JD 4520 cruise control "max speed" problem

[eepete]

I recently had the "enhanced" or "automotive type" cruise control added to my 2009 Deere 4520 when it was in the shop for a few "minor" repairs. The cruise control works great, and the ability to incrementally change the speed by pushing a switch is great :thumbsup:.

When mowing with my MX6, I tend to be in the B range with load match on and auto throttle off. In fact, in general, I only use auto throttle when driving around the place. Seemed like a nice way to vary the engine speed and help with the break-in process. Most of the time, I don't want to mow faster than 4-5 MPH. The cruise control has the "max speed" setting where you can bring the tractor to the desired max speed, throw the cruise control switch to max speed, hit the "set" button, and then release the foot control. Then (in theory) no matter how far you depress the foot peddle the max speed the tractor will move will be what you set it to. That max speed can also be incrementally adjusted using the + and - button while you have the peddle fully depressed.

The max speed function doesn't work very well. If you're going up a hill, it slows way down, if you're going down a hill, it speeds up. So today I did a test. I have a section of my driveway where it is level, then goes downhill a bit, then goes uphill. I first went up the hill with the autothrottle at 5.5 MPH and noted the RPM. For the test, I was 200 RPM above that point. I had no implements on the tractor, PTO off, load match off, auto-throttle off.

With cruise control on and setting the speed to 4.7 MPH, for both the downhill and uphill sections the speed stayed within +- .2 MPH (i.e. 4.5 to 4.9 MPH) and most of the time was within .1 MPH. I then went back and tried the same course with the max speed function, setting a speed of 4 MPH as the max speed. So I was going on a level surface with the pedal all the way down doing 4 MPH with max speed cruise control enabled. When going down the hill, the speed went up to 5.6 MPH. When going up the hill, the speed went down to 2.6 MPH. I went back again and when going up the hill I turned off the max speed function and _quickly_ went to over 6 MPH , so this is not a power issue.

So my question is: Is anyone else seeing this with the max speed function :confused3: ? Clearly the control system can maintain and regulate the speed fairly precisely, and this is not a case of being underpowered on the hill. The max function just doesn't seem to work well.

Sorry the details were not as short as the question My next step is to tear into the service manual. I've talked to my Deere Dealer and will give him this same information.

Pete

 

[jenkinsph]

Pete,

Haven't even tried out this function yet BUT it would seem you are setting the max speed (in rpms') and are expecting the mph to remain the same. I will have to check mine but I think it is a matter of interpetation of the function that is the issue. If you set a maximum rpm of 1500 then the hydro will fluctuate in its mph substancially depending on the slope.

You need to decide which is of importance, either engine rpms' or ground speed. If you wan't to maintain 4.0 mph then the engine may need to increase rpms' to climb the hill or reduce rpms' to decend the hill

 

[eepete]

In the service manual, the "max speed" function is described as:

"The max speed feature allows the operator to set the maximum speed the machine will travel when the drive pedal is fully depressed."

So I'm thinking the max speed is not a max RPM but a max travel speed. Later paragraphs in the section also use the phrase "specific travel speed".

All the switches go into the TCU (Transmission Control Unit). So this is a "drvie by wire" function just like speed control. I'll bet the dealer will have to stop by some time and give this a try, then bounce it of Deere. Because the cruise control works, and because the unit tries to do the max speed functionality, all the pieces work. I know us hardware guys love to say it's a software problem, but I think it is

Pete

 

[jenkinsph]

I got your point now, probably a computer screw up.

So are you gonna sell it?:laughing:

 

[eepete]

So are you gonna sell it?:laughing:

Nah, it took all winter to get it "broke in" :laughing:

It will be interesting. Could be a firmware update or new TCU. I think I am reading all the pieces correctly. It will be interesting to see how Deere handles it.

Mowed today in 100 degree heat, the AC just barely kept up with it. If it goes up to 105 I'll have to take the sweater off.

Pete

 

[jenkinsph]

Pete,
Let us know what you find out, we may all need a fix for this. I will play with mine today as I am working on a site with some steep slopes.

 

[eepete]

Got the word back from the Deere mothership via my dealer:

Thanks for the feedback.

While cruise control monitors mfwd speed and tries to maintain it, max speed does not. When you set the max speed, the controller snapshots the voltage of the pedal potentiometer at the point when the switch was pressed. After setting max speed the controller will only allow voltages up to the voltage that the max speed was set at. Voltages above this, the controller ignores. Max speed will not take into account vehicle loading conditions.

So the term "max speed" is a bit misleading. When you lock in a speed, it is really equivalent to mechanically keeping the pedal from going any further down than it is. So it's more like a "max pedal push" feature done electronically.

Not as useful as a max speed function, and clearly this could be done- the software and hardware are there . But, it does explain the behavoir and now the "bug" is a "feature." Good for flat, not so good for hills.

Pete

 

[jenkinsph]

Pete,
Sounds like they are using the pedal potentiometer as the sole reference instead of referenceing the output shafts speed, They could easily modify this I think. They could increase or decrease the input voltage to maintain a steady output shaft speed.

 

[AKfish]

Given the limitations of the "max speed" function - I can't imagine why anyone would ever use it..

Stick with the cruise control function to establish the operating speed and go to work.

AKfish

 

[eepete]

What it does is easy to implement in software. I'd be surprised if it was more than 1 or 2 lines of code at some point where it picks up and uses the pedal value:

if(max_speed_on && (f_pedal_value > max_speed_setpoint)f_pedal_value = max_speed_setpoint;

Doing it "right" would require having the cruse control code, which is more complex since it's a close loop feedback system, run with two modes. One for regular cruise, and one as a max speed limiter.

Marketing was writing checks that engineering wasn't allocated the resources to cash. That or the programing department hired a bunch of ex-Microsquash programers (Motto:"Do what's easy, not what's right") :laughing:.

AK, on level but rough/bumpy areas, it is really nice to leave your foot on the pedal so it's all the way down and cruise along. When you get in a bumpy spot, you can just lift your foot to slow down to an appropriate speed. When you are through the rough area, just rest your foot back down all the way. On a slope I can use the cruise, but it's harder to shut it down with the switches, get your foot at an approximately correct position, go through the rough area, then turn the cruise on and restore. You have to look at switches/hold your foot just right vs. just lift your foot and set a good speed.

Now in a few years when I've had the chance to box blade/grade most of my mowing surfaces it might be OK but...(insert over-analysis here ->[__])

Pete