John Deere 6420 suspected electrical problem

wildnwackywill10 · Jun 9, 2014

I have a John Deere 6420 and I am having some problems with it. The tractor runs fine most of the time but occasionally the tractor will decide to limit the maximum revs at exactly at 1400 rpm, I initially suspected that there may be a fuel problem as recently i replaced all the fuel lines but every time the accelerator is maxed out the engine revs settle at 1400 rpm which leads me to suspect and electrical problem.

The engine is a common rail engine and has a fuel moisture sensor which has had the two wires broken, in the past they have been soldered back together but this time the soldering has failed and there is a constant light flashing in the cab to indicate this. The error code shows that the sensor has been shorted out but i do not see how this could be affecting the engine rpm. I am determined not to replace the sensor as it is crazy money for all it is, something like £130 for a very simple sensor.
(Part No. re519696)

Could anyone provide me with a solution to this problem, a lot of money was just spent on new tyres and I would be keen to have it in perfect working order.

fbagg · Jun 9, 2014

Highly unlikely to be a solution without changing the sensor, unless you still have enough of the wires to make another repair. This is very common on our 6430 tractors and we keep a spare sensor on hand. The computer is not getting the signal from the sensor so it is derating the tractors motor, that's why you are only getting 50% of your throttle.

wildnwackywill10 · Jun 10, 2014

Do you have problems specifically with the fuel moisture sensor and with the two wires breaking?

fbagg · Jun 16, 2014

we have problems with the fuel moisture sensor where it just quits working, sometimes I find a broken wire and can fix that other times it seems to be corroded where it threads onto the see thru bowl on the bottom of the filter. We have had broken wires on other sensors also such as the cam sensor and the crank sensor. Usually I find that it is rodents who chew on the wires that is the cause.

wildnwackywill10 · Jun 16, 2014

I have resoldered everything back together and have checked there is a solid connection with the multimeter, there is a 5v difference between the 2 terminals in the moisture sensor yet there is still a constant warning in the cab and the error code is still showing that is is shorted out somewhere. The tractor is running fine though and has been working hard the past few days despite the constant flashing light. It's looking like a new sensor may be the only fix as i can't seem to get this one working despite there seeming to be a normal connection.

fbagg · Jun 17, 2014

How are you reading the code? Are you sure the moisture sensor is the one sending the code? My tractor doesn't have a monitor it just has the light so I have to get the dealership to read it. But every time I have a moisture sensor code the engine derates so if your's is not derating the engine it makes me think that its something different.