To Solenoid Valve

Red Wire
16 AWG.

Red Wire
(Terminal 87a)
16 AWG. +12v DC

D

Black Wire
(Terminal 85)
16 AWG. GROUND

NOTES
1. Electric Override Switch is supplied with all Outside-Mounted air kits.

Red Wire
16 AWG.

OM-H719

Black Wire
16 AWG.

2. For default to lift kits including HAC-MCA and HAC-MSO,
remove red wire E and discard.
3. For non-steer, do not connect wires A (Terminal 86) or wire E (Terminal 87).
This disables lift in reverse function but maintains Lift Override function.

15

Plumbing & Wiring Diagram

HAC Air Kits
SECTION 7

Suspension Operation
RAISING YOUR LIFT AXLE
1.
2.
3.
4.
5.

If vehicle is already running, please proceed to Step 6.
Set vehicle parking brakes.
Turn your vehicle ignition to on position.
Press start switch and release when engine is started.
Allow the vehicle to idle until the vehicle air system pressure has reached the compressor cut-out pressure (usually 120 psi).
6. If controls are Inside-Mounted – Pull the lift / lower control knob out to immediately lift the axle.

7. If controls are Outside-Mounted – Ensure the vehicle is stopped and parking brake is set. Exit the vehicle,
go to air control enclosure and open it. Pull the lift / lower control knob out to immediately lift the axle.
■■ Equipped with Electric Override Switch Part No. R-009791 – Turning the switch to the ON position
will override the push/pull knob and raise the axle. Turning the switch off will immediately redeploy
the axle. Likewise terminating power to the switch, such as turning the vehicle off will immediately
redeploy the axle.
8. Visually confirm the axle is lifting.
NOTE

Vehicle system air pressure may drop during suspension lifting process.
9. Lift axle should be completely lifted when the vehicle’s air system pressure returns to the air compressor cut-out point (usually 120 psi).
10. Hendrickson recommends raising the lift axle when not in use and when off-road.

LOWERING YOUR LIFT AXLE
1. If the vehicle is already running, please proceed to Step 6.
2. Set the vehicle parking brake.
3. Turn your vehicle ignition to the ON position.
4. Press the start switch and release when engine has started.
5. Allow the vehicle to idle until the vehicle air system pressure has reached the compressor cut-out (usually 120 psi).
DO NOT LOWER LIFT AXLE WHILE THE VEHICLE IS MOVING IN REVERSE OR TRAVELING AT MORE THAN 15 MPH.
FAILURE TO DO SO CAN CAUSE COMPONENT DAMAGE.

6. If the controls are Inside-Mounted — Push the lift / lower control knob in to immediately lower the axle.
7. If the controls are Outside-Mounted — Ensure the vehicle is stopped and the parking brake is set. Exit
the vehicle, go to air control enclosure and open it. Push the lift / lower control knob in to immediately
lower the axle.
■■ Equipped with an Electric Override Switch Part No. R-009791 — If the axle fails to deploy toggle
the override switch in the cab to the OFF position.
8. Using the regulator, adjust air pressure on the gauge to the appropriate air pressure for vehicle load
conditions. See air pressure load charts in the applicable Hendrickson Auxiliary Lift Axle Owner’s Manual
Literature No. OM-H754 (Steerable) or Literature No. OM-H757 (Non‑Steerable).
NOTE

Air system pressure may drop during the suspension lowering process.
9. Lift Axle should be completely lowered and supporting the pre-determined load when system air
compressor cut-out point is reached (usually at 120 psi).

Suspension Operation

16

OM-H719

HAC Air Kits
SECTION 8

Preventive Maintenance
HENDRICKSON RECOMMENDED MAINTENANCE INTERVALS
FIGURE 8-1
■■

Air Control Filter System – Every six
months remove the filter (Part No.
R-013462) from the Air Control Filter
System, see Figure 8-1. Visually inspect
the filter for contaminants and replace
as necessary to help ensure efficient
operation. An indication that the filter may need replacement is when a
notable reduction in the axle retract or
deployment time has occurred.

AIR CONTROL SYSTEM - DISASSEMBLED

FILTER
Part No. R-013462

FIGURE 8-2
■■

Spool – Lubricate as needed. If the
valve is leaking air or failing to change
state from lifted to lowered the spool
(Part No. R-009796-Z) may need to
be lubricated or replaced. Refer to
the Troubleshooting Guide in this
publication.

SPOOL
Part No. R-009796-Z

SECTION 9

Tech Tips
SUBJECT

TECH TIPS

1.

HAC-UCO & HAC-USO

Air kit knob must be in and lift axle down before activating override switch.

2.

Gauge Pressure

a. The circuit schematic indicates that the gauge is reading the regulator’s output pressure.
b. When the valve is in “lift mode”, the regulator output port is still under pressure but is
blocked at the main valve. This way, the operator always knows what the ride spring pressure
is going to be when the control shifts from “lift mode” to “ride mode”.
c. When the main valve is shifted to “ride mode”, the pressure indicated on the gauge is
exactly what is in the ride springs. Otherwise, what the gauge indicates is what will be in the
ride springs once they have been lowered.
d. The gauge is a constant indicator of the pressure that will be going to the ride springs.

