At work I have a Morbark tub grinder with a JD 6.8 L. It started going into derate and the codes that come up are SPN 28 and FMI 3. I looked them up and found them but I’m not sure what they mean. The SPN is “Digital Throttle Signal” and the FMI is “Out of Range and High. Would anyone be able to help me get this thing going?
620 John
Member
What's the serial number to the engine??
I'm not familiar with a Morbark tub grinder but I am familiar with the 6.8L engine. Wiring and diagnostics vary depending on when the engine was built. Ideally I'd like to know what level ECU it has (engine control unit which is the computer that makes everything work) because the tech manuals are based on which level ECU the engine has. If you can find the ECU and look at the identification tag on the ECU itself that would help a lot, but I think I can figure out what ECU it has with just the serial number. The tag should say either "L21" or "L23" or possibly "L33"
I'm not familiar with a Morbark tub grinder but I am familiar with the 6.8L engine. Wiring and diagnostics vary depending on when the engine was built. Ideally I'd like to know what level ECU it has (engine control unit which is the computer that makes everything work) because the tech manuals are based on which level ECU the engine has. If you can find the ECU and look at the identification tag on the ECU itself that would help a lot, but I think I can figure out what ECU it has with just the serial number. The tag should say either "L21" or "L23" or possibly "L33"
Last edited:
620 John
Member
What style throttle control does this engine have??
From what I'm reading there's 3 different digital throttle control options for this engine which are: dual state, tri-state, and ramp throttle.
The dual state throttle control has 2 position rocker switch on the control panel that controls 2 set speeds.
The tri-state throttle control is similar to the dual stage except it has an adjustable intermediate speed.
The ramp throttle is a spring loaded rocker switch that changes the RPM's in small increments.
What the code means is the ECU is reading too high of voltage from the digital throttle than what the maximum specification is. The digital throttle sends higher or lower voltages to the ECU depending on the desired throttle position. For an example: Lets say the digital throttle operating voltage range going to the ECU is anywhere from 1 volt to 5 volts. If there's a loss in resistance somewhere in the circuit, the maximum voltage could increase to 5.5 volts. This would throw the code because the max spec is only 5 volts. I do not know if this is a 5 volt circuit because the tech manual does not say what the voltage is supposed to be but that's the idea of what the code means.
In a case like this the problem could be caused by a number of things:
1. bad digital throttle switch
2. a wiring issue from the switch to the ECU
3. a bad ECU
4. software needs to be updated.
Unfortunately you're gonna be limited to problems 1 and 2. ECU testing and software updating can only be done by a JD technician. I checked the software payload for this particular engine and it does have new software available that could be downloaded to the ECU. Whenever I deal with issues like this at the shop I go ahead and update the software first because it's quick and easy to do and sometimes fixes the issue.
If you're comfortable using a multi-meter and unplugging a couple of wiring harnesses I can give you specifics steps on checking the harness and switch.
From what I'm reading there's 3 different digital throttle control options for this engine which are: dual state, tri-state, and ramp throttle.
The dual state throttle control has 2 position rocker switch on the control panel that controls 2 set speeds.
The tri-state throttle control is similar to the dual stage except it has an adjustable intermediate speed.
The ramp throttle is a spring loaded rocker switch that changes the RPM's in small increments.
What the code means is the ECU is reading too high of voltage from the digital throttle than what the maximum specification is. The digital throttle sends higher or lower voltages to the ECU depending on the desired throttle position. For an example: Lets say the digital throttle operating voltage range going to the ECU is anywhere from 1 volt to 5 volts. If there's a loss in resistance somewhere in the circuit, the maximum voltage could increase to 5.5 volts. This would throw the code because the max spec is only 5 volts. I do not know if this is a 5 volt circuit because the tech manual does not say what the voltage is supposed to be but that's the idea of what the code means.
In a case like this the problem could be caused by a number of things:
1. bad digital throttle switch
2. a wiring issue from the switch to the ECU
3. a bad ECU
4. software needs to be updated.
Unfortunately you're gonna be limited to problems 1 and 2. ECU testing and software updating can only be done by a JD technician. I checked the software payload for this particular engine and it does have new software available that could be downloaded to the ECU. Whenever I deal with issues like this at the shop I go ahead and update the software first because it's quick and easy to do and sometimes fixes the issue.
If you're comfortable using a multi-meter and unplugging a couple of wiring harnesses I can give you specifics steps on checking the harness and switch.
I believe it’s a ramp throttle. If I push it it in quick increments it bumps up. If I hold it for a few seconds it goes right to the highest rpm setting. I can pick up a multi meter from Home Depot on my way to work. I called one JD service place (Highway Equipment) in Millstone NJ and they were of no help.
620 John
Member
I would agree that it has a ramp throttle based on your description.
Hopefully this stuff makes sense. Without looking at the machine I have no way of confirming all the harnesses and connectors the manual is showing is actually what you have. Sometimes having a Deere engine in an off brand machine can lead to a lot of confusion. Hopefully the off brand manufacture did not change or modify the harnesses that Deere designed.
Diagnostics for this code starts as follows:
1. Unplug the main wiring wiring harness connector on the back of the control panel that contains the throttle, key switch, gauges, ect. Use your multi meter and set it to Ohms to measure the resistance at the "R" and "S" terminals. The resistance should not be more than 4K Ohms (or 4000 Ohms pay attention to the reading on the meter, some automatically measure the Ohm range and some you have to manually set to measure K Ohms).
Step 2. will be determined by what Ohm reading you get in step 1. If the Ohm reading in step 1 is MORE than 4K ohms then it leads to checking the return wire which is "S". Measure from the "S" terminal of the harness connector to battery ground. Resistance should not be less than 10 Ohms. It says to go to the single point ground which usually has a bunch of ground wires getting grounded at a stud on the engine or frame. But I'd say just use the battery cable ground because who knows where the single point ground is gonna be at. Small jumper wires with aligator clips work good with the multimeter for measuring long distant connections like this.
Further steps depend on what these first two tests show.
Hopefully this stuff makes sense. Without looking at the machine I have no way of confirming all the harnesses and connectors the manual is showing is actually what you have. Sometimes having a Deere engine in an off brand machine can lead to a lot of confusion. Hopefully the off brand manufacture did not change or modify the harnesses that Deere designed.
Diagnostics for this code starts as follows:
1. Unplug the main wiring wiring harness connector on the back of the control panel that contains the throttle, key switch, gauges, ect. Use your multi meter and set it to Ohms to measure the resistance at the "R" and "S" terminals. The resistance should not be more than 4K Ohms (or 4000 Ohms pay attention to the reading on the meter, some automatically measure the Ohm range and some you have to manually set to measure K Ohms).
Step 2. will be determined by what Ohm reading you get in step 1. If the Ohm reading in step 1 is MORE than 4K ohms then it leads to checking the return wire which is "S". Measure from the "S" terminal of the harness connector to battery ground. Resistance should not be less than 10 Ohms. It says to go to the single point ground which usually has a bunch of ground wires getting grounded at a stud on the engine or frame. But I'd say just use the battery cable ground because who knows where the single point ground is gonna be at. Small jumper wires with aligator clips work good with the multimeter for measuring long distant connections like this.
Further steps depend on what these first two tests show.
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