Big Fuse - Quick Logic Check please
Discussion
Ok I am a bit of a numpty when it comes to under bonnet stuff but learning as we go. Am I correct in saying that the big fuse sits between the alternator and the battery, as such when big fuse goes you don't die instantly but you start eating into your battery power and then die, hence the little electrical gremlins giving you a headsup as you lose power. Changed mine a few weeks ago following failure and charged battery which was flat, all ok. Since then have been keeping it on trickle charger and done a few short journeys with no probs (daytime). Just gone out tonight, and died withing 10 mins, presumably because I was using headlights.
So if my logic is correct, I may have a simply not fitted the new fuse properly i.e. with a good enough connection and have been getting away with it because of the trickel charger or I guess worst case I have a bolloxed alternator?
So if my logic is correct, I may have a simply not fitted the new fuse properly i.e. with a good enough connection and have been getting away with it because of the trickel charger or I guess worst case I have a bolloxed alternator?
From the wiring diags i have the big fuse supplies the fuse box not the alternator, there should not be a fuse between the batt and the alternator. From your symptoms you may have a duff alternator. You need to see approx 14.4v (measured across the batt terminals) when running, 12v when stopped and 14v when running with loads switched on (lights, heater blower etc)
350matt said:
What happened to me recently was the wiring broke running from the red light on the dash to the back of the alternator, now this lead actually switches the alternator onto charge so if the wire breaks or the bulb fails then the alternator won't charge.
Matt
Matt
he is correct not many people know that it triggers the charging have you noticed the light flickering or staying on ????
So in summary a badly fitted or blown big fuse will not stop the alternator charging the battery and, as my car will run (albeit for a short time), then we assume the fuse is fitted ok? As the Batt light IS coming on we assume it in itself and the wiring is ok so this leaves only the alternator as the problem...Am off to buy a meter before condemning the alternator fully. Is there a way to check the alternator output directly or is it only done at the battery end, only reason I ask as this would still not rule out a bad connection somewhere would it. Sorry for all the dumbass questions, as I say trying to learn more without resorting to paying out for every niggle.
Is this the sort of thing that I need:
www.maplin.co.uk/Search.aspx?C=AddonBasketFillers&T=N15BY&criteria=N15BY&DOY=29m8
Is this the sort of thing that I need:
www.maplin.co.uk/Search.aspx?C=AddonBasketFillers&T=N15BY&criteria=N15BY&DOY=29m8
Edited by Touching Cloth on Tuesday 29th August 11:54
The usual quick check is to check across the battery voltage. When charging (i.e. engine running) you should see about 13.5v across the battery, maybe a bit more. This is a good sign the battery is getting a charge.
If you are lucky it might just be the diode pack on the back which is faulty. A good auto electrician should be able to tell you if the alternator is OK.
If you are lucky it might just be the diode pack on the back which is faulty. A good auto electrician should be able to tell you if the alternator is OK.
That meter will be fine.
A blown big fuse will stop the alternator from charging. This was the case with mine.
The first check to do is measure the DC Volts across your battery. Set your meter to DC Volts and a range of 10-20 Volts. Place the meter red lead onto the +ve of your battery and the meter black lead to a suitable earth - the oil cap is normal ok. Do this without the ignition on and note the reading. The car will probably fire up with 11.5V, but not much less. Then, fire her up and take another measurement (be careful with the meter leads). If the reading has gone up by at least 1V then your Alternator is changing ok - anything above 13.5V indicates normal charging (the exact value can change depending on the load). If this is the case, I'd get your battery checked out by a car electrical place as it almost certainly the battery not holding a charge.
If the reading doesn't change, then you need to get a reading either side of your big fuse. This will help you determine if the big fuse has been fitted correctly or has blown. Now this can be fiddly and potentially dangerous and is always useful to have somebody at hand to help. With the car running and your black meter lead on a suitable earth, you need to carefully place your red meter lead on one side of your big fuse - use the nut if you can. You must make sure you don't short between the big fuse and any other metal around the engine - remember this fuse is capable of sinking 100 Amps. Be very careful. Note the reading and do the same again on the other side of the big fuse. If the 2 sides are different (1V or higher) then your big fuse has blown or is not in correctly. Even a hair line crack in the fuse will limit your alternators chanrging ability. If the 2 sides of the big fuse give you same reading, then it can be 1 of 2 things. 1) if the reading is the same as the battery reading you did first, then it's more than likely your alternator (normally it's the rectifier module within the alternator that's blown - not the motor itself and hence not a complete disaster). 2) If the big fuse readings are 1V or more higher than your battery reading, then there's a break in the connection between your big fuse and your battery.
Note that if any of the readings go up by less than 1V, this could still point to your rectifier in your alternator.
Sorry for the long reply, but I hope this helps
A blown big fuse will stop the alternator from charging. This was the case with mine.
The first check to do is measure the DC Volts across your battery. Set your meter to DC Volts and a range of 10-20 Volts. Place the meter red lead onto the +ve of your battery and the meter black lead to a suitable earth - the oil cap is normal ok. Do this without the ignition on and note the reading. The car will probably fire up with 11.5V, but not much less. Then, fire her up and take another measurement (be careful with the meter leads). If the reading has gone up by at least 1V then your Alternator is changing ok - anything above 13.5V indicates normal charging (the exact value can change depending on the load). If this is the case, I'd get your battery checked out by a car electrical place as it almost certainly the battery not holding a charge.