OM-H719

17

Preventive Maintenance

HAC Air Kits
SECTION 10

Frequently Asked Questions
NOTE

All brake plumbing installations must adhere to FMVSS-121 regulations. Modification to a vehicle’s pneumatic system may alter it’s compliance to FMVSS-121 regulations.

QUESTION

RESPONSE

1

Where do I install my
Hendrickson air kit
control panel?

The Hendrickson air kit product line is available for Inside- and Outside-Mounting. Your particular
application will be dictated by regulations in your state(s) of operation.

2

My axle goes down after
the truck is shut down
and it raises the entire
chassis. Can I make
the axle stay in the up
position?

This symptom is an indication that the Electric Override Switch maybe in the ON position and the
outside the cab lift axle control knob is pushed in to the lower position. Hendrickson recommends
■■ If controls are Inside-Mounted – Pull the lift / lower control knob out to immediately lift the axle.
■■ If controls are Outside-Mounted – Ensure the vehicle is stopped and parking brake is set. Exit the
vehicle, go to air control enclosure and open it. Pull the lift / lower control knob out to immediately
lift the axle.
There are two (2) options to have the lift axle stay in the UP position without having to exit the vehicle
to operate an Outside-Mounted control.
Option 1: With your current air kit installed, rewire the +12v DC power source from the switch. Connect it to
the battery instead of accessory ON power. Amp draw on battery is 0.4a or 4.8 watts.
Option 2: Install the HAC-MCA air control, which is designed to default to the lifted position with the
power off. Changing to the HAC-MCA will keep the axle up when the vehicle is shut down

3

Can I plumb my air kit inline with my ride height
control valve?

Complete vehicle assessment should be conducted considering primary suspension load share and
vehicle handling performance. Specific instances can be reviewed by Hendrickson Technical Service
Department.

4

At what pressure should
the regulator be set?

Typically 70‑120 psi. – All new lift axle and air control installations should be verified at a certified
scale to determine correct air pressures vehicle loading. See air pressure load charts in the
applicable Hendrickson Auxiliary Lift Axle Owner’s Manual Literature No. OM-H754 (Steerable) or
Literature No. OM-H757 (Non-steerable). Improper vehicle loading can cause handling irregularities
and component damage.

5

How do I identify my air
kit?

For Inside-Mounted air kits, the identification tag should be located on the top of the control module.
For Outside-Mounted air kits, the identification tag should be located on the inside of the door panel.

6

Why do steerable
suspensions require auto
lift in reverse?

Due to the positive caster angle built into Hendrickson steerable lift axles they are designed to track
with the vehicle in forward motion. Moving in reverse the wheels will lock sideways and scrub the tires
imparting high stress on the axle and leading to potential damage.

7

How does the reverse
locking option engage?

The reverse locking option requires both an axle with the lock straight optional components installed
and an air control specified to control the lock straight. The relay in the air control kit is wired to sense
the activation of the reverse lights when the vehicle transitions to reverse gear. The relay will energize
the lift axle control solenoid to switch the valve to lift mode.

9

Why does my pressure
gauge show ride pressure
when axle is raised?

The gauge will show a constant value for pressure dialed into the regulator. This makes technicians and
operators aware of both ride pressure and stored pressure, which can cause severe personal injury and
component damage. Refer to the Tech Tips Section in this publication for more information.

10

How do I determine the
functionality of each kit
component?

■■
■■
■■
■■
■■
■■
■■

Frequently Asked Questions

Green fitting to supply
Actuator knob pulled out for lift axle raise
Actuator knob pushed in for lift axle lower
Blue fitting to lift springs
Red fitting to ride springs
Yellow exhaust for lift springs
Black fitting for applicable lock components

18

OM-H719

HAC Air Kits
SECTION 11

Troubleshooting Guide
HAC AIR KITS
TROUBLESHOOTING GUIDE
PROBLEM

POSSIBLE CAUSE
Lack of air kit lubrication
Air control system not properly installed
Solenoid on air kit control panel is not
being energized

Automatic lift
does not function
or manual lift
does not function
with push / pull
valve on air kit
control panel

Does not lower or
does not inflate
ride springs
with panel knob
pushed in

OM-H719

Kinked, pinched, or broken air line between
air kit control panel and lift springs
Kinked, pinched, or broken air line between
air kit control panel and supply tank

CORRECTION
Service air kit by removing and replacing the spool with Part No.
R-009796-Z or adding lubrication to existing spool (Dow III only) with Part
No. R-010880.
Connect black wire to vehicle ground and black wire to backup light power
wire. Confirm +12v DC signal when reverse lights are on.

Replace pinched, kinked or broken air lines.

Supply air pressure insufficient to operate
lift mechanism

Verify that you are receiving 100 psi minimum at the lift axle control panel.
Use calibrated gauge at supply line inlet.

