Hello, I am having an issue with a CAT 312C excavator. The hydraulic system is pulling the engine RPM down excessively to the point of stall under a 2 pump flow function when held over relief. Moderate black smoke is seen from the exhaust. Turbo has been checked not seized but have not checked boost pressure. A single pump function bucket or swing over relief does not pull RPM down. The powershift pressure has been checked and calibrated. The main relief has been set. No codes in ET. One thing I find odd is the right pump has almost double the standby pressure as the left pump. Is the negative flow control pressure not the same pressure feeding both pumps at standby? Wondering if maybe right pump is not coming right off of stroke and under a high load function like arm in or boom up it is enough to stall the engine.
Another bit of info- while tracking fwd/rev with both tracks and simultaneously holding arm or boom over at end of stroke it will not pull RPM down, however if only running 1 track it will
You’re probably going to need the service manual testing & adjusting information. To get that requires the machine Serial Number.
The serial number is CBA04062 we have cat sis not sure what test is relevant
In that case my first suggestion would be to read and understand the System Operation for the hydraulics. I speak under correction here but IIRC your symptoms are indicative of a bad pump. One of the excavator specialists may be able to give you clearer guidance.
What is the actual powershift pressure and NFC when it is bogged down? Don't use the value from the display because it's calculated. Use an actual gauge.
Actual powershift pressure high idling arm cylinder end of stroke is 520psi engine about to stall. Seems if I cycle the arm fast it has lower PS pressure and doesn’t work engine as much, if I slowly arm in it pulls down rpm then cylinder reaches end of stroke and wants to stall engine. NFC pressure is there a tap or T in at pump regulators? Thanks
I have a twin to this machine swapped the powershift valve between the 2 and did not make a difference.
That doesn't sound great. I'm guessing if you turn the pump controls behind the seat to manual, it gets worse? I think that runs like 350psi fixed. You likely have an issue in a pump regulator. I've seen some internals come loose and come unscrewed but that is rare. If you repair/replace a regulator, you need to reset min/max and nfc flow rates on the pumps. This is a pretty involved procedure.
You have a separate left and a right NFC line to the left and right regulators. I dont remember if you have a pressure tap or if you have to tee into the hose. It kind of doesn't matter because at 520 psi powershift, the pumps should be at near minimum flow.
You are correct in that if I put the machine in backup mode the symptoms are worse. What I am not understanding is why when using a dedicated pump function such as bucket does the powershift seem to properly control the pumps? Did some reading in SIS no simple task swapping out a regulator as you say. Would you suspect the higher or lower standby pressure pump to be the bad one? Left pump standby low idle 4200kpa right pump 9000kpa
I have a couple questions Does it have any kind of a mistrack? If you watch p1 and p2 discharge pressures, what do they do if you feather left and right travel individually?
It does not mistrack in either high or low speed P1 and P2 both react similarly when feathered but right track engages much faster presumably because there is higher standby on right pump
Arm in held at end of stroke will not pull rpm down while tracking full tilt both tracks, will stall engine if not tracking
If pin 27 comes unthreaded from the swash piston your pump will be at high displacemdnt even when unstroked, which will cause high discharge pressure from that one pump in neutral. It'll also cause a stall under load as your powershift wont be able to destroke that pump. Usually though, you get a bit of a mistrack from that pumps displacement being higher than spec. Also what ive found is if i feather functions that pump will hit relief pressure way before it should or before the other pump due to the high flow its putting out. Ive seen this on 312c's. You can usually hear the extra oil hissing over the slow return check valve back at the control valve. Im not saying this is what you've got going on, but id be playing with the machine for a bit and getting a really good feeling for the symptoms and if they match pill your regulator. Its an easy fix
Thanks for this information. If it does come down to removing the regulator is that something that can be done with the pump in place and tank full of oil?
Yep, put vacuum on the tank and you'll loose minimal oil