SUMMARY
This article describes how to fix a chiller with a microprocessor that is not receiving proper input signal from the pressure transducer.

SYMPTOMS
Microprocessor shows an “Err” on the top screen and a pressure transducer error code on the bottom screen. Pressure transducer errors include:

"tL1" for compressor 1 low-pressure sensor error.

"tL2" for compressor 2 low-pressure sensor error.

"tH1" for compressor 1 high-pressure sensor error.

"tH2" for compressor 2 high-pressure sensor error.

CAUSE
The Microprocessor is not receiving proper input signal from the Texas Instruments (TI) pressure transducer(s).

RESOLUTION
Several items, that are easy to check as follows, can cause transducer errors:

*** Before starting, make a note of THE transducer error code you are reading in the lower microprocessor screen.
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Suction / Discharge Transducer Terminal Identification:

RED (Top) = "+5" Terminal on Board
WHT (Mid) = "Sig" Terminal on Board
BLK (Btm) = "COM" Terminal on Board

1. With power OFF to the chiller: Check wire connections from all transducers at the back of microprocessor board. Proper locations and color-codes for transducer wires can be found on the chiller wiring diagram located on the back of the electrical compartment door.

2. With power OFF to the chiller: Remove the molded plug from each transducer, blow out any dust or dirt and reconnect plug.

Note: In most cases, the steps above will correct the problem. After performing the above steps, turn the chiller and microprocessor on and check for the error. If the transducer error code returns, proceed to the following steps.

3. With power OFF to the chiller: Using the codes above, determine the physical location of the transducer that is producing the error and remove the “White” signal wire for that transducer from the back of the microprocessor.

4. Turn the power ON to the chiller and turn the Microprocessor ON. Using an AC volt meter make sure you have from 24 Volts AC and 28 Volts AC between the “24 VAC” and “COM” terminals at the back of the microprocessor control. If you do not have AC power in the range above, STOP as further troubleshooting of the chillers control system will need to be performed.

5. Using a DC voltmeter in the DC scale, measure volts between the “+5” and “COM” terminals at the back of the microprocessor. You should read 4.8 to 5.2 Volts DC. If you read NO volts or volts below 4.8, your microprocessor is defective and needs to be replaced.

6. Using a DC voltmeter in the DC scale, measure volts between the “+5” and “SIG” terminals at the back of the microprocessor. You should read 2.0 to 5.0 Volts DC. If you are not reading voltage between 2.0 to 5.0 Volts DC, then your pressure transducer or gray connection cable are not allowing proper signal to return to the microprocessor board.

7: Remove the connection cable plug from the transducer creating the error and swap the cable with another transducer cable on the system. If the error code moves with the cable swap, then the problem is in the cable. If the error code remains at the original location then the problem is in the transducer.

8. If the problem is in the cable, re-inspect the plug end. If everything seems to be in order, you will need to order a replacement cable.

9. If the problem is with the pressure transducer:

- Remove cable plug from the pressure transducer
- Using TWO wrenches, of the proper sizes, remove the transducer and inspect the inlet port. Make sure the internal (refrigerant side) of the device is clean and free of debris. Using WD-40 or ,other cleaning agent, clean the transducer. Once clean, re-install and test. If error code returns, the transducer must be replaced. WHEN ORDERING MAKE SURE TO REQUEST EITHER A HIGHSIDE OR LOWSIDE TRANSDUCER. THE TWO ARE DIFFERENT.

10. If you are getting intermittent "ERR" codes and intermittent "lo1","lo2","hi1" or "hi2" pressure trips, you may have a transducer that is getting ready to fail.

If this condition is suspected:

A. Put the micro-processor in review mode by pressing the "SET" button.

B. Scroll by pressing the UP and DWN buttons until you get to the "lo1","lo2","hi1" or "hi2" values on the top of the screen. The value in the bottom screen will be the pressure the micro-process is seeing.

C. Using your manifold gauge, verify that your micro-process pressures are within 1-3 PSI of what your manifold gauge is reading for the correct refrigeration ckt. If they are not, use the following formulas to confirm that you have a problem with your transducer

> High Pressure Transducer Formula: PSIG = 125 x DC volts between COM and SIG minus 62.5

> Low Pressure Transducer Formula: PSIG = 50 x DC volts between COM and SIG minus 39.7

Using these formulas you can you can determine what the micro-pressure readings should be based on you signal voltage.

If the micro is reading the correct pressure for the voltage returning, than you have confirmed the transducer or the cable is not working correctly.

If the micro is reading an incorrect pressure for the voltage returning, than you have confirmed the transducer is OK and the micro needs to be replaced.

** Procedure ONLY to use in an emergency ** Note: Should equipment damage occur while this procedure is deployed, Legacy Chiller Systems will NOT warranty damage or be held responsible for direct or indirect property damage or personal injury that can happen. ***

Procedure: Installation of a 10k ohm resistor between the +5 and Signal connections should result in a pressure reading within mid range of the transducers rated pressure scale. This can be done on either high or low pressure transducers. Adjustment of pressures in programming mode will be needed. This procedure should only be used as a temporary fix until factory replacement parts can be installed.

APPLIES TO
All chillers systems with PSG microprocessor control.