ABS Diagnostics

[rangerj]

The following is some BASIC information, and some BASIC testing that can be done, regarding the Anti-Lock Brake System (ABS).

The word BASIC is capitalized because the Ford Helm Service Manual has approximately 97 pages in the ABS section, and 80+ of those pages are test proceedures. Most of the proceedures require the "Super Star II (Rotunda) tester.

The ABS system consists of the following components;
Vacuum booster and master cylinder assembly,
Hydraulic Control Unit (HCU),
Electronic Control Unit (ECU),
Wheel Sensors,
Pedal Travel Switch.

There are two brake system WARNING lamps. The red "BRAKE" warning lamp will go on when the parking brake is applied or when the master cylinder resevoir is low on fluid.

The Fluid Level Indicator triggers the red "BRAKE" lamp if the fluid gets below a predetermined point. If the parking brake is not on, and the parking brake switch is not stuck or grounded, then a leak in the system is the most likely cause of the red "BRAKE" light going "ON".
This will usually trigger the AMBER ABS light also.

The AMBER "CHECK ANTI-LOCK BRAKE" warning lamp will come on for numerous reasons. It tells you that there is a failure in the brake system, AND that the Anti-Lock brake system has been turned OFF. When the Anti-Lock Brake System is OFF, your brakes operate as a non ABS hydraulic brake system.

If only the AMBER ABS light is ON you have working non-ABS brakes. If both the Rad BRAKE light, AND the AMBER ABS lights are ON, You have a brake system problem that needs attention ASAP.

PRE-TEST CHECKS:
1.) Verify that the parking brake is fully released. If the red BRAKE light does not go out check the switch.

2.) Check the brake fluid level. Brake fluid does not evaporate, so if the fluid level is low find out WHY. Is there a leak? The low fluid level indicator (sensor) will also trigger the AMBER ABS warning light.

3.) Verify that all of the following connectors are connected and that the terminals are secure in the connectors:
a) 55-pin connector of the ABS computer module.

b) 19-pin connector of the HCU valve body

c) 4-pin and 7-pin connectors of the pump motor relay.

d) 3-pin connector of the master cylinder resivoir.

e) 2-pin connector of the HCU resivoir.

f) 5-pin connector of the main power relay.

g. 2-pin connector of each wheel speed sensor.

h) 2-pin connector of the pedal travel switch.

i) 2-pin connector of stop lamp switch.

4.) Ensure that all fuses and diods are not damaged.

5.) Check Battery (Full charge, no shorted or dead cells, etc)

6.) Check ground connections for anti-lock system located near the computer module and pump motor relay.

It is important for you to look at a (your) service manual and identify the components and connections, and their locations, for your model year. You will also need the electrical wiring diagrams for your model year SHO. (The SLO/Sable have some differences)

Most public libraries have Automotive service manuals in the reference section, for example Chiltons, Mitchels, Motors, Haynes, etc. Bizzy has graciously provided wiring diagrams that you can download (I think, Bizzy is that right?).You will need the diagrams to locate the right "pins", connectors, wire colors, grounds, etc.

ON BOARD SELF TESTS:

Each time you start the car the systems does a "self test". The AMBER ABS light and the red BRAKE light come on for a few seconds, and then go out, if all is well.

There is another, more formal "Self Test" that can be performed with a Rotunda SUPER STAR II Tester 007-00041, or its equivalent. If you have one of these, or access to one, you would not be reading this so I'll skip this section.

The following will be a few simple tests, for ohms resistance and continuity (short, open, or grounded wires, components, or connections).

At each axel is a sensor and a sensor ring. The ring looks like a gear. Check the ring for broken teeth, a break in the ring, a missing ring, or any other damage.

Check the wheel bearings for excess "play" or wabble, and check to see if the ring rotates through approximately the middle of the sensor. I have seen several that have moved outward on the C/V joint, and the teeth hit the steering knuckle causing damage to the teeth or breaking of the teeth.

Testing the sensors (left front, right front, left rear, right rear:

At the connector nearest to the sensor, that is the sensor plug, disconnect the sensor plug.
Measure the resistance, in ohms, at the terminals or "pins". The reading should be between 800 and 1400 ohms (0.8 to 1.4k ohms). Any other reading and the sensor should be replaced.

