flat battery

[yellt350t]

just jumped started friends car ,multi meter showing a -ve 14.69v,checked on own car showing +ve on meter so meter ok.car stayed running ,does the -ve indicate an alternator problem,as wiring looked ok ie -ve to body work etc.

cheers andy b

[prtvr]

yellt350t said:

just jumped started friends car ,multi meter showing a -ve 14.69v,checked on own car showing +ve on meter so meter ok.car stayed running ,does the -ve indicate an alternator problem,as wiring looked ok ie -ve to body work etc.

cheers andy b

the only time I have got a negative voltage at the battery is when the tester is connected the wrong way round,
IE if the positive probe is on the negative terminal of the battery, the voltage looks what I would expect from output from the alternator ( but not in the negative )

I made the same mistake a few weeks ago checking out a friend's corsa, it had a reddish brown lead for the earth, and not having my glasses on proceeded to put red to red, and then spent some time working it out.

Edited by prtvr on Wednesday 20th September 12:02

[Byff]

Is it an old car? Some old cars run a positive earth, but that's very rare these days. If it has a flat battery, has the battery been fitted incorrectly, would explain why it's gone flat.

[yellt350t]

no put tester to +ve on battery and -ve double checked car is an citeron ax and it kept its charge overnite???

[prtvr]

yellt350t said:

no put tester to +ve on battery and -ve double checked car is an citeron ax and it kept its charge overnite???

just to check out whats happening,

what is the reading from the battery, engine off tester set to dc volts ,red to plus black to minus

and then with the engine running, same set up,

[yellt350t]

initial charge on battery was 5.6v ,put jump leads on and started car, put multimeter on and it read -14v checked on own car and it read +ve, been told by workmate it could be a diode on alternator making current run wrong way, for the time being at least the car is still running, bit puzzled as i thought it would have flattened the battery quite quickly.

andy b

[prtvr]

yellt350t said:

initial charge on battery was 5.6v ,put jump leads on and started car, put multimeter on and it read -14v checked on own car and it read +ve, been told by workmate it could be a diode on alternator making current run wrong way, for the time being at least the car is still running, bit puzzled as i thought it would have flattened the battery quite quickly.

andy b

Puzzling to say the least... I can't see how it is still working.. do you get +ve 14v if you change round the tester on the battery ?.
I'm am no auto electrician, but running a reversed voltage in a car sound a bad idea, none of the electronic components should work, and what it is doing to the battery is any body's idea.
I have had a diode go down, but in my case all it did was give me a reduced output voltage, but in your case it could be the source of the problem,
If when the car is not running your battery is showing +VE and when it is running -ve, I think the problem must be with the alternator/voltage control components, that are usually built in to the alternator.

[yellt350t]

i did turn the tester round and it did read +ve.???
just waiting for a phone call to tell me its broke down,fingers crossed the phone wont ring..!
andy b

Edited by yellt350t on Thursday 21st September 19:17

[prtvr]

may be an idea to give a few places that repair alternators a ring and pick there brains,
in the yellow pages under car electrics there is a few, one I would start with is Teesside Start Parts
as they do 24 hr repairs