Non-functioning Quick-Exhaust valves
between ride springs

A non-functioning Quick-Exhaust valve can keep air trapped in ride springs.
Engaged ride springs will overpower the lift springs to keep axle down.
Replace non-functioning Quick-Exhaust valve.

Exhaust port on back of air kit control panel
plugged

Remove obstruction to exhaust port on back of air control panel.

Air kit control panel not properly plumbed

Confirm that the air kit control panel is plumbed per the appropriate
diagram in this book.

Non-functioning air kit control panel

Call Hendrickson Warranty Department.

Lack of air kit lubrication due to
contamination

Service air kit by removing and replacing the spool with Part No.
R-009796-Z or adding lubrication to existing spool (Dow III only) with Part
No. R-010880.

Regulator turned down too low

Increase air pressure at regulator until desired load is carried at wheels.

Truck in reverse gear

Place transmission in forward gear or neutral.

Solenoid valve on back of air kit control
panel energized

Place transmission in forward gear or neutral. If problem persists verify the
source of the solenoid power is a reverse signal and not some alternate
source.

Kinked, pinched, or broken air line between
air kit control panel and ride springs
Kinked, pinched, or broken air line between
air kit control panel and supply tank

Replace pinched, kinked or broken air lines.

Supply air pressure insufficient to operate
lower mechanism

Verify that you are receiving 100 psi minimum at the lift axle control panel.
Use calibrated gauge at supply line inlet.

Exhaust port on back of air kit control panel
plugged

Remove obstruction to exhaust port on back of air control panel.

Air kit control panel not properly plumbed

Confirm that the air kit control panel is plumbed per the appropriate
diagram in this book.

Non-functioning air kit control panel

Call Hendrickson Warranty Department.

19

Troubleshooting Guide

HAC Air Kits
HAC AIR KITS
TROUBLESHOOTING GUIDE
PROBLEM

Slow lift or lower
times

Quick-Exhaust
valve inoperable

Suspension does
not carry rated
load

Lift axle control is
leaking air

POSSIBLE CAUSE

CORRECTION

Lack of air kit lubrication due to
contamination

Service air kit by removing and replacing the spool with Part No.
R-009796-Z or adding lubrication to existing spool (Dow III only) with Part
No. R-010880.

Insufficient air flow or volume being
delivered to air kit control panel

Increase incoming air line size or increase air reservoir capacity.

Insufficient air flow or volume being
delivered to the air springs

Increase air line size from 3⁄8" to ½" going to air spring air lines.

Insufficient ride spring exhaust through
Quick‑Exhaust valve.

Verify Quick-Exhaust valve is operating correctly. Move Quick-Exhaust valve
closer to the ride springs. If problem persists install one (1) Quick-Exhaust
valve per air spring to increase exhaust flow.

Clogged or plugged quick-exhaust valves

Remove obstruction or replace quick-exhaust valve, Part No. R-004348-1.

Insufficient air pressure in ride springs

Increase pressure in the ride springs by increasing the regulator setting.
Check pressure in the ride springs at air spring inlet.

Supply air pressure insufficient to carry
rated load

Verify that you are receiving 100 psi minimum at the lift axle control panel.
Use calibrated gauge at supply line inlet.

Kinked, pinched or broken hose between air
kit control panel and ride springs
Kinked, pinched or broken hose between air
kit control panel and supply tank

Replace pinched, kinked or broken air lines.

Non-functioning air kit control panel

Call Hendrickson Warranty Department.

Incorrect lift axle ride height

Call Hendrickson Technical Service Department.

Leaking from little red button on the
solenoid

The red button is a small on / off valve. Using a small flat head screw driver
turn the red valve from the open position to closed. O / I markings molded
into the solenoid indicate the positions.

Leaking from an air line fitting

Remove the air line and reinstall confirming the line is fully seated. If
problem persists, cut off a short section of line per the direction on line
cutting and reinstall to confirm a clean cut that will interface properly with
the fitting.

Leaking from the exhaust port

The spool inside the valve may be stuck in the incorrect position or shifted
part way between positions due to lack of lubrication or swollen O-rings.
The spool O-rings may also be damaged. Service the spool with Service Kit
R-009796-Z.

Leaking from the regulator

Faulty regulator. Return kit to Hendrickson for service replacement.

www.hendrickson-intl.com

Specialty Products - Auxiliary Axle Systems
Hendrickson Canada
277 North High Street
250 Chrysler Drive, Unit #3
Hebron, OH 43025-8008 USA
Brampton, ON L6S 6B6 Canada
1.800.660.2829 (Toll-free U.S. and Canada)
905.789.1030
1.740.929.5600 (Outside U.S. and Canada)
Fax 905.789.1033
Fax 1.740.929.5601

OM-H719 Rev D 08-18

Information contained in this literature was accurate at the time of publication. Product changes may have been made after the copyright date that are not reflected.
© 2018 Hendrickson USA, L.L.C. All Rights Reserved

Printed in United States of America