Every UNDERHOOD™ air compressor comes with a control box that helps the system communicate with the operator. While most operators are familiar with the very basic controls—“on” and “off”—there are dozens of unique messages the control boxes can share with you.
To learn about specific control boxes or error messages, please select the system below:
UNDERHOOD™ 70 & 150 Control Box
UNDERHOOD™ 70 & 150 Control Box Features
On & Off Functions
The control box’s ability to turn the UNDERHOOD™ air compressor on and off is its most obvious feature, but on and off functions are just the beginning of what the control box can do…
The control box helps ensure your maintenance stays on schedule, providing 200-hour and 400-hour service alerts. If you receive the “HRS:XXXX200HRSVC” or “HRS:XXXX400HRSVC” message, it’s time to service your air compressor. The control box also records the last 10 service times.
The UNDERHOOD™ control box plays an important role is temperature control. The control box will use the “COMP TOO COLD” error message to let you know if the air compressor is too cold for operation and displays elevated temperatures or “TEMP XXF/XXC” when the internal temperature of the air compressor is between 130°C (266°F) to 150°C (300°F). If the air compressor reaches 150°C (300°F), the control box will turn the compressor off and turn the warning light on.
Battery Voltage & Clutch Current
If the battery isn’t adequate or the clutch current isn’t compatible with the air compressor, the control box will let you know. “BATTERY LOW” indicates that less than 11 volts have been detected, while “CLUTCH HIGH,” “CLUTCH LOW,” and “CLUTCH SHORT” are used to share information about faulty clutches, and broken, pinched or disconnected clutch wires.
The UNDERHOOD™ control box logs error codes when errors occur. This data log allows operators and service techs to diagnose problems and accurately reference the UNDERHOOD™ system manual and technical support documents, when needed.
To access the error logs:
- Turn key ON. Do not start engine.
- Press and hold “OFF“ button until “DIAGNOSTICS” is displayed.
- Press “OFF” to scroll down. Press “ON” to scroll up.
|Control Box Message||Explanation||Compressor State|
|HRS:XXXX:XX||Main Screen hour meter||On/Off|
|Park Brake||Park Brake is not applied or bad signal||Off|
|HRS:XXXX200HRSVC||200HR service is needed||On/Off|
|HRS:XXXX400HRSVC||400HR service is needed||On/Off|
|COMP TOO COLD||Compressor too cold for operation||On|
|TEMP XXXF/XXXC||Displays elevated temperature 130°C (266°F) to 150°C (300°F)||On/Off|
|Wait xx Seconds||A 30 second delay to allow for compressed air blow down||Off|
|Code||Display||Fault||LED Code||Possible Problem(s)|
|01||OVER TEMP||Compressor is too hot||Red LED on||-Low compressor oil level.
-Faulty or crushed cooler lines.
-Exceed cooling capacity of compressor cooling system.
-OEM changes in cooling.
|02||TEMP PROBE SHORT||Short to ground in temp probe circuit||Flash code |
1 red LED
|-Faulty temp probe.
-Crushed temp probe wires.
-Pinched or bared wires that are grounded.
|03||TEMP PROBE OPEN||Open circuit in temp probe circuit||Flash code |
1 red LED
|-Faulty temp probe.
-Broken temp probe wires.
-Unconnected temp probe.
|04||BATTERY LOW||A battery voltage less than 11V has been detected||Red LED on||-Faulty power line connection.
-Bad or intermittent fuse.
-Broken or poorly crimped connecters.
-Truck charging system problems.
-Ensure VMAC Control box is sole device on 12V supply wire.
-Bad ground connection
|05||CLUTCH HIGH||Too high of current draw on clutch 5-10A||Flash code |
2 RED LED
-Broken or pinched clutch wire.
|06||CLUTCH LOW||Too low of current draw on clutch <2A||RED LED |
-Broken or pinched clutch wire.
-Disconnected clutch wire.
|07||CLUTCH SHORT||Too high of current draw on clutch >10A||Flash code 2 red LED||-Faulty clutch.
-Shorted to ground or pinched clutch wire.
UNDERHOOD™ 40 Control Box
The UNDERHOOD™ 40 control box is a simpler system than the 70 and 150 models, using a series of flashing lights to indicate any issues that occur.
In the event of an error, the “Compressor Status” LED on the interface panel will flash red until the system is powered down. After the system depressurizes, you may turn the vehicle ignition on without starting the engine. If the problem is resolved, the light will be green.
If an issue is still present, a red light will blink one to eight times in a row, with a pause in between. The number of blinks in the sequence corresponds to the following errors codes:
|# of Blinks / Error #||Error||Description|
|1||Clutch Open||No current detected flowing to the clutch|
|2||Clutch Short||Wiring to the clutch or coil has shorted to ground|
|5||Compressor Temperature Probe Open||Compressor temperature probe disconnected or failed|
|6||Compressor Temperature Probe Short||Compressor temperature probe wires shorted or failed|
|8||Compressor Temperature Probe Hot||Compressor has shut down due to compressor over temperature (145 °C / 293 °F)|
UNDERHOOD™ Green Series Control Box
The UNDERHOOD™ Green Series shares many of the same features as the 70 & 150 control box, including:
- Service reminders
- Low battery indicators
- Temperature issues
- Clutch current issues
- Engine RPM issues
- Errors detected
In addition, the Green Series control box has one particularly cool capability: the ability to modify the default system parameters. You can use this feature to adjust the factory settings for pressure, standby delay, cab temperature, and more.
