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ebnel, Technician
Category: Heavy Equipment
Satisfied Customers: 282
Experience:  master certification diagnostics and repair
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We have a john deere 6420 tractor that has a problem..reverse..levers

Customer Question

we have a john deere 6420 tractor that has a problem with forward reverse. it has had 4 new levers in it's past and had issues with the fuse that is by the battery last year. now it can stop going forward or reverse when you are travelling along - any suggestions of where to look? electrical??
Submitted: 5 years ago.
Category: Heavy Equipment
Expert:  ebnel replied 5 years ago.

ebnel : Hello and Thank You for selecting Just Answer,Get back with me if you need more help after reading my answer. Sounds like could be related to power to shifting solenoids, if you can wait till am I will check schematic and give better answer.

okay, any help in telling us what solenoid to look at will be helpful, ps. changed the fuses and made no difference

ebnel :

Make sure you check trans.system pressure first,would be good to to check while driving and failure is active.Need to verify pressure is maintaining 170-190 psi. Have additional info listed below.

ebnel :

. Check enable relay.
a. Check for switch voltage at pin 30. If no voltage, check fuse F04/15 and wire 572.
b. Replace enable relay.

3. Check EPC controller for voltage and pin 2 (battery) and 29 (switched).
a. No battery voltage at pin 2, check fuse F04/16 and wire 503.
b. No switch voltage at pin 29, check fuse F04/15 and wire 572.

4. Check BCU address 17 and EPC 07 to make sure FNR lever is working properly.
a. Inputs not correct, check out FNR lever and wires.
b. No problems with wires, replace FNR lever.

5. Place tractor in come home mode using relay K10/4. See if tractor moves, may have to hold FNR lever is desired direction and may have to clutch twice.
a. Tractor moves – go to step 7
b. Tractor does not move – continue diagnostic.

6. With come home relay in, check EPC pins 39 and 40 for switch voltage.
a. No voltage – check wire 553

7. Check EPC addresses 9 – 12 for clutch switches.
a. When looking at the addresses, should be the voltage change with the clutch pedal. Channel 2 should be half the voltage of channel 1.
b. If there is no voltage change, need to look at clutch pedal potentiometer and clutch switch.

8. Check EPC address 13 for enable pressure
a. Put come home relay in and FNR in forward or reverse. Should see pressure.
b. If no pressure, then remove enable pressure sensor and install pressure gauge.
i. See pressure, then need to check wire 243 for 5 volts and check wire 241 and 549. If good, replace enable pressure sensor.
ii. No pressure, run a battery jumper wire to pin A of enable solenoid and ground pin B. Should see pressure in pressure gauge. If not, replace enable solenoid. If still no pressure, check transmission for proper pressures looking for a hydraulic leak.

9. Check EPC address 14 for top shaft speed. Need to have tractor in gear 4, 1000 engine rpm, and enable pressure. Reading should be 1000 at address.
a. No reading – check wire 564 for 5 volts at top shaft sensor and wire 973 for 12 volts. Check for good ground.
b. No problem with wires, replace sensor.
c. No 5 volts – continue to step 10.

10. Download the latest BCU and EPC software per instruction MHM983. Make sure you record all input addresses before programming software. This will allow you to reset the addresses after programming.


Thank you for replying. The suggestions showed that the problem was too technical and we did not have the knowledge or equipment to check "wire 503" or pin 29 ..... We paid a John Deere specialist attend to the job. Using their computer diagnosis, it was discovered the pressure sensor was faulty, and the speed sensor had a bad connection.

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