1988 Mack RD 690

[David-L]

Lookin for some truck owners here on the site. ?, I have chased all the air leaks I could find and the air dryer purge valve  keeps going off to frequent. It looks to be a bendix unit with six or eight bolts holding a dome down on top. Would that be the filter under that, is there a purge valve spring gone bad. How can I test it or should I get a new air dryer. I have 409 'd all the lines going to the compresser and the tanks with no leaks found.   New to me so advice is appreciated. My air psi does stay at 120 and bleeds off really slow as the tag stays up for hrs no problem, I did fix any air leaks I could find and have 409'd every air fitting on the truck I could find.



 

[snowstorm]

the dryer will spit at whatever psi the governor on the compressor is set to. 120 ish. you can change the filter and heater if you want. it dosent take much of a leak. with it running do you loose air quicker with the maxis released??if so its time to buy brake cans. if it has the old cans that you could replace the rubber on the spring side.....dont....toss it buy new. that spring can be deadly

[snowstorm]

what do you call to often? air leaks usually are in cold weather a valve with ice in it. a worn out or corroded valve rubber in a brake can. the plastic air will wear through a piece of steel before it would the plastic. if you replace any line it must have the insert

[David-L]

Dosen't seem to loose air running Snowstorm. stays at 120psi just is always purging so I was thinking the purge valve was compromised. can't hear any air at the cans and yes I will replace a few that look suspicious. I chocked the truck released the brakes and put air out into the system and soap fittings and listen to find air leaks. If it won't hold the 120psi for long in the air dryer should I get a rebuild kit and replace that spring.

[snowstorm]

new to trucking? you will learn. a few basics all brake cans have a rubber diaphragm. air applied to one side pushes the slack adjuster applies brakes. the rear cans also have a spring great big mean one and 2 diaphragms.parking brake spring applied air released. anytime the button is in it needs air to hold the spring back.

[snowstorm]

so with the button out no air to the maxi can to release the spring brake. button in air to the can compresses spring releases brake. loosing air with the button in ? bad diaphragm common 

[snowstorm]

you could put a new governor on the compressor they are cheap. if you think it cycles to often 

[Skip]

When was last time you bleed the air tank ?

[David-L]

 Thanks all, I set up a bleeder and did bleed the tank. I saw the governor and it looks like a quick change out and not that pricey. Right now I am working on the Mack Dyna-tard system to see what the glitch is. I will start at the solenoids under the valve covers to see if they are working then start at the switch and go forward if they are. I suppose the wire going through the covers could be compromised also. Its work in progress and just want to make sure its safe to travel loaded.

[bushmechanic]

Sounds to me like the seals on the unloader valve are bad. That will make the purge be really frequent, seems like it is constantly blowing off.

[David-L]

You would mean unloader valve on the compressor Bushmechanic.

[millcreek40]

My Mack was doing that last winter. I bought a new dryer . Works just fine now

[Oliver05262]

  Dynatard switch on the back of the injection pump is a good place to start. The system sends power from a relay to the solenoid valves on the rocker arms when the accelerator is released (the rack goes to no fuel position and the rack touches that screw and gives a ground to the relay).as the fuel comes back in to keep the engine running, that ground circuit is broken.
  In use, there is a little arcing every time that circuit is made and broken, and that erodes the tip of the screw. There should be power on the wire at the governor switch with the dynatard switch on; ground that wire and see if the Dynatard comes on. If so, loosen the lock nut with a 3/8 " wrench and turn the screw in a little at a time until you get the brake to cut out where you want it; usually below 1200 rpm, but not low enough to stall the engine like a Jake Brake will do. Don't forget to lock the adjustment after you're done.
  Some drivers use the engine brake to make the engine rpm's drop quick to shift faster, but  that's not a good idea except in a pinch, like to gain a gear on a steep grade. It's hard not to cause sudden, sharp, torque reversals, and that raises all kinds of problems with the driveline. It sucks if you drop the driveshaft in the road !!!!

[bushmechanic]

Yes the unloader valves are in the compressor. When the system reaches max pressure the governor applies air to the unloader valve so the compressor doesn't keep pumping. If there is air loss around the unloader it will start pumping again and then it will blow off again when it reaches max pressure, the worse the seals the more frequent it will be. I wouldn't rule out a bad purge valve either, seen some funky crap with those also.

[snowstorm]

if the dryer spits every 2 min. something is wrong if it dose it every 10 its fine. you cant put a time frame on how often it should spit. it all depends on what the truck is doing. sitting in one spot or going down the road. as long as the compressor keeps up with what air is needed its all it takes. yes you should drain the air tanks. should be 2 one wet one dry maybe another 1 or 2 towards the rear so it has plenty of volume for the tag. to change the unloader you need to pull the head. so start with a govenor. under $20 at traction truck parts they will have the unloader parts also

[David-L]

Got the Dynatard working somewhat, I bypassed the solenoid on the injection pump and you just don't want to fuel it when shifting with the brake on or it bellows.  Runs off the switch well. I will look at the screw that probably arced and set it up right. I think this will help although I haven't tried it loaded up yet down a hill. Thanks for the advice. I will be asking alot of questions

[David-L]

I meant Dynatard switch on pump I bypassed. Gonna check that at.

[s grinder]

There's a plate on top of the compressor head,if you remove that carefully ,if i can remember there is two small springs and plastic poppets,if a spring or plastic poppet is broke your compressor will constantly cycle.Also if you have a leak on small plastic line from governor to air dryer will cause it to cycle.I have also seen many times the purge valve in dryer.If you drain your tanks and get an oily mess, i would change out dryer for a recon.

[David-L]

Thanks everyone, I enjoy working on this truck and appreciate all the good info. It won't be long now and maybe I will deliver some pole wood to customers. Mud season is upon us though.

[David-L]

after looking at my injection pump and pics of some on line my pump doesn't have a wire going to that connector ( 3/8's nut ) off the back of the pump. I goes to a plate with two screws in it on bottom.. Hot wire into one side and then the other wire out to rocker covers. When I tested it for 12 v power it was only on one side, thats why I bypassed it. Pretty sure that swith is to change the rack position to no fuel as said. It is at the very bottom of the back of the pump. It is a 300L motor and I will get what pump it is later to confirm and take a pic of this switch. Were there different switches on these pumps? Can I damage anything by just running manually off the switch on the dash as long as I remember to shut it down and not use it for shifting as the fuel will keep rolling? Thanks. The learning curve is real.

[David-L]

Well, got the truck running ok with the dynatard and things are going ok. Got some loads delivered. The tanks were not an oily mess when drained, just some clear water. I am going to start to concentrate on the purge valve going off frequently. Sprayed the lines from the gov and can't come up with any leaks, no oil in tanks. Bought a governor as it was cheap and worth trying. Took the old one off and went to plug the same holes in the new one and the third whole on the bottom of the old one was not plugged. Is that plug missing or just suppose to be that way. Might have to check the compressor unloader valve as mentioned before if this gov change does not do anything. My dryer is and old one, AD4. anyone know what this third port is for or should I plug it.



 

[Oliver05262]

Was there anything in the left hole in the old governor, or was it open? I think the three ports are for pressure sense (of the air system), one for a line to a remote function (the blow-off on the air dryer), and the third I believe is a relief port for when the governor kicks back in to load the compressor.
One or the other governor should have those ports marked in some way; but if in doubt, I'd just make sure that the plugs were the same as in the old governor.

[David-L]

Time to look at the unloader valve as the Govenor did not change a thing. Thanks Bushmechanic.