To test that my alternator is charging should I be able to find 14.2 volts etc when I put the multimeter on the terminals when engine is running?

At the moment I'm only finding the current battery reading which is 12.4 and decreasing after start ups as the alternator charge light won't go off when I reach 1000 rpm?

I'm thinking maybe it's not charging I would think I should be able to find the charge rate on one of the terminals when over 1000rpm?

Any help would be great..... Cheers

Na mate in a boat but its the same principle what I'm saying if I disconnect the 3 wires from the alternator and then test the terminals Should I find the charge rate there when the engine is running? Kinda do my own bench test

There's a regulator in there somewhere that could complicate things when you disconnect the 3 wires.
If the regulator is still in circuit you should see the 14 odd volts.
If it's not then you could get anything, from a small voltage, if there's no current being fed to the field coils, to high if there is.

Alternator light on and only battery surface voltage present shows the alternator is not charging. Run the engine and give the side of the alternator a sharp tap where you can,t damage the stator or connections. This may free up the brushes spring tension if they are getting short. If you have one plug with 3 wires then you possibly have an external regulator. If so bridge power from battery positive terminal to the "F" terminal and see if the alternator charges flat out (14-18v) depending on engine revs. Or take off both units and get them to your local auto electrician

dinsdale said... I don't really know, but I would guess that that the AC output, directly from the alternator windings, would go well over 100VAC at high revs - just guessing.

No need for it be anywhere near this high, so why would they design it that way? It would just make things a lot more trickier for the voltage regulator.

The battery charger will boil you batteries but won,t damage your alternator. Do you get your alternators marinized before you install them to stop any internal salt corrosion issues. Also what's size batteries are you running. If the output of the alternator isn't, high enough it may struggle to recharge batteries that have been flattened which shortens the life of the alternator.

Not so much salt water directly but rather condensation from engine room temp. You need to get a larger output alternator if you have 300ah worth of batteries. A 40a alternator is too light for the job required of it especially if you flatten the house banks. The alternator can,t get damaged by the battery charger as the diodes in the alternator protect it from this problem.

dinsdale said...

Harrow said...

dinsdale said... I don't really know, but I would guess that that the AC output, directly from the alternator windings, would go well over 100VAC at high revs - just guessing.

No need for it be anywhere near this high, so why would they design it that way? It would just make things a lot more trickier for the voltage regulator.

The AC output from the alternator windings will simply increase as revs increase. So will the frequency, but that doesn't interest us here. Most normal automotive systems will charge the battery (just) at idle. So, just plucking figures, allowing for losses in rectification and regulation it's probably delivering its output at around about 20VAC to 22VAC at idle. You'll recall that the rectified output from an AC input will be the Root Mean Square (RMS) - 0.707 x peak. So, I reckon my first guesstimates are somewhere in the ballpark. This doesn't make anything at all "trickier". It's no more difficult rectify 100VAC than 10VAC. Given how absolutely agricultural automotive electrical systems are, even the frequency makes no practical difference. That's why sensitive electronic devices have to protected when powered from your car.

You're probably thinking of a dynamo which has permanent magnets. Fundamental electrodynamics dictate that the open circuit voltage of a dynamo will increase linearly with speed.

Cars switched to alternators in the 60s. Alternators use an exciter coil to replace the permanent magnet. The regulator acts on the exciter coil so it's up to the electronics what the open circuit voltage might be. There is no reason why it should increase linearly with revs unless the designer of the regulator wanted it to. I'd guess they'd cap it before 100 volts just for the safety of mechanics under the hood, if nothing else.

It would be designed as a current generator rather than a voltage generator, so open circuit characteristics are just incidental.

dinsdale said...

The AC output from the alternator windings will simply increase as revs increase. So will the frequency, but that doesn't interest us here. Most normal automotive systems will charge the battery (just) at idle. So, just plucking figures, allowing for losses in rectification and regulation it's probably delivering its output at around about 20VAC to 22VAC at idle. You'll recall that the rectified output from an AC input will be the Root Mean Square (RMS) - 0.707 x peak. So, I reckon my first guesstimates are somewhere in the ballpark.

No it's not. The regulator regulates the field voltage -- so once there are enough revs the regulator will keep the output constant by reducing the field current as the revs rise. This will be 14.2V x 0.707 AC (so the peak of the wave does the work of providing the battery voltage). In other words, around 10 volts RMS AC (3 phase), regardless of revs or load. That's the whole point of having a regulator.

Getting back to the original question, you need field current to get the alternator to output voltage -- you can't just bung a multimeter across the terminals and spin it to see what happens.
Follow jenkz's advice and see what you discover. I'd also check that you have battery voltage on the large screw terminal on the alternator (even with the engine off). If not then you have a blown fusible link, a break in the wire between the battery and the alternator, or a bad connection between the eye terminal and the alternator itself.