M936A1 Hydraulic crane pressure

[gstirling]

936 guys
a while back i install a pressure gauge to monitor pressure at the supply side (hi pressure) of the control valve body. after reviewing the TM's they showed a location to do exactly this - to monitor pressure - TM ( TM 9-2320-272-24-2 , section 3-389. PRESSURE RELIEF VALVE MAINTENANCE) shows the tap point on the right (as you look at it from the seat) center "tunnel" of the valve body. I installed a gauge at that point (the only tap on my valve body). the gauge (0 to 3000, new gauge) shows near zero pressure when running at all times - thought it should be seeing about 1350 psi. i have looked at the parts TMs to see if that showed anything else, no dice that i could see? back ground info, crane functions, but at times seems to be "slow" then run at other times run at what i think of as "normal". trying to figure out if pump is weak or i have a bad hose somewhere. i have check and replace the filter.

any ideas - bad gauge?


since the gauge was not working i had removed it (in picture) and was going to move it to the correct tap.... ha ha.. that's the only tap point.... so that's my dilemma. tap point is the shinny bolt

thanks


update on my post - a bit of playing around, verified the gauge is working and it turns out that tap point only sees pressure when one of the four controls is activated. reads zero with the pump running but no control is activated. If i crowd out - it reads about 150 psi, hoist up or down (no load) maybe 100 psi, will try the other controls and up date.

 

[KN6KXR]

Valve block will show zero when spools are at inactivated position. Pressure is only read from that port when valves are actuated. In order to set relief you have to max it out. I do this by raising the boom until it's at max travel. Then the pressure hits the relief and I set it there. Piece of cake just the capscrew, a lock nut and adjuster.

The TM is not correct. I found mine at 2200 or so from RSMS and tried the 1350 stated in the TM it runs like crap. Set it to around 2250 it seems to work as designed there. Before raising the pressure be sure to inspect your hoses if something goes it'll be a hose. Not fun. My hoses are nice and rated at 3000 so I felt fine about this. I also installed a hydraulic quick connect and some elbows/pipe so I can connect a gauge when running and glance at it once in awhile. Helps me make sure I'm not over working the crane.

Absolutely make sure you are in top gear. Mine does this about 1600rpm. If you engage at under 1000rpm and then throttle up with the hand throttle as you pass the 1500 or 1600 rpm point you'll hear it shift. Then you know. Or just run at 1700rpm and call it good. This is a different animal from the 816 and I think the TM's were not updated but copy and pasted for the 936. Hence the different data and tribal knowledge around this.

 

[gstirling]

Valve block will show zero when spools are at inactivated position. Pressure is only read from that port when valves are actuated. In order to set relief you have to max it out. I do this by raising the boom until it's at max travel. Then the pressure hits the relief and I set it there. Piece of cake just the capscrew, a lock nut and adjuster.

The TM is not correct. I found mine at 2200 or so from RSMS and tried the 1350 stated in the TM it runs like crap. Set it to around 2250 it seems to work as designed there. Before raising the pressure be sure to inspect your hoses if something goes it'll be a hose. Not fun. My hoses are nice and rated at 3000 so I felt fine about this. I also installed a hydraulic quick connect and some elbows/pipe so I can connect a gauge when running and glance at it once in awhile. Helps me make sure I'm not over working the crane.

Absolutely make sure you are in top gear. Mine does this about 1600rpm. If you engage at under 1000rpm and then throttle up with the hand throttle as you pass the 1500 or 1600 rpm point you'll hear it shift. Then you know. Or just run at 1700rpm and call it good. This is a different animal from the 816 and I think the TM's were not updated but copy and pasted for the 936. Hence the different data and tribal knowledge around this.

thanks for the response, you confirmed what i see on my 936 gauge. when i engage the crane PTO, my engine auto throttles up to ~1500-1700 and the gov holds it as load varies. so by "top" gear - my shifter is in 1-5 position??? can't say i've heard it shift, just hear it throttle up to rpm when it is first engaged. i have never used the hand throttle. by the way my hand throttle must need some adjustment, basically you pull it out one click at a time and there is zero response and then it hits one click and roars to ~2000rpm and if you click back one click it idles down to ~800rpm? since i don't use it i have never trouble shot it.

 

[KN6KXR]

thanks for the response, you confirmed what i see on my 936 gauge. when i engage the crane PTO, my engine auto throttles up to ~1500-1700 and the gov holds it as load varies. so by "top" gear - my shifter is in 1-5 position??? can't say i've heard it shift, just hear it throttle up to rpm when it is first engaged. i have never used the hand throttle. by the way my hand throttle must need some adjustment, basically you pull it out one click at a time and there is zero response and then it hits one click and roars to ~2000rpm and if you click back one click it idles down to ~800rpm? since i don't use it i have never trouble shot it.

You have an auto throttle then. From reading a lot of posts I gather the A1 or A0 was equipped with these. I have an A2 and it has no auto throttle. I have to engage the power divider then use the hand throttle to crank it up to proper RPM. Yes I mean the 1-5 drive position. On mine without the auto throttle I engage the power divider in neutral and idle, then put it in 1-5, then hand throttle up (sometimes I have to jiggle back and forth between neutral and drive to get the power divider handle to move over). If I slowly throttle up I can hear it shift up. You have to listen and feel for it. Sometimes if I'm in a rush I just crank it out and go.....

If I had an auto throttle I think I would disconnect it. Sometimes I WANT to move the crane slowly and don't care about power. At full RPM the valves can be touchy. I like having control over the hydraulic flow. There are folks here who'll say you'll damage the machine by running low RPM but that's not my understanding of hydraulics. You just won't be able to develop full speed and power.

