Friday, August 5, 2016

Thomas NBO

Today's job was a Thomas Nut Bank Out cart.  Intermittently it would stop moving and sometimes would go again after cycling the key off then back on.
This machine is hydrostatic drive with electronic control. It has two joysticks for transmission control. One is for transmission ranges, and one for forward, neutral and reverse. The forward and reverse is the one giving us fits today.
I called the manufacturer for a schematic or manual to get some direction but was told that it wasn't happening because it is a 2008 machine and they had not written any.
It took me a while to find out how the system worked because of the lack of any books or diagrams but it is not that complicated. It can be traced out by hand from connector to connector. The fact that it would work for a long time and then just stop also added to the time it took to figure out the cause.
The expert on the system was available by phone but the machine was located in a really bad service area so I had to keep leaving the machine to call him. He was quite convinced that one of the six micro-switches in the joysticks were malfunctioning. I wish he would have been right it would have saved me a lot of time and frustration.
What I found was that the back up alarm was spliced into the harness between the joystick and the controller. The wire that supplied the current to the controller to tell it to shift to reverse also turned on the back up alarm. Once I got the machine to act up, I captured this wave form at the hydrostat controller on the pump unit.
This capture is with the machine shifted to reverse and the throttle pedal depressed (the throttle must be depressed to turn off the anti-coast valves.  That is not relevant to this problem.) Forward is the yellow trace and green is the reverse trace. As you can see both forward and reverse are getting powered. This causes the transmission control to just say forget it, I'm not doin' nothin' until you make up your mind. Even when shifted back to neutral the pattern continued. I had to find what was feeding the transmission control with this bad signal. It was a pulsing signal about one second long and 3/4 second off. My first thought was turn signals but they checked out as normal.
I had the machine shifted to forward and the foot pedal depressed with the key on and engine off when I walked around to the back of the machine and heard the faint buzz coming from the back up alarm. BINGO! I had found it!  The back up alarm had a ground stud with a metal tab connected to it and a mounting screw to ground it to the frame. The nut was loose causing a less than perfect ground. I tightened the nut and recorded the following captures.



Shifting from reverse to forward.
This was a tough one mostly because of the lack of service information but also the environment. It is much harder to find these bugs when it is 105 degrees outside and there are harvesting machines buzzing around you with the dust that goes along with them. I'm glad it is finally fixed!