How long trans light on after complete hydraulic rebuild

[MtBaldyBobcat]

I bought a Bobcat 742 (SN# 11809) from a friend who said the hydrostatic pump was bad. Don't know a thing about hydraulics, but I managed to pull the hydraulics completely apart, clean up hoses, port block, etc., had the hydraulic control valve & lift rams & backhoe stabilizer rams rebuilt, installed a rebuilt hydrostatic/hydraulic pump, and put it all back together. I just fired it up for the first time yesterday. It works, but the transmission light is on. There's certainly a ton of air in the system (started/stopped 4 times so far and lots of bubbles in the oil tank), so maybe the air needs to bleed itself out before the pressure can get high enough to make the pressure sensor happy. I'm a little concerned, though, since I obviously don't want all my work to be for naught by frying the rebuilt hydrostatic pump. Lift, tilt, and stabilizer rams work fine (I know... separate system), and it does drive (I've probably only moved it 100 feet total), though seems it could have more power. I've never driven anything hydraulic in my life, so I'm as green as they come. Does the trans light staying on until the bubbles stop appearing in the tank sound reasonable, or should I start troubleshooting even before that happens?

 

[Tazza]

It could indeed be the air. Lift/tilt and drive it a little. Let it sit over night to let the bubbles settle out.
If you remove the pressure sender and install a pressure gauge, it should be around 90 psi.
You did clean or replace the filter in the port block?

 

[MtBaldyBobcat]

It could indeed be the air. Lift/tilt and drive it a little. Let it sit over night to let the bubbles settle out.
If you remove the pressure sender and install a pressure gauge, it should be around 90 psi.
You did clean or replace the filter in the port block?

Thanx Tazza. I did clean the port block filter. That was my plan: lift/tilt and drive sparingly for a few days. If the light's still on after the bubbles are gone (and I can't find an obvious electrical issue), a pressure gauge is the next step.

 

[Tazza]

Thanx Tazza. I did clean the port block filter. That was my plan: lift/tilt and drive sparingly for a few days. If the light's still on after the bubbles are gone (and I can't find an obvious electrical issue), a pressure gauge is the next step.

A pressure gauge sooner than later is really your best bet. Just to be sure.
If it does not scream or bite at the steering levers you should be ok, but better to be safe than damaging your pump.
The senders do go bad.

 

[MtBaldyBobcat]

A pressure gauge sooner than later is really your best bet. Just to be sure.
If it does not scream or bite at the steering levers you should be ok, but better to be safe than damaging your pump.
The senders do go bad.

Actually, it is screaming as I drive around... more pronounced on turns and full forward or backward. My routine is to come home from work, drive around a 10-15 minutes and lift/tilt bucket a bunch, turn it off, and come back after dinner and do the same. I've done this about 3 or 4 times (so 6-8 on/off cycles). The trans light has been solid the whole time. Does it sound like this should have cleared the bubbles yet (still lots of bubbles in the tank w/ each cycle), or might it take longer? Am I blowing it and in danger of frying the hydrostatic pump? I haven't been pushing it by lifting anything or running it more than 15 minutes at a time. Thanks very much for your feedback.

 

[Tazza]

Actually, it is screaming as I drive around... more pronounced on turns and full forward or backward. My routine is to come home from work, drive around a 10-15 minutes and lift/tilt bucket a bunch, turn it off, and come back after dinner and do the same. I've done this about 3 or 4 times (so 6-8 on/off cycles). The trans light has been solid the whole time. Does it sound like this should have cleared the bubbles yet (still lots of bubbles in the tank w/ each cycle), or might it take longer? Am I blowing it and in danger of frying the hydrostatic pump? I haven't been pushing it by lifting anything or running it more than 15 minutes at a time. Thanks very much for your feedback.

