Yes I still try to control the rear lift 
Could you do a simple test like this:
You just need 5V from PCB or Arduino! and a shift switch.
Connect the center of switch to middle pin on tractor “PCB”, and one side to middle from lift potentiometer and the other to 5V.
Start tractor with switch in normal position, then shift to 5V.
Does fault number show or does lift react?
The idea was in the end to have a power supply of 10 V (maybe 12V) and let the arduino control relays that shifts between different voltage dividers to match the 3 or 5 states you need.
For testing this would be a good idea, will do that when the tractor is back from repair. Instead of using multiple voltage dividers I thought of “recreating” the different grounding and something 5V above that with two voltage dividers and then a digital potentiometer. Should work because the relative voltage is in the specifications of those potentiometers. This way I could set the voltages more precisely
Or maybe instead of a digital potentiometer, use a 0-10V PWM converter and send a specific voltage to the ECU?
Just saw this for the first time. Good information! Thank you
I tried to use two voltage dividers to get a measurable voltage under 5V out of the signal pin of the original switch, but even that fails. I dont really understand whats going on there, with the voltage divider I get something around 8V and the switch seems to do nothing. Im kind of stuck, this doesnt make sense to me…
today I tried to do that, I recreated the lifting voltage with a voltage divider. I started the tractor, initialized the hitch and disconnected the switch while it was in lowering position. Then I connected the voltage divider, but nothing happened. There must be some kind of fault detection that prevents this kind of workaround.
if you then reconnect the original switch does it work again? it could be that when disconnecting it, the control unit goes into error until the tractor is restarted
I have to restart the tractor, simply reconnecting doesnt work
Does the fault appear immediately, after 1 second or?
Try unplug and replug original potentiometer as fast as you can.
Or try another way. Imitate the outside switch on fenders. Probably a bit slow reaction though.
Maybe this pdf gives you new ideas.
https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.farnell.com/datasheets/686036.pdf&ved=2ahUKEwj8gOyN_a3uAhXNC-wKHeEXBsYQFjALegQIBhAB&usg=AOvVaw3Dqa2hrXfXAul4RwUJhb1P&cshid=1611264924456
I wonder if I could control it with can bus, although I dont know how it works. In the manual it looks like its possible to control the hitch with a laptop and “win metadiag”, but it would probably be very challenging to implement that. I try to avoid any permanent changes to the control panel
can you measure the power supply voltage of the potentiometer and the resistance values in the various positions? I mean with disconnected potentiometer, if you can do let’s say a table would be the maximum
The fault seems to appear immediately. Using the outside switch is no option, it disables the internal switch, Id always have to initialize it every time I want to manually control it. Also I think it ignores the set limit for lowering. And as you said, it is really slow.
You mean completely replace the potentiometer? First of all, Id like to keep its original function, sometimes I will be using the tractor without any automatic lifting. When I use a digital potentiometer, Ill always need an arduino to run and since there is no power source in the hitch control Id alway need external power, I think thats not a good solution. Next thing is, every 5k digital potentiometer I found is only rated for 5V max and the original power supply has 7V. Adding any extra resistance seems to cause problems, even trying to measure the output voltage with a voltage divider does that.
if you can do let’s say a table would be the maximum
What do you mean by that?
I thought that maybe playing with two or more resistors you could match the value on the signal wire with what the control unit expects for the various positions, without removing the original switch
sorry I expressed myself wrong, forget that part
To keep potentiometer with normal funktion it might be possible to shift by using a stereo potentiometer, and avoid the disconnect fault.
And it is possible to get a digital pot that allows 18 V on the A W and B terminals
I added one resistor in series into the signal wire: The voltage drops and it doesnt seem like theres any fault. That way the original switch would have to be in high position when using automatic lifting, because in that position I read 4V, in the other positions lower voltages. I think its not a good idea to use it that way, I might somehow forget to turn automatic lifting off and it lowers the implement on the street. Also I guess most of the time it will be used to lift when the implement is in working position, so raising would need several movements of the original switch, might also cause problems…
with a resistor of adequate value, in parallel between the supply and the signal of the original switch, you could raise the voltage even with the switch in neutral position, obviously this resistor would only be inserted in automatic mode
That was it! Added a 1K in parallel as you suggested, now I was able (still by hand, but anyway) to lift the hitch when the switch was in lowering position! Maybe Ill adjust the resistor a bit, but in general, it works. Now Ill hook all that to AgOpenGPS with the machine control module. Will take a bit time, but nothing too complicated anymore. Thank you very much!
if you can post the resistance values of the switch in the various positions you could calculate the values to be entered for raise and lower with the switch in neutral position