Sounds like the ebd valve has gone.

This is a valve in braking system that will cut much of the rear braking if the abs system feels the rears are going to lock up before the fronts. The side effect is that this dramatically affects the feel of the brake pedal and can give the impression of a very light pedal or even failed brakes (do a search for ice mode)

disconnecting the abs controller (by unclipping a front wheel sensor) will also disablethis valve.

 

Maybe the ABS sensor itself. Had one of those go bad on my car, but didn't exhibit the pedal characteristics you're observing. Sounds like the guy above me has the better theory.

 

Sounds more like Servo/vacuum leak to me -- could be bad check valves -- you have two of them, one in engine bay near throttle body and the the other one is connected to the brake booster/servo (driver side under front panel where brake fluid reservoirs are), OR the booster/servo itself is bad (less likely).

Check your check valves -- these "capture" vacuum (that's how I think of it) which is used to assist in brake pressure -- makes it easier to push the pedal. You can check them by sucking/blowing (you should only be able to do one, not both, either suck or blow pending which end you have).

Reason I say check valves is because you describe inconsistent even under moderate braking. The ABS issue is hard braking exclusively. If any of your wheel sensors are bad it will trigger idiot lights and/or error codes and dissable the ABS system completely. What you describe sounds more like a check valve issue as when the booster/servo runs out of vacuum it will make the pedal feel very hard.

Rob.

 

+1 on the vacuum/server/check valve, that's my vote.

 

Thank you all for your input, very helpful (as always on LotusTalk). If it is related to the vacuum check valves, would the brakes work dramatically better after I disconnected the ABS?

 

No, would be the same. Pull the ABS fuse or disconnect one of the wheel sensors -- this will turn off the ABS ... if you still have the problem, then it's not the ABS. If the problem goes away completely, then it's likely to be the ABS ... I have an ABS unit (for 07 Exige S) if you want to buy it

Cheers, Rob.

 

I thought the servo assistance system works the same even when the ABS controller is disabled.
However this is pretty easy to check so worth doing.

Agree unlikely to be a wheel sensor as the ABS controller will be specifically looking for dodgy signals and will flag this up quickly.

 

Every time I read his I am glad I have a pre-ABS version.
Every time I am in the wet, I wish the other way.

 

As a former ABS engineer, I think I can give you some ideas. I haven't delved into the Elise ABS other than to study the service manual, so consider this just general experience offered up for your consideration.

Are you ever seeing the ABS indicator light illuminate in the gauge cluster (other than the initial check during power-up)?

Do you ever see unusual speedometer fluctuations?

I would also have a dealer perform an ECU scan to see if any diagnostic codes are logged.

Make sure your stop lamp switch works reliably - it is connected to the ABS ECU and may be used for some diagnostics and plausibility.

As you have already figured out, you can disable the ABS by electrically disconnecting some components. If I were trying to disable the ABS, I would pull the ABS fuse. On former street/track cars I owned I would install a "kill switch" in the line to the ABS fuse so I could disable ABS for the track.

The most likely problem causing the symptoms you are describing is a wheelspeed sensor or the exciter ring (toothed metal ring the sensor "looks" at). I have never removed one on an Elise, but it is important that the sensor gap be within specification, the exciter ring teeth be undamaged and clean, that the wiring harness be securely routed with the insulation in tact. Check for accumulations of iron filings on the end of the wheelspeed sensor (it is magnetic). If your wheel bearings are wobbly or have failed and been replaced, this often can cause one or more of the aforementioned issues.

The other possibility is the ABS valve module itself - but the ABS dump circuit must be holding pressure fine if you are having no issues when electrically disabling the ABS. That would suggest the valve is closing fine after dumping pressure, but it is still possible that it could be slow or sticking enough to case performance issues.

If your symptoms are happening with light pressure in normal street driving, lean toward the sensors and wiring. If your symptoms are happening only during very aggressive auto-x braking, lean toward a valve problem. If your problem happens more at some speeds than others or especially at low speeds (i.e. when the sensor signals are their weakest), lean towards sensor or wiring problems.

The least likely problem is the hydraulics, rotors/pads, or vacuum servo, based on your reporting no problems since disabling the ABS.

Let us know what you find out.

 

Update ... well, as I said I had purchased SS lines, power bleeder and RBF600 fluid. Everything is installed and one word ... WOW. The car stops like it never has before. The ABS is hooked back up and seems to work perfectly now. I did 6 60 to 0 hard stops, the brakes performed consistent everytime (something it has never done). I guess the real test will be at the next autox. At the end of each hard stop I could hear some springy type sound for about a second after the car came to a complete stop, I assume that is related to the ABS function.

Thank you all for your assistance (I am going to check the check valves tomorrow evening), I'm not sure I'm out of the woods yet, but I will know for sure after the next autox.