When i did my 743, i found letting it sit longer was better. An hour or so isn't enough for the bubbles to really settle out.
How smooth is the lift/.tilt operation? does it jitter a littel still? mine took about a week to clear, running it for less than 5 minutes a day. Now its solid as.
Pump noise is normal, its just hard to say if its a bad noise or not without hearing it. I'd suspect if the charge pressure was low enough to be causing damage, i would think that it would not drive, or be really sloppy in the levers and have very little to no power. Does the pump get hot really fast?

 

[MtBaldyBobcat]

When i did my 743, i found letting it sit longer was better. An hour or so isn't enough for the bubbles to really settle out.
How smooth is the lift/.tilt operation? does it jitter a littel still? mine took about a week to clear, running it for less than 5 minutes a day. Now its solid as.
Pump noise is normal, its just hard to say if its a bad noise or not without hearing it. I'd suspect if the charge pressure was low enough to be causing damage, i would think that it would not drive, or be really sloppy in the levers and have very little to no power. Does the pump get hot really fast?

I check the tank, and the bubbles seem to be gone in the 2ish hours between the first and second running of the evening. The lift/tilt seem just fine. Driving doesn't jitter, but it does groan as I've said. With all the air I'm seeing, it's easy for me to believe that that's the issue, but sometimes we believe what we want to believe. It has occurred to me that I could be sucking in air, but since I see no leaks, hopefully not. I don't really know about the pump temperature... it's under the cab and I haven't flipped it since I've gotten it running. The engine compartment and back door seem pretty warm though, but the other light hasn't come on (which I think is the engine light, no?). Both radiator hoses are warm, so presumably water is circulating (the engine isn't that old and it fires up fine, so things should be good on that end).

 

[MtBaldyBobcat]

I check the tank, and the bubbles seem to be gone in the 2ish hours between the first and second running of the evening. The lift/tilt seem just fine. Driving doesn't jitter, but it does groan as I've said. With all the air I'm seeing, it's easy for me to believe that that's the issue, but sometimes we believe what we want to believe. It has occurred to me that I could be sucking in air, but since I see no leaks, hopefully not. I don't really know about the pump temperature... it's under the cab and I haven't flipped it since I've gotten it running. The engine compartment and back door seem pretty warm though, but the other light hasn't come on (which I think is the engine light, no?). Both radiator hoses are warm, so presumably water is circulating (the engine isn't that old and it fires up fine, so things should be good on that end).

Update: the bubbles had pretty much dissipated, and still the light, so I took further action. I pulled the pressure sending unit and discovered that the switch was normally-closed (should be normally-open, I believe). @%$!!#%^*!! Bobcat helpfully sold the wrong sensor (said it had been replaced by a different model). I still had the old one on which the connection tab had broken off, so soldered a new connection. While I was at it, I plumbed in a "T" and ran a pressure gauge to the cab. I fired it up and no transmission light, but still squealing when drive it. The charge pressure was mostly very low (15-20 lbs), though it would rise to 55 at times while lifting/dropping bucket, and sometimes it would drop to the bottom of the gauge at 10 psi. On lifting, I'd see the trans light flicker, and after running it for 15 minutes or so, it was on more often (at least it works!). I'm surprised it wasn't on more often w/ the low pressure (though I saw it on more than once during a 50 psi spike). From what I can tell from an earlier thread on this forum /www.skidsteerforum.com/DesktopDefault.aspx?tabid=54&frmView=ShowPost&PostID=14504>, it looks like my next step will be to replace the bronze 40-micron filter in the port block. When I tore down the system, I cleaned it w/ gas and compressed air, but the previous owner ran it out of oil (thus the rebuild), so maybe some fine metal particles got in there that I couldn't clean out. Hopefully my next and last post to this thread after I replace the filter will be that it's got 100 psi charge and flys up hills without squealing.

