Hello all,
I'm in dire need of an adjustable circuit to simulate the square ware of a two wire ABS, so I can trick the ECU into thinking the vehicle is in a skid so the computer will activate the solenoids in the ABS Controller. This way I can bleed the ABS Controller of air. This can be done with a high dollar scan tool that does this function, problem is I'm not that rich. I know that I can simulate the square wave using an the Astable Operation of a 555 timer but after looking in the forum, under the VSS(Vehicle Speed Sensor) Simulator project help post, I'm a little concerned about some of the comments. I quit designing circuits back in 2001 after I had two stroke and have forgotten most everything, but continued reading, I seem to be recollecting some, but not all, so hence forth, I need help. The circuit in the fore mention post is kind of what I think I need, but I'm not sure, so any and all help would be very appreciative.

 

Welcome to AAC!
Do you want to simulate an analogue (variable reluctance) or digital (Hall effect) type of VSS signal?

 

These are the WSS (Wheel Speed Sensor), they are two wire. As each front wheel spins, a toothed ring gear on the wheel hub interrupts a magnetic field in the WSS, which in turn cause the WSS to generate an AC signal. The EBCM (Electronic Brake Control Module) then uses that AC signal to calculate the wheel speed. I'm trying to produce that AC signal and in explaining this I feel that the circuit that I showed only produces a DC signal and I need an AC signal. But to tour question, I'd like both and I think that it's back to the drawing board, with help, I hope.

 

No scan tool or other device needed for this. All that is needed is a open gravel road to bleed the ABS. I've done it a few times for cars I've fixed.

1. make sure there are no other vehicles around when doing it.
2. get the vehicle up to around 20mph, on the gravel road.
3. when up to speed, apply the brakes hard. This will make the ABS kick in.
4. repeat the above a few times, until the pedal is back to normal pressure hardness.

I'm lucky in that I have a private gravel lane going into the back of my property to do this on. But it does work, the idea came from a GM service manual, one from the factory, not a Haynes manual.

 

..... but if you do want a circuit to sim both types of sensor you could try this. Only one gate of six in the 74HC14 IC is used. With the component values shown you should get pulse frequencies adjustable from ~20Hz to ~150Hz. The analogue output is ~ 0.7V peak-to-peak: the digital one is 0v-5V.

 

To Shortbus, thanks for your advise. I've done this, but while the vehicle was on a lift. Didn't work all that great, still sponge!

Thanks Alec_t, I'll build this circuit and try it, again thanks. I'll look it over and see if I can duplicate you wave for sale in spice.

 

Here's my .asc file if it's of use :

 

I need 4 channels, would I repeat the circuit 4 times or add 4 outputs?

 

To Shortbus, thanks for your advise. I've done this, but while the vehicle was on a lift. Didn't work all that great, still sponge!

I don't know how you could do what I suggested on a lift? I would also be worried about injecting a 'foreign' signal into the brake module. You say you need an AC signal, but with the wheel sensor being a two wire type, shouldn't that just be a square wave output? Not AC which implies going from plus to negative? And you would need to figure out what the expected frequencies would be, and give two different frequencies to simulate a skid.

I've also found that with modern ABS brake systems you can't get a full pedal when doing the old fashion brake bleed way. By this I mean pumping the brake pedal and opening the bleeder. After fighting this too many times I finally broke down and bought a hand pressure bleeder. The Phoenix bleeder is hand pumped and bleeds from the wheel end of the system, driving all of the air in it toward the master cylinder. This means the air in the ABS module too. For brake jobs it is the best money I ever spent, and always get a good pedal after using it. The old style pressure bleeders worked from the master cylinder down, and they still allowed air to be trapped in the ABS. I just bought the cheapest one because didn't know if they would work, and I'm just working on mine and friends cars. But am glad I bought one.
http://www.brakebleeder.com/

 

Shortbus,
I put it into 4WD and took it up to 50 MPH then hit the brakes. The front wheels of the Tahoe are WSS and the rear wheels are VSS. The service tech manual say that the WSS generate and AC signal and looking at my AllData, they are two wire.





Now as far as bleeding the brakes, I used a very big vacuum bleeder, one that does what you have mentioned, but not having to squeeze a hand pump. I have found that as you do this, fluid will bypass air bubbles in the controller sine they are trapped by the solenoids, so are you telling me that you supply fluid to each wheel and pump it to the master cylinder?

 

I need 4 channels, would I repeat the circuit 4 times or add 4 outputs?

Do all 4 channels need the same signal?
Do they all feed into typical microcontroller analogue inputs?

 

Shortbus,
I put it into 4WD and took it up to 50 MPH then hit the brakes. The front wheels of the Tahoe are WSS and the rear wheels are VSS. The service tech manual say that the WSS generate and AC signal and looking at my AllData, they are two wire.

Now as far as bleeding the brakes, I used a very big vacuum bleeder, one that does what you have mentioned, but not having to squeeze a hand pump. I have found that as you do this, fluid will bypass air bubbles in the controller sine they are trapped by the solenoids, so are you telling me that you supply fluid to each wheel and pump it to the master cylinder?

Your talking two different sensors here. WSS - wheel speed sensor. VSS - vehicle speed sensor. VSS is an AC signal(usually but not always) WSS is for ABS, VSS is for traction control, and speedometer.

