Komatsu D21A-6 powershift hesitation

[chuckbuck]

Hi, I'm new to the forum.

I just purchased a 1992 Komatsu D21A-6 with the two speed powershift. The machine is in nice condition with 3150 hours (suppose to be correct).

When I shift from first to second, there is a hesitation of a couple seconds. The hesitation is enough that the machine comes to a stop if going up hill or has any load on it. I serviced the transmission oil and cleaned the strainer. Found some clutch disk material in the strainer, not a lot but about 30 pieces about ¼ in by 3/8 inch. No metal what so ever in the strainer. These machines have a large area strainer.

I check the pressure at the top of the transmission control valve and in neutral there is zero pressure and when I shift to first gear the pressure goes up to 350 psi. When I shift to second, the pressure drops to zero for a second of two (tracks stop spinning) then goes up to 350 psi and the tracks start spinning. (machine is on jacks stands)

Does any one have any ideas what is going on with my dozer? Is 350 psi the correct pressure? Why is the pressure dropping to zero between first and second.

I took the control valve off and cleaned it real good. Didn't seem to have any ware on it and it was clean inside. No debries in any of the orfices.

Any help will be appreciated. I need to get a service manual but don’t have one as of yet.

Regards,
Chuck

 

[Wulf]

350 psi is on the low end of spec for this unit but compare the pressure at idle and HFI as it will vary with engine speed.

I think you should have transmission pressure in neutral position... are you sure you are measuring at the right test port? There should be two ports - one for checking transmission pressure and another for inching pressure.

I would confirm this and at the same time also ensure that the inching pedal/linkage is set up correctly.

 

[chuckbuck]

350 psi is on the low end of spec for this unit but compare the pressure at idle and HFI as it will vary with engine speed.

I think you should have transmission pressure in neutral position... are you sure you are measuring at the right test port? There should be two ports - one for checking transmission pressure and another for inching pressure.

I would confirm this and at the same time also ensure that the inching pedal/linkage is set up correctly.

I must have the wrong test port, since it is tapped into the part of the valve body that has the spool for the inching pedal.

The linkage is free and adjusted properly on the inching petal. This assumes the spring loaded is holding the spool all the way out and the petal has a little bit of play in it.

I will connect the gage to the other test port and report back.

Regards,
Chuck

 

[chuckbuck]

350 psi is on the low end of spec for this unit but compare the pressure at idle and HFI as it will vary with engine speed.

I think you should have transmission pressure in neutral position... are you sure you are measuring at the right test port? There should be two ports - one for checking transmission pressure and another for inching pressure.

I would confirm this and at the same time also ensure that the inching pedal/linkage is set up correctly.

I tested the pressures at the other port on the valve body. At idle, in netural, the pressure is 350 to 375 psi, tends to fluctuate. at high power it goes to almost 400 and is steady. whenn i go into first, the pressure drops to about 50 then slowly climbs to about 350 to 375 at idle, same when i shift to second gear. drops to 50 then climbs. at higher power same thing happens except pressure is close to 400 psi.

Does this sound like a bad pump? Seems like a pressure accumulator tapped into that port would help the pressure drop. I have used the machine and it has good push power and the tranny does not appear to be slipping.

Any help would be appreciated. I'm using this machine for personal use on my 10 acres and it doesn't get run 10 hours a day by any means.
Regards, Chuck

 

[Wulf]

Chuck,

It's normal that the pressure will drop when shifting but the modulation valve will control the rise in pressure to enable smooth shift in about a second and a half at higher engine speeds


I know you said you had the valve off but I would double check the small spools within the modulation valve and quick return valve as there could be a blocked orifice. Also confirm that the problem occurs in F and R

 

[chuckbuck]

Chuck,

It's normal that the pressure will drop when shifting but the modulation valve will control the rise in pressure to enable smooth shift in about a second and a half at higher engine speeds


I know you said you had the valve off but I would double check the small spools within the modulation valve and quick return valve as there could be a blocked orifice. Also confirm that the problem occurs in F and R

Hi Wulf,
I actually had the valve out again today and the small orifice in the modulation valve is perfectly clear. The orifice I'm speaking of is about .040 to .050 inches in dia and goes in one side of the spool and come out the end.

The quick return valve has a smaller hole in the side and it comes out the end of the spool. The spool actually has a small threaded brass plug with a orifice in it. The plug is staked in via a couple center punch marks. I blew compressed air thorough the orifices in both valves and they appeared clear. I reinstalled the valve and the same thing is happening. Pressure drop to zero when shifting from 1 to 2 in both Fwd and Rev. The stablized pressure is about 350 at idle and almost 400 at higher power.

Any ideas as what to do next? Does both valves relate to this problem or just one, if so which one?

Thanks, Chuck

 

[chuckbuck]

Hi Wulf,
I actually had the valve out again today and the small orifice in the modulation valve is perfectly clear. The orifice I'm speaking of is about .040 to .050 inches in dia and goes in one side of the spool and come out the end.

The quick return valve has a smaller hole in the side and it comes out the end of the spool. The spool actually has a small threaded brass plug with a orifice in it. The plug is staked in via a couple center punch marks. I blew compressed air thorough the orifices in both valves and they appeared clear. I reinstalled the valve and the same thing is happening. Pressure drop to zero when shifting from 1 to 2 in both Fwd and Rev. The stablized pressure is about 350 at idle and almost 400 at higher power.

Any ideas as what to do next? Does both valves relate to this problem or just one, if so which one?

Thanks, Chuck

Hi Wulf,
After i made the last post, i decided to take the Dozer out and test it. To my amazment it works fine now. I kept the gage on it and even though the pressure drops to zero it comes back quickly and the hesitation is not there any more. When I had it in shop on jack stands, and the pressure was droping to zero i just assumed i still had the hesitation. Sure glad i took it for a ride. I now feel good about my purchase of the D21.
Thanks for your help, Regards, Chuck

 

[Wulf]

That's great news Chuck... before I saw your latest post I was thinking that the next thing to watch out for was to check with your dealer and make sure you have the right grade of oil in the transmission.