Freightliner Opti-Idle cooling. Why it fails, and a workaround.

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Farmerbob1, Jul 28, 2021.

  1. Farmerbob1

    Farmerbob1 Road Train Member

    Jan 17, 2017
    First, this write-up addresses issues where sleeper booth cooling fails regularly, especially in high temperature environments.

    Second, let's verify your system is of a type and configuration to match the system configuration that this solution addresses.

    The flaw here is that Freightliner has combined the function of a sleeper area thermostat and output air temperature into a single device.

    Let's see how to test and verify your system has combined the two.

    In my system, the temperature controls in the sleeper look like this. Yours might look different, but follow the instructions I give and you can test if the functionality is the same.


    In my truck's case, the right side knob serves TWO purposes, the white dot indicates BOTH the thermostat setting AND the output temperature of the sleeper cooling system air.

    TEST this. Verify that if you adjust the knob during Opti-idle operation, the cooling system output temperature changes. When I adjust mine full left, I get really cold air. In the setting pictured above, I get barely cool air.

    This, is, in a nutshell, why many Freightliner Opti-idle systems fail to cool properly. If you set the sleeper to 70 degrees, for example (in my truck what you see is a 70 degree setting) then the truck tries to cool the sleeper bunk to 70 degrees with output air that is ALSO 70 degrees.

    Anyone with any understanding of cooling system will recognize that barring outside assistance, a sleeper booth cooling system designed this way will be VERY slow to cool the last few degrees at it approaches the thermostat set point. Even worse, the thermostat set point in Freightliner Opti Idle systems are not a fixed point, they are a range. The setting of the right knob above is set for 70 degrees, but is supposed to hold the truck between 67 and 75 degrees. Simply put, you absolutely will NOT ever cool a system to 67 degrees with 70 degree air.

    So, why does Opti Idle occasionally work, you might ask?

    It is very simple. Outside influences from beyond the sleeper bunk. Outdoor temperatures lower than the setpoint, or, more commonly, the cooling system from the cab of the truck.

    We cannot control the weather, so I won't bother going into detail, but in places where nighttime temperatures get down into the low 70s or below, the opti-idle might appear to be working, but in reality it is being allowed to work due to heat being drawn out of the truck into the night air.

    Here, is where I discuss how to work around the broken Opti Idle system variants. In a nutshell, we will use the cab cooling system.

    Here is what my cab cooling system looks like.


    In this case, you can see the leftmost dial's amber light is on. This indicates the truck is recirculating cooling air, not drawing all air from outside. Typically this is best when it is hot outside. If it is cool outside, you might want to turn this off.

    You can also see the middle knob has it's blue light on. This indicates the system is operating in max cool operation. If the light were green, that is eco-mode, which is less aggressive cooling. That is fine in some situations, but for Opti-idle cooling we want the most cooling in as little time as possible, so I am set to max cooling operation.

    The right knob is set for dash vent cooling, because the dash vents push air towards the sleeper, where we need it.

    So, I suggest when you set Opti-Idle, you need max vent speed, recirculating, on the left knob, max cool, lowest temperature on the middle, and 100% dash cooling on the right.

    DO NOT TURN ON THE AMBER LIGHT ON THE RIGHTMOST CAB A/C LIGHT. That should not be possible in opti-idle mode, but if it allows you, it changes the way your whole cooling system works, and will make things worse by conceding some control to the sleeper controls.

    NOW, on to how to make (Edit spelling) the system actually work, despite the apparent Freightliner engineering design failure.

    Some companies tell you to close the front dash curtains to keep heat off the dash and out of the cab. This is sound advice. Do it, especially in hot daytime weather.

    Some companies also tell you to close the blackout curtains. DO NOT DO THIS.

    If you have a single-setting sleeper bunk control as discussed above, the only reason your opti-idle system APPEARS to work on some days is that the cooling from the CAB is bleeding through the blackout curtain!

    What WE want to do is open the sleeper to the cab so the cab can cool the sleeper.

    This works because the cab controls are NOT also thermostat controls. The sleeper controls are looking to maintain 67 to 75 degrees (based on my setting pictures above) but the cab controls are working as hard as they can to push as much cold air as possible.

    If the cab and sleeper are divided with the sleeper bunk curtain, some of the cooling from the cab can bleed through to the sleeper, but that will only work to maintain comfortable temperatures in very mild heat.

    If the cab and sleeper are open to each other, and the dash curtains are closed, it will be a little brighter in the sleeper in the day, maybe a little louder, but the opti-idle system will actually WORK because the cab air system is providing the cold air required to cool the truck.

    There might be ways to fix this in the electronic controls, but I do not have access to the truck software. The simplest, most obvious fix to try would be to set the sleeper bunk output air temperature to always be max cold when in cooling mode, which MIGHT work (would need to test.) Not sure if that is an option.
    Last edited: Jul 28, 2021
    Reason for edit: Spelling
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  3. Farmerbob1

    Farmerbob1 Road Train Member

    Jan 17, 2017
    Three things I think are worth mentioning.

    First, in some newer trucks, there are three settings for cab cooling modes. Off (no light in the dial) Energy saving (green light in the dial) and Max cool (blue light in the dial.) If you are blue-green challenged, the light cycles from off, to green, to blue (in my truck at least.)

    Second, make sure all the dash vents are fully open and blowing towards the sleeper. That's where the cool air needs to go.

    Third, when in max cooling mode, at the coldest setting, your cab cooling should chase you out of your truck in an hour of driving down the road with the curtains closed unless you are wearing a coat. If it doesn't, you might have an AC problem that has nothing to do with Opti Idle settings.
    Last edited: Jul 29, 2021
    Reason for edit: Added info.
  4. Farmerbob1

    Farmerbob1 Road Train Member

    Jan 17, 2017
    New information.

    If your truck Opti-idles at 600 RPM and the truck isn't pushing really cold air from the cab vents **consistently**, check to see if it has a new or old model AC compressor. Freightliner recently downgraded their AC compressors, both replacement units and new-build units. The old units worked well at 600 RPM. The new units need around 900 RPM to work properly.

    If you have a new model AC compressor and opti-idle is set for 600 RPM, it is very likely that the method described above will not work consistently.
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