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  1. #1
    Silver Member Malvern's Avatar
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    Oct 2004
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    139
    Location
    OH
    Tractor
    JD 3120

    Default 3120 problem

    Hi all,

    I have a 3120 with 480 hours, cutting grass Saturday afternoon with the cruise control on, the tractor speedometer started to go a little crazy, I was on the flattest part of the yard, cruise was set at 5 MPH middle gear range at full PTO RPM's was getting readings +/- .3 and felt the surging. The RPM on the engine didn't drop, no smoke what so ever didn't feel like it was engine related. The surging wasn't effected by turning the motion match on or off.

    All fluids from the hydro are clear and clean except for the engine oil, was changes approx 50 hours ago. No water in the fuel separator. Did a complete hydro oil and filter changed about 75 hours ago.
    Sunday I checked the battery and grounding leads clean and very tight, battery is about 3 years old appeared fine. Didn't check the alternator output tractor starts like it has since I've owned it, couple revolution and fire right up, all lights work as usual.

    The thing I find odd is no engine codes and no surging during testing Sunday, she ran great. I can get the engine to pull a code like stepping on both for/rev pedals at the same time so I know the computer works or the light works and shows a code on the read out.

    I'm stumped what do you guys think?

    Malvern
    JD 3120 ehydro with 300CX, 72" 7 Iron MMM Land Pride RB-1572, Land Pride BB-1560, 2005 Yamaha Rhino.

  2. #2
    Super Star Member
    Join Date
    Aug 2001
    Posts
    11,496
    Location
    Upper Midwest USA
    Tractor
    JD 4300, JD X485 JD 4x2 Gator, JD 425, JD455

    Default Re: 3120 problem

    You say the speedometer started to go crazy, but then said the engine rpm didn't change.
    You didn't say if the actual speed of the tractor changed at the same time as the speedometer.
    I'd suggest trying to replicate the fluctuation at different settings of the cruise.

    And if there is a ground speed sensor, then I'd look to clean any dust or debris off of it that may have built up.

  3. #3
    Member
    Join Date
    Jul 2011
    Posts
    25
    Location
    North Mississippi
    Tractor
    John Deere 4720

    Default Re: 3120 problem

    This is the same symptoms when my MFWD sensor was defective on my 2004 4720, as long as I was bushogging with the cruise set it just surged. Then while trying to remove a tree that had blown down with the FEL it threw a code 06, I have pictures of the sensor location on mine if you are interested.
    04 4720 W/FEL, MX6, 7' Box Blade
    99 4100 W/Land Pride FDR 1660
    36B, 39B, 50M

  4. #4
    Silver Member Malvern's Avatar
    Join Date
    Oct 2004
    Posts
    139
    Location
    OH
    Tractor
    JD 3120

    Default

    Quote Originally Posted by beenthere
    You say the speedometer started to go crazy, but then said the engine rpm didn't change.
    You didn't say if the actual speed of the tractor changed at the same time as the speedometer.
    I'd suggest trying to replicate the fluctuation at different settings of the cruise.

    And if there is a ground speed sensor, then I'd look to clean any dust or debris off of it that may have built up.
    Quote Originally Posted by deere36
    This is the same symptoms when my MFWD sensor was defective on my 2004 4720, as long as I was bushogging with the cruise set it just surged. Then while trying to remove a tree that had blown down with the FEL it threw a code 06, I have pictures of the sensor location on mine if you are interested.
    Beenthere correct, the speed of the tractor changed with the speedometer but the engine didn't lug down with it, I didn't notice any change in engine rpm and watched it very closely, thinking what the he**
    Where is the ground speed sensor as I'm thinking this is electrical in nature

    Deere36 would it matter if I was only in 2wd, strange I had no code
    Sunday I had no issues
    JD 3120 ehydro with 300CX, 72" 7 Iron MMM Land Pride RB-1572, Land Pride BB-1560, 2005 Yamaha Rhino.

  5. #5
    Member
    Join Date
    Jul 2011
    Posts
    25
    Location
    North Mississippi
    Tractor
    John Deere 4720

    Default Re: 3120 problem

    I was in two wheel drive till I tried to move tree with FEL then I was in 4WD and I wondered if the code was an error or not related to the surging but I talked to the Service Manager at our local dealer and he was confident it was the sensor which fixed the problem. On my tractor the sensor is below the seat on the left if you are sitting in seat, might want to see if the pigtail is plugged in good, etc. If you decide to replace it you will have hydraulic oil above the sensor so you will have to be prepared and do it quick, also the sensor has an o-ring on it so make sure the old one comes out and the new one is on the new sensor (cost is $65.00 if I remember correctly).

