M936 hydraulic system question

[gstirling]

I have been chasing a hydraulic issue (M936A1) on the main crane system and have a general question on how the system operates that i can't figure from the TM's. Does the hydraulic pump run full flow through the crane control body and back to the tank when no action is given. then for example: when you actuate the swing control does it close off the return flow and direct flow to the swing motor? I'm trying to figure out why i have a "slow" operation one day and a "normal" speed operation on another (all controls). if this is the way it operates then there is no pump bypass or internal relieve that could be leaking by (trying to eliminate possible failure points). if so then i am likely looking at a failing pump?? i put a pressure gauge on the valve body and on a slow day it basically reads near zero when an action is given (no loads). on a fast day it reads over 100 psi during an action (depends on load). if say the cylinder hits end of motion, it can spike to over 1000 psi ). i haven't held it to see what max is (what the TM shows for setting system relief at ~1300 psi if i remember right).
any info or thoughts would be apricated.

 

[KN6KXR]

First off make sure you are in top gear. You'll hear it kick into 5th as you throttle up. I just was using mine this morning and went back and thought "this is slow" then went back to the cab, one click out maybe 100rpm more to 1700rpm heard it kick down, back to the crane operates at full speed. I've gotten an "ear" for my machine the hoist motor makes a particular whine when it's at full speed and operated. If I don't hear the whine it's probably not in 5th....

The 1300 psi number isn't right (for my machine anyway). I found it at ~2200, set it at 1300 ran like crap, turned it back to about 2300 runs fine now. Install a hydraulic line where the 1/8" tap is with a couple elbows and pipe nipples. Terminate in a quick coupling and make up a valve. My local shop had all the parts it wasn't much. Now when I want to check what's up when operating I just stick it on there and can see what I'm running. When I hit 2300 on the gauge it's time for plan B! The relief is super easy to tune it's under the little acorn nut on the valve block. Needless to say one should inspect all the hoses and their rating before diddling with it. Unless you like oil showers I guess.

To answer your original question yes the valve block spool valves are pass through in their neutral positions. The system does not ride on the relief it recirculates. This is why to set your relief under the little acorn nut you need to two block it while looking at the pressure. Use the boom up or extension at the stops. Super easy.

Not sure about your machine but mine is rather idiosyncratic about getting the power divider engaged or going into high/low range. I need to bump the shifter quite often to get these items to drop in. Also the thing about getting it in 5th. I think I'm there but I'm not. I'm working on a M542A2 rescue and the crane on that one is the same but in a whole different way. It's an Army truck thing. No matter I'm still grinning when I'm running them.

 

[gstirling]

First off make sure you are in top gear. You'll hear it kick into 5th as you throttle up. I just was using mine this morning and went back and thought "this is slow" then went back to the cab, one click out maybe 100rpm more to 1700rpm heard it kick down, back to the crane operates at full speed. I've gotten an "ear" for my machine the hoist motor makes a particular whine when it's at full speed and operated. If I don't hear the whine it's probably not in 5th....

The 1300 psi number isn't right (for my machine anyway). I found it at ~2200, set it at 1300 ran like crap, turned it back to about 2300 runs fine now. Install a hydraulic line where the 1/8" tap is with a couple elbows and pipe nipples. Terminate in a quick coupling and make up a valve. My local shop had all the parts it wasn't much. Now when I want to check what's up when operating I just stick it on there and can see what I'm running. When I hit 2300 on the gauge it's time for plan B! The relief is super easy to tune it's under the little acorn nut on the valve block. Needless to say one should inspect all the hoses and their rating before diddling with it. Unless you like oil showers I guess.

To answer your original question yes the valve block spool valves are pass through in their neutral positions. The system does not ride on the relief it recirculates. This is why to set your relief under the little acorn nut you need to two block it while looking at the pressure. Use the boom up or extension at the stops. Super easy.

Not sure about your machine but mine is rather idiosyncratic about getting the power divider engaged or going into high/low range. I need to bump the shifter quite often to get these items to drop in. Also the thing about getting it in 5th. I think I'm there but I'm not. I'm working on a M542A2 rescue and the crane on that one is the same but in a whole different way. It's an Army truck thing. No matter I'm still grinning when I'm running them.

Thanks for the info, question? when you say it needs to be in 5th, mine has the Allison automatic, so TM has it in 1-5 position. the HI-Lo shifter is in neutral, and i then engage the transfer case power take off control lever (beside the seat) which auto throttles the engine to about 1500-1700 rpm. i have no way to really know what gear it is in (or am i missing something?). Since the "slow" days affect all controls i'm becoming more inclined to think the pump is failing rather than any other source of internal leakage (relief valve leaking etc..). just trying to eliminate/identify all possible problems before i start repairing/replacing parts. and since all my hoses are probably the original hoses, i suspect many should be replaced when i drop the pump. so in for a pump, in for pump and hoses.....LOL

Oh and yes my Hi-Lo shifter is a bear to get back into Hi. I did notice the linkage back on the transfer case was bent on mine, i straightened it once years ago, and it became easier (i.e. maybe two tries rather than 6 or 7). its getting harder to engage again so i need to check it out.

 

[KN6KXR]

Pretty sure they all have the automatic (the 936's). Do you have an auto throttle? Mine does not (A2) but I think yours might. I have a hand throttle. So I put it in neutral, engage the power divider, put in in drive 1-5, then throttle up using my hand throttle. It's when I'm doing that where I can hear a very mild "thunk" as I pass about 1600 rpm. That's when I know it's in top gear (5th I assume). If I don't do this I get a slow crane. Once I'm in 5th I can actually let it settle back to about 1600 or even 1500 without it shifting back into 4th. Also it's not consistent; like on warmer or colder days or maybe the slope of the way the truck is parked (haven't figured that out) it's a little different. Anyways I see how if you have an auto throttle engaged by the power divider, and it's right on the edge where sometimes it engages top gear and sometimes it doesn't, that could cause some confusion. Maybe try running it up to 2000 every time you run the crane just to be sure it's in top gear?

