What’s Your UNDERHOOD™ Control Box Telling You?

underhood control box

UNDERHOOD™ Control Box

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
UNDERHOOD™ – 40
UNDERHOOD™ – Green Series

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…

Service Alerts

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.

System Temperatures

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.

Error Logs

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.

Warning/Information Messages

Control Box MessageExplanationCompressor State
HRS:XXXX:XXMain Screen hour meterOn/Off
Park BrakePark Brake is not applied or bad signalOff
HRS:XXXX200HRSVC200HR service is neededOn/Off
HRS:XXXX400HRSVC400HR service is neededOn/Off
COMP TOO COLDCompressor too cold for operationOn
TEMP XXXF/XXXCDisplays elevated temperature 130°C (266°F) to 150°C (300°F)On/Off
Wait xx SecondsA 30 second delay to allow for compressed air blow downOff

Error Codes

CodeDisplayFaultLED CodePossible Problem(s)
01OVER TEMPCompressor 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.
-Non-standard application.
02TEMP PROBE SHORTShort 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.
03TEMP PROBE OPENOpen circuit in temp probe circuit Flash code
1 red LED
-Faulty temp probe.
-Broken temp probe wires.
-Unconnected temp probe.
04BATTERY LOWA 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
05CLUTCH HIGHToo high of current draw on clutch 5-10A Flash code
2 RED LED
-Faulty clutch.
-Broken or pinched clutch wire.
06CLUTCH LOWToo low of current draw on clutch <2A RED LED
ON
-Faulty clutch.
-Broken or pinched clutch wire.
-Disconnected clutch wire.
07CLUTCH SHORTToo 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 #ErrorDescription
1Clutch OpenNo current detected flowing to the clutch
2Clutch ShortWiring to the clutch or coil has shorted to ground
5Compressor Temperature Probe OpenCompressor temperature probe disconnected or failed
6Compressor Temperature Probe ShortCompressor temperature probe wires shorted or failed
8Compressor Temperature Probe HotCompressor has shut down due to compressor over temperature (145 °C / 293 °F)

UNDERHOOD™ Green Series Control Box

underhood green series control box

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:

SettingDescriptionDefaultAdjustment Options
Standby DelayDelay time to standby10 minutes1, 2, 5, 10, 15, 30 minutes, Never
System PressureOperating system pressure150 PSI80 to 175 PSI @ 5 PSI increments
Top-up PressurePSI below system pressure to allow inlet to open and build pressure5 PSI1 to 10 PSI @ 1 PSI increments
High Air Use RateAir rate set-point for pressure-based restart-3.0 PSI/sec0 to -30 PSI @ 0.5 PSI increments
Restart Pressure PSIAir pressure set-point for pressure-based restart120 PSI80 to 175 PSI @ 5 PSI increments
Disable Auto Restart DelayTimer for disabling automatic engine restart30 minutesAlways, 30, 60, 90, 120 minutes, Never
Low Battery RestartControls if system will restart when battery voltage gets too lowEnabledEnabled or Disabled
Batt Lower ThrsRestart point for battery voltage12.1V10.0 to 14.0V in 0.1V increments
Batt Upper ThrsFully-charged set point for battery voltage12.8V10.0 to 14.0V in 0.1V increments
Low Batt Restart DelayTime delay before going back to STANDBY after battery fully charged2 minutes2, 5, 10, 15, 20, 30 minutes
Cab Temp RestartControls if system will restart when cab temperatures drops too low (accessory kit required)DisabledEnabled or Disabled
Cab Lower ThrsRestart cab temperature10°C/50°F-10°C/14°F to +20°C/68°F in 0.1°C/1.18°F increments
Cab Upper ThrsFully warmed cab temperature20°C/68°F10°C/14°F to +20°C/86°F in 0.1°C/1.18°F increments
Low Cab Restart DelayTime delay before going back to STANDBY after warming cab5 minutes2, 5, 10, 15, 20, 30 minutes

Warning/Information Messages

The following information messages have been programmed to show on the UNDERHOOD™ Green Series air compressor display box:

Error CodeMessage DisplayedCause
32BATTERY VOLTAGE TOO LOWThe battery voltage is too low (<11.8V).
-200HR/6MTH SERV DUE IN xx HOURSThis warning will show up 10 hours before the 200-hour service interval.
54COMPRESSOR SERV 200HR/6MTHThe system is due for a service. This warning will show after every 200 hours of running the compressor. Clearing the compressor service reminder is required through the Diagnostic menu.
-400HR/12MTH SER DUE IN xx HOURSThis warning will show up 10 hours before the 400-hour service interval.
55COMPRESSOR SERV 400HR/12MTHThe system is due for a service. This warning will show after every 400 hours of running the compressor. Clearing the compressor service reminder is required through the Diagnostic menu.
62LOW BATT RESTARTThe truck failed to start when requested to due to a low battery voltage condition.
63CAB TEMP RESTARTThe truck failed to start when requested to due to the cab temperature getting too cold.

Error Codes

Error CodeMessage DisplayedCause
1CLUTCH FAILED – OPENClutch is unplugged or wire to control box has been cut (<1.0A).
2CLUTCH FAILED – SHORTClutch has shorted or wire to control box is shorted (>10.0A).
3CLUTCH CURRENT TOO LOWUnder-current reading on clutch (1.0-2.0A).
4CLUTCH CURRENT TOO HIGHOver-current reading on clutch (5.0-10.0A).
5COMP TEMP SENSOR FAILED – OPENTemperature sensor on the compressor is unplugged or wire to control box has been cut.
6COMP TEMP SENSOR FAILED – SHORTTemperature sensor on the compressor is shorted.
7COMP TOO COLD XX.X°F/XX.X°CCompressor is too cold to start. The temperature is displayed (<-40°C/104°F).
8COMP TOO HOT XX.X°F/XX.X° CCompressor is too hot to start. The temperature is displayed (>130°C/266°F).
9CAB TEMP SENSOR FAILED – OPENAccessory temperature sensor in the cab is unplugged, enabled but not installed, or wire to control box has been cut.
10CAB TEMP SENSOR FAILED – SHORTAccessory temperature sensor in the cab is shorted.
12AIR PRESSURE SIGNAL VOLTAGE LOWLow voltage reading on the air pressure transducer (signal <0.5V).
13AIR PRESSURE SIGNAL VOLTAGE HIGHOver voltage reading on the air pressure transducer (signal >4.6V).
14AIR PRESSURE SENSOR TOO HIGHAir pressure is higher than expected (>180PSI).
14AIR PRESSURE XXX.X PSIIf the previous error is present, the air pressure is displayed.
22PARK BRAKE NOT ENABLEDPark brake needs to be engaged or wire is disconnected.
26ENGINE RPM TOO LOW TO STARTEngine RPM is too low to start system.
27ENGINE RPM DURING OFF STATEEngine RPM detected during an OFF state.
34ENGINE RPM NOT DETECTEDEngine RPM is not detected. Wire may be cut.
35ENGINE CRANK TIME OUTThe engine did not start during the crank cycle.
41CLOSE HOOD TO STARTThe truck’s hood is open. It must be closed to operate the system.
-DISPLAY CANBUS CONN ERRORA CAN bus error occurred between the control box and display box.
-End of Errors OK to RetryEnd 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
1-888-241-2289.

Discover More Articles In Our Control Box Series