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1994 Mack CH trailer coupling and uncoupling not working


usrjcro11
Go to solution Solved by Hans Remmers,

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forgot when moving the arm make sure it's still connected to the valve NOT the rear end. might sound like "i'm not that stupid". not implying anyone is ; after 50 yrs of road calls it's the question that sound "stupid "make a difference  when not being there to physically check an issue.  a quick one;; call in liftgate doesn't work :: reply is the power cord plug in the trailer ? response ;; yes it is ;;    OK 1 1/2 later arrive to find power cord UNPLUGGED back of tractor..   driver==you didn't ask about the other end!!

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Will test tomorrow. Where should the air exhaust from if switch is working properly? Leveling control valve? Behind dash?

Wondering if I may have a plugged exhaust line. Dirt Dobbers are bad here plugging lines. Had a international truck with vacuum assist brake. Brake pedal hard but no brakes.put new booster on no better. Found exhaust line plugged from dirt dobber. Learn expensive lesson.

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I have also seen the filter screen inside the air input port on the leveling valve, blocked up with red dirt...  I guess southern red dirt can travel throught the smallest places...  :)

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40 minutes ago, Joey Mack said:

ok....  there is a leveling valve on the front drive axle..  make sure there is a supply of air to it.  if not, let us know..  you can un hook the supply line after you reduce air pressure below 85 psi,  then crank the truck and after 85-90 there should be air coming out of the line you un-hooked.. if not... the regulator at the tank is probably junk...  also, remove the dash panel and see if the switch is an air switch or all electric..  post the results.  jojo

The air bags do not start raising till about 90 psi. Same time as air bag on cab starts raising.

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well that is correct...  they will not rise until tractor protection valve opens up to supply air to the other systems.....  only the brakes get the air above 25 PSI...  It is a safety feature in big trucks...

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3 minutes ago, mechohaulic said:

am I correct then the only situation when flipping the toggle was ; you weren't sure what the valve did ?possibly there is nothing wrong with the system . the bags work properly ?

I knew what it was suppose to do but my air bags never purged air to assist uncoupling from trailer.

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53 minutes ago, mechohaulic said:

going back and forth on your other post about the air lines to trailer.  a "94 is too new for me to know its air schematic.  red button out shouldn't be air coming out trailer emergency line (red) during brake application. between that issue and air bags doing nothing with the switch ;first thought is someone has lines crossed.

Sorry to confuse. Two separate issues.

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the red button is not part of this issue Mech..   It is a seperate switch, that only controls the rear air bags..   its actually a simple system, especialy on a 90's truck... 

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trying to cross reference the two topics he wrote earlier dealing with same truck . earlier stated with trailer emer button out , air comes out the red hose when stepping on tractor brake pedal.  now the suspension issue seems A OK , working on the air / emerg line

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