[havoctec]

Recently my 2012 3720. on several occasions would barely move when first started in B range and seemed to be a little better in A range.
Each time after a couple minutes it started working normal. At first I thought it was something in the transmission but finally realized the tach was not displaying correct engine RPM.. It was purchased used with 450 hours two years ago and I noticed right away that the tach would stay at 1000 RPS for a brief moment when throttling up but would alway jump up to correct RPM after a couple seconds. Recently on several occasions it has taken longer for the tach to jump to correct RPM and from what I can tell load match is sensing that the RPMs are to low and limiting forward movement.

Engine will be at 3/4 throttle while tach displays only 1000 RPM. Tractor barely moves with load match on. Turning off load match allows the transmission operate normally.

So, seeing the expertise on this site I figured this is a good place to start diagnosing the problem. What should I look at first?
I believe I have a good understanding of how the whole system works but I do not know exactly how the input and output signals are getting to and from the processor.

Grateful for any help.

 

[Superglidesport]

It "sounds" like a classic case of a bad engine speed sensor. The engine speed sensor and MFWD sensor have been known to be troublesome.

 

[Kennyd]

It "sounds" like a classic case of a bad engine speed sensor. The engine speed sensor and MFWD sensor have been known to be troublesome.

^^^^
This.

 

[havoctec]

Great. That is what I was thinking.
However, while talking with my nephew who has a lot of knowledge about gas engines was thinking I would be having more issues if the crank sensor was not working. I reminded him that this is a diesel and there is no ignition timing. 😄 However was not sure if the fuel injections was getting input from the engine speed sensor.

 

[Superglidesport]

Great. That is what I was thinking.
However, while talking with my nephew who has a lot of knowledge about gas engines was thinking I would be having more issues if the crank sensor was not working. I reminded him that this is a diesel and there is no ignition timing. 😄 However was not sure if the fuel injections was getting input from the engine speed sensor.

Nope. The fuel injection events are mechanically timed on this engine.

 

[havoctec]

Thanks to all of you for confirming my thoughts.

 

[tvtegtmeier]

What do other electrical things do when it does this?

do the headlights dim?

does the engine seem to be running normally? - just the tach displays 'wrong' and it doesn't want to move?

I've had this happen when the Alternator belt was slipping. in my case the belt was a little loose and when moving snow, it would slip when it got wet.

Tightened it up, sprayed a little belt dressing on it and no problems since.

The key symptom is that the lights dim at the same time as the tach glitch and 'no move'. All 3 together.

Tim