Next, check for continuity between each sensor plug pin (sensor side) and vehicle (body) ground. If you get continuity at either pin replace the sensor (it is grounded out).

If the sensor is OK, you should check continuity between the harness at the 55-pin connector and the sensor connector at the harness end for the sensor you just tested. (Is a wire shorted, broken, or have a poor connection?)

You will need to look up the two pin numbers and their location at the 55-pin connector in the wiring diagrams. It helps to confirm the pins by using the wire colors. Again, this information could vary between model years.

That is the first installment regarding the ABS diagnostics. You can now test the sensors at the wheels and the cables from the 55-pin connector to the sensors THROUGH any connectors in between.

More than half of the ABS "trouble codes" are related to the sensors, and sensor problems are the most common cause of the ABS AMBER light staying on. That is why I chose to address the sensor tests in this first installment.

I'll dig through the ABS information I have and see if there are more test that can be done by the average "Shade Tree Mechanic" with simple inexpensive diagnostic tools (multi-meter re. volts A/C & D/C, ohms resistance, Amps, and continuity).

I would encourage any other forum member to call to our attention any errors in the above (other than spelling errors and typo's), and add anything that might be helpfull.

Would the forum administrators consider this for posting in the technical reference section?

The next installment will be for the pump motor, pump relay, pedal travel switch, and fluid level indicator. There will be more continuity and ohms tests.

Does any of the forum contributors know how we can do the various "Volts" tests without going through the Super Star II. If battery voltage is applied incorrectly it could ruin the ABS computer, so I am reluctant to post these tests.

I'll see what I can find out from my local Ford Dealer's service technitions. Don't laugh, I know a few who own and work on their own SHOs, AND know their "stuff". Not all of them are "parts replacement artist". rangerj

 

[Bizzy]

Thanks rangerj for the helpful info. To the top and featured! thumb

 

[pjtoledo]

Hey RangerJ, sorry I didn't introduce myself at the meet last week. I didn't get around the other table much. I did want to meet you. Anyway, on many dual braking systems a loss of fluid in part of the system will allow the master cylinder pistons to travel too far and cause a plunger to lock in a detent or something to seal off the bad part of the system. Usually turns on a brake warning lamp too. Do our systems have this feature?

Perry

 

[rangerj]

Perry,

I am also sorry I did not get a chance to meet you. I got embroiled in a fascinating conversation about an engine build.

Anyway, because our cars and their systems span 11 model years, I'll qualify my answer regarding our brake systems.

The answer to your question lies in the following quote from the 1990 Helm Shop Manual;
"The Fluid Level Indicator replaces the pressure differential valve used in the previous brake systems. It is contained inside the body of the master cylinder plastic resevoir and activates the brake warning lamp whenever fluid level is low".

The 1989s may have a pressure differential valve in the proportioning valve, or elsewhere, but I do not know this for sure.

Many of the old proportioning valves were made so that the lack of pressure in one of the lines (keep in mind the rear brakes were supplied with one line which was split left and right), due to a leak, would allow the pressure side to force an internal piston over to the low pressure side, and seal off that line (one of the fronts or THE rear) , that is not allow any fluid to be directed to the leaking line.

When this happened the brake lamp switch would be triggered. After the failed line, cylinder, hose, caliper, etc. was repaired the warning lamp switch had to be removed, and the proportioning valve had to be reset.

The other system you are referring to was a physical limitation in the master cylinder that limited the travel of the master cylinder pistons (duel piston master cylinder). IMHO this never did serve a worthwhile purpose.

Our system is a duel piston master cylinder, with a rear brake (pressure) proportioning valve, that is split diagonally, that is the primary piston supplies pressure to one front wheel and the opposite rear wheel, and the secondary piston supplies pressure to the other front and rear wheels.

The warning about the loss of fluid is provided by the fluid level indicator. The RED BRAKE light will be illuminated if the fluid level gets below a preset point. Note that the illumination of the red BRAKE light will also turn on the AMBER CHECK ABS BRAKES light, AND the ABS brakes system will be shut OFF.

