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[Webby the Bear]

Afternoon chaps, hope you're all well. 

So, I gets round to my buddies yesterday to look at his motor before the footy.

He's got a 2003 Renault Megane 1.9 disease-al. The car has an intermittent ABS light coming on. It goes off then comes back on etc. so I told him that as I don't have an ABS reader I can't see the code (or erase) but I can test his ABS sensors.

Now, my understanding was that if I back probe the sensor and switch the ignition on and spun the wheel I would see a production of A/C voltage.

First issue was I couldn't back probe lol cos the insulation goes in to a little housing its impossible to do. So being a resourceful kinda chap I disconnected it, probed the two pins on the harness side and touched the sensor end connector to my pins.

Nothing!

I tried it with key on, key off, engine running.

What am I doing wrong?

TIA chaps 

[Simon270172]

Before you do anything else, I would check the tyre pressures are not wildly different as that can fool the abs into thinking that there's a problem.
The lower the pressure the smaller the effective diameter and the faster that wheel turns, which means more revolutions than the opposite or other wheels and brings on the abs light. If it's boarder line, the light can come on and off as the tyre heats up and cools down giving an apparently random fault. Some cars are more sensitive than others.
You'd be surprised how often this gets overlooked, especially by garages eager find expensive faults for a simple fix.
I'm probably telling you what you already know, I just thought I'd try and help in case you did'nt

[chrisgixer]

I'd guess that you need the wheel turning to see a pulse.

I'd also guess you need a reader to see live data on which sensor is a at fault.

Ime with an intermittent fault, your looking at a wiring issue, rather than a failed sensor. Is there any alterations or extra bits of insulation tape in the loom to ecu or sensor wiring.

Does the fault affect braking? If the system sees open circuit it can assume a locked wheel and let the brakes off, even if the driver is hard on the brakes.

[dbug]

You usually need the wheel to be turning equivalent to about 10mph to see live data.
Check visually the reluctor ring is present/intact/in correct position - its not unknown for rust on hub to split a reluctor ring

[Marks DTM Calib]

Check visually the reluctor ring is present/intact/in correct position - its not unknown for rust on hub to split a reluctor ring

First port of call for such ABS faults 

[Webby the Bear]

Thanks for the input guys.

Re tyre pressures.... I will ensure he checks them. thanks for the tip.

Re testing the sensor..... so basically I was spinning the brake disc by hand..... so we're saying that i wasnt spinning it fast enough to priduce a voltage?

Re the reluctor ring. I tried to have a look at it but it seems to be buried inside the hub as i couldnt see it at all. As opposed to the drive shaft reluctor rings i'm used to seeing.

[henryd]

A lot of cars now use the back of the wheel bearing with a magnetic seal as a reluctor ring instead of the toothed rings we are used to seeing

[Webby the Bear]

Thanks Henryd

So in terms of testing.... should I be running the car up to say 15 mph off the floor (with the front up on jack stands?

As it's a two pin connector i'm presuming i hook my DVOM to the two pins... or should i be gounding the black lead and probing just the one signal wire?

I can't understand why it wont produce a voltage though cos surely the fact that its a magnet passing over a coil will produce volts?

If anyone can confirm the test procedure (and for my future reference learning) i'd much appreciate it

[Webby the Bear]

Hi again lads,

Ok, I think I've figured this out using the following info from the net...

http://www.underhoodservice.com/Article/105757/wheel_speed_sensor_diagnostics_for_meters_and_scopes.aspx

So. I'm looking at either a passive or active sensor. It would appear that the one on the Renault is active due to the tone ring being buried in the hub.

I note that I should be looking for a permant battery voltage with the black lead on my DVOM grounded to the chassis (or battery -) and the other lead probing the harness connector. I think I was getting nothing because I was using both leads to probe the two wires... that's WRONG I think.

So assuming this is a 'active' sensor I'll see a constant battery voltage on the supply pin and with the leads connected to a good ground and one lead on the signal wire i'll get a voltage with the disc being spun.

[Webby the Bear]

...just to add, I did test the resistance of the sensor and got nothing! That also supports the theory of it being an 'active' sensor as the resistance on these is 3,500,000 ohms lol hence why i didnt get anything!

[Webby the Bear]

Anyone agree with my new revised approach?