Thanks for the pointer on the linkage. That's my next stop. I can hear the torque converter pitch change when I push the button on the end of the stick but it still want me to "bump" the tranny into drive and back into neutral to drop in. Same deal with the power divider a lot of the time. Also on my "to do" list is a transmission fluid change the original owner swapped the tranny fluid to standard ATF I think I'm going to go to something like the Castrol Transyn fluid and see if I get a little better performance.

 

[gstirling]

Pretty sure they all have the automatic (the 936's). Do you have an auto throttle? Mine does not (A2) but I think yours might. I have a hand throttle. So I put it in neutral, engage the power divider, put in in drive 1-5, then throttle up using my hand throttle. It's when I'm doing that where I can hear a very mild "thunk" as I pass about 1600 rpm. That's when I know it's in top gear (5th I assume). If I don't do this I get a slow crane. Once I'm in 5th I can actually let it settle back to about 1600 or even 1500 without it shifting back into 4th. Also it's not consistent; like on warmer or colder days or maybe the slope of the way the truck is parked (haven't figured that out) it's a little different. Anyways I see how if you have an auto throttle engaged by the power divider, and it's right on the edge where sometimes it engages top gear and sometimes it doesn't, that could cause some confusion. Maybe try running it up to 2000 every time you run the crane just to be sure it's in top gear?

Thanks for the pointer on the linkage. That's my next stop. I can hear the torque converter pitch change when I push the button on the end of the stick but it still want me to "bump" the tranny into drive and back into neutral to drop in. Same deal with the power divider a lot of the time. Also on my "to do" list is a transmission fluid change the original owner swapped the tranny fluid to standard ATF I think I'm going to go to something like the Castrol Transyn fluid and see if I get a little better performance.

yes i have the auto throttle. and when i engage the transfer case power take off control lever (beside the seat) the auto throttle will rev the engine up - it will hit ~2000 rpm then quickly settle to 1500 rmp or so. also when i disengage the transfer case power take off control lever (beside the seat left side) the engine will rev up to 2000 rpm and then drop to idle. this lever is also "sticky" to engage or disengage. i have lubed it but it needs more attention.

Since you brought up the manual hand throttle, mine does not work in that you can pull it out one click at a time and there will be no effect, then you will hit this one click and the engine will rev to 2100 rpm, if you back click one click rpms drop to idle. since i don't use it, i have not spent any time figuring out what is wrong or what needs reset. any ideas?

 

[KN6KXR]

yes i have the auto throttle. and when i engage the transfer case power take off control lever (beside the seat) the auto throttle will rev the engine up - it will hit ~2000 rpm then quickly settle to 1500 rmp or so. also when i disengage the transfer case power take off control lever (beside the seat left side) the engine will rev up to 2000 rpm and then drop to idle. this lever is also "sticky" to engage or disengage. i have lubed it but it needs more attention.

Since you brought up the manual hand throttle, mine does not work in that you can pull it out one click at a time and there will be no effect, then you will hit this one click and the engine will rev to 2100 rpm, if you back click one click rpms drop to idle. since i don't use it, i have not spent any time figuring out what is wrong or what needs reset. any ideas?

OK so I think the A0 and A1 came with the auto throttle but for some reason the A2 did not. When you say "transfer case power take off control lever (beside the seat)" I'm thinking what's also called the "power divider" which engages the PTO drive to the hydraulic pump under the deck. It's the lever in the rear corner of the cab by the driver. So anyways it sounds like your auto throttle is working as designed. That said is the design very good or working properly? Is the throttle held high enough, long enough, to get it to shift to top gear reliably every time? I think the only way to tell is to duplicate the issue then hand throttle to 2000, let it sit and see if you can hear/feel the shift, then let go the hand throttle (back on the auto throttle) and go check to see if the crane is running at rated speed.

Another option is to ditch the auto throttle at least temporarily. Not having the item on my truck I don't have a clue BUT I did just defeat the auto front axle engagement by unbolting the air switch and using a zip tie. I'll bet it's similar. You may have to move the actuator to get true linear hand throttle but I'm betting this was an add on item so removal will likely be just as easy as the front axle auto switch.

I like my hand throttle and have no intention of adding anything. Partly it's KISS but there's another good reason: I need it sometimes while in rough terrain. Several days ago I got a day to go "rescue a wrecker" (I'm working on a M543A2 stuck in the woods that has to be driven out and took my M936A2) and I went in the back way. Well the back way is steep, narrow, dirt, mountain on one side and cliff on the other. Some of the switchbacks took 4 points and the road is off camber by about 20-25 degrees at those points (ever walk your wrecker sideways?). I came out after dark and downhill. I sure was happy to have that hand throttle because as I had to back up I could keep my foot on the brake, shift into reverse, hand throttle up and ease off the brake. 37,xxx pounds of truck with a half dozen 5-ton NDT tires and rims binder chained to the boom on the back facing a cliff a couple feet from the edge.... Yeah I like that thing. Also I can't afford a Chinook because that's what it would take to get my rig out. If it was worth anything after the drop.

But I digress..... I like it. One of those "you'll have to pry it from my cold dead hands" things. Give it a shot you can always put it back.

 

[KN6KXR]

This older one is cool it has a hand throttle and an engage/disengage just forward of the drag winch on the back deck. Got your control right there! Why the heck did they get rid of that?!?