First, cut the electrical to the system off, tag out and lock it out. Safety first!  You can ohm out the compressor and check for good windings and ensure the compressor is not grounded, provided your volt/ohm meter is working properly and you have fresh new batteries in your meter.  Follow the fault in the literature. Once you have confirmed a good compressor electrically (hopefully), look back at the literature again. Follow the offerings in the literature under this fault:

“Alert A321: The cause of the alert is usually compressor wiring causing reverse rotation or a faulty compressor.”  

Begin asking questions to the owner such as:

• Has anything happened recently to the building electrical wiring or system electrical wiring?
• Has there been a recent thunder storm which knocked out electrical power in the area?
• Have you called an electrician to do some work on your building electrical systems or panel box? Or was a main electrical transformer changed by the local power company in the area?”

Next, what does the literature state?  

“The cause of the alert is usually compressor wiring causing reverse rotation or a faulty compressor.”  

Now begin checking for reverse rotation of compressor which could happen with one of these episodes described above that could have caused a change of the 3-phase building wiring.  So, reverse the line voltage by switching any two leads of the 3-phase wiring of the system at the system.  Cycle the system back on, and your system begins operating. After a time delay, the compressor begins operating. Your suction pressure and head pressures are beginning to look normal. You have made the repair. You are the Friday afternoon hero! You repaired the system and it is now cooling down your customer’s building.

So, the next time you get a Friday Afternoon call on a 48JC system, don’t panic. Remember go to HVAC Partners. Look up the literature for this system, find your fault listed on the system Vu, and make the repair.

So, what we learned with this lesson:

1. The SystemVu is common to 48JC systems
2. You can work thru your control issues on the SystemVu Control
3. You can find out and read your system faults on SystemVu
4. Go to HVAC Partners to find the systems literature
5. Most of the information you need will be located in HVAC Partners
6. Safety first: Tag out, Lock out the disconnect to any system you are working on
7. Always make sure your voltage/ohm meter has fresh new batteries before you start the day
8. “Fail To Pressure”, means the compressor wiring causing reverse rotation or a faulty compressor
9. Reversing any two wires on a 3-phase system will reverse rotation, if needed

Source:

1. https://www.carrier.com/commercial/en/us/systemvu.
2. HVAC Partners: 48/50JC 04-06 Ultra High Efficiency Single Package Rooftop Units with SystemVu™ 48JC0; page 34
3. Carrier Service Tech App