Starfire GPS receivers have a built-in terrain-compensation module (TCM) which adjusts the reported GPS position to account for the roll and pitch of the tractor. Since the receiver is usually mounted on the roof of a machine, anytime a while rides over a bump it causes the GPS receiver to pitch and roll, which if uncompensated for, would cause the tractor to veer to the right or the left. The TCM requires some setup and calibration so it knows how far above the ground it is, and how far from the pivoting axle of the machine it is. Also it needs to know where dead level is. This is all done through the Greenstar monitor.
Occasionally the receiver will report a TCM failure error. As long as this error code exists, Autotrac will not allow itself to engage. Typically a receiver with a failed TCM is sent to John Deere or AgExpress to be repaired. I've wondered what this TCM is, exactly, and how is it replaced, but since the Starfire receivers are hermetically sealed units, no one wants to break open a perfectly-servicable receiver to find out what's inside and I can find no information online about the internals of these units.
Fortunately, or unfortunately, I had an opportunity to open one up and peer inside, and this post has pictures of what I found. It was very interesting and educational.
Failed TCM
A couple of years ago in preparation for the end of life of the Starfire iTC receiver which has served us well for many years, we bought a used Starfire 3000 online. The seller indicated he worked for a John Deere dealer and repaired receivers. This receiver that we purchased was one he had recently repaired to replace the TCM. Because of the work required, he had to cut open the receiver, do the repair, and then glue on a new dome. After we got the unit, for various reasons it sat on the shelf for about a year after which I decided I probably should just replace the iTC with it. Unfortunately the brown box monitor reported that there was a problem with the TCM and thus autosteer wouldn't work. I'd long since forgotten who the seller was, and for various reasons, put it back on the shelf until the demise of the iTC forced me to revisit the issue.
Since I knew the unit had had its TCM module replaced, I was curious why it was reporting failure. I'm sure shipping is hard on any package and since the heart of any TCM-type module is a set of MEMS gyros and accelerometers, I'm sure they can be break. They are little physical mechanisms that move after all. Since I knew the receiver had been worked on and had a replacement dome installed (a taller dome than the original dome), I wondered if I could soften the glue and remove the dome and have a look inside. I reasoned that the "TCM" had to be some kind of plug-in module, or at least easily replaced by technicians, so I wanted to have a look. Sure enough I was able to open it without damage and this post contains pictures of the internals and documents what I learned.
Opening a Starfire globe
Starfire globes are hermetically sealed, and the only way to open them if they've never been opened before is to cut the dome off. This unit had been cut just below the rim of the dome previously. The replacement dome was slightly taller and glued on. I've heard the AgExpress used to cut the yellow part of the dome, and then plastic welded it back together with acetone when they were done. Now AgExpress has a source of replacement yellow domes. Perhaps similar to the taller dome this unit now has.
Antenna
The first thing you see under the dome is the antenna.
The anntena can be removed by undoing the four screws that line up with the antenna leaves.
The TCM
Under the Antenna is the top of the aluminum chassis that encloses the main circuit board. This thing is build ruggedly, and the aluminum chassis sandwhiches the board for maximum RF shielding. Attached to the top of the chassis, and connected to the main board by a small ribbon cable, is the heart of the TCM, the gyro/accelerometer unit. In the Starfire 3000 I have, it is an Analog Devices ADIS16364BMLZ six-axis innertial sensor. It consists of three gyros and three accelerometers.
Technically the TCM is a program running on the receiver's CPU that takes innertial information from this sensor, and adjusts the GPS position to compensate for any roll and pitch of the machine. Roll is the most critical. However when the receiver reports that the TCM has failed, it's referring to this innertial sensor. When AgExpress or others replace the TCM, this is the module they replace. This module can be purchased from Digikey, Mouser, and from other component vendors. The data sheet can be found here.
Other Starfire models may have other sensors or different versions of this sensor. Eventually I'll crack open the old iTC and see what sensor it used.
The main board
Once the screws holding the chassis together are removed, we can see the main board. It is a very complicated board, entirely built from surface-mount components and chips attached with tiny ball grid solder arrays. To repair anything on this board requires fancy and expensive rework equipment and Louis Rossman-type skill. After seeing that, I knew that the "TCM replacement" was entirely that innertial sensor.
Oddly enough there were two bodge wires on my receiver. I assume they were done by the most recent repair. The little bodge wire was not soldered very well, but until it causes problems I won't try to re-solder it.
Final analysis and "repair."
After seeing what the TCM was, I was all set to order a new one from Digikey. However I got wondering if the problem wasn't something more simple, like the little ribbon connector not being properly seated, or being jarred in shipping and losing connection, even though the connector is held down by a little bracked with a piece of foam. I popped off the ribbon connector which came off very easily and put it back on. I put the whole thing back together again and tried it out and it worked just fine! Later on I decided to tear the receiver apart again to take pictures for this blog, since there just aren't any pictures out there. This time it was really difficult to get the connector unplugged. So that confirms that the problem all along was the connector. I double checked the pins and none were bent. After taking pictures I pushed it back on and put everything back togther.
All I need to do is re-glue the dome on. And I noticed that this receiver happens to be unlocked for SF2 and RTK if I ever want that. Bonus! But I have other plans for RTK.
Should you replace your own TCM?
If you're well past warranty and if you're not afraid of cutting the dome open and have a plan to re-seal it when you're done, I don't see any reason why you couldn't do it your self. Have a look first at the gyro/accelerometer unit inside the receiver to make sure you can get an exact replacement (different part numbers have different measurement characteristics).
However, given the cost of the replacement gyro unit, the price AgExpress charges to do the replacement and replace the yellow cover is quite modest.
