Many 85-92 Mark VII’s have a very simple yet mysterious way to turn on the ABS warning light and disable your ABS brakes instantly.



Introduction

What I found to be the common problem of ABS problems in Mark VII’s are the ABS wheel speed sensors. These sensors are the primary feed from the wheels to the ABS computer, which then calculates the rotation of each wheel to see which one is locking and which one is not so that the proper valve in the ABS valve body gets engaged and you don't skid into the bushes or another vehicle on a slippery surface.

The most important thing to know about ABS wheel speed sensors is that they are permanent magnets - meaning that they collect metal chips and other unnecessary debris that is made up of metal. Brake pad dust will stick to the sensor and prevent it from reading the sensor trigger which is wheel mounted on the rotor/hub/drum assembly.

Cleaning your sensors will in most cases will fix your ABS problem. If cleaning doesn’t do it then you have to test them.

NOTE: Test the ABS sensors first, even if you retrieve error codes from the ABS computer. It will display codes stored in memory first, even before current error codes.

Tools You Will Need

To test the ABS Wheel Speed Sensor(s) you will need:

• DVOM (Digital Volt/Ohm Meter)

Each sensor has it's own location. The front right plug is located on the front right shock tower. The front left plug is located on the front left shock tower. The rear left/right plugs are located behind the rear seat (under the carpet in the rear-most place in the trunk).

NOTE: You will have to trace by wire the front left ABS sensor plug from the bottom to the top next to the ABS pump assembly.

--- CAUTION --- REMOVE YOUR KEYS FROM THE IGNITION AND PUT THEM IN YOUR POCKET BEFORE DOING ANYTHING TO PREVENT ELECTRICAL DAMAGE TO ANY COMPONENTS.

Let’s Get Started

Disconnect the ABS sensor plug, set the DVOM to the 'Ohms' resistance setting, and measure the resistance between the two wires in the sensor plug. The reading must be '800-1400 Ohms' in resistance. If the DVOM reading is NOT steady in range, then the sensor has an open circuit and it must be replaced. Before replacing the faulty sensor, please take a good look at the wires to see if they are cracked or broken. If the wires are cut, simply strip the insulation and solder them together. It's just another simple copper wire with extra ground next to it.

To Get Codes From The ABS Computer

If all is ok, when you turn the ignition key to ON the ABS indicator light will come on for a few seconds, and then go out. If it stays on, then the power-on diagnostics have detected a problem.

To Read The Error Codes From The ABS Unit

Ground the orange/white wire at the ABS diagnostic connector (with the ignition turned OFF), and then turn the ignition key to ON. The ABS indicator light will flash the number of times to indicate the first digit of the first error code, followed by a short pause, and then flash the number of times to indicate the second digit of the first error code.

There will then be a longer pause, then it will repeat for the second error code, and so forth. The ABS unit holds the error codes in memory for a certain number of power-on/power-off cycles even if the problem has been corrected. This messed me up trying to hunt down problems that weren't there. So, as long as the indicator light goes out after a few seconds following ignition ON, the system is fine, even though you can still read error codes out in diagnostic mode. Look at the diagram below to see which wire to ground.



After you get your code(s), take a look at the list below to find out what your ABS computer is pointing you to.

11, 12, 88 & 99	ECM fault
21	Main valve
22	LF inlet valve
23	LF outlet valve
24	RF inlet valve
25	RF outlet valve
26	Rear inlet valve
27	Rear outlet valve & ground
31 to 48	Wheel sensors must be checked
51	LF outlet valve
52	RF outlet valve
53 & 54	Rear outlet valve
55 to 78	Wheel sensors must be checked

I hope this will resolve problems or restore your ABS braking in your Mark VII as it did in mine, and many others.


Good luck -

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