Quick Xantia ABS query

andmcit · Post by **andmcit** » 26 Apr 2007, 21:02

I've finally managed to get my 1995 2.0i VSX going again - turns out the
fuel pump sender in the tank packed in - decided one day never to work
again - no warning signs whatsoever - took ages to work that one out...

The process of elimination was a slow and torturous one with many things
tried and swapped over (but reinstated as they were again when no
change was evident).

Only thing now is, I've got an ABS warning light on permanently. Basically
the system is reading that it's 'lost' a key signal and instantly brings the
light on after start up.

Checking the resistance with a multimeter I've found the sensor pins at the
ECU are totally dead so it strikes me the earth isn't there? Is this a correct
diagnosis? Where do I look - would a missing/duff power relay stop the
circuit through to the sensors?

Any suggestions?

Andrew

AndersDK · Post by **AndersDK** » 26 Apr 2007, 21:31

how did you test the sensors ?

If you unplug the ECU multiconnector, the only way testing the sensors is on the 2 pins leading out to each sensor. The sensors are not connected to ground on either side, but may have a low ohmic ground path on one side internally in the ECU.

It makes no sense trying to do any measurements on the ECU multiconnector, when the cable part is disconnected. In fact your ohm-meter may very well fuse some of the circuits on the ECU pins if you try.
Because ohms measurement in the low ohm range sends quite heavy currents from the ohm meter.

Did you set the ohm meter to the correct range ? It will read disconnect if set to a lower range than 0-2K ohm (0-1999 ohms), because ABS sensors are in the 1K ohm region.

deian · Post by **deian** » 26 Apr 2007, 21:40

Would I be correct in saying...

1. you need to spin the wheel to get a reading from the abs sensors, as the sensor detect the teeth?

2. this works without power because the ohm meter gives a little bit of a current to read sensor readings when a condition is met... i.e wheel turning

3. ...hence why ohm meters can blow ECU's, the current they give off is enough to fry some internal components

4. resistance in the current fluctuates... but how? magnetic field?

andmcit · Post by **andmcit** » 26 Apr 2007, 21:46

Hi Anders - so i read you're in the UK too at the moment!! - You've
managed to catch some rain after we've had such a good hot/warm
spell too...

I've done this very test yesterday on another Xantia with the same
meter and pin out and got the correct readings on the corresponding
pins that run to each sensor.

Yes, I accept I need to check a wheel sensor at the first connection on the
wheelarch, but the fact that I may have managed to lose a connection to
all FOUR wheel sensors simultaneously when they were working 100%
before my other fuel pump issue, tends to point to something being
disturbed...

I have checked I've not managed to undo the whole loom!! The car
otherwise works well.

Andrew

AndersDK · Post by **AndersDK** » 26 Apr 2007, 21:48

deian wrote:4. resistance in the current fluctuates... but how? magnetic field?

Yes -
The magnetic field in the ABS sensor coil winding generates a small current each time a tooth passes the sensor. This is not a DC current, but an approximated sine wave which passes zero to give positive then negative, then positive etc. etc. small currents. These currents are added or subtracted to the DC current from the ohm meter, which then gives a fluctuating ohms reading.
Because the ohm meter works by transforming the current into an ohms reading ...

AndersDK · Post by **AndersDK** » 26 Apr 2007, 21:54

andmcit wrote:Hi Anders - so i read you're in the UK too at the moment!!

That was supposed to be a joke mate

- because DK is east of UK 8)
Its common practice over here to refer to UK as the far, far away West Denmark, and the english language as (deep) west DK dialect

andmcit · Post by **andmcit** » 26 Apr 2007, 22:11

AndersDK wrote:
andmcit wrote:Hi Anders - so i read you're in the UK too at the moment!!
That was supposed to be a joke mate

- because DK is east of UK 8)
Its common practice over here to refer to UK as the far, far away West Denmark, and the english language as (deep) west DK dialect

Bit lost on me I'm afraid! Thought I caught a reference to a Hotel that's all.
Don't worry I am losing my grasp on things.

As it will evidently be seen, my grasp of auto electrics is a beligerent love/
hate relationship and a few 'dodges' picked up along the way have been
learnt to assess the potential damage to my wallet! The pin out to the
sensors is the first test I try as 9 times out of 10 I find the duff sensor
- no wheels in the air or dirty hands - I wore my work cream chino's with
an open engine bay and DIDN'T get oil on them!!

Didn't get any reading from the ECU multiplug either!

Andrew

Mandrake · Post by **Mandrake** » 26 Apr 2007, 23:28

Are you sure you weren't counting the pin numbers from the wrong end or something silly like that ?

Chances of all 4 wheel sensors going faulty at once is basically NIL. (You have a better chance of winning the lottery!)

It's possible that there could be another plug further down the loom, besides the individual plugs to each sensor, but I wouldn't have thought so.

By the way as Anders mentions, please don't try to measure the ECU socket - it won't tell you anything of value and it may cause damage.

Regards,
Simon

andmcit · Post by **andmcit** » 26 Apr 2007, 23:54

Hi Simon.

I was actually thinking the same thing!

I can count the multiconnector's pins from the left along and
then upwards where they're moulded on the multi plug.

Just to confirm I've got the right Xantia pinout (s1 -1995):

front right 1 6
front left 15 30
rear left 19 28
rear right 29 31

Andrew

DickieG · Post by **DickieG** » 27 Apr 2007, 05:59

To help you indentify the number of each pin/which way round to read it, if you look carefully at the ecu connection block you'll invariably see a very small number moulded into the plastic alongside each pin.