M985 No Air Pressure

[matt2491]

M985 HEMTT won't build any air pressure. Starts and runs fine, but will not drive as the brakes (and apparently throttle) are air actuated. Air pressure gauge reads zero.

Thinking compressor but can't find any spare replacements. But maybe it's not even the problem. Any ideas?

 

[ericp351]

First check if your auto purge is unseated. (if applicable) May get lucky!

 

[Floridianson]

Were is the unloader valve on that model? Quick Google looks like it is on the other side of your pic under a plate? Governor easy enough to check, the unloader valves a little harder but not bad you would think as you have some room to work on it with compressor installed.

 

[matt2491]

Thanks for the replies.

What/where is the 'auto purge'?

Isn't the unloader the same as the governor? Google is seeming to suggest that to me, and I don't see anything on the other side of the compressor.

 

[ericp351]

My 1991 5 ton was fitted with an auto purge, under the rig. When it did not seat, I could not build air press. Your air comp needs to unload, by keeping the intake valves from closing so it can "coast" at pressure (lines confined to comp, or a line from purge valve back to comp.). If your comp spins it should make some air, even if worn out IF the valves seat and it isn't dumping air (auto purge, busted pipe, relief valve.) Most of the industrial compressers I ever worked on had unload trouble, or valve issue. Rings and pistons fairly realiable. Some let ya lift out the valves to fix and clear the plates and swap the little springs with the heads on. They may have a cheap or semi reasuable copper gasket. Other guys may know your rig well, I mostly just have this general stuff...

 

[simp5782]

Ok for those who have never worked on a 92 series please exit stage right. Thanks.

First step. Get an air pressure gauge. Rig it up to the discharge from the air compressor that feeds the truck system right there before it goes to the governor in your picture.

See what your air pressure is. It should wrap the gauge around past 150. This will see what the governor is doing. IF YOU HAVE NO AIR PRESSURE FROM THE COMPRESSOR remove the governor from the compressor itself and any lines going to the governor and see if your compressor starts putting out air.

Then report back

 

[Floridianson]

Ok for those who have never worked on a 92 series please exit stage right. Thanks.

Yea my bad been 40 years since I worked on my 6v92. The truth never had compressor problems with my truck I bought in 1981. O well hope you get it figured.

 

[matt2491]

So the truck went from building no air pressure to plenty of it. My friend messed with the governor I think.

But now the truck makes an awful noise once fully up to pressure. It sounds like its coming from this thing:

 

[MrMikey4026]

Loosen the bolts for the air governor and crack the seal, If it builds air replace the air governor.

 

[matt2491]

delete

 

[matt2491]

We replaced the governor and air dryer purge valve since both were cheap so why not. The bad noise (from my last post) went away. The truck built around 100-120psi and we heard the automatic air release. All seemed well. But then we went to drive the truck and noticed the air pressure was now steadily dropping. Revving it didn't help. And now we have this marbles-rattling-around noise near the compressor / governor. I tried to redneck stethoscope it with a screwdriver but couldn't exactly place the noise.

So the current status is that we're back to no pressure and this funky noise.

What do you guys think?

Thanks for all the help so far.

 

[MrMikey4026]

Is there a hole in the compressor discharge hose?

 

[matt2491]

Turns out it was something to do with the unloader valves in the air compressor. We bought a rebuild kit and threw it in and now everything works as it should. I think one of the little springs in there was busted.

This was back in May. I just forgot to update this thread until now.

Also, this whole time I thought the truck was a M985. It's actually a M978.