Low Pressure Readings - Yale GLP050 Forklift Transmission

[Blueacre]

I recently bought a Yale forklift Model GLP050RENUAF089. I have been experiencing problems with the transmission. When first started and shifted to forward or reverse nothing happens – don’t feel clutch engage and no movement when engine is revved up. After 5 minutes or more of idling I start to feel the clutch engage when shifted into gear and a little movement when the engine is revved. If I keep shifting back and forth between forward and reverse and revving the engine it starts to move better and eventually drives OK - see my earlier post for more details.

I finally the fitting I needed to connect pressure gauges to the transmission test points this morning with the following results

Test port 1 - pump pressure - Installed gauge and started engine - after about 5 seconds gauge climbed quickly to 65 psi and stayed steady there at idle. I revved engine part throttle and pressure went up to about 80 psi - I didnt try full throttle. My manual says specs pump are 156 to 200 psi

Test port 2 - torque converter - installed gauge and started engine - had zero pressure, revved engine up part throttle but still zero pressure. Shifted transmission into forward and reverse - could feel clutch packs engage and it would move a little if revved up but still zero pressure Specs for converter are 43 to 100 psi.

Test port 3 - clutch pressure - installed gauge and started engine - shifted to reverse and got 65 psi (same as pump pressure). Pressure would climb to about 80psi when I revved part throttle.

Next I hooked up gauges to the pump and converter test points with hoses so I could drive with them both connected. I did my routine of shifting back and forth between forward and reverse and revving the engine and it gradually moved better – the pump pressure climbed to about 80 or 90 psi but still felt like it was slipping a little. I took it out of my shop to the driveway and started to move forward a little faster. I felt a surge like a car shifting gears and when I looked at the pump gauge it had gone up to 175 psi. Forklift was now driving normal with good clutch engagement when shifted and lots of power – I have about an 8% grade in my driveway and I was able to drive up it with no problem other than spinning the tires in the packed gravel in a couple of spots. The torque converter pressure stayed a zero when the machine was stopped or moving slowly but as it sped up with full throttle the converter pressure gradually climbed to about 40 psi at full speed.

I drove it up and down the hill for about 15 minutes to get the oil hot -after that the pump pressure dropped to about 125psi at idle but just revving it slightly above idle it went back to 175psi same as when cold. Converter pressure still at zero when stopped and climbs to 30 psi at full speed.

Next I shut the engine off and restarted it right away – the pump pressure went to about 50 psi at idle and stayed there. I started to slowly rev it up – it climbed to about 80 psi and then suddenly shot up to 175 psi and when I went back to idle it was at 125psi – same as it had been at idle before with hot oil. Tried shutting off and restarting several times – same thing every time – approx. 50 psi at idle – rev it up slowly and it slowly climbs to 80 or 90 psi and then suddenly shoots up to 175psi and drops back to steady 125psi at idle.

I am not very knowledgeable on transmissions, but I came up with the following thoughts.

Pump seems to be OK – meets spec of 156 to 200 psi when cold at idle (175 psi) and when hot at just above idle rpm – the slightly low pressure when hot probably due to its age and pump wear.​

There seems to be something that is causing low pump pressure for an extended period of time when the machine is cold and for a very short time when restarted after it is hot – could it be a sticking pressure valve or something else in the transmission control valve that is bypassing oil flow but them closes when it heats up or when the engine is revved up when hot ???​

Not sure what to make of the torque converter pressure being zero most of the time and the pressure only going up when the machine is moving faster ???​

Any suggestions on what to do or check next will be greatly appreciated.

 

[tractormech]

I'm not there so I can't see what you have-but it sounds like maybe an inching valve is hanging up and dumping tranny pressure. Look and see if when you push the brake pedal there's something going in the transmission control valve moving. Put it in gear and pull up on the brake pedal and see if it makes pressure and takes off. Edit-I looked at the schematic after I answered and you do have an inching valve. I'd start there. Good luck!

 

[Blueacre]

thanks for the reply - the inching valve is operated by brake fluid pressure, not mechanically - a 1/4 brake line tees off of the main brake line from the master cylinder and goes into the transmission control valve to an internal piston so you cant see if anything is moving. It could still be the problem but what puzzles me is that even once the machine is driving properly with full pump pressure if I push the brake pedal the inching it disengages the clutches and when I release the brake pedal it takes off again like normal but the whole time the torque converter pressure is zero. Also when stopped in neutral and the pump pressure is at full pressure if I shut it off and restart it the pump pressure is low but I haven't touched the brake pedal so I wouldn't think the inching valve position would have changed.

Anyways I will probably pull the inching valve spool out of the transmission control valve and check to see if the seals are OK and it moves freely to eliminate that possibility.

Looking at the hydraulic schematic the inching valve, cold relief valve, anti-plugging valve and pressure sensing valve (sequencing calve) can all dump hydraulic fluid back to the reservoir if they are leaking or not operating properly.

I had someone tell me yesterday I should be using Hy-Tran type fluid not Dexron (red) transmission fluid - my service manual calls for either Dexron if a yale transmission or Allison C-3 type fluid if an Allison transmission - not sure which one mine is but it had red fluid in it when I got it so I stayed with that.

 

[Blueacre]

Followup to my last reply - When the pump pressure was normal and I pushed the brake pedal to actuate the inching valve the pump pressure increased by about 20 psi. in the schematic the line to the torque converter tees off from the main pump line before the inching valve so I cant figure out why I am not getting pressure at the torque converter test point.

 

[tractormech]

Going down the list I'd go to the pressure regulating valve next as you pressures are sort of all over the place, then the cold relief valve. Does this thing have a visible external charging pump or is it in the front of the tranny and driven by the flat lugs on the torque converter? To be honest it sounds like a relief valve is dumping your pump and consequently your clutch pressure. Something else to consider is a partially plugged suction screen. When the oil gets warm it's thinner and flows better. If it's starving for oil it will do what you have going on. If you can access the screen I'd start there. Good luck.

 

[TD24]

"Something else to consider is a partially plugged suction screen. When the oil gets warm it's thinner and flows better. If it's starving for oil it will do what you have going on. If you can access the screen."

I have also wondered about the possibility of that sump strainer being partially clogged.
Or an actual suction/vacuum leak between intake and pump, if connection would allow.
Not sure of the intake feed from the schematic shown. Is there any "filter" in this other than
the strainer? Sporadic function usually indicates starving fluid supply; by vacuum bypass,
a clogged or partially clogged filter, and in some cases a partial inner collapse of a feed
hose. Inching is usually erratic, its on and you go, stop or reverse and lose it. Correct and then goes, but with power, Never saw any "halfhearted inching".