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Kevin, Technician
Category: Agriculture and Farm Equipment
Satisfied Customers: 732
Experience:  Over ten years of expirence in Ag Electronics and Hydraulics
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John Deere 8320 tractor with code PCU 27 and trans &

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John Deere 8320 tractor with code PCU 27 and trans & exclamation mark flashing. I try to calibrate transmission, but it seems to freeze part way through. corner post display shows bc, then --- and alternates between them. ocasionally CLU will come up for a split second but not any error code. I cycle the clutch pedal, but never advances past this point. Also automatic air purge doesn't work when I install F10 fuse after heating transmission.Transmission drives in all gears and doesn't slip, customers only complaint is the flashing lights.
Other test I did are;
Adjust trans system pressure, it was low 25psi
PST shift solenoid test - there is no pressure "dip" on elements Ab, bC, cC, dC
Lube pressure test at oil cooler - I have 10psi at 2000rpm (very low according to specs)
PST element leak test - these seem to be in spec range but unsure because of low lube psi.
The tractor has 9314 hrs and a reman transmission I don't know when it was installed. Found some slivers of rubber (packing?) and a piece of gasket material when I pulled trans screen. Thank you for your help.

Hi my name is***** will be glad to try to help. Low lube pressure always flags a problem of excessive internal leakage or pump/relief problem. Lube is non adjustable as you are aware and is everything left over for pressure used for lubrication and cooling. Low lube will cause transmission failure over time. your lube pressure will be back into spec when all other pressures are back into specs. If there was no particular clutch low or dropping off pressure and they are all low within the same range, it points to a weak pump. Let me know if you need more help

Customer: replied 1 year ago.
what about the code 27 not being able to calibrate and auto purge not working. should these not still work?

It will throw that code due to the mechanical internal problems as the PCU is no longer seeing the calibrated and expected values. You could have the dealer reflash the PCU to its original settings and start fresh but you will likely run into the same problems with the internal problems you are describing.


Transmission not calibrated

Have your John Deere Dealer calibrate as soon as possible

Are there any other codes? Click here for the list

PCU codes

Customer: replied 1 year ago.
Thank you for your reply. I would expect codes to be generated if fill times were not in spec so wondering if I can do more tests before tear down. I have talked with a mechanic, Kevin, in the past and am wondering if he could comment on this issue as well. Can you forward this to him, I don't know how? Attached is his info. Thank you.

Sure I will opt out and maybe he will pick this up on the board.


I'm not really surprised that you can't get the auto purge feature to work. High hour tractors often won't conform to all the parameters needed for the controller to allow the auto purge to work. When auto purge doesn't work just refer back to the manual purge procedure for the early serial number 8000 series tractors then start the standard calibration procedure.

The lube pressure is below spec which could indicate a potential problem as could the fact that you had to adjust the system pressure. The fact that the element leak test passes successfully is good news however. I would suggest repeating both system pressure test, and lube test with the MFWD both on and off. Leaking MFWD clutch packs are a common fault and it will not pass a trans cal with a leaking MFWD clutch pack. While the transmission may have a problem it could take some time before it shows itself in full. If the transmission shifts decent and doesn't creep it should be able to pass a calibration.

Heat the transmission to at least 65 degrees that's 10 degrees above the spec required by the CCU to start the calibration. Use the manual purge method and see if the transmission will pass a cal. It has most likely been some time since a good cal has been done which mean that the fill volume and time's could exceed the maximum change values stored in the CCU. After two unsuccessful calibrations the CCU is supposed to default to a non valued calibration (meaning it ignores the old cal values). If it fails at the same point each time then it may be identifying a element with a fault or wheel motion as detected. In some high wear tractors you have to stand on the brakes during calibration to keep the tractor from rocking (which the controller sees as wheel movement). Other causes of faults is the controller sees the engine RPM's droop too far kicking the calibration out. This can be due to an engine power problem as the rpm droop is based on expected engine power output.

I will note that if the controller kicks out the calibration and displays CLU, it will allow the calibration to continue but will not show as successful.

