Battery drain - alternator
Discussion
Hi all,
I wanted to pick the collective PH brain about a electrical issue with my 20 year old (996) 911.
It's been draining the battery whilst left on the drive for a few weeks now.
Took it into a nearby garage, explained the symptoms and they said "alternator is faulty" we need to replace it.
£700 later and they've done that, now say there's still a drain and need to investigate further.
Im not even convinced the alternator was faulty, I suspect it may have been the alternator's regulator, but I needed the car fixed, so I let them get on with it.
They said they couldn't diagnose what caused the drain before replacing alternator because it could have been the alternator causing it with a faulty diode reversing polarity.
Sounds like BS to me, but I'm not knowledgeable enough to know for sure.
What do you guys think?
I wanted to pick the collective PH brain about a electrical issue with my 20 year old (996) 911.
It's been draining the battery whilst left on the drive for a few weeks now.
Took it into a nearby garage, explained the symptoms and they said "alternator is faulty" we need to replace it.
£700 later and they've done that, now say there's still a drain and need to investigate further.
Im not even convinced the alternator was faulty, I suspect it may have been the alternator's regulator, but I needed the car fixed, so I let them get on with it.
They said they couldn't diagnose what caused the drain before replacing alternator because it could have been the alternator causing it with a faulty diode reversing polarity.
Sounds like BS to me, but I'm not knowledgeable enough to know for sure.
What do you guys think?
Edited by youngsyr on Tuesday 17th March 17:06
I think they are rubbish.
Current drains are really easy to find. I don't know why people have so much problems with them
would take five seconds to decide whether the drain is through the alternator. You just measure the current drain through the battery with a meter and disconnect the alternator. (I think its very unlikely to be a short through the alternator and diode packs don't usually short circuit, mainly they open circuit).
I would ask for your old alternator back t keep as a spare
Current drains are really easy to find. I don't know why people have so much problems with them
would take five seconds to decide whether the drain is through the alternator. You just measure the current drain through the battery with a meter and disconnect the alternator. (I think its very unlikely to be a short through the alternator and diode packs don't usually short circuit, mainly they open circuit).
I would ask for your old alternator back t keep as a spare
They are normally in the black plastic box at the back of the alternator.
As others have said if a new alternator hasn’t fixed it then it was unlikely to have been the alternator.
Could be central locking motors jamming or an alarm issue but they are wild guesses- as others have said you need to put an ammeter in the battery feed to see any parasitic drain and then try to find the source by taking the fuses out until the drain stops.
As others have said if a new alternator hasn’t fixed it then it was unlikely to have been the alternator.
Could be central locking motors jamming or an alarm issue but they are wild guesses- as others have said you need to put an ammeter in the battery feed to see any parasitic drain and then try to find the source by taking the fuses out until the drain stops.
Could be a tracker draining the battery. They have a built in battery which normally means it only recharges when the car is running, but after a number of years the internal battery degrades such that it recharges from the main car battery more often and will do so when the car is not running.
Connect a multimeter across the battery with the car running and set the meter to read AC volts.
If the meter reads above 0 volts, one or more of the rectifier diodes is bad.
With the engine off remove the ground cable from the battery. Set the meter to read DC amps and set it to it's highest range, normally 10 amps. Put one lead on the battery ground post and the other on the ground cable so the meter bridges the two. Do not start the engine. You should see the higher than normal amperage draw on the meter. Start removing fuses until the amperage drops to between 10 and 50 milliamps.
Gives you somewhere to start looking.
If the meter reads above 0 volts, one or more of the rectifier diodes is bad.
With the engine off remove the ground cable from the battery. Set the meter to read DC amps and set it to it's highest range, normally 10 amps. Put one lead on the battery ground post and the other on the ground cable so the meter bridges the two. Do not start the engine. You should see the higher than normal amperage draw on the meter. Start removing fuses until the amperage drops to between 10 and 50 milliamps.
Gives you somewhere to start looking.
GreenV8S said:
You should not need to pull fuses or disconnect battery cables.
A typical modern digital multimeter has enough resolution to measure the sort of current drain you're looking for via the voltage drop across its fuse.
Why would you have a voltage drop across a fuse. Its a straight bit of metal about 1cm long. It will have no resistance across it and therefore no voltage drop?A typical modern digital multimeter has enough resolution to measure the sort of current drain you're looking for via the voltage drop across its fuse.
julian64 said:
Why would you have a voltage drop across a fuse. Its a straight bit of metal about 1cm long. It will have no resistance across it and therefore no voltage drop?
Fuses have a known and carefully controlled resistance. It's integral to their operation; how would a fuse blow if the resistance was zero?C4ME said:
Isn't the theory that fuses have a low resistence so as not to intefere with the operation of the circuit but a slightly higher resistence than the rest of the wiring so they heat up and melt first?
Different fuse types have different resistances. The standard figures are published in various places, but for example a typical standard 15A automotive blade fuse would be in the region of 5 mOhms. A parasitic current that flattens a 50Ah battery in a week would be of the order of 0.3A. Ohms law says the voltage drop for that combination will be a little over 1mV, which is well within the range that can be measurable by a typical modern digital multimeter.GreenV8S said:
C4ME said:
Isn't the theory that fuses have a low resistence so as not to intefere with the operation of the circuit but a slightly higher resistence than the rest of the wiring so they heat up and melt first?
Different fuse types have different resistances. The standard figures are published in various places, but for example a typical standard 15A automotive blade fuse would be in the region of 5 mOhms. A parasitic current that flattens a 50Ah battery in a week would be of the order of 0.3A. Ohms law says the voltage drop for that combination will be a little over 1mV, which is well within the range that can be measurable by a typical modern digital multimeter.Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff