I have a BX TZD with abs. The ABS is functioning correctly, all sensors are OK, the relay is OK, the ECU is OK (I have 2 and changed them). My problem is that the dash light does not come on when the ignition is switched on (it should then go off after checking). In addition I can't make it come on by disconnecting a sensor. Yes I have checked the bulb and all the wiring.
Any ideas? I have found the diode which is near the relay. This also seems OK. Operates one way only with the correct resistance etc.
Any ideas?
Thanks Dave
BX abs
Moderator: RichardW
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tomsheppard
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Get onto the far ends of the wires that eventually lead to the bulb. I guess that one goes from either earth or supply and the other gets powered by the black magic box. Use a meter (if you don't have one, buy a cheap digital one it is £10 to 15 well spent,) and see if there is any sign of 12 volts across the two wires when there should be. If there is, (and I reckon that there will be because you say that the system works and that you have substituted the ECU) then one of the wires to the bulb is not making contact. Probe the wires from junction to junction and you will eventually find the break or corroded connection. If you can break into the wiring at any point and exercise the bulb on the dash just to see that it hasn't gone then so much the better. A word of caution: If you try to bridge the supposedly broken wire to the bulb, make completely sure that the wire you are parallelling is the one you think it is. If you short something out, you may blow fuses or send the ECU's transistors to their ancestors. Good Luck.
Thanks Tom
However. The bulb wiring is fine. I put 12V across it and it lights.
The feed to both wires is 12V. That is what puzzles me. In other words both get the supply, which I believe is the correct function when the light is to be out. However one should switch to earth to light the bulb during test.... but it don't.
I'm baffled. I can only suspect the diode but as I said it seems fine under test off the vehicle. Maybe I will substitute it and see what happens. I've tried everything else. All else fails.. at least the abs works. Normally it is the other way round, you can't get the light to go out.
Dave
However. The bulb wiring is fine. I put 12V across it and it lights.
The feed to both wires is 12V. That is what puzzles me. In other words both get the supply, which I believe is the correct function when the light is to be out. However one should switch to earth to light the bulb during test.... but it don't.
I'm baffled. I can only suspect the diode but as I said it seems fine under test off the vehicle. Maybe I will substitute it and see what happens. I've tried everything else. All else fails.. at least the abs works. Normally it is the other way round, you can't get the light to go out.
Dave
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tomsheppard
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"The feed to both is 12V" If the wire that was being grounded was open circuit then there would be 12 volts on both sides of the bulb because you are drawing (next to)no current in the meter so the voltage stays constant. If you know which is the feed to the bulb, try removing or cutting the switched wire (which I guess returns to the ecu's connector) and earthing it out. If the light comes on then normally it must be in the ECU or a corroded connector. This diode intrigues me though. Where is it connected?. One end will have a stripe and this should be at the lower voltage with respect to earth to make it conduct eg: if no stripe end 13.2 volts, thne stripe end is likely to be 12.1 to 12.9 volts if the diode is conducting. The diode is not the problem. It is working all right. the thing on the other end of it may be duff which is why I am seeking this info from you. If the stripe end goes to the end of the abs bulb that is to be pulled down then +12 volts on the non stripe end would inhibit the light by pulling the supply up. If the non stripe end goes to the bulb then pulling the stripe end to ground would light the bulb but if there is another connection (non stripe +bulb+another wire) then the diode is probably not the thing that is pulled down.
Tom
I'll have a look later. When I said both feeds were 12V, I mean when I put a meter between earth each of the 2 wires that lead to the bulb in turn I read +12V.
The diode does as you suggest. One way current flow. It is in the feed to the bulb but there are 2 wires in (both from ecu, I think)and one out ( prob to relay) I'll check which way round the diode is and if it is as given in the circuit diagram.
I could always wire it into the heater lamp, that goes on and off as required!!!
Dave
I'll have a look later. When I said both feeds were 12V, I mean when I put a meter between earth each of the 2 wires that lead to the bulb in turn I read +12V.
The diode does as you suggest. One way current flow. It is in the feed to the bulb but there are 2 wires in (both from ecu, I think)and one out ( prob to relay) I'll check which way round the diode is and if it is as given in the circuit diagram.
I could always wire it into the heater lamp, that goes on and off as required!!!
Dave
Only just seen this and although I don't have a BX, two observations.
If your system is similar to a CX (Bosch), and it may well be, then what turns the light out on start up is the oil pressure switch. ie the two lights, oil and ABS should should go out together. If this is your system then the fault must be with the connection to the oil pressure switch.