Modifying System Parameters
Operators can modify the following Green Series system parameters:
|Standby Delay||Delay time to standby||10 minutes||1, 2, 5, 10, 15, 30 minutes, Never|
|System Pressure||Operating system pressure||150 PSI||80 to 175 PSI @ 5 PSI increments|
|Top-up Pressure||PSI below system pressure to allow inlet to open and build pressure||5 PSI||1 to 10 PSI @ 1 PSI increments|
|High Air Use Rate||Air rate set-point for pressure-based restart||-3.0 PSI/sec||0 to -30 PSI @ 0.5 PSI increments|
|Restart Pressure PSI||Air pressure set-point for pressure-based restart||120 PSI||80 to 175 PSI @ 5 PSI increments|
|Disable Auto Restart Delay||Timer for disabling automatic engine restart||30 minutes||Always, 30, 60, 90, 120 minutes, Never|
|Low Battery Restart||Controls if system will restart when battery voltage gets too low||Enabled||Enabled or Disabled|
|Batt Lower Thrs||Restart point for battery voltage||12.1V||10.0 to 14.0V in 0.1V increments|
|Batt Upper Thrs||Fully-charged set point for battery voltage||12.8V||10.0 to 14.0V in 0.1V increments|
|Low Batt Restart Delay||Time delay before going back to STANDBY after battery fully charged||2 minutes||2, 5, 10, 15, 20, 30 minutes|
|Cab Temp Restart||Controls if system will restart when cab temperatures drops too low (accessory kit required)||Disabled||Enabled or Disabled|
|Cab Lower Thrs||Restart cab temperature||10°C/50°F||-10°C/14°F to +20°C/68°F in 0.1°C/1.18°F increments|
|Cab Upper Thrs||Fully warmed cab temperature||20°C/68°F||10°C/14°F to +20°C/86°F in 0.1°C/1.18°F increments|
|Low Cab Restart Delay||Time delay before going back to STANDBY after warming cab||5 minutes||2, 5, 10, 15, 20, 30 minutes|
The following information messages have been programmed to show on the UNDERHOOD™ Green Series air compressor display box:
|Error Code||Message Displayed||Cause|
|1||CLUTCH FAILED – OPEN||Clutch is unplugged or wire to control box has been cut (<1.0A).|
|2||CLUTCH FAILED – SHORT||Clutch has shorted or wire to control box is shorted (>10.0A).|
|3||CLUTCH CURRENT TOO LOW||Under-current reading on clutch (1.0-2.0A).|
|4||CLUTCH CURRENT TOO HIGH||Over-current reading on clutch (5.0-10.0A).|
|5||COMP TEMP SENSOR FAILED – OPEN||Temperature sensor on the compressor is unplugged or wire to control box has been cut.|
|6||COMP TEMP SENSOR FAILED – SHORT||Temperature sensor on the compressor is shorted.|
|7||COMP TOO COLD XX.X°F/XX.X°C||Compressor is too cold to start. The temperature is displayed (<-40°C/104°F).|
|8||COMP TOO HOT XX.X°F/XX.X° C||Compressor is too hot to start. The temperature is displayed (>130°C/266°F).|
|9||CAB TEMP SENSOR FAILED – OPEN||Accessory temperature sensor in the cab is unplugged, enabled but not installed, or wire to control box has been cut.|
|10||CAB TEMP SENSOR FAILED – SHORT||Accessory temperature sensor in the cab is shorted.|
|12||AIR PRESSURE SIGNAL VOLTAGE LOW||Low voltage reading on the air pressure transducer (signal <0.5V).|
|13||AIR PRESSURE SIGNAL VOLTAGE HIGH||Over voltage reading on the air pressure transducer (signal >4.6V).|
|14||AIR PRESSURE SENSOR TOO HIGH||Air pressure is higher than expected (>180PSI).|
|14||AIR PRESSURE XXX.X PSI||If the previous error is present, the air pressure is displayed.|
|22||PARK BRAKE NOT ENABLED||Park brake needs to be engaged or wire is disconnected.|
|26||ENGINE RPM TOO LOW TO START||Engine RPM is too low to start system.|
|27||ENGINE RPM DURING OFF STATE||Engine RPM detected during an OFF state.|
|34||ENGINE RPM NOT DETECTED||Engine RPM is not detected. Wire may be cut.|
|35||ENGINE CRANK TIME OUT||The engine did not start during the crank cycle.|
|41||CLOSE HOOD TO START||The truck’s hood is open. It must be closed to operate the system.|
|-||DISPLAY CANBUS CONN ERROR||A CAN bus error occurred between the control box and display box.|
|-||End of Errors OK to Retry||End of errors screen. Pressing ENTER will try to restart the system.|
If you have questions about any of the control box error messages, including inquiries on how to resolve them, please visit the Knowledge Base or call VMAC’s technical support team at
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