ZF 325-1 transmission- pressure drops to idle (~5 bar) intermittently

[rorourke]

Having a problem with my 2006 ZF 325-1 transmission.  It will typically start up fine, but at some point while underway (up to hours after continual running), the pressure will drop to idle.  Resetting the electronic controls and/or swapping the filter usually gets it running again, but only temporarily.  The issue recurs eventually.  Oil filter and screen filter are both good.  Pulled the ECV apart today and didn’t see anything incredibly suspicious, aside from odd discoloration (shiny parts in the pics) and mild scoring on both the piston and valve.  Could it be the springs?  Or something else?

Topic

[Tony Athens]

You do not necessary need the rubber cover.. Just make a gasket, use a good modern sealant   and reinstall the square cover plate ( wait 48 hours) . 

[rorourke]

Got the lay shaft cover apart no problem.  Pics attached.  Everything looked pristine.  The main shaft has a rubber cap over it, which the repair manual says will be destroyed once removed.  Without a spare handy I don’t want to yank it unless there’s a way to get it out without damaging it.  

[rorourke]

I see.  Yes, that looks doable.  I’ll tackle that tomorrow.  Those rings being in worn/poor condition could cause a sudden loss of pressure while engaged/running?  And then regain pressure when it has a chance to reset (via breaker disconnect or oil filter change)?  I’m a quick learner with mechanical stuff but never had to deal with a transmission before, so I do appreciate the advice.

[Tony Athens]

Look at these pics..   

These usually can be accessed from the back of the gear without splitting the cases.

 

 

[rorourke]

Yep you’re right, we’re talking about different things.  The repair manual refers to the part in the picture (inside the ECV) as a piston too.  I was thinking earlier that the spring or seats in the control valve might be worn.   Cracking open the main gear case feels a bit more intimidating.  I’ll take a look at the manual and see if that’s something I feel comfortable tackling.   Thanks for the help so far!

[Tony Athens]

I think we are talking about totally different piston seal rings.. They have NOTHING to do with the control shifting valve.. They are on the gear main shaft,  and on the pump lay shaft– 2 on each–  They are hardened & ground steel rings– very precision. with interlocking ends-– maybe 1.5″ OD, 1.3″ ID x .125″ wide.    

[rorourke]

Downloaded the repair manual yesterday and pulled the piston today.  Pics of the piston and valve, including o-rings were in my original post, not sure they’re visible.  There seems to be slight wear on them, but not sure if that’s enough to cause the loss of pressure.  It’s certainly possible, but I haven’t worked on this enough to know by sight.

[Tony Athens]

Let’s assume your pressure readings are right.

Since you seem to comfortable with the valve, then next would be worn piston sealing rings (10,000 hours).  Depending on access and your talent, should be replaceable without pulling the gear.  You need to order a repair manual. 

 

 

[rorourke]

Readings are being taken from a Siemens PLC, which pulls info from the transmission by a new Danfoss pressure sensor.  Readings are in bar, not PSI, so closer to 72.5 PSI.  Normal operating range is 23-24 bar.

Installed new in ‘06.  Around 10,000 hours on it.  

[Tony Athens]

Your reading are being made using an accurate mechanical gauge, right?

“Idle” means like 50 PSI or ? 

Total hours on the gear since new ?

 

 

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