Are you getting wide and narrow rows when using your GPS?
Your receiver could have a bad TCM calibration. Don't worry. It is easy to check your calibration to see if it is correct, and it is easy to re-calibrate if you are not lining up properly.
How to check your current calibration
The easiest way to check your current calibration is by getting on a patch of ground where you can make some fresh tracks that are easy to see. Set a new AB line and let the machine drive itself for 40 or 50 yards. Turn around and let the machine drive itself back down the same tracks you just made.
If your tires are falling back in the center of your tracks your TCM is calibrated correctly and you will need to verify you do not have any center offsets in your machine settings that are causing you to not be centered. You may also have to inspect your plow and/or 3-point hitch and make sure everything is centered and tight.
If your tires are offset from the tracks you just made, you will need to recalibrate your TCM.
How to calibrate your TCM
Menu Button -> Starfire Receiver -> Setup
The first thing you will need to do is move your machine to a piece of level ground where you have enough space to turn the tractor around. The more level the ground is the easier it will be to get a good calibration.
Once you are on level ground, you will need to set your implement down on the ground if you have one attached to the machine. Hit your Menu Button and then choose your Starfire receiver in the menu. At the top of the page you should see a tab labeled Setup. Touch the Setup tab and you will see the TCM section located on the bottom right quarter of the page. You will be able to get in to the calibration pages by hitting the button with the black traiangle labeled CAL.
Once you are in the calibration pages you will need to follow the on screen instructions. They will walk you through the calibration step by step. It should only take you a minute to complete the calibration.
After you are completed the steps, you can check and see if you have a good calibration by going through the steps outlited at the first of the article. You should see that your tires match up better with your tracks then they did when you first checked them before recalibrating.
You may have to calibrate your TCM a couple of times before you get it tuned in where you want it.
Once you get your TCM calibrated, you should see that your issue of wide and narrow rows has been resolved. If for some reason you can not get your TCM calibrated properly, you may want to contact your local John Deere dealer for assistance.