If the reading doesn't change, then you need to get a reading either side of your big fuse. This will help you determine if the big fuse has been fitted correctly or has blown. Now this can be fiddly and potentially dangerous and is always useful to have somebody at hand to help. With the car running and your black meter lead on a suitable earth, you need to carefully place your red meter lead on one side of your big fuse - use the nut if you can. You must make sure you don't short between the big fuse and any other metal around the engine - remember this fuse is capable of sinking 100 Amps. Be very careful. Note the reading and do the same again on the other side of the big fuse. If the 2 sides are different (1V or higher) then your big fuse has blown or is not in correctly. Even a hair line crack in the fuse will limit your alternators chanrging ability. If the 2 sides of the big fuse give you same reading, then it can be 1 of 2 things. 1) if the reading is the same as the battery reading you did first, then it's more than likely your alternator (normally it's the rectifier module within the alternator that's blown - not the motor itself and hence not a complete disaster). 2) If the big fuse readings are 1V or more higher than your battery reading, then there's a break in the connection between your big fuse and your battery.
Note that if any of the readings go up by less than 1V, this could still point to your rectifier in your alternator.
Sorry for the long reply, but I hope this helps
thegreengoblet said:
That meter will be fine.
A blown big fuse will stop the alternator from charging. This was the case with mine.
The first check to do is measure the DC Volts across your battery. Set your meter to DC Volts and a range of 10-20 Volts. Place the meter red lead onto the +ve of your battery and the meter black lead to a suitable earth - the oil cap is normal ok. Do this without the ignition on and note the reading. The car will probably fire up with 11.5V, but not much less. Then, fire her up and take another measurement (be careful with the meter leads). If the reading has gone up by at least 1V then your Alternator is changing ok - anything above 13.5V indicates normal charging (the exact value can change depending on the load). If this is the case, I'd get your battery checked out by a car electrical place as it almost certainly the battery not holding a charge.
If the reading doesn't change, then you need to get a reading either side of your big fuse. This will help you determine if the big fuse has been fitted correctly or has blown. Now this can be fiddly and potentially dangerous and is always useful to have somebody at hand to help. With the car running and your black meter lead on a suitable earth, you need to carefully place your red meter lead on one side of your big fuse - use the nut if you can. You must make sure you don't short between the big fuse and any other metal around the engine - remember this fuse is capable of sinking 100 Amps. Be very careful. Note the reading and do the same again on the other side of the big fuse. If the 2 sides are different (1V or higher) then your big fuse has blown or is not in correctly. Even a hair line crack in the fuse will limit your alternators chanrging ability. If the 2 sides of the big fuse give you same reading, then it can be 1 of 2 things. 1) if the reading is the same as the battery reading you did first, then it's more than likely your alternator (normally it's the rectifier module within the alternator that's blown - not the motor itself and hence not a complete disaster). 2) If the big fuse readings are 1V or more higher than your battery reading, then there's a break in the connection between your big fuse and your battery.
Note that if any of the readings go up by less than 1V, this could still point to your rectifier in your alternator.
Sorry for the long reply, but I hope this helps
A blown big fuse will stop the alternator from charging. This was the case with mine.
The first check to do is measure the DC Volts across your battery. Set your meter to DC Volts and a range of 10-20 Volts. Place the meter red lead onto the +ve of your battery and the meter black lead to a suitable earth - the oil cap is normal ok. Do this without the ignition on and note the reading. The car will probably fire up with 11.5V, but not much less. Then, fire her up and take another measurement (be careful with the meter leads). If the reading has gone up by at least 1V then your Alternator is changing ok - anything above 13.5V indicates normal charging (the exact value can change depending on the load). If this is the case, I'd get your battery checked out by a car electrical place as it almost certainly the battery not holding a charge.
If the reading doesn't change, then you need to get a reading either side of your big fuse. This will help you determine if the big fuse has been fitted correctly or has blown. Now this can be fiddly and potentially dangerous and is always useful to have somebody at hand to help. With the car running and your black meter lead on a suitable earth, you need to carefully place your red meter lead on one side of your big fuse - use the nut if you can. You must make sure you don't short between the big fuse and any other metal around the engine - remember this fuse is capable of sinking 100 Amps. Be very careful. Note the reading and do the same again on the other side of the big fuse. If the 2 sides are different (1V or higher) then your big fuse has blown or is not in correctly. Even a hair line crack in the fuse will limit your alternators chanrging ability. If the 2 sides of the big fuse give you same reading, then it can be 1 of 2 things. 1) if the reading is the same as the battery reading you did first, then it's more than likely your alternator (normally it's the rectifier module within the alternator that's blown - not the motor itself and hence not a complete disaster). 2) If the big fuse readings are 1V or more higher than your battery reading, then there's a break in the connection between your big fuse and your battery.
Note that if any of the readings go up by less than 1V, this could still point to your rectifier in your alternator.
Sorry for the long reply, but I hope this helps
Now if I can get clients to stop bothering me with pointless phonecalls I can get down to Maplins
Just to update on this, think I have sorted it, big fuse when it blew had burnt out quite a nice black mess and think this was causing the problem with getting a decent charge through, cleaned up as best I could and now showing a definited increase up around 13v with the engine running compared to when switched off. Only odd thing is that if you hold the meter on there the voltage drops slowly back down to around 12v, if you remove and reattach it shows 13+ again. Anyhow no sign of flickering battery light and the gauges are working normally again, will get out for another prolonged blat and see how we go. Thanks for the advice peeps.
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