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Can't build air


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By Peter Mullin - Sunday, June 10, 2012 2:56 PM
We are having a presistent problem with my father's Transtar II refusing to build air pressure. We are getting tired of throwing governors at it although when we do the probelm seems to go away for a while.

Today we started pulling lines off (checking for plugged lines - found none) and we get a vacuum from the inlet side of the compressor and some pressure (you can fairly easily stop the flow) on the discharge side.

We have no apparent external leaks except the alcohol evaporator line.

We swapped regulators and disassembled the most recent one. All internal components looked good. Nothing stuck and no crap inside. I adjusted the regulator with no improvement.

Any ideas? The truck has an NTC 300 Magnum and a dual circuit system.
By David M. Holt - Sunday, June 10, 2012 3:29 PM
Check on top of compressor not sure What kind u have but some have a plate on top held on with 1/4 bolts i think that has springs under it that are check valves or see if it has reeds they might be stuck. if has a air dryer make sure the control line is sending signal to the governor.
By glenn akers - Sunday, June 10, 2012 3:30 PM
To keep from buying a new governor and installing to do a test is simply unbolt the governor from the compressor with lines still connected and see if compressor will charge air. The governor do not cause the compressor to pump air but only keeps if from pumping air when its time to stop. I would connect a shop air hose to the hose or line that comes from the compressor to were ever and air it up with shop air. This will show you if you have a leak between the comp and air tanks.If not and it is a cummins comp then pull the unloader out and clean and check it out.It may be sticking and not compressing air. If it is a two cy compressor same thing can ahppen but not as easy to look at the valves.
By Tony Bullard - Monday, June 11, 2012 12:13 AM


"you can fairly easily stop the flow) on the discharge side."

If this is the case something is wrong in the compressor. Check and clean as stated above.


By Peter Mullin - Wednesday, June 13, 2012 1:50 PM
I unbolted (two bolts) the boss on top of the compressor that the governor bolts to (I assume that accesses the top of the unloader). Everything under there is free but got pulled out and cleaned up anyway.

I tried running it without the governor and it still won't build air.

It is a Cummins compressor. The truck does not have an air dryer but it does have an alcohol tank.

The next component down on top of the compressor goes on with 4 - 1/4 inch bolts. It appears to be above the actual cylinder head as the section with the coolant lines running into it sits below it. I will need to get a gasket set from the local Cummins dealer if I need to go down into that.

Could it be as simple as a plugged line to the tanks? But then what could plug a1/2 inch line??

If I hook shop air to the primary tank it will not charge the secondary. I am assuming that is normal as there must be check valves in the system.
By FBHOMER - Wednesday, June 13, 2012 3:11 PM
Hook shop air to the discharge line from the compressor like Glenn said to check the rest of the system for leaks. It sounds like a compressor problem to me. I would also bypass the air dryer and check for any difference before I condemned the compressor
By Geoff Weeks - Thursday, June 14, 2012 2:41 PM
could be a simple as a leaking discharge line. With it all hooked up spray soapy water on the discharge line from the compressor to the 1st tank and look for it to bubble. I have seen lines that look ok from the outside, but spray them and they turn all "fury" with bubbles. It was leaking along the whole line, so you don't hear a leak.
By FBHOMER - Saturday, June 16, 2012 2:39 AM
I also have learned to rely on soap instead of my ears too LOL!
By wayne graham - Saturday, June 16, 2012 2:55 AM
What they said. Use the soapy water in a windex spray bottle. Wayne
By Aaron - Saturday, June 16, 2012 4:31 AM
If I hook shop air to the primary tank it will not charge the secondary. I am assuming that is normal as there must be check valves in the system.


I'm corn fused the first tank has to feed the next two, the first tank is the wet tank as it catches all incoming air and mosture from the compressor the next tank is the dry tank and the 3rd tank is emergency, all of which are protected by acheck valve that doesn't let air work its way back to the compressor except the wet tank there is generally no check valve on it, if hooking shop air into the first/wet/primary/ tank and air does not travel to the dry tank then there is a problem between those two,did someone put a check valve in backwards.
By wayne graham - Saturday, June 16, 2012 4:53 AM
Aaron, I wondered about that too but we seem to be doing the basic first which is leak on discharge line. You are right of course if you charge the compressor discharge line with shop air and all the tanks do not fill something is blocking the show. This started as them saying cannot build air but if compressor is filling the wet tank the primary guage should show psi. I think the advice was to start at the beginning which is the best way to learn how it all works anyway. Wayne
By glenn akers - Saturday, June 16, 2012 5:58 AM
Are you say that your getting loud hearing.I think its cause the air ant as loud any more.
By Geoff Weeks - Saturday, June 16, 2012 5:59 AM
IHC used a divided tank, on one of the tanks, the front part was the "wet tank" and the back half was the "primary" on the split system (two tanks, two brake line system) with a soild divider between the two halves. If that is what he has (and I think so) chargeing the back half or the secondary tank (either one) will not charge the other. Also the gov line for the compressor is fed off the wet tank (so it will tell if either of the downstream tanks need air) so not chargeing the wet tank will tell little about the system. The split tank had a check valve between the two (on mine it is always external, but I have heard that some were internal, which would mean a failed valve would require a tank replacement!)

