7720 Display Problems - The Combine Forum
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Old 02-02-2010, 06:04 PM   #1 (permalink)
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Default 7720 Display Problems

I finally made the switch from a New Holland TR70 to a John Deere 7720. When I first got the machine home the digital display worked in the cab. Now it only shows 8888 and the green light is lit up next to all four buttons. Is the unit shot? Also, I don't have any operators/service manuals yet but the computer for the dial-a-matic controller is not in the machine right now, where does this go? Could this be related to the display problems?
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Old 02-02-2010, 08:36 PM   #2 (permalink)
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Default Re: 7720 Display Problems

check your grounds for the cab for the display problems. as for the dam i will have to take a look when i get to the farm tomorrow and will get back to you.
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Old 02-02-2010, 09:56 PM   #3 (permalink)
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Default Re: 7720 Display Problems

The DAM controller is bolted to the left side of the grain tank. Open the door on the left side of the machine at the top of the steps, were you check hydraulic oil. There are two self tapping screws that hold it to the grain tank wall. As for the digital display its a self contained system. Power and ground in and leads to the four speed sensors. I have the service manual in the shop, will take a look tomorrow and post back. I know Deere wants close to a grand for a rebuilt one. All States Ag Parts has rebuilt ones for about half.
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Old 02-03-2010, 08:44 PM   #4 (permalink)
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Default Re: 7720 Display Problems

Thanks for the replies. I will check the grounds next time I have the thing out. If that doesn't work I much appreciate the heads up from mnscott about getting one from All States. I will be putting the DAM controller in this weekend so it's out of the cab.
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Old 02-04-2010, 01:04 PM   #5 (permalink)
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Default Re: 7720 Display Problems

I took a look at the service manual. There are only two pages on the tach. There is a test for the display segments in the digital display and a Deere special tool test for accuracy. If one or more sections are giving no RPM reading it would be wiring to or a sensor. Yours sounds like a bad unit tho. No mention of flashing display or all lights lit up. There are four switches under a round plug in back that set the calibration for your tire size and final ratio. If you replace make sure your new unit's switches are in the same position as the old one. There is a six prong plug in the rear. On the face of the plug there is a number beside each terminal. No.6 white wire is the ground and should have no resistance when checked with a ohm meter. The cigarette lighter, hour-meter and a few other things use the same ground circuit. No.5 purple wire is the hot wire from the ignition switch and should have battery voltage with key on. The other four wires go to the sensors. If the ground circuit and power circuit check out I would say you have a bad tach unit. You can PM me and I can scan and email you the pages if you want. Look at it this way at least the 20 series only have three spendy electronic box's compared to the dozen or more on the newer ones. BTW you can Google the PN. AH99763. Its the PN for the tach and there is another company that rebuilds display units.
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Old 02-05-2010, 10:25 AM   #6 (permalink)
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Default Re: 7720 Display Problems

Thank you for the help mnscott. I figure if only one or two of the readings didn't work it would be sensors, but all 4, with all 4 lights on I figure it's the unit itself. The heads up is much appreciated for the ground speed. I figure I'll take it out, check the ground in the machine, power it up and if it does the same thing with none of the sensors attached, I need the tach itself. From there it only leaves the 4 sensors wich it's easy enough to check the wiring to them.

Thanks again. This has been a huge help in getting me going in the right direction.
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Old 06-11-2010, 08:48 PM   #7 (permalink)
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Default Re: 7720 Display Problems

i think ag xpress electronics can fix your tach or exchange yours for a rebuilt one google them and you will find them
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Old 07-31-2010, 12:08 AM   #8 (permalink)
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Default Re: 7720 Display Problems

Well, I finally got the tach out of the combine. Powered it up and it did the same thing as in the combine. So I sent it to Ag Express. $185 to get mine rebuilt. Made sure the switches were in the same spot, put it back in and it works like a dream. Got the computer for the dial-a-matic back bolted up where it belongs, one of these days I need to find out if that works.
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