I'll bet you only have ABS on the front brakes. This is pretty common for pickup trucks, the rears aren't ABS because the bed is usually empty and will lockup the brakes when not hauling a load. People got upset when the ABS kicked in all the time so it was changed to ABS only in the front.

And as far as 50mph in 4wd to do the ABS bleed, WOW. No wonder it didn't work. You need to only go about 15 -20 mph in 2wd. But it has to be on a lose surface, like gravel over a packed substrate. Too fast and the brakes just lock and ABS won't kick in, hard surface and both wheels get the same braking force.

Like I said before a vacuum bleeder won't work. That's what I used to use, and no luck with ABS Has to be an injection bleeder, puts the fluid in at the caliper and drives it up and out of the system into the master.

 

Shortbus,
Thank you for the clarification on the bleeding process, I even thought about doing that, but on another note, It is a Tahoe not a pickup. As I said before and showed in the schematic diagram, the front wheels are a two wire WSS. As I said before and shown in the next schematic diagram, the VSS circuit, you will see that the reluctor and coil are the same as the WSS reluctor and coil.



This is a snip from AllData, which is the top of the line Automotive Repair Software:

<Wheel Speed Sensors (WSS) - As the front wheels spin, toothed rings located at
each wheel hub interrupt magnetic fields in the wheel speed sensors. This causes
each wheel speed sensor to generate an AC signal. The EBCM uses these AC
signals to calculate the wheel speed. The wheel speed sensors are serviceable only
as part of the wheel hub and bearing assemblies. Any imperfections in the toothed
ring, such as a missing or damaged tooth, can cause an inaccurate WSS signal.
Vehicle Speed Sensor (VSS) - The input signal for rear wheel speed originates at the
VSS. The Powertrain Control Module (PCM) receives rear wheel speed input from
the information to the EBCM>

As you can see, it says that the WSS generates an AC signal to the EBCM (Electronic Brake Control Module), I don't see how you can argue with that! An old adage applies here, "No matter what you think or feel, if someone tells you that you are drunk, you'd better lie down."

Now traction control is controlled by the by the EBCM not the VSS:

Traction Control System (TCS)
When drive wheel slip is noted while the brake is not applied, the EBCM will enter
traction control mode.
The EBCM uses a 5-volt Pulse-Width Modulated (PWM) signal to request the PCM to
reduce the amount of torque to the drive wheels. The PCM reduces torque to the drive
wheels by retarding spark timing and by commanding the throttle actuator control. The
PCM uses a 5-volt PWM signal in order to report to the EBCM the amount of torque
delivered to the drive wheels.

The EBCM uses various inputs to control various applications, the VSS sends its signal, which is AC, to the PCM, which in turn controls the amount of pressure that is applied to the rear brakes.

I will try reversing my pump, so I'll apply pressurized brake fluid to each wheel and flush out the air that way, but I still want to complete this circuit that activates the automated bleed of the ABS Controller, I so invested into it now that I want to complete it.

 

Won't continue the argument with you. That said the aldata is written for people that aren't electrically savvy. They say AC (alternating current) because most people don't know it is really a sine wave DC, it never goes below zero volts into negative volts. Unless you can show how a single polarity input can make an AC out put I rest my case.

And without reluctor rings on the rear wheels, which neither of your schematics show, the rears aren't ABS, but are proportionally controlled. Meaning they both get the same pressure at the same time. Not individually controlled like ABS does.

 

Isn't that what I said? I don't want to argue with you either, all I want to do is build a circuit that does what the scan tools do to activate the automatic bleed of the ABS controller.

 

Hi All,
Been gone for awhile working on a house that I bought for the wife and now getting back into trying to design this circuit. Shortbus, I did take your advise and drove on gravel parking lot, it is a big parking lot, dove the Tahoe at 20 mph and hit the brakes. Seemed to active the ABS controller but didn't fix problem, so I'm back to square one. So Alec-t, trying to get this circuit to work universally seems to be a monster. I'm thinking that the use of that circuit 4 times but only using however many leads are needed for the specific vehicle. i.e., the Tahoe would only use two leads with the generated signal going to the controller, then all that's needed is to stop the signal on one. Where as to test my Explorer, it would take all four leads but what is needed is to ground on lead. Thoughts????

 

Thoughts? I don't know what the various control modules do in the way of processing the sensor signals, so can't really advise. The reluctance type sensor outputs will vary widely in amplitude from millivolts to many volts, so the modules are obviously designed to cope with that. The analogue input to the modules may need to be of greater amplitude than my circuit provides. Also, the modules may not like the fact that one terminal of the circuit is grounded. Perhaps it would be advisable to use an independent, non-grounded supply (say 3 AA cells) for the circuit. Use the circuit at your risk, bearing in mind that the modules will cost arms and legs to replace.

 

hmmm...seems like a need to emulate the signal output of the VSS adapter..

 

Shortbus, I did take your advise and drove on gravel parking lot, it is a big parking lot, dove the Tahoe at 20 mph and hit the brakes. Seemed to active the ABS controller but didn't fix problem, so I'm back to square one

Doing it one time isn't enough. Do it 2 or 3 times in a row. The air in the ABS unit is hard to get out, but the gravel thing has always worked for me. And since the air is going into the calipers, the lowest point in the system they will also need bled again, after the pedal no longer gets firmer from the gravel/skidding.

 

very curious to see if this worked out for you. im in a similar situation and having a hard time finding all the necessary parts to build the simulator myself.