does not explain why the system aired up (after bypassing the air dryer) and maintained 125 PSI in both tanks for the next 4 hours driving home? I'm not convinced there's a problem with the compressor, the governor, or the unloader valve. If the unloader valve functions as the name implies, it is relieving back pressure on the compressor once the pressure builds up.

I gotta agree with ya there, but I don't know what the "unloader" really does either. Maybe the unloader is a back-connect from the dryer, somehow relieving pressure from the dryer? Your description said the unloaded is mounted on the compressor, but that doesn't mean it is part of it.

 

[RedT]

After talking again this morning with Haldex, they gave me a troubleshooting procedure that confirmed bad dryer.
1. Connect air lines original configuration, plug control line.
2. Start engine
Air up = Bad governor or unloader
No air = Bad dryer
Rebuild Kits (DQ6026 and DQ6020) = $194.99 plus tax with no fix guarantee.
NEW replacement dryer = $306.56 plus tax

Think I'll go with a new dryer.

 

[Sean G.]

Thats what a lot of my customers do is replace the dryer.  When parts, labor and the risk of error is calculated, swinging the dryer makes more sense. 

The operation of the governer.  Usually the governer is mounted to the compressor.  It senses system pressure via a remote line that usually comes from the wet tank.  When the governer wants to unload the system it pressurizes a port in the compressor it is mounted over and holds open the compressor intake valves via the unloading pistons in the compressor head.  There is also a line that goes from the unloader to the dryer to trigger the purge cycle when the compressor is unloaded. 

This is a traditional setup, some vehicles use a system that has the governer on the dryer and unloads by simply allowing what the compressor is pumping to exit the dryer when the system is unloaded.  The compressor itself is never unloaded and is pumping air at 0 psi when the system is unloaded. This helps to cool the compressor in a high capacity system.  MCI has used this style for the past 10 years. 

 

[Jeff]

I gotta agree with ya there, but I don't know what the "unloader" really does either. Maybe the unloader is a back-connect from the dryer, somehow relieving pressure from the dryer? Your description said the unloaded is mounted on the compressor, but that doesn't mean it is part of it.

Gary


There are two air lines between the compressor and the dryer. the main line supplies air t the dryer and from there the air systems of the coach. The second line is a return line that sends air back to the compressor unloader to stop thee pump from building pressure until it is needed. The valve that controls the air returning to the compressor is called the turbo valve and is one of three in the dryer. Haldex has upgraded that valve but if it doesn't work properly one of two things occurs: 1. The valve doesn't close and no air pressure is built up. (My first problem) or 2. The valve never opens and the unloader never is signaled to stop pumping and the compressor runs continuously up against its relief valve. (My second issue after the campaign on the turbo valve was completed) You can also tell if this is happening if the ejector is dumping air too often.


After spending about $300 for the turbo and ejector valves to be replaced at separate Service Centers plus replacing the desiccant cartridge in 3 months I spent the cost to replace the dryer and have had no further problems.

 

[Gary RV_Wizard]

Thanks to both Sean & Jeff. It's a bit clearer now.

Note that Haldex makes multiple different dryer systems, e.g. the Dryest, the Purest, the Pure Air and Pure Air Plus. There may be some differences in how they are plumbed, plus I know the dessicant mechanism differs.

 

[Jeff]

Mine is a Purest.

 

[Utclmjmpr]

My Pure air plus had a simular problem two years ago , I disassembled it to find it dirty and the turbo valve was bad. I replaced the turbo valve after a good cleaning and its been problem free since.>>>Dan