[SOLVED] 1997 M1088 pressure release valve at 30psi?

[ke6rwj]

hello all,

my "new" M1088 has problems getting pressure above 60psi. it takes over an hour to get there. also if i touch the brake petal at all it drops to 40psi.

i found a leaky inversion valve and replaced it, also replaced the roached air dryer assembly with a new haldex unit and replaced the governor / unloader on the compresser.

im going through the troublshooter in the manual but wanted to know if that presure relief valve blowing off at 30 psi is normal. seems to me that if the pressure is low it shouldt do that.

also, i think this truck always had this issue as there was tape over the buzzer to quiet it.

it will build enough pressure to release the parking brakes eventually, but you cant use the foot brakes even once or it will lock up with low pressure..

so what say you about the pressure relief?

chris

 

[coachgeo]

....

so what say you about the pressure relief?

chris

sorry can't answer that but have found that if I have the trailer brake supplied from dash..... with no trailer on it.. I get similar issue. Not sure if this indicates I have leak on trailer side..... but anyway.... it might help you.... so check your trailer supply valve and make sure it is NOT set to supply air

 

[ke6rwj]

sorry can't answer that but have found that if I have the trailer brake supplied from dash..... with no trailer on it.. I get similar issue. Not sure if this indicates I have leak on trailer side..... but anyway.... it might help you.... so check your trailer supply valve and make sure it is NOT set to supply air

its def not set to supply..

 

[ke6rwj]

an update, it looks like this release is coming from the bottom of the haldex air dryer.

also if i let it sit and run for over an hour i never get more than 60 psi.. no matter what.

doing the troubleshooter now..

chris

 

[ke6rwj]

update...

i followed the troubleshooting and it takes me to "replace compressor"... i vaguely remember something about these compressors in early versions having some sort of issue where the drive gear wasnt properly secured to the compressor shaft.. or am i delusional at the thought of having to replace the compressor? (can they be rebuilt?)

 

[simp5782]

First step. Rig and air gauge in so you can see what the pressure is out of the compressor output. If it's reading over 130. Then move to the input on the air dryer and check there and work your way down the line if the pressure is ok at various points

If its not reading at the compressor then go into it and look for a stuck unloader. Or a governor is not letting the compressor full open.

If the pressure release is open on the air-dryer then that is probably due to overpressure at that point

 

[Ronmar]

The unloader senses wet tank pressure. If there is a blockage between the dryer and wet tank, the dryer would see High pressure/overpressure And vent thru that safety release, but the unloader would only see the reduced pressure at the wet tank and keep the compressor engaged.

Some air is getting to the wet tank and the rest of the system, but the fact that the system doesn't go above 60ish PSI also indicates you are probably pissing air away somewhere.

do you have an air compressor? On the Front emergency gladhand cover, there is a screw-on vent. You can remove that vent and add a quick detatch air fitting to connect an air compressor to the emer port. This will pressurize your system without the engine running(chock the wheels as it also releases the parking brakes). No engine noise makes it real easy to find leaks. This will also take the engine compressor out of the equation

 

[ke6rwj]

The unloader senses wet tank pressure. If there is a blockage between the dryer and wet tank, the dryer would see High pressure/overpressure And vent thru that safety release, but the unloader would only see the reduced pressure at the wet tank and keep the compressor engaged.

Some air is getting to the wet tank and the rest of the system, but the fact that the system doesn't go above 60ish PSI also indicates you are probably pissing air away somewhere.

do you have an air compressor? On the Front emergency gladhand cover, there is a screw-on vent. You can remove that vent and add a quick detatch air fitting to connect an air compressor to the emer port. This will pressurize your system without the engine running(chock the wheels as it also releases the parking brakes). No engine noise makes it real easy to find leaks. This will also take the engine compressor out of the equation

when i first got the truck i hooked the front emergency air output on my M923 to the front emergency gladhand on the 1088 and did find the severely leaking rear inversion valve. once i replaced that i did it again and didnt hear air hissing anywhere

i did replace the air dryer and governor on the compressor.

after studying the air system schematics (20-5, FO-2) i noticed that the airdryer is immediatley after the compressor so it seems the compressor is good enough building pressure to force the dryer to vent. and since both front and rear air is low, it points me to the primary air tank check valve being faulty.. if that valve was stuck or barely open i think we could have the same thing. I ordered a couple to replace but wont be able to donthe work till later this week.

im open to any other ideas ya'll have.. i just got a M373A2 shop trailer to put behind this thing, cant wait!


at any rate i need to redo the air check at the front gladhand and see what we get.

thanks all

chris

 

[Ronmar]

the primary and secondary tank see wet tank pressure simultaneously, so it would have to be 2 failed valves, or one system would be reaching a greater pressure than the other. Also If the inlet to the primary and secondary tank were the problem that means the wet tank would Still be receiving full pressure and sending that pressure to cutoff the governor, cycle the purge valve on the dryer and not over-pressurize the dryer enough to lift the relief.

I think your problem has to be between dryer and wet tank. This could be as simple as a bad airline between dryer and wet tank. I have seen a Braided hose fail like this where the inner liner collapsed And blocked most all flow thru the line... you could disconnect that line where it connects to the wet tank to see if the dryer safety stops venting...

I would hook up the alternate air source again to look for leaks. Since the system only builds partial pressure...

an easy way to check tank pressure is using a Compressed air blowgun with a rubber tip. Screw a pressure gauge where the airline would attach, hold the rubber tip against the tank drain port and open the valves To check the pressure...

 

[ke6rwj]

a quick update. i ran down to the shop and pulled off the two check valves at the primary and secondary air tanks and both were completley stuck shut. caked in rust. i have some on order should be here saturday..

more to follow...

