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Discussion Starter #1 (Edited)
After originally posting a thread about a battery drain on my Rover 25, I have now posted this new thread as the fault is now more specific.

After buying a new and decent multimeter :bgrin2: I found the following:

When doing the usual current draw measurements, and removing the fuses I got a 0.8 - 0.9A draw when I replaced the 15A alarm/ central locking fuse. When I delved deeper, I traced the current loss to a purple wire that leaves the engine compartment fusebox and terminates at the immobilser unit (according to Haynes wiring diagram). I am assuming that the wire carries
the trigger current to activate the central locking, but the diagram that I have only shows the immobilser control unit as a box, so I don't know what it does really.

:dunno:
Has anyone come across this fault before?
Can someone please tell me where the alarm control unit and immobilser are located - I am guessing that they will be hard to get at?:cus:
 

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Discussion Starter #2
I have found the alarm control unit (Lucas 5AS) and removed it and this cleared the 0.9A drain from the battery. It has been suggested to me that the alarm relay my be leaking to earth, but I cannot find that. Does anyone know where it is please?
 

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Mmmm....Rover..
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I have just connected up a 5AS and the quiescent current draw is 9 milliamps, 0.009A!

The alarm relay is located behind the fascia above the bonnet release lever.

However the relay draws about 0.145A and it's power isn't sourced through the 5AS ecu so it wouldnt show up if you measure the 5AS current draw.

I would guess that the 5AS itself is faulty, you could try disconnecting the various circuits that draw power directly from the 5AS/15A CDL fuse.

These are the CDL motors, the Indicators (flash or arm/disarm) and the Interior ultrasonic sensor.

The last item would be in serious trouble if it were drawing nearly an amp and as the first two are relay driven it would be unlikely that they could have any "leakage" current.
 

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Agreed with Technozen the 5AS is solid state ( well sort of) and unlikely to consume nearly an amp whatever is doing it it must be getting warm at 12.42w.I would be with the above at isolating the the subsequent circuits driven by the 5AS first.
 

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Discussion Starter #5 (Edited)
Well I am in well educated company with you guys - thank you!

Would I be right in thinking that if the 5AS was disconnected, the alarm relay would be isolated from that circuit too or at least from drawing current? I did actually feel the relays where you said and one appeared to be warm to the touch.

Would it be of any consequence that the ignition transponder appears to get warm when the car is reset to the condition before I disconnected anything? With the transponder being a coil of thin copper wire I thought that it would generate some heat? Am I clutching at straws in shear desperation here lol?

As a point of interest also, I disabled the immobilser using the ignition key and the code, but when I replace the 15A fuse, the fault is still there?

I'm sorry if I am appearing thick here, but it has got the better of me now and I may be guilty of overlooking the obvious :eek:(
 

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Mmmm....Rover..
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Disconnecting the 5AS will stop the alarm relay from operating.

My mistake, the 12v feed for the relay is from the 15A CDL fuse, but if it was energised through some fault within the 5AS then the sounder would be operating all the time.

The transponder coil is only energised for a brief period when the Ignition is switched on and it's driven with a 125KHz waveform, it shouldn't be getting the least bit warm!!

Disabling the immobilser doesn't switch the entire unit off, it will still draw current.
 

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Discussion Starter #7 (Edited)
Thank you again for your time on this one.

Right, I put the car back to fault condition and checked the transponder. It got extremely hot, almost too hot to touch near where the two wires connect to it.
I disconnected the transponder and the current draw disappeared ,well dropped to below 0.1A which is acceptable.
I have no circuit diagrams for this circuit, so I am still none the wiser. Could it just be the transponder, or could it be whatever is supplying it? I presume that the 5AS supplies the 125 KHz to it?

Update: I have just disconnected the transponder and disabled the immobiliser. The alarm still activates and the central locking works on the fob - so my wife will be a happy bunny! I think we can live without the immobilser, but I am just wondering if the immobilser can be re-activated by accident and leave my wife stranded because the transponder isn't fitted to the circuit!!
 

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Even though you have disconnected the coil I think the immobiliser will still work. The coil is for passive immobilisation. You should still be able to disarm and arm the immobilser with the fob buttons.

Get in the car wait for a minute and then try and start the car. It shouldn't start as the immobilser usally kicks in after 30 seconds.
To disarm it simply press the unlock button on your fob.
 

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Mmmm....Rover..
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The transponder coil just stops you having to press the disarm button each time you go to start the car.
The feature wasn't present on the earlier version of the 5AS.

Disconnect the coil and re-instate the immobiliser.

Having had a quick look inside a 5AS it seems that one side of the coil is connected to a 12v supply, the other side is connected to a transistor which switches to ground when driven by a 125KHz signal. My guess is that the transistor is short and thus the coil is permanently connected, I am surprised the coil hasn't burnt out.
 

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Discussion Starter #10
Thank you very much for the interest and time that you have all taken to help me with this nightmare of a problem. I have the transponder on my desk at this moment, and can't believe that such an innocent looking piece of technology could have caused so much grief! -well that it could be affected so badly by a tiny transisitor.
I will re-instate the immobiliser, if I can remember how to do it lol! (sure I've seen that somewhere recently on here) After that, I will do one final current check, hopefully for posterity ;o)

I will keep an eye out for other people with a similar problem and share my experience with them - lord knows this fault could have kept the battery and recon alternator industry going alone with us mere mortals!

Thanks again guys
 

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Discussion Starter #11
Still not sure

Although I have resolved the battery drain issue on this car, there still seems to be a problem.
When the car is started and running, the ignition barrel gets quite hot and so does the key.

Could this be a faulty ignition switch?
 

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mg_zr
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CL problems MG ZR 51 plate 5 door 1.4

I also have this same problem, I bought the MGZR from my mother-inlaw, it had the battery drain problem. A mechanic cut the purple cable from the fuse box and ran a cable directly into the igniton circuit and said this would stop th battery dain problem. It seemd to have worked, but the central locking would not work from then on.

I now have the car and would like the central locking to work again as I want to sell the car and would like everything to work so I can gt th best price.

Could any one describe the part I need to change?

Any help would b great, thanks in advance.
 
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