abs module

[smdk2500]

So i was driving my 95 with the 6.5 today. As soon as i let of the throttle to stop I'm pretty sure that I heard the abs module cycle with out pressing the brake pedal. When I did go to hit the petal it was hard as a brick for about 1/4 inch down then went to its normal pressure. If the module did do its thing why would it do it before I even touched the pedal? I did have the abs light come on couple days ago and has been off since. I don't have my scanner to check for abs codes. I will be getting it later today. Does anyone have any thoughts?

 

[Chewy11B]

Sounds like a speed sensor losing signal. If module see a sensor drop it will activate the abs thinking the wheel locked up.

Have a scanner to watch values??

 

[delta_p]

The basic operation is the arming of the electronic brake module by receipt of a brake switch activation input, and then the comparison between each wheel speed sensor signal vs. the vehicle speed sensor signal. The ABS won't activate without a brake switch input. check there first.

 

[Supercharged111]

Do these things warm up? I know on my Corvette and Envoy the module will do weird **** when you first start moving sometimes.

 

[smdk2500]

well drove over to get my scanner i had the abs module activate again like it did earlier and i noticed that i had my foot slightly on the petal. I had boots on earlier and this time shoes and i could feel the pedal through the sole this time so im sure that is what happened the first time. Any way when i shut the key of and start it again the abs light goes out. After I went through the procedure of hooking up to pull the code it showed a RF wheel sensor. I didn't catch whether it was a loss of signal or erratic signal before it switched to no codes present.

I tried reading the live data but the screen would be up for about 4 seconds and it would go to a no communication screen. I went to my old job and used there scanner and got the same results on the live data. Any one know why it would only let me read live data for 3-4 seconds before it would go away. I can read engine and transmission data live with no problems with my scanner and my old jobs scanner but cannot read abs info.

Another thing can the module cause one brake to not work at all? When i headed home I stepped on the brake and the truck pulled badly to the left like the right front brake was not doing anything. But I did not hear the abs module and did not feel anything in the pedal like I have in other vehicles when the abs kicks in.

 

[delta_p]

Another thing can the module cause one brake to not work at all? When i headed home I stepped on the brake and the truck pulled badly to the left like the right front brake was not doing anything. But I did not hear the abs module and did not feel anything in the pedal like I have in other vehicles when the abs kicks in.

It might feel like that, but if it is the ABS on that wheel it is attempting to get the wheel spinning again at the desire relative between the wheel sensor signal and the vehicle speed sensor signal. And it will feel out of sync with the other wheel operating off the master cylinder fluid path.



The ABS, if activated on the circuit for a particular front wheel, is trying to get the wheel to recover to its desired speed. There are two solenoid valves per front wheel in the module, and a single pump for both wheels. One valve normally isolates the ABS path from the normal master cylinder path to the wheel, so you just operate off the master cylinder and U foot on the pedal in normal mode. Under ABS active, the iso valve closes and now your on the ABS circuit. The other valve is dumping off the locked up fluid (due to the iso valve closing) to an accumulator closing back again to allow the pump to build pressure. The two valves work in conjunction with each other along with a pump to pulse on and off for the dumping and reapplication of pressure to the wheel.

So the scenario goes like this. You hit the brakes and wheel relative signal for one wheel isn't what the computer likes and it goes into ABS. It closes the isolation valve to get off the master cylinders building of high pressure due to your foot on the pedal. You are now on the ABS circuit for that wheel. But the computer still doesn't like the signal and thinks the wheel is locking up, so it dumps the ABS pressure to try and get the wheel going again (usually rapid on/offs of the dump valve). The dumped fluid also feeds a pump which builds pressure against the iso valve. The pump output path is also in MC path to the iso valve. The pedal can feel mushy as the pump operates and takes fluid from the MC. The iso valve will also pulse open to allow this reapply of pressure, and the dump valve will pulse to dump pressure. All of this to try and get the wheel speed back but closely controlling brake pressure it needs to apply braking force. The ABS wants the wheel rolling but it also wants to have pressure available to slow the wheel when it needs it.

 

[smdk2500]

It might feel like that, but if it is the ABS on that wheel it is attempting to get the wheel spinning again at the desire relative between the wheel sensor signal and the vehicle speed sensor signal. And it will feel out of sync with the other wheel operating off the master cylinder fluid path.



The ABS, if activated on the circuit for a particular front wheel, is trying to get the wheel to recover to its desired speed. There are two solenoid valves per front wheel in the module, and a single pump for both wheels. One valve normally isolates the ABS path from the normal master cylinder path to the wheel, so you just operate off the master cylinder and U foot on the pedal in normal mode. Under ABS active, the iso valve closes and now your on the ABS circuit. The other valve is dumping off the locked up fluid (due to the iso valve closing) to an accumulator closing back again to allow the pump to build pressure. The two valves work in conjunction with each other along with a pump to pulse on and off for the dumping and reapplication of pressure to the wheel.

So the scenario goes like this. You hit the brakes and wheel relative signal for one wheel isn't what the computer likes and it goes into ABS. It closes the isolation valve to get off the master cylinders building of high pressure due to your foot on the pedal. You are now on the ABS circuit for that wheel. But the computer still doesn't like the signal and thinks the wheel is locking up, so it dumps the ABS pressure to try and get the wheel going again (usually rapid on/offs of the dump valve). The dumped fluid also feeds a pump which builds pressure against the iso valve. The pump output path is also in MC path to the iso valve. The pedal can feel mushy as the pump operates and takes fluid from the MC. The iso valve will also pulse open to allow this reapply of pressure, and the dump valve will pulse to dump pressure. All of this to try and get the wheel speed back but closely controlling brake pressure it needs to apply braking force. The ABS wants the wheel rolling but it also wants to have pressure available to slow the wheel when it needs it.

Thank you for giving me a better idea of how the system works. I know what it does and didn't fully understand how it did it. I knew there where valves and such in the module but didn't know how they all worked together

 

[delta_p]

Probably the reason your pedal felt hard at first is because when the iso valve closed and your MC output and your foot to the pedal is against the closed iso valve and locked fluid. Then it got soft when the pump engaged.