We bought a D9H over the winter to level out a 50 acre field that we plan on putting back into crop production. When we first started using the dozer, we noticed that it was building a lot of pressure in the hydraulic fluid tank. We discovered that some parts were missing from the cap, making it unable to vent any pressure that builds. These parts were replaced, and now the cap vents. However, any time you lower the blade (even slowly) a large quantity of hydraulic oil squirts out the vent hole on the cap. The level was right in the middle of the sight glass with it running and the blade lowered, so it doesn't appear to be overfilled. Last time I ran it for 4 hours, and I bet it lost about 2 gallons of fluid in that time. My suspicion is that previous owners were having this problem, so they just removed parts from the cap so that it wouldn't vent, essentially sealing it off. We were concerned that could be dangerous without a way for that pressure to vent from the tank. Any idea why this is happening or a possible fix?
I could be wrong, but it seems like the cap is supposed to seal when tight, and vent when loosened. When removing the cap, you loosen it a bit, let the pressure bleed off, then remove all the way. If not, it is not uncommon for oil to erupt out of the tank when the lid comes off.
The hydraulic tank is supposed to be pressurized when the oil is hot. That helps push the oil into the implement pump inlet so that the pump is always working on a flooded suction. The tank should not vent when the machine is operating. The cap assembly should be the following - imagine the cap is turned upside down and start assembling it working upwards: - 1. 7H-1447 Cap. This cap does not have a vent hole in it like you describe in your OP to the best of my recollection. 2. A small ball bearing 4B-9782 that sits in a detent in the centre of the underside of the cap. 3. A circular plate 6H-5139 that is installed on top of the ball. The plate is almost as big in diameter as the inside of the threads in the cap. 4. The rubber gasket 9H-6454. 5. Finally the lock ring 6H-9361 (circlip) that holds everything together. Hydraulic oil level should be checked with cold oil, the machine on level ground, blade on the floor, and the engine stopped. In other words before starting work in the mornings.
Agree with Jerry & Nige, a sealed system is the only real way to keep dirt out. Need to fix the cap and do not overfill
It could be possible that the holes that let the fluid into the sight glass are clogged up. We had one on a D8N that was always showing nothing in the sight glass but if you opened the cap you could see fuild close to the top
Agree, cap should not normally be vented. Cat has used this system since way back....actually depends on the air cushion to counter the affects of hydraulic shock.
The vent lets air into the tank when the level goes down which happens when you pick up the blade. When you set the blade down the pressure stays in the tank. This way you don't need an air compressor putting pressure on the top of the oil level. Maybe on the other side you got the machine with the optional hydraulic oil synthesizer. In that case the level keeps going up all the time. Just get the optional cab with the shut off valve in the top and hook a hose to it. When the level gets high and you can just run the excess into a bucket and use it in your other machines.
Thanks for all the replies. That answers my question.
I'll be impressed when CAT work out how to have the hyd tank on their dozers stop blowing back oil once you get upto 1/2 way up the sight glass when your filling the tank with oil!!
It has been many years since I ran a service truck. But if I remember correctly Caterpillar equipment had a short pipe welded inside the hydraulic tank filler opening. That pipe made it impossible to overfill the tank and there was always room for the oil to expand.