No system checking happens until you are on the move.
What happens if you disconnect a wheel sensor and go for a drive? This should still put the light on.
The system fitted fitted to my "H" reg XM's is bendix and the light is supplied with 12v from the dash and ECU. Checking does comence with the ignition on and the car stationary. This is why it appears to happen twice ie, three flashes as the ignition is turned on and again when the engine is started. This is because the the ECU is diconnected momentarily as you move the key from ignition on to engine start back to ignition on. All my toubles with this system revolve around the valve block.
This will be good too know the answer.
Regards Richard
If your system is similar to a CX (Bosch), and it may well be, then what turns the light out on start up is the oil pressure switch. ie the two lights, oil and ABS should should go out together. If this is your system then the fault must be with the connection to the oil pressure switch.
No system checking happens until you are on the move.
What happens if you disconnect a wheel sensor and go for a drive? This should still put the light on.
The system fitted fitted to my "H" reg XM's is bendix and the light is supplied with 12v from the dash and ECU. Checking does comence with the ignition on and the car stationary. This is why it appears to happen twice ie, three flashes as the ignition is turned on and again when the engine is started. This is because the the ECU is diconnected momentarily as you move the key from ignition on to engine start back to ignition on. All my toubles with this system revolve around the valve block.
This will be good too know the answer.
Regards Richard
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RichardW
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I had a look at the wiring diagram in Haynes last night. The bulb should be fed +ve from the igntion, and then runs to the +ve side of the diode. IIRC the ABS computer switches the -ve side of the diode between +ve (light out) and earthed (light on). You might have a problem with the +ve feed to the light. Try earthing the connection from the light to the +ve side of the diode and see if the light comes on. Otherwise there might be a track break in the ECU, or a faulty connection that is not allowing the earth to be made. You could try rigging a secondary light from a +ve source, and then connecting it to the +ve side of the diode, to see if this lights up as it should. If it does, then your problem is somewhere on the power side of the dash light, if it doesn not then the fault is on the earth side of the ECU light system.
Richard
Richard
The oil pressure switch is not involved in BX ABS systems. It is not shown in the curcuit in the wiring diagram and on my car (as apparently on Richards) the light comes on with the ignition in the auxilliaries position (ie engine not running) while the computer checks the system.
How long have you had the car? Has it ever worked properly? The reason I ask is that I have heard of people messing with the wiring in an attempt to fool the MOT inspector by extinguishing the light. What a wicked thought! - On mine the bulb had been removed. On reflection this is probably unlikely as you have 12 volts at both sides of the bulb.
From what I remember the multi pin plug comes apart quite easily. Is it worth trying to trace the pins on the control unit and metering the terminals controlling the light to see if they behave as expected? I would expect the 'earth' terminal to show nothing while the unit is checking and the bulb should be illuminated and then to rise to 12 volts as the bulb is extinguished.
Having had a look at this in some detail a couple of years ago I'm sure it is not an error signal but a complete malfunction.
Good luck
Jeremy
How long have you had the car? Has it ever worked properly? The reason I ask is that I have heard of people messing with the wiring in an attempt to fool the MOT inspector by extinguishing the light. What a wicked thought! - On mine the bulb had been removed. On reflection this is probably unlikely as you have 12 volts at both sides of the bulb.
From what I remember the multi pin plug comes apart quite easily. Is it worth trying to trace the pins on the control unit and metering the terminals controlling the light to see if they behave as expected? I would expect the 'earth' terminal to show nothing while the unit is checking and the bulb should be illuminated and then to rise to 12 volts as the bulb is extinguished.
Having had a look at this in some detail a couple of years ago I'm sure it is not an error signal but a complete malfunction.
Good luck
Jeremy
Jeremy
It is not an ecu fault as I have 2 units and both do the same.
The bulb and wiring are fine.
The feeds from the diode does change from 10.5 to 12V during start up without the engine running, but I think this is more to do with the voltage drawn by the heater plugs. I should try the same with the engine running. I did check that the voltage rises to 14+V when the alternator is charging but I didn't put it through its startup.
I really don't think it is has been extinguished on purpose. If it has I'd love to know how they did it.
In answer to a previous question, no it doesn't light up when driving with a faulty sensor (or disconected)
It is not an ecu fault as I have 2 units and both do the same.
The bulb and wiring are fine.
