Cat 3126 air compressor

jcross

Seasoned Expediter
I have a 1998 FL70 with a CAT 3126. The air pressure wont build up over somewhere around 10psi and it sounds like there is air hissing from inside the compressor. I had the old governor in my toolbox and swapped that out and also an old unloader valve kit. I also noticed on oil leak on the front of the engine in front of the compressor that I haven't noticed before.

Is it time for new compressor? Thanks in advance.
 

davekc

Senior Moderator
Staff member
Fleet Owner
Depends on how much of a leak. Small seepage shouldn't render it useless. May be frozen. how warm is it where you are at? If below freezing you need to drain the water out of the tanks, drop the pressure to zero, and put some airline anti-freeze in the primary line attached to the compressor. Just prior to doing that, you can run a hair dry on the compressor to heat it up if that is available.
Restart and run truck at a high idle for 10 to 15 minutes. If nothing above say 50PSI, repeat proceedure. That should get you going if working in below freezing temps.
If it happens fequently, replace dryer or the cartidge depending on what type of dryer assemply you have if moisture and freezing is a problem. Outside of that, may be time for a compressor if you already got a new governor on.
The hissing is air escaping somewhere or a stuck/frozen valve.
 
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jcross

Seasoned Expediter
davekc, thanks for the reply. I'm in south Texas, cold but not freezing. Truck ran fine yesterday.....builds up air pretty fast. I have noticed lately that the truck is leaking off air relatively fast. I heard it leaking one day and it seemed to be coming from the valve on the firewall where the air hoses from the dash valve come from....like a purge valve looking thing. Thoughts?
 

davekc

Senior Moderator
Staff member
Fleet Owner
The purge valve will be where the dryer is. If it is that, you need a purge valve kit. Any truck stop with repair will likely have it for a Freightliner. If it is leaking out of the air block, that is seperate. That can be identified by smaller colored coated air lines going in to it. If one of those lines is leaking, those can be tightened up or if the hose is long enough, you can cut and refit.
Since numerous small hoses go to it and you are in above freezing weather, hit with some cleaner/soap to determine which one is leaking.
 

jcross

Seasoned Expediter
Yes, the air block is what I was describing.....there is a rubber flap on the bottom (end) air was coming from that but not all of the time. But, air compressor won't build more than 10psi right now and when I shut the truck off I can hear air that sounds like its inside the compressor.
 

davekc

Senior Moderator
Staff member
Fleet Owner
Probably compressor then. That sounds like a bad valve inside the compressor. On Cats they sometimes will give you a early irritating warning like a knocking sound before they go out.
Probably should add, if the valve is bad it sounds like it is backfeeding through that block. That is why you have little air pressure.
 
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zorry

Veteran Expediter
Could he be looking at the foot valve ? The rubber flap ?

My father dove many hours to release breaks on a truck that had a pebble under the foot valve.
 

davekc

Senior Moderator
Staff member
Fleet Owner
Could he be looking at the foot valve ? The rubber flap ?

My father dove many hours to release breaks on a truck that had a pebble under the foot valve.

Tell you the truth, I'm not sure. In his case it is releasing air at the compressor and backfeeding through the air block.
 

jcross

Seasoned Expediter
Zorry, the valve on the inside of the firewall that the airbrake hoses that go through the firewall attach to. But, the truck doesn't build up enough air (around 10psi) and it sounds like it is leaking internally in the air compressor....its pretty loud and I've soaped everything around it and inside the engine compartment. I'm thinking air compressor is done.....worked good yesterday though.
 

zorry

Veteran Expediter
If you can get air to it, fill the tanks, from a good source( shop air or another truck).
You may determine a leak that way.
Pull main line coming out of compressor and start truck. Is it pumping air ? Enough ?
 

zorry

Veteran Expediter
If you do change the compressor, be careful.
It's probably heavier than it looks. And always be careful when unhooking any air lines. Compresed air is dangerous. Even a little.
 
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