Mr.Alex

Occasionally my brakes have a mind of their own, sometimes when I'm driving I press the brake and it goes down normally but no braking at all, I then have to let it up and then once I depress it again it brakes like nothing ever happened.

I have the vacuum pump from checking Adrian's book and I checked around to see that I have no leaks anywhere and everything seems normal.

I was then going to check the relay only to find out I have no relay at R51.

Could it somehow be air in the system? I'm slightly out of guesses.

alexjc4

Sounds like an abs issue to me.

Rocket Rob

Check & clean your ABS sensors

h_____

Check your hubs are tight, I've had this sensation (through pad knock off), with hubs not quite torqued up much. I assume no recent work on the car?

Mr.Alex

Il try to clean the sensors. No recent work, but I have been taking the wheels of a lot lately to raise the car a bit and small check ups, etc. so I'l give that a shot too.

The sensors are behind the hubs correct? Also, silly question, but are there four sensors behind each hub or just on the fronts?

Vandit

There's a sensor per hub. It's a 5mm hex head bolt and an O-ring holding each in place. I have mine out at the moment.

What's the best method to clean the sensors? Brake cleaner?

HiWind

yes brake cleaner works well - wear goggles!
then wipe clean and blow brake cleaner and then air into the hole while turning the wheel to clean the teeth on the ring inside the hub which the sensors reads. Then replace sensors - gently with the o ring.

I just did mine - sadly ABS light still on tho and ABS not working judging by the blue smoke behind me when I lock up brakes

Hopefully OPC can help tomorrow -- problem is I have a custom fuel mgmt system which came with the 3.8, so orig ECU isn't plugged in, hence no error codes. Hopefully a wheel speed test directly on the ABS Ecu will reveal something. I hope its just one of the sensors are faulty and not damage to the abs ecu or those teethy rings.

Mr.Alex

Thanks guys, life savers. Il report what I find.

Mr.Alex

They were a bit dirty so I sprayed them down with brake cleaner and it took 95% of the dirt off. Should I have used a rag or something to completely get them spotless?

Nevertheless the issue is still there when I took the car out last night, any other tips?

Cheeksyboy

Have you held the system, and checked all the pipes, hoses, nipples, etc?

How old is the fluid in the system?

alexjc4

If the pedal feels normal, it must be moving the m/c piston, which means it must be generating line pressur. The pressure is not getting to the caliper pistons or it is but its not moving them. The later wkuld seem unlikely as an intermitent fault. If there is no braking at all then all four wheels are effected. That would require a fault in all four calipers or all three hard lines. That's why i think abs unit. Abs works by isolating the calipers from the m/c. Blocking the path, holding pressure so it doesnt build further the realising and reapplying as needed. It uses uses sensors to choose when to do this. Faults in the sensors, mechanism or control unit could give you this kind of issue.

That said I have a very poor record when it comes to diagnosing things so that may all be nonsense.

Mr.Alex

I checked as much as I know and can't find any leaks or faults anywhere, the brake fluid was flushed sometime last year.

The pedal gets a bit harder when there are no brakes, and then once I release and reapply its back to its progressive soft-ish self.

Could it be the sender switch on the abs?

alexjc4

Its not air in the system for sure.

If the pedal goes very hard it could be a fault with the servo assistance, vacume on a c2 or hydraulic on the c4. Without assistance the pedal effort is so high it would feel like no brakes at all.

I hate to say it but you need to take it to a man, abs unit needs to be plugged in to a tester. You have a dangerous fault there, no time for us amatures to play Columbo.

I had a similar fault (bad accelerometer) but with different symptoms. The first time it happened i nearly had a low speed fender bender, second time it caused me to run a red light and cut across traffic. Luckily the symptoms were recognisable from other chaps experiences on here so i could fix with a high degree of confidence and allow me to drive around safely again. But in your case I don't think you will be able to diagnose and fix it with a sufficient degree of confidence without special equipment.