ABS light

[chrissporte30]

Hey all,

I’ve used the wiki which doesn’t seem to describe my issue and search proved unsuccessful.

The ABS light comes on the instrument cluster when you rev the vehicle at idle and remains illuminated, stationary with out moving on first start up. If you start the car and let it idle the light will stay off until you raise the RPM to approximately 1500 with out moving/in gear.

Light comes on with key at position 2 as normal and goes off when vehicle is started. Only thing to note is I replaced the thermostat and potentially some water may have got down by the alternator or from the over flow/expansion tank. But I would think these would dry out after a couple of days. Also engine was misfiring which cleared up now.

I thought it may have something to do with overvoltage or surge from the alternator but it puts out a constant 14.2v even when revs are raised.

Any ideas what makes the light behave this way? Or what water could of gotten into that could have this effect?

[Brianmoooore]

Strange one, that I've never come across before. My first thought, when I read the first part of the post, was that it might be an alternator problem, and then I read that the alternator might have been damaged, so that would be the first thing to try.
The alternator uses nine diodes to turn three phase AC into two DC outputs, and while a meter might still give the output voltage as correct with one of these dud, the actual waveform might be far from smooth DC.
Unfortunately, you'd need an oscilloscope to check both output waveforms, rather than a simple voltmeter.
The diodes are mounted in the main body of the alternator, rather than the regulator pack, so you can't check simply by swapping that over.
One thing you could try, is to disconnect the blue wire from the alternator and temporarily connecting it to a separate, fully charged 12 volt battery or 13 volt power supply, start the engine, and see if the warning light behaves.

[chrissporte30]

Thanks Brian,

The alternator was my first thought but with the voltage remaining constant on the meter it put an air of doubt in my mind. I do have a oscilloscope so I will see if that yields any result.

Also if the ABS system is not connected to RPM/engine system the only thing that would change for it when revving the engine would be voltage change, again pointing to that.

But you’re correct I don’t think many people have come across it and documented, I searched all over the web.

[paultv]

I'd remove the alternator belt and run the engine to see if behavior is the same without charge voltage and possible spikes from failed or faulty diodes....that is assuming a good codition battery which is fully charged is in the car.

Paul

[Brianmoooore]

I'd remove the alternator belt and run the engine to see if behavior is the same without charge voltage and possible spikes from failed or faulty diodes....that is assuming a good codition battery which is fully charged is in the car.

Paul

Without any voltage on the blue wire from the alternator, I'd expect all the warning lights that are legally required to have a lamp test function (handbrake, ABS, etc.) to simply remain in test mode. That's why I suggested connecting a power source in place of the alternator's output on the blue wire.

[paultv]

Good point Brian!

Paul

[chrissporte30]

Just resurrecting this one.....

Did some more investigation and put a power supply/charger unit that I use for programming on the blue wire out of the back of the alternator, set to 13.8v, i disconnected the wire from the alternator and attached the power supply revved it up and its still the same, the abs light came on. I totally disconnected the alternator in the end inc large red wire and the abs light still came on when revved. So I think the alternator is ruled out.

So back to the same question why would revving the engine cause the ABS light to come on, the alternator blue wire is the only connection to the ABS regarding the engine so how else would it know its revving?

Again scenario is...

1) turn on ignition, ABS light is on as normal
2) start engine and ABS light goes out as normal, stays out if you leave engine running.
3) dont put it gear and rev the engine on the spot and the ABS light comes on and stays on till key is turned off.

The above can be repeated at will every time.

At step 3 if instead of revving you put in gear and pull off with the clutch/no revs and get the car moving the light wont come on, if you drive 20ft and then stop and put it in neutral and just rev the engine like in step 3 the light still does not come on so system operates as normal till car is turned off.

if you turn the car off and repeat 1 to 3 again the light comes on.

Any suggestions on what to check next? or what would cause it? The fact it can be repeated at will and manipulate to fault or not fault makes me think its not wiring.

Edit - I also checked the alternator output with an oscilloscope and all looked normal, no big dips or spikes, voltage was high at some points at around 14.8v. i also scoped the blue output wire and it looked normal although there was quite a bit of noise on that line.

[chrissporte30]

Fixed - ignition coil lead was too close to the abs sensor wiring

[paultv]

Well that's a new one - it just shows how sensitive the ABS system really is.

Glad you got it fixed - I was thinking about engine vibration as a possible cause!

Paul

[chrissporte30]

Yeah it was an unusual one to find. Must be the fact the ABS Was getting a reading from the sensor when the engine was revved and non of the other sensors so triggered the fault, or what it was seeing made it fault.
But when moving the real value from the sensor overrides the interference or because it’s seeing a value from the other 3 it doesn’t fault.

[Brianmoooore]

Definitely one to add to the data banks there! Ignition HT lead or black wire to coil neg.?