C250td battery ok but car electrically dead

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fwd

Active Member
Joined
May 27, 2007
Messages
83
Location
East Midlands
Car
C270 Estate - Smart ForTwo
Good evening all - bit of a puzzler here and any guidance from the learned forum members appreciated...
Battery OK but car TOTALLY dead electrically (ingnition key fob does not even turn in lock or make the usual click)! !
Happened briefly twice before but eventually started ok.
Suspected a loose battery terminal but found it tight and secure, disconnected the battery nevertheless and reconnected the terminals but noticed there was NO spark (like the one you get normally on connecting a battery to a car with various live consumer units like alarm, radio memory etc).
Out of habit! Checked security of all visible relays and fuses under the bonnet and boot and all looked ok.
Connected a jump battery and there was not the normal associated spark at the contact areas which leads me to believe that the positive wiring within the car is somehow electrically disconnected either by a stuck relay or some other misbehaving 'clever box'.
Resorted to momentarily touching the earth lead to the battery terminal several times until the 'proverbial' spark reappeared and hey presto, all is almost well and the car started on the button but with the interior lights flickering and ABS, BAS and OIL LEVEL lights on - flickering lights diminished on RPM increase but other lights remained on.
As this happened at a friend's house, we are taking the car to my place while all seems working.
One thought that springs to mind is that once the car is running it is fed electrically by the alternator (flickering lights) with the battery STILL isolated as the cause of the original problem..
Anyway, would appreciate any thoughts on this and in the meantime will attempt to get the car home and report back in a while
Many thanks
 
Your diagnosis is exactly right. The flickering is caused by the battery not being in the circuit. Don't run it this way as you could very well damage electronic components.
The good news is that faults like this are very simple to diagnose.
Get a test light (you can make one if you want). Don't use a voltmeter as this will give misleading results.
Now find if the problem is on the supply or earth. Find a good clean point on the engine or chassis and see if the testlight lights up when you put it onto the positive terminal. If it does, the fault is prolly in the supply side. If not, and it illuminates across the battery the the problem is in the earth circuit.
Now you need to trace where the main feed goes - probably straight to the fusebox, possibly via a master fuse or circuit breaker. A wiring diagram will help here.
Just take your time and be methodical and logical and I guarantee you will find the problem.
 
Thank you Ted, will put that into practice as soon as the problem re manifests itself as at present all was well on the way home - the curse of intermittent faults, SOD has a lot to answer to :)
If anything, I suspect a supply issue as the problem was the same with jumper leads (direct connect to chassis and car battery positive terminal/cable).
Thanks again and will keep you posted
 
No problems so far, have not had any re occurrence yet, and that's not for the lack of trying.:dk:
 
update...
finally failed at home so had time to have a good look. Turned out to be VERY high resistance between the +ve clamp and +ve post!!!
Took the clamp off to reveal a thin concentric band of 'crud' between the clamp and the post that has built up over many years.
All cleaned and reassembled and no flickering lights apparent with the only consequence, accidental or otherwise, being the OIL LEVEL warning comes ON within a minute or so after starting - oil level is on the full mark.
Am I right to assume the oil level switch is not an ON/OFF affair and it communicates with a clever box elsewhere handling the level rate and illumination delay?
 

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