ABS MOD I
TROUBLESHOOTING GUIDE
L20302
Rev. 11/03
No special diagnostic tools needed. For trailers built between 2/28/98 and 2/28/01.
Before Getting Started
This guide lists the most typical installations/problems only. Consult Haldex Brake Systems for anything not addressed here.
Power is required at stop light circuit, pin 4, and air is needed to both the supply and service gladhands.
Do NOT use a battery charger to power the ABS.
A Volt/Ohm Meter is needed for some of the troubleshooting procedures.
Before disconnecting anything, remove electrical power and air pressure from the system.
The numbers in parentheses ( ) refer to the wiring diagram shown on the reverse side.
Start Here
Don’t overlook the obvious. Check to make sure that you have power and that everything is connected properly.
I. ABS Light Comes On And Goes Off
While the trailer is sitting still only, there is also a valve blow down (air exhausts from ABS relay valve).
This is normal and indicates that the ABS system is Ok. The ABS light will come on and go off. The valve blow down is an audio check
of the solenoids (located on the top of the ABS relay valve). If the ABS is powered up when traveling above 6 mph, the ABS light will
stay off unless there is a fault.
II. ABS Light Never Comes On Or Is Very Dim
This troubleshooting procedure requires that electrical power be connected. Older tractors power ABS through the stop light circuit
and the ABS light will come on when the brakes are applied.
1. Verify that there is adequate power at the trailer 7-way, pin 4 (1). There should be 12-14 volts for test purposes. If there is no
power then the tractor wiring or coiled power cable needs repair.
If there is power at the 7-way, check for 10-14 volts at the power cord connector (2). If no power exists, then the trailer ABS
power cord needs repair. (Most trailers have integral ABS power in the main trailer wiring.)
2. Check the bulb to verify that it is functional. If not functional, replace it.
3. Disconnect the ABS wire harness at the power cord. Check for continuity through the light wiring (3). The resistance should
be 3-6 Ohms. If this is not the case, then the light wiring needs to be repaired. Check connections for excessive pin spread
and/or corrosion.
4. If the problem remains, disconnect the ABS ECU connector (4). Check for bent pins, repair and test. Use a Volt/Ohm Meter
to test continuity between ABS light and pins in ECU connector. See wiring diagram on reverse side for pin assignments.
Replace harness if bad, and ECU if problem continues to exist (5).
III. ABS Light Stays On When There Is Power To The ABS System
This troubleshooting procedure requires that power be applied to the trailer for 30 seconds/off to verify. The ABS is powered through
the auxiliary circuit whenever the key is in the “on” position and the stop light circuit with the brakes applied. NOTE: For some sensor
faults, the trailer must be driven above 6 mph for the light to go off.
1. If the light stays on, the ABS may be wired incorrectly. One of the ABS light wires should be grounded. (The ABS light ground
wire may attached to pin #C on the ABS power harnesses 5-pin connector.) The other ABS light wire is attached to pin #D on
the ABS power harness 5-pin connector. See wiring diagram on reverse side.
2. Verify the voltage at the 7-way (pin #4) is 12-14 volts and that a valve blow down occurs when powering the ABS. If there is no
blow down, then verify that air pressure was applied to the service brakes.
If there is air pressure to the service brakes, then disconnect the solenoid connector (6) and check the resistance at the solenoid
pins. Readings across the two bottom pins should be 7-9 Ohms. Readings between either bottom pin and the top in should be
3.5-4.5 Ohms. If resistance is okay, then check the female pins in the connector for excessive pin spread and/or corrosion.
Repair and test.
3. If there IS a valve blow down, then a sensor is the most likely issue. Sensor gap could be too large. Try pushing the sensors
in until they touch the exciter (7).
If the problem remains, disconnect the sensor connectors from the sensors and measure the resistance between the two pins
in the sensor connector housing (8). The reading for each sensor should be between ~980 and 2350 Ohms. Check for pin
spread and/or corrosion also. The sensor should be replaced, if an issue is found.
Measure the AC voltage at each sensor while rotating the wheel at a rate of ~1 revolution every two seconds. The reading
should average >.200 Volts. If this is not the case, replace the sensor.
If the problem persists, disconnect the ECU connector (4). Check for bent pins. Repair and retest. Use a Volt/Ohm Meter to
test continuity between pins in valve connector and ABS ECU connector. See wiring diagram on reverse side for pin assignments.
