ABS Warning light

[skrappi]

The ABS (Wheel sensor?) indicator on my 97 came on after the front hubs were replaced in a shop. Long story short, they aren't fixing anything and it is now on me. The light was not on before. Is it just a connector that is loose at the hub? Exactly what would I look for?

Thanks!

[abaird]

Well if a connector was loose that would be easy enough. Otherwards you will need a scan tool to help pinpoint the problem. Sounds like kind of a shade tree Joe kinda shop.

[Negatoro]

Exactly what would I look for?

You should be able to check the connectors with the car on the ground -- turn the wheels all the way to see behind the knuckles.

There is one sensor per side in the front, each contained within the wheel bearing/hub assembly. Wires from the sensor run through to the inboard side [behind wheel] of, and clip onto, the steering knuckle.

Seperate each connector, inspect contacts, clip connector back together using dielectric grease between the plastic halves [not the electrical contacts] to keep water out.



-Mark T.

[Ed-o]

My 2001 LS also has the ABS warning light on. I checked and cleaned both front connectors, and it's still on. I will check and clean the rear ones as soon as this cold front passes by and I can get outside again. I was wondering if there is a way to analyze which sensor is the bad one? Can it be sucessfully diagnosed with an ohms meter or something? If so,, how? I would like to get my can back to it's orginal operating condition and keep it runnning great. Feel free to post here or email me @ [email protected] Thanks in advance.

P.S. I also had the A/C cutting out and the lights blinking. I did the contact clean job according to the post and it works great ! This board rocks ! I am glad I did a search and found it ! Thanks again !

[madmedix]

My light came on too. Sensors replaced. Still lit. Turns out the wiring harness has a break in it from the right front to the junction block (which conveniently snakes around the front of the engine...) on the left side of the engine bay. Gobble a couple of Tums before you ask how much that wiring harness is . Only GM makes it. At least I got some of the $$ back from the sensor job.

Cheers,
Andy

[Ed-o]

Andy,
How did you know which sensor to replace or did you just go for a crapshoot and pick one ? I hear the front ones are expensive since you have to replace the whole bearing hub, but what about the rear? I might be able to check my rears tomorrow if it warms up more. I'd kinda like to find a broken wire in mine. I think I would just splice in a fixer wire without replacing the whole harness. This site is really helpful, and I am sure my car loves the extra attention ! Thanks for the reply.

[Negatoro]

Originally Posted by Ed-o

I think I would just splice in a fixer wire without replacing the whole harness.

[madmedix]

The codes popped up for the left sensor. replaced. About a week and a half later (about 480 miles); the abs light came on again. This time code for the right one. changed. Another 800 miles (I um, have a long commute) and the light comes on again.
Another mechanic (don't ask) tells me there's somethin' fishy going on here whilst he is working on the problem. The nasty part of this is that the abs sensor wires (both sides) worm into a rather large wiring loom just inside the engine compartment. On my car, anyway, that is one HUGE job to get into that loom and isolate the abs wires to the junction blocks. Perfectly suited to someone with 8 knuckles per finger, if you know what I mean. I'm going to tackle it when I can get near a hoist for a spell to make the (splice) job easier. It's been said this is a delicate (physically and regarding RF Interference). If that's true, why isn't it given a dedicated bus with shielded wiring...anyone?

[Blue Bowtie]

If the sensors are properly connected and functioning, they are actually fairly noise immune. The signal is at a relatively low potential but is referenced to a bias voltage above ground, so noise is reasonably easy to ignore (again, so long as the sensor circuits are complete). Oxygen sensors have operated reliably this way (and at a lower bias voltage) for over 25 years now with no special shielding. I'm guessing that the "technician" who made that remark may have also been guessing, but, I digress.

Since we have about 4-5 different vehicles being referenced in the thread, I'm going to pick on the original poster's 1997 Malibu, just for clarity.

The '97 should have the second series ABS Delco VI system. To determine if the problem is a wheel speed sensor or something else (system power, EBCM, motors, sensor wiring, or solenoids) you need to monitor the ABS warning lamp at system start-up.

The EBCM will also turn on this light as a self-test when the vehicle is started, and how the lamp responds is important:

1. The ANTI-LOCK or ABS light will illuminate for about 3 seconds when the ignition is first turned to the KOEO (Key On, Engine Off) position but the engine is not started. The warning lamp should turn on. During that 3-4 seconds, you should hear the ABS motors run and solenoids cycle, after which the light should turn off and remain off;
2. This ABS light will again turn on while the ignition switch is in the crank (engine start) position;
3. The ABS light will remain on for about 3-4 seconds after the engine starts, whereupon you should again hear the modulator motors run briefly, then the ABS lamp should turn off and remain off.

