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General => The garage => Topic started by: sam j on 18 November 2013, 19:08
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Hi all. Firstly, apologies for the first post / thread being a call for help, only bought the mk2 last weekend and trying to get it back on the road. Its a LHD 1991 4-door.
So, took it for its annual test today, and it failed on the ABS warning light not working. Bit of investigation and someone has disconnected it. Reconnect it and the light comes on with the ignition and stays on.
Now comes the problem, I know that ABS on these Mk2s is pretty rare, my Haynes doesn't even mention ABS. So does anyone have a guide / wiring diagram / anything on the ABS system and how it works?
I'm guessing that as the light comes on and stays on it isn't a wheel sensor. When you turn the ignition on you can hear the brake pump kick in and then go off a few seconds later. Assume this is the system coming up to pressure?
Any tips as to where I should be looking?
Thanks
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Hi, checking all the sensors have adequate resistance using a multi meter would be a good start as this will need to be eliminated first. But it could also be caused by dirty fluid or moisture in the fluid. Poor electrical connections or relay. The pump itself could be deffective. Does it have an abs ecu anywhere.
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I'm sure the abs ecu is in the boot. If you go onto VAGCAT there is a harness guide.
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Thanks for the quick replies.
I shall check the wheel sensors this evening. The pump kicks in and runs when you turn the ignition on, and kicks off again later, and seems to pump fluid (i.e. if you open a bleed nipple on a wheel cylinder it happily pumps fluid out)
As dragonfly says, the ECU is in the boot with a single multi-pin connector. Guess there is going to be quite some wiring to trace, I shall grab the harnass guide from vagcat (thanks for the tip).
Is there any special technique for bleeding these systems for a full fluid change?
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For fluid change your best just opening the nipples and let it drain while topping up until clean fluid appears. You don't want pump to go dry. You should be able to check all the sensors via the ecu if you can find out what goes where. I had all these issues with my early 92 vr6 and I still could never to get the abs light of all the time but it works 85% of the time. The light going out is always acompanied by a relay click. This assuming we run the same pumps. Teves 2 pump.
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I shall have a look, thank you. I had the rear calipers off and rebuilt at the weekend and flushed quite a lot of new fluid through at the same time, will run some fresh through the front whilst I have it in the air later.
Do you know if it is possible to get the ECU to display error codes on the dash? from googling the Teves II pumps it seems that in some systems you can get the error code "flashed out" on the warning light.
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I must be missing something - on the wheel sensor connector there is two conductors, and I am guessing I should see some connection either between them, or between one / both and ground? but I see nothing, on either pin, on any wheel... either all 4 sensors are dead, I'm doing something wrong, or my multi is dead (quite possible, had issues with it before...)
I have tested both relays and they work, all fuses are good.
Shall borrow the test meter from work tomorrow and nip home on my lunch break, see if I can get some kind of reading with that one.
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I must be missing something - on the wheel sensor connector there is two conductors, and I am guessing I should see some connection either between them, or between one / both and ground? but I see nothing, on either pin, on any wheel... either all 4 sensors are dead, I'm doing something wrong, or my multi is dead (quite possible, had issues with it before...)
I have tested both relays and they work, all fuses are good.
Shall borrow the test meter from work tomorrow and nip home on my lunch break, see if I can get some kind of reading with that one.
Your testing for resistence yeah? Not voltage. I know a healthy sensor on a mk3 should hold steady at anything between 1 and 1.3 K Ohms.
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Yeah, turns out I have a dead meter. Borrowed the decent one from my lab at work...
FL - 95 kΩ
FR - 1.05 kΩ
RL - 1.08 kΩ
RR - 1.03 kΩ
So dead sensor front left. Managed to source and replace the sensor yesterday, and whilst the light now goes out i have another problem. Pulling away from a stop / at low speeds the brake pedal judders and the ABS relay kicks in and out rapidly as if the ABS thinks a wheel is locked. This happens when my foot is off the brakes only, the abs light stays off, and it stops by the time you get over about 20-25 kph. Any ideas what would cause such an issue?
edit to add: the brakes don't actually engage when the pedal jitters. The car drives fine and pulls away as it should, the brake pedal just randomly moves down an inch or so and then jumps back. Does this anything up to 2 or 3 times a second, then stops.
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I'd inspect and clean up all the abs rings and that should sort it mate.
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Thats my plan. Going to drag the oscilloscope out some point this week and see what kind of signals I'm getting off the sensors. I assumed that if a wheel sensor was giving a crap signal the warning light would come on again?
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Thats my plan. Going to drag the oscilloscope out some point this week and see what kind of signals I'm getting off the sensors. I assumed that if a wheel sensor was giving a crap signal the warning light would come on again?
The light will only come on again if the abs system shuts down and reverts back to non-abs mode. But since you were getting juddering through the pedal that suggests all is well electrically and its a dodgy ring causing the pump to compensate for an innacurate reading.
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Not had time to look yet, but one thought... on the end of the ABS unit there is a 2-pin connector on what looks to be some kind of sensor / switch. Any idea what resistance this should read and when it should change?
(the 2-pin connector on the far-left end of part #15 here (http://www.jimellisvwparts.com/images/parts/VW/fullsize/092116300.jpg))
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Just to update quickly...
Went through all for wheel sensors with the oscilloscope, and all gave pretty good signals. Just to make sure I went through each sensor in turn and bypassed it, connecting the sensor on the other side of the axle to both ECU inputs (ie, connect front left wheel sensor to both front left and front right connectors on the abs wiring loom, bypassing the front right, etc etc), and still the pedal jumped around of its own accord when driving slowly...
So, each sensor has good resistance, seems to give a stable signal on the oscilloscope, and makes no odds if its bypassed... I am swaying towards it not being a wheel sensor. And if it is, its got to be at least 2 of them!
So, question now is what is left:
ECU,
Pump / valve unit / master cylinder
Is there a speed sensor anywhere?
Guess a lot more cable tracing needs to be done, check all earths and make sure there are no damaged / shorting wires anywhere. Don't know why this would only affect the system at very low speeds though...
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Just to update this again.
I double checked myself, and bypassed the front right sensor again, using a bit better wiring this time, and it all worked fine... so, I've now changed both front wheel sensors, and all is good. All working as it should
My theory on this, front left sensor was dead, which was keeping the light on. Replaced that and light goes out Front right sensor the insulation around the connector in the engine bay was all brittle and falling apart / held together with tape, so whilst the sensor measured okay from a resistance point of view, the signal coming from it was poor with a lot of crosstalk etc. Both changed, all working, job done
Now on to the rest of the little jobs... starting with the rear wiper motor being crap...