If you look at the master cylinder there are four brake lines coming out of it, two out of the primary piston end, and two out of the secondary piston end. There are no "common" lines like there was in the old days.

The point is that you do not have to worry about pumping all of the fluid out of both sides (primary or secondary), as long as one of the master cylinder pistons is working.

So, are all of the model years from 1990 forward the same? shrug The way the Helm is worded implies that the 89 was different. If I remember right the 89 did not have ABS brakes (not sure).
rangerj

 

[NWGRN94MTX]

Ranger,

Good response. I have a question about the Rotunda ABS scanner, Do you know if you have to use the Rotunda scanner to bleed the ABS system. Snap-on, MAC and OTC all offer ABS/Air Bag Scanners. But do these readers allow for bleeding the system?


(I also posted this question on the "Pedal to the floor" posting)

Thanks

 

[rangerj]

NWGRN94MTX,

I'm only familiar with the Rotunda Super Star II, but I would think that the others are capable of doing the bleeding setting.

If you are going to buy one of these units make sure that it is capable of setting the ABS in the "Bleed" mode.

I find it very difficult to justify owning one of those units because the use is so infrequent, and the cost is high. It is the kind of tool that a group, such as a club, could buy and make available for use by the members.

I think you will find that the OTC tool will be the least expensive, and OTC more than likely manufactures the tools for MAC and Snap-On. Snap-On sells great tools but they are very overpriced IMHO.

I find it is cheaper to pay a dealership service department $100 every few years to bleed the ABS. I pump fresh fluid through the four brake lines and calipers, then have the dealership push my fluid (Castrol GT LMA) through the ABS, by doing the ABS bleeding process.

If you are going to buy an ABS scanner you should also find out about the "programs" for the different manufactures (Ford, GM, Chrysler, etc) that come with it, or cost extra.

Then you will also need connection cables for the different manufactures. And last, but not least, are the program updates for additional model years.

IMHO these scanners are not worth owning unless you do a lot of commercial brake work.

Hope this helps, rangerj

 

[Shuey]

i tested a '93 MTX abs with a Ford Code Reader. i was not able to receive any code at all. any ideas what's wrong?

thanks

 

[rangerj]

We need more information. Is the ABS light on? Is the red brake light on also? what problems are being experienced. What tester did you use, The Rotunda Star Tester II?

 

[Shuey]

I used a Ford Code Reader.

well, after idling for a while and driving the sho around, i tested the ABS with the code reader. No ABS lights or brake lights. maybe the bulb is out?

worse come to worse, the bulb is out and the sensors need placing, which is not too bad to replace.

 

[rangerj]

A "code reader" will not read the codes in the ABS computer! If the ABS light is not "ON" check the bulb.

When you first start the car the ABS light should light up for a few seconds while the computer does a self test. Does yours do that? If it does, then the bulb is OK.

What makes you think you have an ABS problem? rangerj

 

[Shuey]

this code reader has that capability. the abs codes are also defined in the manual it came with.

i was just pulling codes from the eec, i then decided to do the abs as well, thats all.

 

[rangerj]

OK, you are able to read the ABS codes. Did you find any codes stored in the ABS ECU (Electronic Control Unit aka the computer)? Whether the light works, or not, will not affect your being able to read the codes if there are any stored.

If you have an ABS failure and you are not getting any codes from the ECU, then there is a possibility the ECU has failed. Does your scanner have the ability to run the ECU through a "self test"? rangerj

 

[SHOfun 93]

Sorry to bring this back from the dead...

After the 96 Upgrade was completed, I have noticed that a couple times upon startup the ABS light will come on and after I shut the car off and restart, it will go back off again. Yesterday it came on and stayed on. Obviously something is up so I will pull the codes today and let you guys know what I find. The above info is very helpful, but I tell you I am no Ford Master mechanic by any means, and some of that first post intimidates the crap out of me!

Also, the previous rotors were rusted on and I had to bang on them pretty hard to get them off. Could I have damaged some ABS stuff while doing that? I will check the fluid level today also...