M35A3 Air Pressure Problems

[glcaines]

I went to drive the A3 to the coffee shop this morning and it let me down. I ended up driving the A2, which seems to never have any problems. After starting the A3 engine, the air buzzer came on and never went off. The air pressure gauge never built up any pressure. After 2 - 3 minutes, a loud air purging sound started from under the truck. Air was being released every 15 seconds or so in 2 - 3 second bursts. I crawled under the truck and found that the air was being purged from the safety relief valve shown as #36 on the attached diagram from the -24P TM. This relief valve is located in a hard to get to spot directly over the secondary air tank. I thought perhaps the relief valve had failed and was not letting pressure build up. I cracked open the drain cocks on both the primary and secondary air tanks. Althugh the pressure wasn't measured, both were obviously at very high pressure, much higher pressure that normal when draining water. Apparently the relief valve is actuating to relieve excess pressure, as designed. I checked, and the brakes work fine - there is definitely enough air pressure to handle the brakes. The symptoms involved in the problem are not covered in the troubleshooting section of the TM. This compressed air system is very complicated when following the diagram and it is not entirely clear how this system actually operates. The CTIS is turned off and has been during this entire issue. I know the compressed air system is designed to prioritize air pressure to the brakes in the event of a CTIS failure or massive leak on the CTIS. There are no air leaks anywhere in the entire compressed air system. I believe the problem lies in the part of the system that prioritizes air to the brakes, but cannot find the cause. Has anyone else experienced this problem? I searched, and couldn't find the same issue in any other posts.

 

[goodguyzy]

sounds like your on the right track, post up when you figure it out.

 

[jcappeljr]

Mine did the same thing,I unhooked the CTIS where it plugs in at the steering wheel.Problem solved.

 

[bullfrog1234]

just disconnect the CTIS and you should be good. my experience with the CTIS has not been good. I saw a truck with all tires flat in the yard. The mechanic later unplug the box and air the tires back up with a compressor. No problem after that. The CTIS Box crapped the bed

 

[glcaines]

Update. I disconnected both connectors from the CTIS on the steering column. This had no impact on the problem, so I'm still trying to understand exactly how the compressed air system operates.

 

[motomacguyver]

Mine does the same thing about 1 out of 3 starts. If I drain all of the air from the tanks and start again at zero psi, it usually shuts off the buzzer. But if I cheat, and backfill the system with shop air at the emergency glad-hand, the buzzer shuts off. I think it is the 60 psi failsafe valve myself. But I don't know where to find it.

 

[AceHigh]

Quite a mystery! I always wondered where all the air was till the gauge suddenly pops up.

 

[bullfrog1234]

Indeed, it you disconnected the CTIS, then it could very well be the pressure switch.

 

[doghead]

There is a "priority valve" shown in figure 114, in TM9-2320-386-24P

 

[G-Force]

Sounds like the unlaoder for the compressor isn't working

 

[CobraCDR]

Every determine the cause?

 

[Rustygears]

agreed. Check the compressor unloader attached to the compressor. They commonly wear out and only cost about $15 at a truck stop or Napa

 

[glcaines]

There is a "priority valve" shown in figure 114, in TM9-2320-386-24P

DH, I saw thr priority valve on the TM and it appears to me to be a valve that cuts off if the line to the emergency gladhand valve in the event of failure of that line or anything attached to it. I have fully traced out all the pneumatic lines on both the truck and in the TM to understand how the system works. It is actually fairly simple, but looks complicated. I traced out each circuit in the TM with colored hi-liters and that made it very easy to understand. I have decided the problem lies either in the Haldex air dryer or the compressor governor. For other reasons, I already have an NOS air dryer ordered. I plan to replace the air dryer and if the problem remains I will tackle the comressor governor. It it is the governor, it is likely the unloader not functioning properly. I'll keep everyone posted on the progress as others may have a similar problem. I plan to drive the A3 to the Georgia Rally this year, so I need to get it fixed. I already have another issue with the truck involving replacement of one leaking axle seal before I can drive it.

 

[R Racing]

If you notice on a A3 the guage will show no pressure at all for about a min. Then jump to 90 PSI . I believe there is a solinoid valve ( Could be priority valve, I don't have a -24 or -20- TM ) That opens and when it does it allows air to both the gauge and pressure switch.

 

[goodguyzy]

DH, I saw thr priority valve on the TM and it appears to me to be a valve that cuts off if the line to the emergency gladhand valve in the event of failure of that line or anything attached to it. I have fully traced out all the pneumatic lines on both the truck and in the TM to understand how the system works. It is actually fairly simple, but looks complicated. I traced out each circuit in the TM with colored hi-liters and that made it very easy to understand. I have decided the problem lies either in the Haldex air dryer or the compressor governor. For other reasons, I already have an NOS air dryer ordered. I plan to replace the air dryer and if the problem remains I will tackle the comressor governor. It it is the governor, it is likely the unloader not functioning properly. I'll keep everyone posted on the progress as others may have a similar problem. I plan to drive the A3 to the Georgia Rally this year, so I need to get it fixed. I already have another issue with the truck involving replacement of one leaking axle seal before I can drive it.

ive got an leaky axle also, looks to be an inner seal. from what i have read, not fun.

 

[glcaines]

Update - I replaced the air dryer with an NOS dryer I got on Ebay. My dryer was really corroded and so I decided to just replace the whole unit. I didn't really think the dryer was causing my air pressure problem as I had settled on the compressor governor / unloader as the culprit and planned to replace it. I replaced the air dryer yesterday, but didn't bother starting the truck until today. My problem has now gone away or I hope it has because the system is now 100% functional. The truck is still down due to a hub seal problem and I can't finish the repair until I receive new parts.

 

[BadMastard]

Glad it's fixed! I found out that they modified my A3, so where the compressor feeds to the main air line now had a T with a safety valve on one side instead of the "factory" elbow. All the other air lines seem relatively the same, but it makes for fun troubleshooting when your parts don't match!

 

[CobraCDR]

It always adds to the fun when it doesn't match. The joys of surplus vehicles!

 

[glcaines]

Glad it's fixed! I found out that they modified my A3, so where the compressor feeds to the main air line now had a T with a safety valve on one side instead of the "factory" elbow. All the other air lines seem relatively the same, but it makes for fun troubleshooting when your parts don't match!

All compressors on M35A3s are supposed to have that modification. Look at thread Installing the Pressure Relief Valve on the M35A3 . What you are looking at is the 4820-01-490-1773 pressure safety relief valve. MWO 9-2320-386-20-2 covers the installation. All M35A3s were required to have this kit installed prior to 30AUG05. My A3 didn't have it installed either. It is a very simple installation. I've seen the kits on E-bay and elsewhere for a few dollars.

 

[BadMastard]

Hey GLcaines,

I don't disagree with you, but I do wish the manual reflected it! The downloads from Jatonka didn't have it. It'd be nice to find a set that is truly up to date.

All in all, it's a very good mod. My compressor though leaks at that T, so it's going to have to come out and get teflon all over it. That's the odd thing, there are a bunch of air connections on mine that were leaking, and it mostly appeared to be simply no teflon tape. Side note, I had to replace my oil pan seal, it had shrunk just a little too much. It was the original pan seal, bolts still had the catepillar yellow on them. Might be that the desert was a little hard on my poor little truck.