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Electrics, Alternators And Multimeters


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#1 Madmitch

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Posted 07 September 2019 - 05:15 PM

We have an ancient but brilliant Micra K11 which will be 25 next year.  I put four new tyres on it a couple of weeks ago and then last week the very old battery died so I replaced that too.  Now find the new battery is going flat overnight and realise I have a significant parasitic drain.  Have pulled every fuse with the meter in circuit on the neg battery terminal and none stopped the discharge, have checked every relay bar one, which is up under the dash and which I just cannot get out, with same result.  The pos battery clamp has two cables plugged into it which both stop the drain when pulled.  When I did a continuity check between the battery end of the cables and the alternator terminal where the main cable into the alternator is bolted, both made a circuit.  Now suspecting I have an alternator diode failure so want to check for a circuit across the alternator terminal and case, in both directions, to see if I get a circuit in one direction only, both directions will confirm diode failure.

 

So, first question.  My multimeter is as old as me and has no position for testing diodes as modern meters do.  Normally I use ohms / resistance for circuit testing but I'm not sure which end of the scale to set it, presumably the minimum but guidance appreciated.

 

Second question.  The two cables that plug into the pos battery clamp vanish into the wiring loom so I can't tell if they do go direct to the alternator or if they connect to something else which might be the culprit.  Can anybody confirm that these are both alternator feeds and likely to run direct.

 

Any other thoughts and suggestions would be much appreciated!

 

Thanks in anticipation.

 



#2 TheHood

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Posted 09 September 2019 - 08:10 PM

One of the positive battery leads is probably going to the starter motor, so it could be worth having a poke around there for a chaffed cable / partial short.

For diode test just use the lowest value resistance range and reverse the probes to see if you get different readings.

Edited by TheHood, 09 September 2019 - 08:10 PM.


#3 Madmitch

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Posted 09 September 2019 - 10:27 PM

Thanks for your thoughts, much appreciated.  Since my original post I have borrowed a multimeter with a diode test setting, that worryingly shows nothing in both directions with everything off and disconnected, however it is charging the battery at 14.2v which is 1v over the resting voltage suggesting that the diodes and regulator are fine.  The other anomaly is that I am getting 31.8vac at the alternator rather than the expected 0.5vac!  Pulling the plug on the back of the alternator does not have any effect on the leak so that, in theory, says the alternator is OK.  So then somebody suggested pulling light bulbs and the first one I pulled, the roof light, cut the leak from off the scale to 55ma, fractionally more than the acceptable limit of 50ma.  On that basis I have left the bulb out and reinstalled the battery in the hope that I have found the problem and tomorrow the battery will be fine...…………...  



#4 siztenboots

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Posted 11 September 2019 - 10:10 AM

lucky find on first pull



#5 Madmitch

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Posted 11 September 2019 - 01:28 PM

Just a bit!!  Put the meter across the battery this morning and got 12.95v so fired up and went off to play Petanque, as we do on Wednesdays, it ran like a top.  Not to forget though that I still have a 55ma leak from somewhere and also is it worth trying to find the big leak in the interior light circuit...……………...



#6 TheHood

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Posted 11 September 2019 - 06:58 PM

If the interior light has a delay function as most do I'd be suspecting something funny with that.

#7 Madmitch

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Posted 11 September 2019 - 09:54 PM

No, it's just a straight forward on/off arrangement from the door switches plus a switch on the light giving on/off/courtesy.  Thankfully because it's old it is a very simple vehicle, no a/c, abs, screens etc, just fuel injection and ignition system of course.  I might take the roof light out of the car completely and run a continuity test to see if that is the problem but rather suspect that it is buried somewhere else in it's circuit which I have broken by removing the bulb. 






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