Location of air compressor & governor

ralphluby · Apr 18, 2016

The governor is located on the outer firewall of the M939 series. I have looked through all the 5 ton posts and TM 272-24-1 & 2. I have replaced the PPV and the pressure relief valve on the wet tank line to the brakes and cannot stop the relief valve from discharging. Would the governor be the next thing to change?

porkysplace · Apr 18, 2016

Download this TM TM9-2320-272-24-1.pdf
Chapter 3
page 3-476
covers the air system

[h=1]TM's for the 939 series trucks[/h]

ralphluby · Apr 19, 2016

Thanks porkysplace. I had seen this page but was looking for more detail. I find it very hard to find a specific thing in the tech manuals without a ton of time being involved especially for newcomers. The minute detail in many cases is ridiculous. yet in other cases there appears to be a lack of general overviews or exploded views of major components. My thanks to those who were able to provide these tech manuals on the Steel Soldiers site they are very helpful.

porkysplace · Apr 19, 2016

ralphluby said:
Thanks porkysplace. I had seen this page but was looking for more detail. I find it very hard to find a specific thing in the tech manuals without a ton of time being involved especially for newcomers. The minute detail in many cases is ridiculous. yet in other cases there appears to be a lack of general overviews or exploded views of major components. My thanks to those who were able to provide these tech manuals on the Steel Soldiers site they are very helpful.

The page I posted above has a list of all major components of the air system , scroll down to the air compressor and to the right is a page number with a red circle around it , click on that and it takes you right to the air compressor section. You need to read the entire air compressor section then simply match the numbers to the parts in the diagrams . If you need more detailed diagrams go to parts TM ( TM's that end in -P ) . This will also give you the the NSN number and part number that you can google to find a parts supplier.

I would really suggest reading the entire air system section of the TM and learn about the system your working on . By your post and not knowing where the air compressor is , I'm guessing this is your first experience with air brakes . So if you are going to try and repair them yourself you really need to understand the whole system , you dealing with a 12 ton truck that the brakes have to be fixed correctly , not guessing if this is it or not .

Have you serviced the air dryer on the truck since you bought it ?

dilligaf13 · Apr 19, 2016

Which "relief valve" is discharging and how many lbs are you able to build your air pressure to?

73m819 · Apr 19, 2016

NOT the compressor, it is just a pump, could be a stuck unloader valve or the governor. CHANGE the governor.

ralphluby · Apr 19, 2016

73m819 --Thanks for your reply. I have replaced the governor and PVC and still have the problem. What is an unloaded valve? I am going to look at the air dryer next.

ralphluby · Apr 19, 2016

dilligaf13 said:
Which "relief valve" is discharging and how many lbs are you able to build your air pressure to?

Dilligaf24 -- Thanks for your reply -- The relief valve that keeps discharging is located between wet tank and the rear brakes booster. I have installed a new PPV, new relief valve and a new governor with no luck. My air pressure has lately been reduced to about 60 psi on the gauges. Probably should now look at the air dryer.

ralphluby · Apr 19, 2016

porkysplace said:
The page I posted above has a list of all major components of the air system , scroll down to the air compressor and to the right is a page number with a red circle around it , click on that and it takes you right to the air compressor section. You need to read the entire air compressor section then simply match the numbers to the parts in the diagrams . If you need more detailed diagrams go to parts TM ( TM's that end in -P ) . This will also give you the the NSN number and part number that you can google to find a parts supplier.

I would really suggest reading the entire air system section of the TM and learn about the system your working on . By your post and not knowing where the air compressor is , I'm guessing this is your first experience with air brakes . So if you are going to try and repair them yourself you really need to understand the whole system , you dealing with a 12 ton truck that the brakes have to be fixed correctly , not guessing if this is it or not .