If the system is responding in that manner, and the ABS warning only turns on once the vehicle is moving at 3-5 MPH, the problem is most likely a wheel speed sensor not generating signal.

If the system warning lamp turns on, the motors run, the light turns off, then immediately turns back on (before moving the vehicle) the problem is more likely a circuit to a wheel speed sensor (disconnected/open, or shorted).

Which situation are you experiencing?

If the lamp remains off until the vehicle is moved at 3-5 MPH, you can test speed sensor outputs one at a time.

1. Raise a wheel and support is safely so that it is free to spin.
2. Disconnect the electrical connector to the wheel speed sensor and connect your AC voltmeter to the sensor pigtail (not the vehicle side of the harness).
3. CAREFULLY (meaning, keep your fingers and meter leads clear) rotate the wheel at about 5 MPH (approximately 1 revolution per second, or 60 RPM). The transmission will have to be in neutral to allow spinning the front wheels on wrong-wheel-drive cars and the park brake released for the rear wheels.
4. Monitor the output signal level at 60 RPM, then at about 120 RPM. Record the reading on your notepad.
5. Reconnect the wheel speed sensor electrical connector and move to the next wheel.

Front speed sensors should generate a minimum of 0.90VAC at 60 RPM (one rev per second). Rear sensors should generate a minimum of 0.70VAC at that rotational speed. Even if the signals are a bit low, all is not lost, I've seen them function properly all the way down to 0.58V and not cause error codes. If the sensors will produce about 1.0V or greater at 120 RPM, they are probably adequate. If you find one wheel generating no signal,or a significantly low signal, there is likely a problem at that sensor or tone ring.

If the signals are present and adequate, the next step is to test the wiring between the wheel sensors and EBCM. Most problems occur right at the wheel connectors and in the first couple feet of wiring from the connector. Not surprisingly, this is the portion of the wiring which is always moving,and is most subjected to road moisture and debris. Replacement connectors are available, and any wire harness damage can be repaired by soldering in new wiring. If you don't intend solder and properly seal the wires, forget about the repair and take is to a professional, or you'll be chasing signal problems forever - Save the crimp connectors for temporarily installing U-Haul trailer wiring.

If the connectors and exposed harness are intact, test continuity between each sensor wire and the EBCM. That's going to take some long meter leads, but there aren't many other options.

[Ed-o]

Wow,, Thanks Blue Bowtie ! That's exactly the kind of information I was looking for. Sounds like you know your stuff.. When doing the individual wheel sensor tests, does this hold true for the 2001 LS also? that's the car I have. As soon as it get's warm again.. (Next week) I am going to get out and do the tests you talked about. This is exactly the kind of information I like to gather from this board. I'm pretty excited to get out and see how mine check.. This was definaltely a good post.. Thanks a million.. If there are any differences between the 1997 you wrote about and the 2001, please inform me about it.. I'm looking forwards to finding my problem and getting it fixed.. Thanks again..

[Blue Bowtie]

Ed-o,

The speed sensor output voltages for your '01 should be the same.

You're certainly an optimistic one. "Warmer next week"? It's supposed to be snowing again here by Wednesday. Where is Al Gore, err, "Mr. Internet Inventor/Global Warming enthusiast" right now? I want to beat him with my heating bill (taped to a pipe).

[Ed-o]

Blue Bowtie,
I just got back in from running a quick test on the first part of the post you made. On my 2001, I turned the key to run position, (engine not cranked or running) and the light came on but never went off. I waited over 30 seconds and it never went off.. I then started the car, and once again the light stayed on and after about 5 to 10 seconds, my in dash beeper went off alerting me there was a warning light on.. The ABS light never went off at all. I also never heard the motors cycle like you talked about.. I remember before whenall was working good turning on the key on and hearing a motor clycle. I always thought it was the fuel pump pressureing up, but apparently not huh! The car was dead quiet the whole time when I checked it. I assume from what you posted, I must have a bad connector or broken wiring in my system. Is there anything else to check while I am tracing wires? I'd like to have a list of things to look for while I am witch hunting wires. This post has really helped me understand what is going on and what to look for. I feel confident now that I can isolate my problem without spending money on un-needed parts. Thanks a ton. If you think of anything else please keep me informed. Oh,, BTW,,, Where is the block located that all the ABS sensor wires come together. I feel like I am ready to start ohm testing my wires unless you have somethng else in mind first.. I really appreciate this !

[Blue Bowtie]

First, please don't hold me responsible for any errors or omissions in what I am transcribing. I don't have all the diagnostic manuals in front of me, and the systems can get rather involved.