 

[Mullaney]

the primary and secondary tank see wet tank pressure simultaneously, so it would have to be 2 failed valves, or one system would be reaching a greater pressure than the other. Also If the inlet to the primary and secondary tank were the problem that means the wet tank would Still be receiving full pressure and sending that pressure to cutoff the governor, cycle the purge valve on the dryer and not over-pressurize the dryer enough to lift the relief.

I think your problem has to be between dryer and wet tank. This could be as simple as a bad airline between dryer and wet tank. I have seen a Braided hose fail like this where the inner liner collapsed And blocked most all flow thru the line... you could disconnect that line where it connects to the wet tank to see if the dryer safety stops venting...

I would hook up the alternate air source again to look for leaks. Since the system only builds partial pressure...

an easy way to check tank pressure is using a Compressed air blowgun with a rubber tip. Screw a pressure gauge where the airline would attach, hold the rubber tip against the tank drain port and open the valves To check the pressure...View attachment 808716

Very cool and easy to make tool. Neat Idea
!

 

[Ronmar]

a quick update. i ran down to the shop and pulled off the two check valves at the primary and secondary air tanks and both were completley stuck shut. caked in rust. i have some on order should be here saturday..

more to follow...

Well thats a cool one, 2 valves failing the same way at the same time Is pretty rare... if those are not flowing, they are only part of your problem Though and dont explain the other symptoms. Like what is causing the overpressure at the dryer, and what is leaking away the air downstream Of the pri/sec tanks(at the same rate) as fast as those bad valves can flow it...

You really need to know what that wet tank pressure really is...

 

[ke6rwj]

Well thats a cool one, 2 valves failing the same way at the same time Is pretty rare... if those are not flowing, they are only part of your problem Though and dont explain the other symptoms. Like what is causing the overpressure at the dryer, and what is leaking away the air downstream Of the pri/sec tanks(at the same rate) as fast as those bad valves can flow it...

You really need to know what that wet tank pressure really is...

I feel much better about understanding how this system works now, so i plan on getting down into it this weekend. my concern is that since the air dryer was so badly damaged that it obviously didn't do much by way of removing the moisture, it could mean the other sensors on the wet tank are likely bad also... So im gonna just go through it all this weekend. Im not afraid of tearing into this stuff, and since this will be pulling my M373A2 shop trailer, I need to make sure its 100%.

Thanks for all the assistance, i appreciate it greatly.. Updates to follow...

Chris in Bama

 

[ke6rwj]

Update...

replacing the two check valves allowed the truck to build 90lbs of pressure in under 2 min.

now however i think it still not shutting off the air compressor as it should. its still blowing air out the air dryer every now and then... also ctis not working, but we are getting there!

i attached a pic of the dash...

thanks all for the help!

chris

 

[coachgeo]

Update...
....

now however i think it still not shutting off the air compressor as it should. its still blowing air out the air dryer every now and then... ..

chris

isn't that normal? I know mine does.... but do not recall how often cause it does not sit and idle long enough for me to hear it happen more than once.

 

[ke6rwj]

isn't that normal? I know mine does.... but do not recall how often cause it does not sit and idle long enough for me to hear it happen more than once.

well im not sure actually, it seems abnormal to me. the guages are pegged at 120psi then blows the relief in the dryer...

chris

 

[Mullaney]

Update...

replacing the two check valves allowed the truck to build 90lbs of pressure in under 2 min.

now however i think it still not shutting off the air compressor as it should. its still blowing air out the air dryer every now and then... also ctis not working, but we are getting there!

i attached a pic of the dash...

thanks all for the help!

chris

Hi Chris,

Not sure what "every now and then" amounts to, but my truck at idle with throw air out of the dryer every few minutes. I have never timed it, but it amounts a couple of times as I walk around the truck during a pre-trip check.

 

[ke6rwj]

Hi Chris,

Not sure what "every now and then" amounts to, but my truck at idle with throw air out of the dryer every few minutes. I have never timed it, but it amounts a couple of times as I walk around the truck during a pre-trip check.

do your guages sit at 120 psi at idle?

 

[Mullaney]

do your guages sit at 120 psi at idle?

No. Mine sits at about 105/110.

It might be worth verifying that your gauge is correct. MV instrumentation on the dash isn't terribly reliable. I did a quick check on eBay and searched "air compressor pressure gauge" and found several for about $8 delivered. For a few bucks you could plug one in and get a second opinion pretty quick...

Heck, so you don't have to do any "plumbing" on your truck, get a gladhand and screw the gauge into it. Then attach the gladhand on the drivers side (red) of your truck - front or back - and check the air pressure reading. It is a good cheap way to diagnose pressure without doing any plumbing If you need a little more info don't hesitate to ask. I have one in the tool pocket on my truck. I can snap a pic and post it tomorrow when it isn't pouring rain...

 

[ke6rwj]

No. Mine sits at about 105/110.

It might be worth verifying that your gauge is correct. MV instrumentation on the dash isn't terribly reliable. I did a quick check on eBay and searched "air compressor pressure gauge" and found several for about $8 delivered. For a few bucks you could plug one in and get a second opinion pretty quick...

Heck, so you don't have to do any "plumbing" on your truck, get a gladhand and screw the gauge into it. Then attach the gladhand on the drivers side (red) of your truck - front or back - and check the air pressure reading. It is a good cheap way to diagnose pressure without doing any plumbing If you need a little more info don't hesitate to ask. I have one in the tool pocket on my truck. I can snap a pic and post it tomorrow when it isn't pouring rain...

yea i felt the pressure was too high simply because its pegged the guage.. i would expect 100-110... im def gonna see if the guages are correct, and also dig into the governor connections..

thanks

Chris