Have you serviced the air dryer on the truck since you bought it ?

ralphluby · Apr 19, 2016

Governor not the problem

Porkysplace -- Thanks for your reply and help. You have made a good point and I will read the air brake section more throughly. I am quite familiar with heavy equipment having owned many bus coaches and motorhomes but never got too involved in the air brake systems since I never had any problems to speak of. I have now installed a new relief valve, new PPV and a new governor. This truck according to tags and dates on filters was serviced in January 2015 and I purchased from GovPlanet in August 2015. I am going to take your advice and now look at the air dryer since that item was probably not serviced. Lately my air pressure has also only got to 60 psi after sitting all winter long in southern Maine. It is amazing the wreath of knowledge within this Steel Soldiers organization and how you and others are so willing to help.

98G · Apr 20, 2016

A definitive diagnosis prior to replacing any parts will save you a lot of $$$.... likely many of tje parts you replaced were fine...

This is not to say that rebuilding the air dryer isn't a good idea, but much of the other stuff may have been fine...

dilligaf13 · Apr 20, 2016

Use shop air to the emergency glad hand (red one) on the front of the truck. This will pressurize the system and since the trucks not running you will be better able to track down any leaks. Also, the air compressor outlet line (large line coming our of the bottom rear of the air compressor) should be hot to the touch (200 + degrees) while the truck is running. If not the air compressor is not producing sufficient air pressure. These are two quick and easy stating points that will get you headed in the right direction.

73m819 · Apr 20, 2016

This started out as a high pressure issue, now it is a low pressue issue, try ADJUSTING the governor since things changed AFTER installing the NEW governor.

MtnSnow · Apr 20, 2016

I would second Ron's suggestion.

Plane Fast · Apr 20, 2016

Edit: this info is for a A2 939 series.

My unloaded was sticking due to carbon build up. (Compressor draws air from the intake, so any oil from the engine, transmission, or transfer case. Gets cooked in there.)

You can pull the intake side off the compressor and look inside at the unloaded valve.( two bolts and a hose clamp) there is a gasket under that, so have a plan if you rip it. Some have just sprayed penetrating oil to loosen the carbon. I did not go this route, because mine had quite a bit of carbon. I knew I would have to take it apart to clean it with that much carbon. I bought a rebuild kit for $99, came with all the gaskets, o-rings and parts.
It works flawlessly now. I was having intermittent over pressure on mine (valve sticking open) I chased my tail changing the PPV, air dyer and governor. All along it was the unloader.

ralphluby · Apr 20, 2016

What is the normal pressure of the wet tank. I think my main problem is that my M923A2 came with a relief valve rated for 65 psi which keeps discharging. It appears that this valve should have a rating over 100 psi, probably as high as 125. Is that correct

ralphluby · Apr 20, 2016

What is the normal pressure of the wet tank. I think my main problem is that my M923A2 came with a relief valve rated for 65 psi which keeps discharging. It appears that this valve should have a rating over 100 psi, probably as high as 125. Is that correct

Plane Fast · Apr 20, 2016

So the over pressure valve on the wet tank is venting? The tank should be at 110 to 130psi, depending on the governor cut off setting.

Plane Fast · Apr 20, 2016

If you can, take a picture of the valve. Some trucks have had some strange things done to them.

98G · Apr 20, 2016

ralphluby said:
What is the normal pressure of the wet tank. I think my main problem is that my M923A2 came with a relief valve rated for 65 psi which keeps discharging. It appears that this valve should have a rating over 100 psi, probably as high as 125. Is that correct

No. That is not correct.

Compressor builds air and sends it downstream. PPV engages once it sees 70psi (or therebouts). Once the PPV engages , air is sent to the governor and everything else on that side of the system (horn, wipers , other 'nonessential' equipment ).

If the PPV doesn't see 70psi, it doesn't send air to that side. The governor never sees pressure so never cuts flow. The air dryer doesn't purge. The horn and wipers don't work. Pressure continues to build. When pressure reaches 150psi the pressure relief valve opens. Driving at steady highway speed, the pressure relief valve will rhythmically open about every 10 seconds.

Question 1) what led you to believe the PPV wasn't working?

2) what led you to believe the governor needed replacing?

3) have you tampered with the manual valves?

4) what have you done to adjust the governor?

5) is the air compressor outlet line hot?

6) what happens when you disconnect the line between the governor and the unloader?

Location of air compressor & governor

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