ABS

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barry badger
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ABS

Post by barry badger » Mon Oct 20, 2014 11:36 pm

I'm trying to sort the abs out on my tzd, it hadn't worked for well over a year, after a few adjustments late spring it's been working fine all summer and early autumn but has started acting up again.
I've just scrapped a tzd hatch and whilst stripping it of anything useful i found a diode in the abs wiring loom behind the glovebox.
Does anyone know its purpose?
Do i test it with a multimeter set to ohms? if so this one is faulty, ( the abs was intermittent on this car too ).
One thing i noticed on the scrapper the coaxial cable to the nsf sensor was routed behind the metal brake pipes on the bulkhead behind the battery and was badly flattened. If it they're all routed this way from the factory then this could possibly cause a fault long term.
1993 tzd estate, on sorn
1992 tzd estate, slowly being raided for parts
1992 txd estate, "doom blue" on the road
1991 4x4 estate, on the road

citronut
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Re: ABS

Post by citronut » Tue Oct 21, 2014 9:14 am

yep its the ABS relay protection diode aparently

regards malcolm
curent ride
K reg BX 17TD TZD est
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K reg D special

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H reg CX saffari 2.5 TRI (now gone to Malaysia)
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electrokid
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Re: ABS

Post by electrokid » Tue Oct 21, 2014 10:20 am

Do i test it with a multimeter set to ohms?
Nope - that will not usually give you any indication of whether it's good or not. Most multimeters have a specific 'diode' test setting which simply has the symbol for a diode next to the switch position. With the probes connected to the diode one way around will appear open circuit (EG: 1.999) and the other way around it will read low (EG: 0.625) This will show the diode is ok. If the reading is high - 1.999 with the probes connected either way around then the diode is stuffed - open circuit - if the reading is low - 0.01 - both ways round then the diode is stuffed - short circuit.

If your multimeter doesn't have a diode test function then you can do the same test using a battery (single cell AA type is fine) and a resistor.

You can use any value of resistor between say 470Ω and 10KΩ - it's just there to limit the current during the test.

Connect the resistor in series with the battery and the diode (in a loop as it were, forming a circuit) then measure the voltage across the diode. Reverse the diode and measure it again.

There will be a marker of some kind on the diode indicating which end is the cathode - when this end is pointing towards the negative end of the battery the voltage reading should be less than the battery voltage - it will be around 0.2 volts or 0.6 volts depending on what type of diode it is. When the marked end of the diode is pointing towards the +ve end of the battery the voltage across the diode should be the same as the battery voltage - if a new AA battery is used that will be around 1.6 volts.


The diode should have a type number written on it - let us know what the type number is - I will almost certainly have an equivalent in stock if you need one.
1992 BX19 TGD estate 228K Rusty - SORNed
2002 C5 HDi SX estate

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barry badger
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Re: ABS

Post by barry badger » Sat Oct 25, 2014 5:55 pm

Thanks for the advice, i originally tested it using the ohms setting, not realising my meter does have a diode setting. The diode is ok!
The abs is very hit and miss, some days i can go out and the light doesn't go out at all, sometimes it comes on when going slowly, yesterday and today it's worked perfectly.
I think damp weather is a factor.
Up to now i've replaced the front cv joints, the osf sensor, checked and cleaned every connection, checked the resistance of every sensor and checked the sensor air gaps.
My next step will be to check the resistance of every sensor back at the ecu multiplug.
1993 tzd estate, on sorn
1992 tzd estate, slowly being raided for parts
1992 txd estate, "doom blue" on the road
1991 4x4 estate, on the road

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Jaba
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Re: ABS

Post by Jaba » Sun Oct 26, 2014 6:27 pm

Intermittent ABS can be time consuming to fix.
As it is almost always a faulty sensor or lead my strategy has always been to first measure each sensor for around 1000ohms. If you can move the leads around at the same time this may reveal a lead fault.
Then if that seems normal replace each sensor and lead one at a time to find the one playing up.
It is easier to flex the lead thoroughly when the sensor is off the car to reveal a bad cable. You need one known good front and rear sensor to achieve this as if you remove each sensor non destructively and the fault remains then you can refit this one to the other side after testing off the car.

By this means I found my last ABS fault to be caused by water getting in to a rear sensor cable and corroding away the outer screen in such a way that is was making contact for most of the time.

It is possible to repair the cable by splicing in some screened hifi type cable.
The Joy of BX with just one Citroën to my name now. Will I sing Bye Bye to my GTI or will it be Till death us do part.