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OT unrelated question sorry :-)


eds
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My son has just come home with a Mitsubishi Colt. Very off topic so please forgive me :-)

Car starts absolutely fine with new battery...

Unbelievably the volt meter reads 0v on the battery terminals with the engine running.

While engine running i disconnect battery and all comes back to life, battery a healthy 12.7v and the disconnected terminals pumping 13.9v.

When you reconnect it it behaves normally for a minute then it all kills to absolutely 0v.

I can't even imagine what is happening unless the ecu is able to reverse the polarity of the alternator and fight the battery until 0v is achieved. Sounds highly improbable to me though.

But the battery recovers instantly so absolutely stumped.

Anybody got a view on this? Sorry to disappoint by lack of land rover involved, or 4x4 even. But knowing how nice everyone is I thought I would put it out there for discussion.

Thanks :-)

Ed

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It does sound like reverse polarity, but there would be a huge spark when connecting or disconnecting the battery. Are you sure the battery is correct? I.E. Polarity, could be faulty. Does the alternator look like it's been changed? If someone has put an alternator on from another car it could be running backwards. How old a car are we talking about?

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It is an r reg 1997 and the polarity is all good.

No alternator change.

No sparks on disconnecting battery with engine running. Recovers immediately. Both alternator and battery healthy. Battery new and starts car fine. Polarity is correct! Just 0v on multimeter...

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I will do try a new battery in the multi meter as that is an unknown variable but the thing does behave when used in all other normal circumstances. I tested it on the other car and it gave all normal readings and when I disconnect the battery with engine running reading immediately behave as expected....

0v is quite an achievement!

Ordinarily I would say this scenario is 100% impossible.

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Changed battery in meter, old battery was 8.6v which may be lower than tolerated.

Waiting for car to return to check situation.

Will report back, hopefully this nonsense has simply been solved as a multimeter error rather than a car error :-)

Thanks for the tips btw...

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Just to report back, new volt meter battery did not make any difference.....

I had to jump start the Colt's dead battery (10.5v) with a good car. Whilst the jump leads were connected it was charging at 13.9v as expected. Disconnected the +ve jump lead on the colt and the voltage dropped back to 11ish v. Then after about 30 seconds the voltage on the Colt started to plummet and got to zero in 30 secs and the car died. Tried taking the Colt's battery out of the circuit whilst being jumped but the moment the leads were removed the Colt died. Could not check the alternator on the back to see what is what as it is dark so waiting to tow home tomorrow.

I think the ECU is somehow reverse charging the battery. Or the alternator is. Not really sure how it gets to exactly 0v though.

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It isn't the ecu (or there would be huge smoke signals!) that is one hell of a current draw

Most likely suspects altenator but to be fair that is still a huge current draw (without a smoke signal), or.... And this is just a WAG... (No basis whatsoever) but have you tried starting the vehicle and disconnecting the starter motor lead just after the vehicle has started. There are normally two or three chunky leads at the battery one of them will go to the fuse box and the other off to the starter motor unfused

If you unbolt the starter lead before you start it then disconnect once started then see what happens and report back

Rob

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Diode pack in the Alternator.

My guess is that the diode pack is not rectifying the current out of the alternator and the AC is superimposed over the battery voltage. Try switching the meter to AC. If I'm right it will read an AC voltage with the engine running and 0v AC when stopped - the reverse of the DC reading.

Si

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I think all things so far are suggesting it is the diode pack as you say Si, makes perfect sense of the 0v which I must admit to not realising myself but now in hindsight seems perfectly obvious!

I have taken the alternator out and am now wondering whether a breakers one or a new one would be best..... £30 plays £150....

When it is all together again hopefully I can report full function and that it was the alternator that was broken.

Thanks for the help guys............

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Reporting back. I don't know exactly what failed but it is all working again.

I got a new alternator and it did not charge. Then changed the charging relay. Still no charge. Finally saw the slow blow 100 amp fuse was blown. When I fitted that it all came back to life.

The alternator was defo broke otherwise it would have shown the battery voltage when running at least and not 0v. It did this with new alternator.

How it blew the 100amp fuse I have no idea.

Anyway alls well that ends well.

Thanks for the support.

Ed

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