JD 7720 Runaway

[pcrh]

Hi:

I am looking at a 7720 combine. Overall in good condition except the motor will run at full rpm and the kill switch does not work. Is this an injection pump issue or a cable issue stuck at full throttle?

The combine has not been used for several years. I am told the unit has been started every year and all working parts engaged until 2024. It has not been on the field since around 2020. In 2024 it was driven out of storage and has been stored outside since then. The combine now will start right up with no problems but it will immediately go to full rpm and cannot be shut off without depriving it of air or fuel.

Any way I can test what the issue is with the runaway?

Thanks

 

[Ray,IN]

Without knowing the engine brand and size, anything said is AWAG. My guess is injector pump.

 

[Fixingfarmer]

So does the throttle move with the lever?

 

[rrlund]

Can you unhook the cable at the pump, have somebody up there to run it by hand and see if it'll idle?

 

[Dieseltech]

On ANY make inline pump just ONE stuck plunger will keep the rack from moving by the governor control. Safest way to check for a stuck rack on any engine that's been setting unused awhile is to loosen ALL injector line nuts so engine WILL NOT start. Now crank engine and operate the fuel ON/OFF control. If rack IS NOT stuck you should see fuel delivery when control is in start/run position, and fuel STOP when moved in the fuel OFF position. If fuel WILL NOT stop injection pump needs attention/repair. If fuel flow will start/stop, THEN it's safe to tighten all line nuts and start engine.

 

[pcrh]

Hi

Thanks for all the responses. I should mention that I have not yet seen the combine and hope to do so soon. The weather is not conducive to do outside work but I hope that next week will be better.

The cable would be my first test to see if it is free to move and then to see if the pump on/off control arm will move.

I am not familiar with this model so how accessible is the injection pump? The information I found on this site says this is a early 7720 turbo with yellow cab top. I believe this would mean a 466 motor.

If the cable and pump control arm move then Dieseltech's test (thank you for the steps to test) would seem like the next test.

Any additional item I should check for on this model combine?

 

[Dieseltech]

Hi

Thanks for all the responses. I should mention that I have not yet seen the combine and hope to do so soon. The weather is not conducive to do outside work but I hope that next week will be better.

The cable would be my first test to see if it is free to move and then to see if the pump on/off control arm will move.

I am not familiar with this model so how accessible is the injection pump? The information I found on this site says this is a early 7720 turbo with yellow cab top. I believe this would mean a 466 motor.

If the cable and pump control arm move then Dieseltech's test (thank you for the steps to test) would seem like the next test.

Any additional item I should check for on this model combine?

Bosch A size injection pump. IF you can get to it there's a front rack cap that could be removed to see the rack FRONT end. Cap can be a steel round cap with two wrench flats for 19MM or 3/4 open end wrench. Later engines use an aluminum hex shaped cap with a magnet inside. With cap removed work the stop control lever and see id the rack will move in/out, it SHOULD if pump is OK. If it DOES NOT move then one ore more plungers are stuck, keeping rack from moving as it should. Just because the STOP lever moves does NOT mean the rack is moving on the Bosch RSV governor.

 

[used red MN]

I can tell you that per the engine layout the injection pump will be on the back side of the engine, as in grain bin side. A “Bosch A sized injection pump” is an inline pump. Per the parts catalog the connection between the throttle lever and the pump is also a push/pull cable.

 

[pcrh]

Bosch A size injection pump. IF you can get to it there's a front rack cap that could be removed to see the rack FRONT end. Cap can be a steel round cap with two wrench flats for 19MM or 3/4 open end wrench. Later engines use an aluminum hex shaped cap with a magnet inside. With cap removed work the stop control lever and see id the rack will move in/out, it SHOULD if pump is OK. If it DOES NOT move then one ore more plungers are stuck, keeping rack from moving as it should. Just because the STOP lever moves does NOT mean the rack is moving on the Bosch RSV governor.

Thank you Dieseltech the extra information will be very useful.

 

[pcrh]

I can tell you that per the engine layout the injection pump will be on the back side of the engine, as in grain bin side. A “Bosch A sized injection pump” is an inline pump. Per the parts catalog the connection between the throttle lever and the pump is also a push/pull cable.

Hi used red MN. I was so hopping the pump would be on the outside. Oh well, is there an access panel in the grain tank? Or, is there a picture to show me what the mounted motor looks like? I am going to need a warm sunny day to pursue this work or remember how to work with frozen fingers.

 

[Dieseltech]

Hi used red MN. I was so hopping the pump would be on the outside. Oh well, is there an access panel in the grain tank? Or, is there a picture to show me what the mounted motor looks like? I am going to need a warm sunny day to pursue this work or remember how to work with frozen fingers.

It’s a tight fit but you might be able to get along side the injection pump in front of the grain tank.

 

[pcrh]

It’s a tight fit but you might be able to get along side the injection pump in front of the grain tank.

Hi Dieseltech:

Thank you for the help.

I hope you don’t mine some dumb questions as I learn more about diesels.

How does a stuck rack allow the combine to even start?
How does a stuck rack allow the combine to run at full rpm?

This is just to expand my knowledge.

 

[Dieseltech]

IF the rack IS in the run or start position by just ONE or more stuck plungers the engine COULD start on the remaining WORKING plungers.
Engine COULD then run FASTER than the governor would control because the stuck rack CAN'T be pulled back to no fuel position.
I've seen several engines with rods come through the block due to NO governor control. That's why it's good to check a few things first on ANY diesel engine that's set unused awhile. If one does not know how to check the pump in question, having ALL the lines loose so engine will not start is the safest way to then see if the fuel delivery will stop/start as it should WHILE cranking engine. I've seen this happen on other injection pumps too, not just Bosch inlines.

 

[pcrh]

Hi Dieseltech:

Thank you for your explanation. It helps my knowledge.