scoping abs sensors

[hipwell]

I had my faults read for my abs/ESP service light which said it was the rear right abs sensor and the break pedal switch.

I then adjusted the break switch so that it came on at the slightest tap of the breaks but the faults are still there.

So I cleaned both rear abs sensors and tested with a voltmeter and both were giving similar reading when the wheel was spun. The error is still there.

I have read you can test the sensors with a scope, I'm pretty handy with a scope but don't know what I'm looking for when testing the sensors.

 

[mr.pink87]

Re: scoping abs sebsors

Sine signal with increasing frequency if you spin the wheel faster.

 

[hipwell]

Re: scoping abs sebsors

So a properly functioning one should give off a nice since wave weras a faulty one will give a square wave maybe?

 

[scotty2hotty81]

Re: scoping abs sebsors

You though about changing the switch? Pin the sensor from the abs unit to check all the wiring, the easiest thing to do is get a decent diagnostic Machine that will show all 4 wheel speed signals so you can compare.

 

[MicKPM]

So a properly functioning one should give off a nice since wave weras a faulty one will give a square wave maybe?

Not exactly no. What you're looking for is a nice, progressive curve as the rotation speed increases and decreases. No output will be either a sensor or wiring fault but chance are the sensor is producing a smooth-ish wave form that has some errors at slow speeds so it'll be a target fault (contamination, cracks, etc).

You though about changing the switch? Pin the sensor from the abs unit to check all the wiring, the easiest thing to do is get a decent diagnostic Machine that will show all 4 wheel speed signals so you can compare.

+1 on the switch, id just wang a new one in and be done. Good idea with the continuity test also but you will need a machine that can graph the data rather than present it as a numerical value as you'll never see it in that format.

Mick

 

[hipwell]

Re: scoping abs sebsors

Not exactly no. What you're looking for is a nice, progressive curve as the rotation speed increases and decreases. No output will be either a sensor or wiring fault but chance are the sensor is producing a smooth-ish wave form that has some errors at slow speeds so it'll be a target fault (contamination, cracks, etc).



+1 on the switch, id just wang a new one in and be done. Good idea with the continuity test also but you will need a machine that can graph the data rather than present it as a numerical value as you'll never see it in that format.

Mick

Yeh I'm thinking about getting a new switch to eliminate the fault but it appears to be working fine. (Break lights come on and off when I press the pedal). Is there something to look out for if the switch really is faulty?

OK I'll get my scope out tomorrow and see what I find.

 

[MicKPM]

The brake pedal switches (depending on type fitted) performs two functions. The first is an "on/off" switch for the brake lamp circuit and the second is a fail safe kind of interrupt for the cruise control program.

Because of the role it performs it's not a simple switch as it has the two separate circuit "switches" incorporated into the single assembly. This means that just because your brake lights are working when you hit the stop pedal doesn't mean the other circuit is fine and this is what (most likely) your code reader is picking up as "Active".

Mick

 

[hipwell]

Re: scoping abs sebsors

The brake pedal switches (depending on type fitted) performs two functions. The first is an "on/off" switch for the brake lamp circuit and the second is a fail safe kind of interrupt for the cruise control program.

Because of the role it performs it's not a simple switch as it has the two separate circuit "switches" incorporated into the single assembly. This means that just because your brake lights are working when you hit the stop pedal doesn't mean the other circuit is fine and this is what (most likely) your code reader is picking up as "Active".

Mick

Thanks. Well I've just been out with the scope. The rear right one which the diagnostics said was faulty was producing a very small wave with the scope set to 0.2v per div. Whereas the rear left was giving a full screen sine on the Same settings. So I'm going to assume that sensor is kaput.

If I change the sensor will the fault clear itself or am I going to have to have the fault cleared?

Also Renault want 140 for a sensor, gsf said 35, will the cheapo one be OK?

 

[MicKPM]

Re: scoping abs sebsors

Where did you take the reading from? It does sound like a duff sensor indeed and the GSF part will be fine.

Mick

 

[hipwell]

Re: scoping abs sebsors

I just unplugged the sensors from their plugs next to the center section of the exhaust on the n/s of the car and put the probes from the scope in and got my dad to spin the wheel.

 

[MicKPM]

Re: scoping abs sebsors

That's what I was hoping to hear. Normally (and for easier access) we'd scope at the ABS unit multiplug but that doesn't take into account possible faults due to continuity or corrosion problems between the box and each wheel sensor. By testing each sensor at its source you take the wiring out of the equation and have not only found but confirmed problem - result

 

[hipwell]

Re: scoping abs sebsors

That's what I was hoping to hear. Normally (and for easier access) we'd scope at the ABS unit multiplug but that doesn't take into account possible faults due to continuity or corrosion problems between the box and each wheel sensor. By testing each sensor at its source you take the wiring out of the equation and have not only found but confirmed problem - result

Cool so hopefully I can get my abs working again. I'm gonna change the brake pedal switch anyway for the sake of a Tenner, do gsf sell those aswel or am I best off going to Renault?

Also will I need the faults reset or should the ecu pick up the fact its fixed and clear it itself?

 

[MicKPM]

Re: scoping abs sebsors

ABS should be self rectifying as soon as a new, working sensor is hooked up and when the ABS module confirms a correct signal

 

[hipwell]

cool thanks mick, ill let the thread know how i get on.

Looks a nightmare to get the sensor out, i managed to get the torx screw out but the sensor did not shift lol. Overnight soak with some wd40 i recon

 

[scotty2hotty81]

They never come out complete.

 

[MicKPM]

Take the rear disc off and smash s**t out of it... Feels good hammering f**k out of something to.

 

[hipwell]

well I just picked up the new sensor from gsf, however the actual nib of the sensor looks a tad different the one currently installed.
Will it be OK?

 

[hipwell]

Oh bugger, apparently its a different one for cars with ESP. Mine is a female connection. This part from gsf is a male. Bulls eye can get me one for 100 or its 140 from Renault.

Gutted lol.

 

[Kirsten]

Did you sort this?
My lights have been on all week, just put a new brake pedal switch in but the lights are all still on gutted!

Looks like I'll have to head down llocal garage!