ABS sensor - circuit open

Having had a dodgy ABS sensor diagnosed by another garage, the dealer have today replaced this, but still found other problems with the wiring etc.

I just want to be sure that I havent just paid to have a sensor replaced when its not that at all. They said, however, that the ABS sensor was 'open'.

First garage charged me £75+VAT to diagnose this problem, but it looks like theres more to it than that because the ABS light is still staying on.

Each sensor should have a circuit through it, so if there is no connection through it then it is 'open' circuit and it cannot work. The computer diagnostic cannot differentiate between a failed connection or broken wire and an open circuit sensor. Which is why all these diagnoses need to be made by an experienced mechanic. Just the same as a PAT test, first thing to do before plugging in the computer is a visual check over, which will often reveal the fault, be it dirt or damage.

Mrcheerful

you need to check the wiring loom for cuts breaks or any shorting.

before you do though get the fault code reset (usally a battery disconnect )

Yep. Garage is doing this next week.

OK. So if the sensor is 'open' circuit its knackered then?

The place that I originally took it too was a proper auto electrician. Looks like they diagnosed this and assumed this was the only problem...

OK. So if the sensor is 'open' circuit its knackered then?

Yes. First check is just an overall visual for damage, then pull each plug apart and have a look/clean it. Buy a multi meter and check the sensors yourself, on ohms setting you should see an approximately equal reading through each of them, probably a couple of hundred ohms, anything massively different is a problem, you would then substitute that one, if the same fault remains on the ecu log then you would then check wiring from the ecu to each sensor by backprobing the connectors at the ecu, where you should find the same ohm reading as you got before directly from the sensor

It's also worth checking for an AC voltage generated from the disconnected sensor with the wheel spinning.

Mathew

It depends on if they're convetional inductive sensors, or more modern Hall effect sensors.

If they're hall effect sensors, then they'll appear to be open circuit when tested with an ohm meter. Only way to test a hall effect sensor is with it connected. One wire should have battery voltage, and the other should see a changing/switching voltage as the wheel is turned slowly.

Will that type have three wires? or two? I expect that most of the older vehicles have a plain coil and would show continuity if OK

OK. So if the sensor is 'open' circuit its knackered then?

The place that I originally took it too was a proper auto electrician. Looks like they diagnosed this and assumed this was the only problem...

They probably changed it without checking because that is the most common fault, then found it was something else.

Steve

Thanks for the help.

So, I understand that generally an ABS system does a self-test when you turn the engine on. Would a sensor in this condition cause an ABS fault light?

On some cars the lights stay on until you are going above a certain (low) speed so it can detect a number of sensor pulses from each wheel.

Having had a dodgy ABS sensor diagnosed by another garage, the dealer have today replaced this, but still found other problems with the wiring etc.

I just want to be sure that I havent just paid to have a sensor replaced when its not that at all. They said, however, that the ABS sensor was 'open'.

First garage charged me £75+VAT to diagnose this problem, but it looks like theres more to it than that because the ABS light is still staying on.

what vehicle is it ?

If the ecu can't see a sensor then the system cannot test correctly and the light will stay on. I have not seen any system that has to have the car moving, I will have to have a look through the abs test sheet at the MoT place.

It is a Hyundai IIRC

Yep. Hyunai coupe F2

Two wires. Kinda screwed me up the first time I came across them. Just aswell I realised the chances of four sensors going open circuit at the same time is highly unlikely!

Land/rangerovers. Light stays on on until the car reaches x mph (think it's something under 10, but not sure)

Update

After having the sensor changed the ABS light was still on. Further investigation found some faulty wiring as well - so not a cheap experience by any means !!!!

I just wanted to be sure that the ABS sensor being faulty was part of the problem and that I hadnt changed it for nothing...