IK1900251 ­ LT Daycab ­ HVAC Performance Concerns

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Countries:

CANADA, UNITED STATES, MEXICO Document ID: IK1900251

Availability:

ISIS, NotSIR

Revision:

1

Major System:

ACCESSORIES

Created:

10/22/2017

Current Language: English

Last Modified: 10/30/2017

Other Languages: NONE

Author:

Viewed:

Jon Pierce

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Title : LT Daycab - HVAC Performance Concerns
Applies To : LT daycabs only

CHANGE LOG
Please refer to the change log text box below for recent changes to this article:
010/22/2017 - Initial Article Release

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DESCRIPTION
This document will guide the user through identifying a customer's concern regarding HVAC performance under certain driving
conditions.

SYMPTOM(s)
Vent temps rise due to extended compressor off times.
Diagnostic Trouble Code(s) & Dashboard Indicator Light(s):
For the scope of this article, fault codes do not apply and it is assumed that all other repairs have already been completed and you are
trying to verify the customer's true complaint.
Customer Observations or Concerns:
The HVAC quits working at somepoint durring the day. They are concerned that the vent temps do not remain stable durring the day and
there is a signifigant fluctuation in vent temps noticed.

SPECIAL TOOL(s) / SOFTWARE
Diamond Logic Builder

SERVICE PARTS INFORMATION
Not applicable

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DIAGNOSTIC STEP(s)

Under certain situations such as high idle (>1000 RPMS) and the engine fan cycles on and off aggressivly and there isn't any good air
flow across the condenser, overall performance of the HVAC system on the daycab may not be as optimal as the customer would like.
This concern can also be duplicated when the customer is sitting in heavy traffic. Always perform complete performance diagnostics on
the HVAC system and make sure that your customer's fresh air filter and recirc filter are not clogged with debris.
If the body controller disengages the compressor clutch because of the high or low pressure switch, the clutch will remain off for 14
seconds after the switches close again. ­ Potential cause for increased cab temps. No resolution at this time.
Durring a request to disengage because of the evap temp, evap core temp will below 42.8 F and remain off until the evap reaches 48F.
If the evap temp rises in under 8 seconds, the clutch will remain off until 8 seconds have passed.

Raising Vent temps from extended compressor off times are cause by:
Low Pressure Switch Cycling, you described this pretty well, fan kicks on and pulls down quickly causing compressor to cycle off LP
switch 2­3 times.
Compressor staying off until Max Initial Head Pressure is below 250 psig.

Step Action
DIAGNOSTIC:
Any active diagnostic faults pertaining to the a/c system?

#1

Decision
Yes. Refer to appropriate service manual,
diagnose and repair the faults.

No. Proceed to next step.

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IK1900251 ­ LT Daycab ­ HVAC Performance Concerns

Step Action
DIAGNOSTIC:
Run performance tests on the a/c system per the LT service manual. Any issue
discovered?

#2

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Decision
Yes: Repair and retest.

No: Refer to graph section below for
explanations.

Figure #: 1 Low Pressure Switch cutout
Item 1: Low pressure switch went from closed to open.
In this example the fan engaged (yellow line) and due to the sudden change in air flow across the condenser the low
side actually dropped so rapidly that the low pressure switch (orange line) opened up and requested the clutch to
disengage (Purple line). As the event started, evap temperatures (grey line) began to rise from the upper 40's to the

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low 60's. As the fan stayed engaged the low pressure dropped out multiple times and the evap temp continued to rise
as a result. Once the system got stable air flow across the condenser the system began to perform much better.

For graphing purposes; in the actual raw data a "1" was replaced with either a "50" or a "100" so it could be viewed easier in the graph.

Figure #:2 Freeze Probe Requesting the Clutch to Disengage
In this example the evap dropped below 42.8F and the clutch did not reengage until the evap temp
reached 48 degrees. The cycle repeats each time the evap temp drops. This is a pretty good
snapshot as the evap temp remains right in between the low and high setpoints.

REPAIR STEP(s)
If the customer is not satisfied with how their system operates, especially in heavy traffic:
1. Advise customer to engage the fan manually while sitting in traffic. The constant air flow will help keep the evap temps down.
a. Consider adding a manual switch if the unit is not already equipped with one.
2. Make sure the driver is using the HVAC system in "max" mode as much as possible.
3. If the customer does not accept that resolution create a case file with tech services and they can possibly adjust some parameters to
get a little better performance.
a. When opening a tech service case file, an A/C performance snapshot must be attached when submitting the case file.
4. Engineering is activley investigating this concern and there will be more updates in the future.

INSTALLATION PROCEDURE:

WARRANTY INFORMATION

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Warranty Claim Coding:
Refer to the Warranty Coding Manual for Group and Noun Codes.

Standard Repair Time(s):
Refer to the SRT Manual for Repair Times

OTHER RESOURCES
HVAC Resource Center
Body Controller Resource Center

Figure #3 - Additional Example
you can see in this example due to the extended off time for the compressor to recover vent temps began to rise. This would have
caused a noticable difference in temp at the vents.
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