 

[Mullaney]

You have an auto throttle then. From reading a lot of posts I gather the A1 or A0 was equipped with these. I have an A2 and it has no auto throttle. I have to engage the power divider then use the hand throttle to crank it up to proper RPM. Yes I mean the 1-5 drive position. On mine without the auto throttle I engage the power divider in neutral and idle, then put it in 1-5, then hand throttle up (sometimes I have to jiggle back and forth between neutral and drive to get the power divider handle to move over). If I slowly throttle up I can hear it shift up. You have to listen and feel for it. Sometimes if I'm in a rush I just crank it out and go.....

If I had an auto throttle I think I would disconnect it. Sometimes I WANT to move the crane slowly and don't care about power. At full RPM the valves can be touchy. I like having control over the hydraulic flow. There are folks here who'll say you'll damage the machine by running low RPM but that's not my understanding of hydraulics. You just won't be able to develop full speed and power.

.
The reason for setting the speed to 1700 rpm is to keep you from "smoking your Allison" transmission.
Nothing to do with hydraulic pump speed and resulting speed.
Just creep the handles if you want to move slowly.

The older wreckers with manual transmissions will operate at idle all day long.
Operating the M936 at idle will burn up the transmission - and that ain't cheap.

 

[KN6KXR]

.
The reason for setting the speed to 1700 rpm is to keep you from "smoking your Allison" transmission.
Nothing to do with hydraulic pump speed and resulting speed.
Just creep the handles if you want to move slowly.

The older wreckers with manual transmissions will operate at idle all day long.
Operating the M936 at idle will burn up the transmission - and that ain't cheap.

So the logic here is that with the power divider in you could overload the transmission at low RPM without it kicking down? A load is just a load whether it be driving the truck or the crane. In driving the truck up a hill as it stalls the transmission should kick into a lower gear; if it doesn't you just sit on a hill without pressing the throttle down you just come to a standstill. Right? So the risk of burning up the transmission is the same if you do this (sit on a steep hill going up just holding the throttle steady and going nowhere).

I have a hard time understanding what the difference is or how it could cause damage. I reckon the machines were designed so the guys could treat them like that. I know around my place I've had to do a bunch of switchbacks on roads uphill and have sat there, seemingly forever, with my foot to the floor waiting for the truck to move. Seems like the same thing to me. The rig seems to handle it fine I keep an eye on the temp gauges.

At the end of the day it's kind of immaterial. The crane won't lift over about 1k pounds at lower rpm and is a real dog. I only throttle down when I'm doing stuff like stowing the hook with the stay cables. Nice and slow so I don't snap the cable hook off the stay. I have some new shiny clevis hooks on my OEM cable....

 

[Mullaney]

So the logic here is that with the power divider in you could overload the transmission at low RPM without it kicking down? A load is just a load whether it be driving the truck or the crane. In driving the truck up a hill as it stalls the transmission should kick into a lower gear; if it doesn't you just sit on a hill without pressing the throttle down you just come to a standstill. Right? So the risk of burning up the transmission is the same if you do this (sit on a steep hill going up just holding the throttle steady and going nowhere).

I have a hard time understanding what the difference is or how it could cause damage. I reckon the machines were designed so the guys could treat them like that. I know around my place I've had to do a bunch of switchbacks on roads uphill and have sat there, seemingly forever, with my foot to the floor waiting for the truck to move. Seems like the same thing to me. The rig seems to handle it fine I keep an eye on the temp gauges.

At the end of the day it's kind of immaterial. The crane won't lift over about 1k pounds at lower rpm and is a real dog. I only throttle down when I'm doing stuff like stowing the hook with the stay cables. Nice and slow so I don't snap the cable hook off the stay. I have some new shiny clevis hooks on my OEM cable....

.
Transmission overheating is why - according to the information shared with me.

Logically, what you mentioned would be the same - and if so - it seems that when a slushbox gets hot it gets unhappy no matter what it is doing.

 

[gstirling]

You have an auto throttle then. From reading a lot of posts I gather the A1 or A0 was equipped with these. I have an A2 and it has no auto throttle. I have to engage the power divider then use the hand throttle to crank it up to proper RPM. Yes I mean the 1-5 drive position. On mine without the auto throttle I engage the power divider in neutral and idle, then put it in 1-5, then hand throttle up (sometimes I have to jiggle back and forth between neutral and drive to get the power divider handle to move over). If I slowly throttle up I can hear it shift up. You have to listen and feel for it. Sometimes if I'm in a rush I just crank it out and go.....

If I had an auto throttle I think I would disconnect it. Sometimes I WANT to move the crane slowly and don't care about power. At full RPM the valves can be touchy. I like having control over the hydraulic flow. There are folks here who'll say you'll damage the machine by running low RPM but that's not my understanding of hydraulics. You just won't be able to develop full speed and power.

yes my 936 is an A1, i had figured all 936's had auto throttle - good to know, as usual, there are lots of variations. I like my auto throttle, as for my machine, i can't imagine wanting it to go slower which is what started this whole thread. i'm trying to figure out why (for no obvious reason) some days my crane operates at what i think of as "normal" speed, and some days it seems to be running "slow" (hydraulically, all crane controls are sluggish). when i installed the Pressure gauge per the TM was hoping for pump discharge pressure to help me determine if my pump is failing or if i have some sort of blockage (maybe a internally collapsing hose?). The location the TM gives for the pressure gauge is pretty much to set the relief valve and doesn't give a pump discharge pressure like i was hoping so i could compare normal days to slow days..
and yeah my hi-neutral-low handle is very "sticky" to get back into Hi. usually takes four or five tries to get it to go back in gear.