The feeds from the diode does change from 10.5 to 12V during start up without the engine running, but I think this is more to do with the voltage drawn by the heater plugs. I should try the same with the engine running. I did check that the voltage rises to 14+V when the alternator is charging but I didn't put it through its startup.
I really don't think it is has been extinguished on purpose. If it has I'd love to know how they did it.
In answer to a previous question, no it doesn't light up when driving with a faulty sensor (or disconected)
Tim
This is most mysterious. When I was having trouble with mine I did considerable research on the web and the one thing that did come out was that ECU unit trouble was not common. I came across various people who claimed to have had excess error codes removed by experts but I doubt if that was necessary, and suspect if it was done it was an 'expert' covering for having wasted his customers money by misdiagnosis and overlooking something basic.
It has always seemed to me that this system will put the warning light on if at all possible and that the light is the most reliable part of the system. The light will even come on (as it should) if the fuse is removed. The only time it doesn't light is if the brake light switch fails as I found out!
The curcuits can be divided up as follows:
sensors - presumably checked for continuity during the test phase by firing a current through them. otherwise dead at standstill?
stop light switch - dead at standstill - switches from monitoring to active when braking - NOT part of the check procedure as the curcuit is open during the test phase.
The relay/operating valve. The relay controls the heavy current required for the operating valve and is switched by the ECU. This can be heard to click (I think) during the startup phase. (there are so many relays clicking on my car I could have wrongly identified this one)
The control feed - from the ignition curcuit. It is in fact on the auxilliaries curcuit so the ABS tests without the need to start the engine. It is in this curcuit that the diode is to be found and also a connection after the diode to the relay.
I don't know which wiring diagram you have but I have found a full page of Haynes dedicated to ABS (p429 1995 edition and left out of the light blue one) and if you are interested I will send it to you by e-mail (assuming it copies properly which I think it will)
I know you have exchanged ECU's but I wonder if there is another fault which has blown both - for example a short curcuit allowing heavy current into one of the sensor connections? Having said that I would have thought the ECU was designed to be resistant to accidents and abuse.
jeremy.
This is most mysterious. When I was having trouble with mine I did considerable research on the web and the one thing that did come out was that ECU unit trouble was not common. I came across various people who claimed to have had excess error codes removed by experts but I doubt if that was necessary, and suspect if it was done it was an 'expert' covering for having wasted his customers money by misdiagnosis and overlooking something basic.
It has always seemed to me that this system will put the warning light on if at all possible and that the light is the most reliable part of the system. The light will even come on (as it should) if the fuse is removed. The only time it doesn't light is if the brake light switch fails as I found out!
The curcuits can be divided up as follows:
sensors - presumably checked for continuity during the test phase by firing a current through them. otherwise dead at standstill?
stop light switch - dead at standstill - switches from monitoring to active when braking - NOT part of the check procedure as the curcuit is open during the test phase.
The relay/operating valve. The relay controls the heavy current required for the operating valve and is switched by the ECU. This can be heard to click (I think) during the startup phase. (there are so many relays clicking on my car I could have wrongly identified this one)
The control feed - from the ignition curcuit. It is in fact on the auxilliaries curcuit so the ABS tests without the need to start the engine. It is in this curcuit that the diode is to be found and also a connection after the diode to the relay.
I don't know which wiring diagram you have but I have found a full page of Haynes dedicated to ABS (p429 1995 edition and left out of the light blue one) and if you are interested I will send it to you by e-mail (assuming it copies properly which I think it will)
I know you have exchanged ECU's but I wonder if there is another fault which has blown both - for example a short curcuit allowing heavy current into one of the sensor connections? Having said that I would have thought the ECU was designed to be resistant to accidents and abuse.
jeremy.
Jeremy
Thanks for your words of assistance.
My Haynes is the blue book, I gave the earlier one away and would appreciate the circuit diagram. david.timms@port.ac.uk.
Have changed the diode and relay, have disconnected sensor, all made no difference.
I suspect that the module is either not switching to earth or can't find earth. I have checked and cleaned all earths. I am still baffled but I will find it eventually.
dave
Thanks for your words of assistance.
My Haynes is the blue book, I gave the earlier one away and would appreciate the circuit diagram. david.timms@port.ac.uk.
Have changed the diode and relay, have disconnected sensor, all made no difference.
I suspect that the module is either not switching to earth or can't find earth. I have checked and cleaned all earths. I am still baffled but I will find it eventually.
dave