Cant Fix Brakes

zachary00 · May 13, 2011

I have posted on here about this problem, so if it makes you angry sorry, but some things have changed. My abs have been locking up at low speeds, so I took it in. Mechanic performed the necessary cleaning as the tsb indicates. I left the dealership and they still locked up. I turned around and he said one of the hubs was going out and when I fixed that, it would stop locking up. I replaced the hub and it didn't help. I took it back. The mechanic said the brake ecu was out and said when it was fixed the brakes should work. He replaced the ecu with no success. He checked the sensors and the new hub was reading low voltage (I watched this part and agreed). He suggested returning the new hub and replacing it. After the second hub of the same brand, it didn't work. I checked voltage with my multimeter. I can't read mV, but mine will read 200 v ac (I dont know how that translates). I read the right (old hub and read .9-1.1 . I check the left (new) hub and it read .3-.5. I took it back in and got a different brand that cost slightly more.. I know genuine gm is by far superior, but also by far more expensive, so I got another generic. I put it on and again it didn't work.

What is going on? I can't afford to take it back in and pay for another failed repair. Is it possible the old is reading to high? I am absolutely lost. The mechanic said it may need to be reprogrammed? Does this sound right. So far I am out $890 and no closer than I was before.

On a positive note, I have got to where I can completely replace a hub in under 20 minutes, taking my time.

Carder09 · May 13, 2011

Well this seems like a problem I might be able to help with, Im in the military and work F-16's and what your ABS is to us is the same as our wheel speed sensors, ABS for airplanes, Now It seems like you need to replace your ABS sensors, they detect wheel speed and send the voltages or differences of each wheel to the ECM and the ECM compensates and activates the brakes or slows the engine down, now if the sensor is faulty then it could possibly be activating your brakes suddenly, what you could do is isolate one end of the sensor and take readings by ohming them and getting the values or you could simply replace the sensors and then see if that helps, being that the mechanics are idiots and want more of your money now and days, see if that helps you out or not.

asc08 · May 13, 2011

350 millivolts AC is the same as .350 volts AC. My multimeter does not read millivolts, but it has a scale for 2 volts (the other scales on the meter are like 20 volts or 200 volts), and that will read down to .01 volts. If that is the case with your meter, you should be getting a reading of .35 volts or more for a good sensor. If not, you may want to get or borrow a meter that has a scale in the low end.

Did you take the truck into a dealer or a local, independent mechanic? I would recommend that you stop buying parts at this point, and see what the voltage readings are.

I cleaned the hub as per the TSB (with a wire brush and a piece of emery cloth), and got lazy and did not read the voltage when I was done. The problem persisted. I then re-did the procedure, checking voltage before and after. Before, t was less than .10, and after it was like .40 (on both wheels). The second time thru, I really got on them and polished them down to bare metal with a dremel tool.

zachary00 · May 13, 2011

the thing I haven't figure out that has me more confused than anything is how the old hub is reading so much higher than the new hub, with a new speed sensor. The only part I have actually paid for was at request of the gm mechanic. It was at a gm dealer wearing the nice gm mechanic shirt. I am concerned that the old sensor is reading to high and that is why the speeds are reading different, and the new sensors are actually right. Is that possible?

Carder09 · May 13, 2011

Is there a wire harness that goes from the ECU to all the hubs and sensors?

markw8 · May 13, 2011

I think you need to find a tech at a different dealer that knows how to trouble shoot.

Mark

05GMCZ71 · May 13, 2011

I agree, time to find an auto shop that knows brakes. I've found that many dealers don't know crap about brake system troubleshooting, only how to put on new pads and rotors. (many times when they don't even need them!)

asc08 · May 13, 2011

Is there a wire harness that goes from the ECU to all the hubs and sensors?

There is, but if the sensor is disconnected from the harness to check the voltage, and the voltage is reading bad, that is the first thing that needs to be looked at.

It has already been expensive for you, and will only get more expensive throwing parts at it. As others have stated, find a good technician who can diagnose this and get it resolved.

You may also want to get the service manager at the dealership involved. Spending $900 on an issue that is not fixed is unacceptable. If you read the posts on this site, the low speed activation of abs is not exactly a one in a million issue. I can guarantee you that the dealer's service department has seen numerous cases of this.

To answer you other question about re-programming, no it does not sound right.

zachary00 · May 13, 2011

After ANOTHER trip to shop I have a new report. I suspect they are right this time. They claim the old hub is reporting to high of voltage, and it isn't matching the other one. It is suspicious, because I had this hub before with no issues. It doesn't seem like it should have been that hard to find.

brianf60 · May 14, 2011

This isn't all that uncommon. Its best to replace both frt hubs at the same time. This way you will be sure to the same reading to the scanner. The only way to accuratly check the two hubs is to use a digital lab scope and do a dual trace ( view both signals at the same time) to see that when driving in a straight line the ABS module is seeing both wheels moving at the same speed. By using the scope you will also be able to see the wave form and se if there is anything abnormal ( and chipped tooth on the tone ring ect.) Chances are you don't have a scope, so buy 2 identical hubs and replace them both.

Goodluck!

GasHog369 · May 14, 2011

Had a very similar , costly problem 1-1/2 years ago when both abs sensors / wheel sensors went out in front... Had them replaced.

Then 4 months ago abs light came back on, took it in and the hub was bad... Replaced with timken and all good now. I found an excellent deal on a new timken on amazon with speed sensor in allready for $120... i have it as a spare now for other side.