Let me know what you are seeing. I'm not available as often as in the past but I will do my best to get back to you as quickly as I can.



Customer: replied 1 year ago.
Thanks Kevin, I will check these pressures and get back to you.
Customer: replied 1 year ago.
Hi Kevin, I had a couple hours today and doubled checked system pressure is 280 psi @ 2000 rpm. lube is 10 psi @ 2080 rpm. MFWD stayed at 10 psi between on and off. Element leak test was within 2 psi. I tried calibrating trans 4 times and it fails at the same point. It doesn't continue past bC element which is the 2nd element to calibrate [Ab is first] . (I waited 20 minutes holding the brakes), CLU will momentarily display, but not long enough to cycle pedal. I still cycled pedal to see if it would continue but doesn't. At this point I shut down tractor and restart to calibrate again and reach the same point. I heated trans over 65 degrees but doesn't start calibration till 63 degrees, and did a manual purge. It's looking like an internal trans fail? I thought we would have more codes to indicate that. Would connecting Service Advisor give more info or would it calibrate trans successfully?Thanks for your help and detailed information. Last time we talked about a 7510 with trans pump housing machined wrong. Much appreciated.

You are far from the only one to be stymied by one of those bad pump housings. I'm glad I was able to help.

I'm surprised it failed to give you a letter associated with the "CLU" indicator. Regardless its time to do some swapnositcs. Swap the transmission BC control valve with another element to see if the problem follows the valve or stays with the element. No need to heat past 55 this time since it did not help the last time. Let met know what it does.



Customer: replied 1 year ago.
I swapped bC valve with Ab and the problem stayed with the element. During calibration the hyd light & exclamation mark flash (it has all along), but quits once out of cal.
Customer: replied 1 year ago.
when it stalls out at bC I get no letter code, just CLU momentarily and --- , and then it tries again. I watched the lube gauge, when it dips below 10 psi (like 8) is when CLU flashes on, and --- .When it was calibrating Ab it never dipped below 10, it ranged between 10 to 15 psi.

Ok, so I've done a bit of digging. It seems that the controller can display CLU if it sees clutch pressure below spec. Ironically BC is used for 5th, 7th, 9th, and 11th forward and acts as the main traction clutch for all four gears. Typically we see more wear on the BC clutch because 7th forward is the default startup gear. You are not getting any other faults which means it is not a fill volume or pressure fault but rather a clutch pressure issue. The diagnostics for the fault has us jumping directly into clutch control pressure and system 2 oil pressure tests. This could indicate that the controller either believes the clutch pedal has been depressed based on the clutch pressure sensor or the clutch pressure sensor is faulty. I believe the controller sees the clutch pressure fall so it assumes the pedal has been at least partially depressed, because the pressure is down for so long the calibration times out hence the ---.

Have you by chance done the clutch pressure tests?



Customer: replied 1 year ago.
I haven't done element pressure test at the respective ports. I replaced clutch pressure switch after checking its status at PCU 11 but didn't help. This test did show that clutch pedal up was not recognized until 1400 rpms (XX1). It stayed at 000 until 1400 then clutch function up/ down worked. bC pressure may be the problem. Your Comments are very interesting.

I agree that the pressure at BC is most likely the problem. The clutch pressure switch readings shows a lack of sufficient pressure thus the controller views the clutch as depressed. This accounts for the CLU during the BC calibration. More then likely the pressure dips enough for the clutch switch to trip and the controller assumes the clutch was depressed. This could also account for why auto purge fails to function. You might check the clutch pressure status to see if it does the same in other gears. Also verify with MFWD off and then on. If it does act up in other gears then it may not be an element but rather a control spool although I can't explain why it makes it through the AB calibration if this is the case.

Another good test of the clutch circuit would be to try pumping the park brake off and see if the pedal remains. Push the button on the top left of the transmission down and pump the clutch until it stays down. I believe the tech manual states that it should take up to 20 pumps but from experience I can tell you it may take more then twice that number of pumps. The pedal will remain on the floor once the park brake pressure has reached its max. If the pedal visibly leaks back up or fails to stay down at all then the problem may be in the control valuing. This test will allow us to verify that portion of the valving.

