Long story but here it goes.... I bought a new to me 2000 IH 4900 single axle truck with a dump bed, air brakes, 4 door cab, everything I've been looking for. Then found a 16 ton pintle hitch trailer to go with it. Hooked the two up and thought I was in heaven... Until the trailer brakes wouldn't work. When I pushed the dash valve in it just exhausts air out the exhaust port of the valve. My brilliant notion was the dash valve was wore out. $175 dollars later I have a new dash valve and the same problem. Hooked the trailer emergency line to shop air and I can release the park brakes. Hold the valve in and I can get reservoir pressure at the emergency glad hand on the truck so I have now isolated it I believe to the function of the tractor protection valve. Only problem is... I can't find the &$#@ thing. I've been three days trying to make it work but now I'm stumped. Can anyone tell me where I'd find the tractor protection valve? Everything I've read says near the back of the truck but I can't find it anywhere. Thanks guys John
Probably mounted inside of cab behind the drivers seat and covered up with upholstery. Are the trailer air hoses connected at back cab? Truck Shop
Yes the trailer hoses come out of the cab through a bulkhead type fitting. I'll pull up the floor mat tomorrow. Thank you John
On my old KW the tractor protection valve was mounted inside the frame rail, kind of beside the transmission. Not sure if IH mounted them like that too but ya never know.
I will also drop dime to donut on the inside the cab behind the seat. The idiot engineers would at times place them in some pretty stupid places as you know, they never fail!
For future reference for some other poor soul the tractor protection valve is located behind the seat in the center of the cab along the back wall of the cab. I got it out and the valve is full of slimy black oily crap and pink slime that looks like tire stop leak... It's no wonder the valve isn't working correctly, with all the crap in it an o-ring could never seal. Anybody running an inline air filter??? Thanks guys John
If the air dryer is functional and maintained and the compressor is not shot, the air in the rest of the system should stay clean and dry. No other place for contamination to enter really. I would blow out the lines if you feel it is necessary, replace it, and chalk it up to something that happened in the past.
One problem I see quite a bit is junk in the trailer lines getting blown back into the TP valve. Mostly bugs, from seasonal use trailers. or the bugs plug up the trailer valves. Gladhand gaskets with screens can be very helpful in a case like that.
Well I got the new tractor protection valve installed this afternoon. Another $175 and still have the same problem. When I push the dash valve in to release the trailer brakes, it just exhausts air out the exhaust port and the valve will not lock in to place. I'm open to suggestions. The guys at the parts store and I are getting to know each other pretty well, and I still don't have a functional truck. Thanks John
What model dash valve is it? How is it plumbed? Have you tried this with the yellow knob already in or is it out? Are you applying foot brake at the same time or not?
Bendix mv3 dash valve. Tried it with tractor supply valve in and out. Tried it with foot brake on and off. Tried applying trailer brakes with the Johnson bar... Attached is a photo of the old mv3 valve. I plumbed the new one with the same line colors to the same ports as they are stamped in the valve body. Yellow is Tractor Delivery, Green and Orange are supply, Red is Trailer Delivery and Black is exhaust. Also attached are images of the tractor protection valve that I replaced and plumbed back just as it was to start with. Thanks John
Have you checked to see if you have air coming back through the service/blue line when it is disconnected from trailer with supply/red line hooked up and red valve pushed in? Truck Shop
My thought too. Have you tried blocking off the red line at the back of the tractor and pushing the red knob in like that and see what happens?
No air returns out the blue/service side of the trailer when it's disconnected. I have deadheaded the emergency line gladhand on the truck with a pipe plug and a gauge. Neither allowed the trailer valve to lock in. With a gauge I got system pressure every time as long as the trailer valve is held in. I'm out of ideas and it's driving me nuts at this point. Thanks guys John
The MV3 is about as simple as they come, operation and plumbing wise. Inside it is a different story. If I was in your shoes I might remove the valve and test it with shop air. Feed air in the supply ports and dead head the delivery ports and see if it works that way. Also perhaps remove the valve from the system and feed air in the trailer supply (red) line at the dash and see if the air comes back out any of the other lines.
Have you checked the slider valve to your johnson bar. In some macks, i have seen where that will allow air to back feed into the MV3 popping the trailer circuit. If you had crap in your ttr prt vlv you may have debris holding the slider in between.
So I got it fixed and working correctly. The fix ended up being to unhook the air line from the exhaust port on the back of the dash valve.... When I did that the whole system came to life and began to function correctly. Not sure why the open line hooked to the back of the valve would have made a difference but I spent $500 in parts and several days chasing it. Johm
Is the line really open or mud dauber nest? So now you get to listen to a big blast of air when you set the brakes, all else is well?
Line was/is open. It would blow exhaust air out the end of it, and even shop air blew through it well. The exhaust port on the dash valve is inside behind the dash so it isn't a huge issue to hear the air blow off, but yeah that solved its issue completely. John
Thanks for posting the outcome. While not able to offer any advice I was following this thread with interest. But I am not clear as to why the line being removed helped it. Something in the valve that plugged it and blew out when you played with it again perhaps?
Had issues with the composite valves in the old CO9670 IHC units, lines in a bind or tight cornered would do that. But that was LONG ago.
But two different valves did the same thing and this valve is used in practically every truck these days...just a mystery I guess.
something to do with the fitting going into it maybe...