raise and lower cab

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m-35tom

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94 A0 VERY hard to raise cab, VERY hard to lower cab. Air pump works very hard and labored, manual pump very hard to operate handle. Has steadily gotten worse over past 10 times of opening. Ideas? will not go over center when open. Will not self close after being over center.
 

m-35tom

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How many restrictor orifice are there and where are they? Should the cab close on its weight or does it have to be pumped all the way down?
Is there a locking valve in the cylinder?
 

m-35tom

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So cab needs to be pumped all the way down is normal? I thought I read it would close with a flow control but that may have been another series of trucks. New fluid, clean, no air, no water.
I tried the suspension circuit and it worked fine as did the spare. Then I tried the cab, no go so I tried the spare again no go on that either just pumps against high pressure. Same not with suspension. Tried several other settings and the suspension worked for 2" and all others still locked. So I should remove and clean each control valve?
 

snowtrac nome

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I had the same problem once it was cold as he double toothpick and dark a few days later I found one of the other control knobs turned a bit. As a note to self check all knobs are straight up and down except for the function you are using.
 

m-35tom

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I set bottom to cab lift and top to raise. Pump hand pump. Pumps up high pressure and will not pump more. Then turn top knob to lower, you can hear the ping as the pressure is released. Play with all combinations of settings and occasionally it will work fine for 5 or 6 pumps until jams again. Sometimes cab will start to raise easily for a few inches then stop, pump will not go any more, nor will it lower. Will lower 1/8" at a time by turning top knob from raise to lower repeatedly.
 
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Suprman

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There are 2 filters in the control box on top there are hex head covers that come off and then the filters unscrew and come out. The parts tm shows the breakdown of the control box. There are 2 different styles. Again any water in the system will cause issues like you are having.
 

NDT

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Sounds like trash is being circulated in the system. Personally I would start at the easy place, the fittings at the lift cylinder. Remove them and look for chunks of crap inside. Usually that is where the flow restrictors are, I don't have TMs handy presently.
 

m-35tom

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System is extremely clean and no sign of water or trash. Pump will operate suspension usually, cab up sometimes but not cab down. Spare tire for 2" then never again. Cab will not open past where the cylinder pushes it. The pin will not slide up in the loop at end of cylinder. So brace never goes over center into safe area. Strange. This truck has been possessed in so many ways I think the devil lives in it.

I took out the 3 top spool valves, fluid pumps easily into which ever is selected by bottom valve.
 

NDT

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None the safety braces on my trucks go overcenter either. Yes these trucks are no G742, they all have these type issues and much worse too. Spend some time searching the FMTV forum for bizarre problems other people have had with the cab lift. There have been several resolutions posted by folks.
 

m-35tom

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Looking at the design, it seems apparent that it was intended for the cab weight to go further forward so the cab brace would fold on itself and lock. I cannot find any reason the cab stops where it does.
 

Suprman

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The cab brace should fold out and lock when the cab is fully open. I had a truck that was giving me hyd function issues I swapped the control box and all was good. I had planned on taking the bad one apart and see what was going on with it but haven’t gotten to it.
 

Suprman

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The arm folds open and locks out. As best as it can for what it is. So if you push on the cab it’s locked open. And to start closing it, takes hyd action to pull the cab and arm back.
 
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