If you want to test the whole system, T into the line from the res to the gov and hook shop air to that. If you have a modern air drier, it will have a check valve in it also (he said it had no air drier) and to presureize the whole system you need to presureize the compressor discharge line.
By Aaron - Saturday, June 16, 2012 6:40 AM
Wayne I'm still in the dark, disconnect the compressor, shop air to the wet tank, now the compressor is out of the loop, if the rest of the tanks aren't building air there is a problem. Geoff may have hit it on the head with the internal valve on a Fast Back Farmall, I haven't had to mess with one of those. I haven't seen anything about a leaking discharge line, course I may be half blind besides being half deaf.
By Peter Mullin - Saturday, June 16, 2012 2:28 PM
Ok, I thought there was a split tank in there somewhere.

I haven't had a chance to work on this anymore in the last few days. I did pick up the necessary fittings to charge the system through the Compressor discharge line (and subsequently check for leaks) and also to check the compressor output pressure.

Hopefully I will have some progress to report on Monday.
By glenn akers - Saturday, June 16, 2012 3:30 PM
On the mack that i run it has a aluminm wet tank that has the divider in it with a flaper check valve inside. I have another one with a steel tank that has a check valve that screws into the side with the valve inside with a divider.
By FBHOMER - Saturday, June 16, 2012 4:38 PM
That's what I'm saying Glenn LOL.
By wayne graham - Sunday, June 17, 2012 5:06 AM
Glenn, I parted out a Volvo and it has a divider with the flapper on steel tanks. I liked the set-up so well I used the tanks on my A model KW that I am building. Eliminated one long tank in the way. Aaron, I'm sorrry but I guess I did not explain that too good. It is an old guy thing.Wayne
By Peter Mullin - Thursday, June 21, 2012 2:39 AM
Ok, attempted to air the truck up from the shop compressor and now I know at least one reason why we can't buils air. The exhaust port on the bottom of the treadle valve is leaking by in a very big way. So I guess we need to either get that unstuck or replace the treadle valve...
By dclerici - Thursday, June 21, 2012 3:08 AM
Air leaking from the exhaust port does not always mean a bad treadle valve.
By Geoff Weeks - Thursday, June 21, 2012 3:17 AM
Could be back feeding from a bad relay valve. Or it could be the treadle. Dis connect or pinch off the outlets from the treadle and see if the leak stops. If it does, it is further down the pipe, if it still leaks then it is the treadle.
By FBHOMER - Friday, June 22, 2012 6:22 AM
Or a brake pot could be blown and feeding back, UN hook your parking brake pots on the pankake side and check for air coming output
By Geoff Weeks - Friday, June 22, 2012 7:10 AM
A brake can can not back feed unless there is more wrong. The quick release valve (either part of the relay, or a seperate valve) will prevent a leak between the the parking brake chamber and service brake from feeding back into the system. The quick release is off its seat when the the brake line is not charged or when the chamber line is at a higher pressure then the feed from the treadle. It is a common misconseption that a brake can can cause (in itself) a back feed but it can not if everything else is working as it should.
By dashby - Friday, June 22, 2012 9:36 AM
Had same problem with the Acar. Replaced relay valve. Fixed it.

Dean
By Geoff Weeks - Friday, June 22, 2012 9:46 AM
Some Internationals don't use relay valves. If this is a the case, then all of this is for naught, and it is the treadle valve. Both my 9670's don't have relays, brakes are fed streight from the treadle.
By glenn akers - Friday, June 22, 2012 12:00 PM
Geoff Weeks (6/22/2012)
Some Internationals don't use relay valves. If this is a the case, then all of this is for naught, and it is the treadle valve. Both my 9670's don't have relays, brakes are fed streight from the treadle.
Many is that way but you notice the service line from the foot valve to the can is larger.
By Geoff Weeks - Friday, June 22, 2012 12:29 PM
Yes, they use 5/8" line IIRC. Still it can cause balance issues if the wrong quick release valave is used. I think they use a 5psi crack valve when no relay is used.
By FBHOMER - Monday, June 25, 2012 8:59 AM
I have seen a lot of valves changed(excluding foot valves) and the problem was still there. I just eliminate the cans first and go from there. Besides my labor is always free LOL
By Peter Mullin - Sunday, July 01, 2012 1:14 PM
Some penetrating lube into the exhaust port and a few gentle taps with a 3/8 drift and small ball pein hammer and the leak is gone. She builds air as it should. Now I just need to readjust the governor and we are good to go.
By wayne graham - Sunday, July 01, 2012 2:49 PM
What exactly did you tap with the drift and hammer if I may ask. Wayne