My process are often a bit different but keep in mind we have to think how it would respond based on the inputs to the controller. Hope this is helping.


Customer: replied 1 year ago.
Hi Kevin, my time spent on tests will be limited for a while during seeding time starting here now in Alberta Canada. But I will keep in touch with you. I did do the clutch pumping test and it stayed down after 39 pumps and remained there. Your diagnostic insight is much appreciated. Thanks, Daniel.

Hi Daniel,

I understand lack of time. Keep me posted when you get time.



Kevin, Technician
Satisfied Customers: 732
Experience: Over ten years of expirence in Ag Electronics and Hydraulics
Kevin and other Agriculture and Farm Equipment Specialists are ready to help you
Customer: replied 9 months ago.
John Deere 8320 tractor with code PCU 27 (order number 16193420-143)
Hi Kevin I'm finally continuing with this tractor issue. The farmer used it for seeding and fall work and had no problem other than the light flashing. He still wanted to rebuild the transmission. I have it it torn down now. I found a ripped gasket on shift valve cover near one of the ports of the clutch valve. Also some rubber slivers from clutch piston on input shaft (2nd driven/3rd drive gear/hub). it peeled rubber on the back side of piston, I don't think it caused leakage though. Do you have time for some more chats? We can open a new case if you want. I can send pictures of the gasket if you like. Thanks.

Hello, my apologies for the very delayed response. I have been away for a holiday vacation and only just returned. If you are still in need of help on this please let me know.



Customer: replied 9 months ago.
Hi Kevin, Thanks for the reply. I am going through and rebuilding trans with new pistons, return springs, discs, snap rings, bearings, all seals and gaskets. All springs for valves and pressure control valve kit. I'm replacing input shaft due to worn splines that couple to engine, and one clutch hub that has damaged snap ring groove. I guess the damaged gasket is the failure point, as the clutches didn't seem too bad and o-rings looked good on shift spools. The fact the farmer had no problem with performance in the field means there won't be big damage in the trans. Is there other things that I should check from your experience with these tractors? Thanks, Daniel.

It sounds like you are doing a very detailed rebuild. The worn snap ring groves is a common issue so any that look worn should be replaced. We have also seen some cracking on the drums at the snap ring groove that will be visible from the outside. Make sure you rebuild the MFWD pack as the pistons are known to fail. As you mention I wouldn't expect to see much wrong if it was performing properly in the field. I have seen gasket failure problems with these transmission in the past so I would agree that the problem most likely started there. Otherwise they are know to be tough, reliable transmissions.

Keep me posted if I can be of further assistance.



Customer: replied 9 months ago.
Do you apply any extra sealant to the gaskets? The new one looks of different construction, maybe its ok. Also in assembling valves in manifold there are three the same (spool & double spring) beside the electro hyd valves. They are called clutch accumulators (pn R155963). The arrangement is different between with MFWD and without MFWD. Mine is set up as without. Should I change this as I have MFWD. Another valve that is poorly explained is R108491. It came in clutch control valve kit RE168161. I installed it the same way the old one came out but tech manual doesn't show it. Have you experienced these issues? Your insight is greatly appreciated.Thanks,Daniel

Hi Danial,

Ok so the gaskets. The new Deere gaskets are infused with sealer that bonds to the housings when compressed. I use upholstery spray glue to tack the gaskets in place but no additional sealer.

The Accumulator valves I wouldn't alter what you have. Late serial number tractors were different between MFWD and 2wd (supposedly) however I have never seen one setup differently then what you describe. We have discussed changing one but have never tried it.

I attached the diagram showing the updated clutch valve and the spring orientation. I believe your most likely correct in putting it back together like the old one came out but a picture is worth a thousand words.

Hope this helps, keep me posted if you need anything further.


Customer: replied 8 months ago.
Hi Kevin,I finished the tractor just before Christmas. Trans & lube pressures are right on. Auto purge & calibration worked great. Thanks so much for your expert help.