 

[Tazza]

Update: the bubbles had pretty much dissipated, and still the light, so I took further action. I pulled the pressure sending unit and discovered that the switch was normally-closed (should be normally-open, I believe). @%$!!#%^*!! Bobcat helpfully sold the wrong sensor (said it had been replaced by a different model). I still had the old one on which the connection tab had broken off, so soldered a new connection. While I was at it, I plumbed in a "T" and ran a pressure gauge to the cab. I fired it up and no transmission light, but still squealing when drive it. The charge pressure was mostly very low (15-20 lbs), though it would rise to 55 at times while lifting/dropping bucket, and sometimes it would drop to the bottom of the gauge at 10 psi. On lifting, I'd see the trans light flicker, and after running it for 15 minutes or so, it was on more often (at least it works!). I'm surprised it wasn't on more often w/ the low pressure (though I saw it on more than once during a 50 psi spike). From what I can tell from an earlier thread on this forum /www.skidsteerforum.com/DesktopDefault.aspx?tabid=54&frmView=ShowPost&PostID=14504>, it looks like my next step will be to replace the bronze 40-micron filter in the port block. When I tore down the system, I cleaned it w/ gas and compressed air, but the previous owner ran it out of oil (thus the rebuild), so maybe some fine metal particles got in there that I couldn't clean out. Hopefully my next and last post to this thread after I replace the filter will be that it's got 100 psi charge and flys up hills without squealing.

That pressure is low, hopefully the new filter will fix it. You technically can run it to test without the filter, but i'd recommend raplacing it after though. The newer machines don't run this filter, just the one in the engine bay from the oil cooler.

 

[MtBaldyBobcat]

That pressure is low, hopefully the new filter will fix it. You technically can run it to test without the filter, but i'd recommend raplacing it after though. The newer machines don't run this filter, just the one in the engine bay from the oil cooler.

Final Update: I put the new 40-micron bronze filter into the center of three ports in the side of the port block. Charge pressure is now running 90-100 psi, driving is pretty good. I say pretty good since I still hear the occasional groan... but nothing continuous like it was before the new filter. Hydrostatic drives are responsive, so I think I'm good to go. Thanx Tazza for your valuable insights.

 

[Cyclone Ranger]

Final Update: I put the new 40-micron bronze filter into the center of three ports in the side of the port block. Charge pressure is now running 90-100 psi, driving is pretty good. I say pretty good since I still hear the occasional groan... but nothing continuous like it was before the new filter. Hydrostatic drives are responsive, so I think I'm good to go. Thanx Tazza for your valuable insights.

Where is this pressure sensor you speak of?

 

[Tazza]

Where is this pressure sensor you speak of?

Excellent news. The slight noise is normal, they all do it.
The sensor, if you have the older style with the square block on the hydraulic pump will screw into the block. It should be a round canister.

 

[Cyclone Ranger]

Excellent news. The slight noise is normal, they all do it.
The sensor, if you have the older style with the square block on the hydraulic pump will screw into the block. It should be a round canister.

I just have a plug where the sender should be. The service manual shows a temp sender where the plug is and I can't find any reference of a hyd pressure sensor in the manual. I'm thinking this might have been added on later models. Does anyone have an extra sender or know a part number for one?

 

[MtBaldyBobcat]

I just have a plug where the sender should be. The service manual shows a temp sender where the plug is and I can't find any reference of a hyd pressure sensor in the manual. I'm thinking this might have been added on later models. Does anyone have an extra sender or know a part number for one?

I have a Bobcat 742 circa 1989. The pressure sender screws into the front end of the port block. That's where I installed the "T" and another line to my pressure gauge. I understand that they did away with this particular port block on newer models, and I think I read that they switched the positions of the temperature and hydraulic charge pressure sensors. Others on this forum could probably explain this more accurately than I have.

 

[wings5j]

I have a Bobcat 742 circa 1989. The pressure sender screws into the front end of the port block. That's where I installed the "T" and another line to my pressure gauge. I understand that they did away with this particular port block on newer models, and I think I read that they switched the positions of the temperature and hydraulic charge pressure sensors. Others on this forum could probably explain this more accurately than I have.

My 1993 742B has the newer style port block and no 40 micron filter. The hydraulic temp sender is on the front of this block. The sender for the pre-charge pressure system is mounted right on top of my hydrostat unit. FWIW.
John