On initial ignition power-up, the fuel pump should cycle for about two seconds, but the ABS motor should also cycle briefly (and rather quietly). It's harder to hear the ABS motor run when starting the engine.

A few different scenarios may prevent the ABS motor from running:

• The lack of system power from the main relay will obviously stop the motor.
• A shorted or grounded wheel speed sensor circuit may stop the motor.
• An open or grounded brake pedal switch circuit can stop the motor.
• An open or grounded motor circuit or solenoid coil (or circuit) can set a fault code and prevent the motor from running.
• Some other hard fault codes in the EBCM can prevent motor running on power up.

SYSTEM POWER
There is a master ABS power relay and a pump motor relay in most installations. The main power relay is frequently located under the hood, inside the Underhood Electrical Center. (Older vehicles mounted the relay adjacent to the ABS hydraulic modulator unit.) The inside of the UEC cover should have a label indicating which relay is where. If you examine adjacent relays, you can often find another with the same part number and can swap them to see if that makes a difference. If the "other" device works but the ABS does not, chances are the relay was good. The EBCM operates the relay to provide main motor and solenoid power to the ABS modulator unit. One other check you can perform is to assure that battery voltage is present at the NO relay contact in the socket for the ABS relay. Obviously, lack of voltage there would interfere with ABS operation.

SENSOR INPUTS
Metering the sensors and wire harnesses at each wheel is easiest, and will reveal the condition of both the sensor signal level and wire harness. With the ignition off, you can check resistance to ground on the sensor harness wires to help reveal any cuts of other harness damage. With the ignition ON, and if the EBCM is powered, you should be able to measure a DC bias voltage across the two harness wires, but I'm not certain what that voltage is.

OTHER INPUTS
Check the fluid level switch, even of the red BRAKE warning lamp is not on. There are probably two sets of contacts in the switch, one N.O. and one N.C. to indicate switch position. The N.C. contacts are typically used to signal the EBCM whether fluid level is correct, and an open circuit here from a wiring problem or defective switch will not necessarily cause the red brake warning lamp to turn on.

Check the brake pedal arm switch for continuity. You may also want to insure that the center, high mounted brake lamp is working, since that can affect other systems on the vehicle, such as the cruise function and possibly the ABS.

OUTPUTS
There is at least one main valve solenoid on the ABS modulator unit, and up to six control valve solenoids. Disconnect the electrical connector and check the resistance of the solenoid coil windings one at a time , then the coil resistance to a good ground. The coils should present about 2-5 ohms through the coil, and show infinite resistance to ground. You can also test the solenoid coil wire harness for grounding if the ignition power is off.

There are many more checks which can be performed, but those get rather involved and nearly require a scanner.

MISCELLANEOUS PROBLEMS
I'm not positive on the '01 Malibu, but the EBCM is likely located mounted in the left front fender area near the ABS modulator. It's probably easier to access by removing the inner wheel opening splash shield, but you may want to save that job for later since there will be little you can do there without a scanner. One problem with that mounting location is that acidic battery fluid can drip or be washed down onto the module and create a lot of corrosion at the connections. Additional road spray can also find its way to the module.

Another, more obscure problem which can occur with this EBCM is a supply voltage problem caused by a failing alternator. If a rectifier in the alternator bridge is breaking down and allowing some reverse current flow, some AC voltage can be present in the system. Even relatively low levels of AC voltage can interfere with EBCM operation and inexplicably set error codes. You can test the output of the alternator directly at the output stud for AC voltage over 0.2VAC. At that level, problems can occur. Obviously, that is not likely the problem unless a hard code in EBCM history is causing the unit to shut down on power-up, since there should be nothing but clean DC voltage from the battery in a KOEO situation.

[Ed-o]

Blue Bowtie,
Thanks for the list of checks and information. I now have my work cut out for me and am sure with time and patients, I can find my problem thanks to you. I'm kinda looking forwards to discovering what the real problem is and getting it fixed. I was just wondering, since you seem so packed with valuable information, do you work as a mechanic at a dealership? If so where? You really seem to know what you are talking about and I think I can learn alot from you. I really appreciate your help and your detailed explainations. I will keep you posted on how this turns out, and very well may be asking you for more advice soon.. Thanks again dude,,

[Blue Bowtie]

No dealership. I work on everything. Chryslers, Hondas, Isuzus, Fords, Mitsubishis, Nissans, VWs, whatever, That's why I have six Chevys. Not to say that GM doesn't have some really questionable vehicle offerings, but generally speaking... I'll leave it at that.