    04 4720 W/FEL, MX6, 7' Box Blade
    99 4100 W/Land Pride FDR 1660
    36B, 39B, 50M

  6. #6
    Silver Member Malvern's Avatar
    Join Date
    Oct 2004
    Posts
    139
    Location
    OH
    Tractor
    JD 3120

    Default Re: 3120 problem

    Thanks for the help you guys, I have some more testing and going to look at the connection under the tractor this week, will let you know what I find.

    Malvern
    JD 3120 ehydro with 300CX, 72" 7 Iron MMM Land Pride RB-1572, Land Pride BB-1560, 2005 Yamaha Rhino.

  7. #7
    Silver Member
    Join Date
    Jun 2007
    Posts
    228
    Location
    Middle of west TN
    Tractor
    JD: LX172, X740, 3520

    Default Re: 3120 problem

    Malvern,
    What timing!!! I read your post last night, then took my 3520 out to mow today and seem to have developed the same (or a very similar) problem.

    I set the cruise and mowed a few rounds normally, then the tractor started to slow down & speed up rapidly - almost a bucking motion. The digital speed readout was doing the same thing but engine rpm was steady. Switched motion match ON/OFF but didn't change the problem.

    I switched to the MAX speed setting and the tractor operated normally but the speed readout continued to go up and down rapidly. I used the MAX function to complete my mowing: a little over an hour of operation after the problem began. At first the speed readout appeared accurate, just fluctuating up & down by sevreal tenths mph, but the longer I mowed the indicated speed dropped and the amount of fluctiation increased. I started with MAX set ay about 5 mph and the readout was fluctuating about 5 tenths mph (4.8 to 5.3). By the time I finished mowing, the readout was bouncing between about 1 and 2 mph (with no noticable drop in the 5 mph ground speed).

    One additional strange thing: the readout appears to be stable and accurate in reverse, but continues to be slow and irradic going forward.

    I will be very interested in what you discover, and any additional input from the experts.

  8. #8
    Silver Member Malvern's Avatar
    Join Date
    Oct 2004
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    139
    Location
    OH
    Tractor
    JD 3120

    Default Re: 3120 problem

    John_in_TN,

    I'm currently reading the technical manual one of our wonderful TBN members sent me last night, my gut tells me this is electrical so I'm starting there first. Pulling plugs and cleaning.

    I still think this is strange it didn't pull any codes. Will let you know what I find.

    Malvern
    JD 3120 ehydro with 300CX, 72" 7 Iron MMM Land Pride RB-1572, Land Pride BB-1560, 2005 Yamaha Rhino.

  9. #9
    Silver Member
    Join Date
    Jun 2007
    Posts
    228
    Location
    Middle of west TN
    Tractor
    JD: LX172, X740, 3520

    Default Re: 3120 problem

    Well, I used the 3520 again this morning and the readout and cruise are rock solid - no problems at all. By the way: it is an '06 model with more than 625 hours on it.

    I didn't use it hard, but did put over an hour on it tending a large burn-pile (at least 20 feet in diameter and 6 to 8 feet tall). Made several trips hauling supplies and last minute items to the pile. Then used the loader to consolidate the pile as it burned. I shut-down and restarted the tractor several times and used the cruise every chance I got - it performed flawlessly???

    I did crawl around under the tractor and looked for loose or damaged wires & connectors. Didn't find anything unusual. I didn't attempt cleaning any connections - most are strapped together with wire ties that I didn't want to cut, yet.

    I did locate the MFWD Sensor (as deere 36 suggested). Seems logical that a failing hall-effect sensor could be the cause of our symptoms. My sensor is located on the right side of the tractor almost directly under the operator's right foot. It is at the rear-end of the Clutch Housing and doesn't appear to be in a location that would have oil inside it (unless it actually extends thru the clutch housing and into the front of the transmission). It appears replacement would be easy.

    I am keeping my fingers crossed for now, hoping the problem was just a fluke, bit feel that is highly doubtful!!!

    Good luck with yours, and lets keep everyone posted on whatever happens. When you have an issue or question it sure is great to have this group for help!!!

  10. #10
    Silver Member Malvern's Avatar
    Join Date
    Oct 2004
    Posts
    139
    Location
    OH
    Tractor
    JD 3120

    Default Re: 3120 problem

    I didn't have any problems Sunday, I only cut maybe 15 minutes more of a test than anything else.
    I'm going back over it this weekend, I agree with you it's got to be the MFWD sensor.
    JD 3120 ehydro with 300CX, 72" 7 Iron MMM Land Pride RB-1572, Land Pride BB-1560, 2005 Yamaha Rhino.

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