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™ 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
-Faulty clutch.
-Broken or pinched clutch wire.
06CLUTCH LOWToo low of current draw on clutch <2A RED LED
-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)

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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