Replace harness if bad, and ECU if problem continues to exist (5).
IV. ABS Light Only Comes On Some Times
If the tractor has stoplight power only, and there is a fault, the light will only stay on with the brakes applied.
1. Intermittent problems are the most difficult to diagnose. The best place to start is at the speed sensors. Sensor gap could be
too large. Try pushing in the sensors until they touch the exciter (7).
Refer to Section III, Paragraph 3, on the reverse side, for further sensor related testing.
Next check for a loose sensor and excess wheel end play. The sensor should fit tight in its mounting. If the sensor feels loose,
replace the sensor clip. If wheel bearing end play is excessive, or if the wheel bearing is worn, the sensor can produce an
intermittent signal and cause a fault. Repair as needed and retest.
Other possible causes include tires that are significantly different in size or a damaged exciter. Inspect the exciter teeth (7)
closely. If any damage is found, replace the exciter.
2. If no problem is found, then the rest of the electrical system needs to be inspected. The most common cause of ABS faults
involve wires and connectors. This can include connectors not latched properly, bad splices, abrasion, spread or bent pins,
pin corrosion, and wire strand breaks. Check for these issues at each connection (1,2,3,4,6 & 8). Look closely at all splices.
Repair or replace as required.
V.
ABS Light Comes On When Powered Up, Goes
Off Above 6 mph, But Wheels Lock-Up
This indicates that the ABS is functioning properly electrically. ABS requires properly operating brakes to do its job.
Some things to consider.
1. MOD I is a 2 sensor, 1 modulator system. On a tandem or tri-axle trailer the ABS can’t “see” what the non-sensed axles are
doing (axles with no speed sensors). The ABS only knows what the sensed axle is doing. Because of this, the non-sensed
axle may lock during braking.
2. A dragging service or parking brake, a pinched or kinked delivery hose, or excess use of 90º fittings, can all cause wheel lock.
A perfectly functioning ABS system may not be able to overcome these issues. Examine the air installation and look for these
types of problems. Replace any defective hardware and retest.
3. If both sensors have excessive gap or if both exciters are missing, the ABS system will ALWAYS assume that the vehicle is
sitting still. There will be no indication of a fault. In this case, the ABS system can not respond to a locked wheel condition.
Try pushing in the sensors until they touch the exciter (7). Inspect for missing exciters.
4. If the problem is still present, contact Haldex Technical Service at (800) 643-2374, Prompt #2.
3
2
B
6
E C
Front View
A: +12V Stop light
B: +12V Ign perm
C: Cab lamp
D: Trailer Lamp
E: -12V Ground
Make sure connector is locked
into place (listen for two clicks
when connecting wire harness)
Hand tighten collar
5
FFABS Valve
With ECU
4
Solenoid Connector
1
2
1
A D
Power Connector
3
1: Solenoid ground
2: Dump solenoid
3: Hold solenoid
Standard (SAE J560)
Curb side 1A
Power cord
(see pin out
below)
red
yel/wht
cab mount
malfunction
lamp
7-Way
Connector
3
-12V Ground
Cab lamp
7
8
14
15
16
17
18
19
11
12
Wire harness
8
blue
Hold solenoid
Trailer lamp
Dump solenoid
Solenoid ground
Open
Open
Diagnostic BSensor 1B Lo
6
white
ECU Connector
2
8
Road side 1B
1
2
3
4
5
6
+12V Ign perm
Diagnostic B+
+12V Stop light
Open
Diagnostic input
Sensor 1A Hi
Diagnostic out
Sensor 1B Hi
Sensor 1A Lo
Exciter ring
Sensor
9
10
Sensor 1A
Connector
1 Hi
2 Lo
7
Diagnostic connection
1 Hi
2 Lo
Sensor 1B
Connector
ABS Light (Note: Light
is mounted on side of
trailer.)
Haldex Commercial Vehicle Systems
L20302W US 11/03 WEB ONLY
North American Sales Division
Haldex Brake Products Corporation
10707 N.W. Airworld Drive
Kansas City, MO 64153-1215
Phone: (816) 891-2470
Fax: (816) 880-9766
North American Sales Division
Haldex Limited
525 Southgate Drive, Unit 1
Guelph, Ontario CANADA N1G 3W6
Phone: (519) 826-7723
Fax: (519) 826-9497
www.haldex.com - www.hbsna.com