Alternator low voltage output

[mikegunn]

Now it’s been established that the main problem is that the alternator does not appear to be charging, there are a series of steps that can be taken to determine the cause.
Firstly ensure continuity between the alternator and the battery’s positive terminal. Easiest way is to place one of your meter’s probes on the alternator’s positive terminal (usually the biggest and will have thick cable running to battery attached to it) and the other probe onto the alternator’s body. You should see battery voltage. If you don’t, then the fault lies either with the positive feed to the battery or the negative feed. This is where it’s useful to do a continuity resistance check. With your meter set on Ohms check the resistance between the battery positive terminal and the alternators positive terminal. Then check the resistance between the battery’s negative terminal and the body of the alternator. In both cases the resistance should be pretty close to zero Ohms.
if you are seeing battery voltage at the alternator you need to eliminate the voltage from the ignition switch as the cause. On the end plate of the alternator look for a spade terminal. Not the one marked “W”, but the other one which should be carrying a voltage from the ignition switch when turned on. Remove the wire from the alternator turn on the ignition switch and measure the voltage between that wire and the alternator’s case. It won’t be 12V but probably around 7 or 8V. It is this voltage that provides the initial excitation current to the alternator to coax it into life. If you don’t see a voltage the problem lies within the feed from the battery/ignition switch/alternator,

Mike

 

[rogerowen]

Now it’s been established that the main problem is that the alternator does not appear to be charging, there are a series of steps that can be taken to determine the cause.
Firstly ensure continuity between the alternator and the battery’s positive terminal. Easiest way is to place one of your meter’s probes on the alternator’s positive terminal (usually the biggest and will have thick cable running to battery attached to it) and the other probe onto the alternator’s body. You should see battery voltage. If you don’t, then the fault lies either with the positive feed to the battery or the negative feed. This is where it’s useful to do a continuity resistance check. With your meter set on Ohms check the resistance between the battery positive terminal and the alternators positive terminal. Then check the resistance between the battery’s negative terminal and the body of the alternator. In both cases the resistance should be pretty close to zero Ohms.
if you are seeing battery voltage at the alternator you need to eliminate the voltage from the ignition switch as the cause. On the end plate of the alternator look for a spade terminal. Not the one marked “W”, but the other one which should be carrying a voltage from the ignition switch when turned on. Remove the wire from the alternator turn on the ignition switch and measure the voltage between that wire and the alternator’s case. It won’t be 12V but probably around 7 or 8V. It is this voltage that provides the initial excitation current to the alternator to coax it into life. If you don’t see a voltage the problem lies within the feed from the battery/ignition switch/alternator,

Mike

Thanks Mike, this is helpful. I will run through these checks during some free hours in the week. If I find the problem I’ll be sure to post it for others. Thanks again.

 

[rogerowen]

Problem solved! Having cleaned and checked all connections I was still getting a very low voltage output from the alternator, so thought I'd drop a line to the company that I bought it from. They have received this question before especially from owners of canal boats and a few sailing boats that have very slow revving engines - to get the alternator charging a strong initial rev of the engine is required. I tried it, and it seems to work - 15 volts!. A mild rev didn't work, but a big one did. Still thinking about installing a voltmeter somewhere in the system to give me the confidence that the alternator is working.

 

[rogerowen]

One thing that still confuses me is that there was mention that there should be 12 volts from the battery measured between the alternator Positive and body of the alternator when the engine is not running. This I still cannot find, even with battery switch and ignition on.

 

[William_H]

One thing that still confuses me is that there was mention that there should be 12 volts from the battery measured between the alternator Positive and body of the alternator when the engine is not running. This I still cannot find, even with battery switch and ignition on.

In some cases of dual battery system a diode isolator will be fitted. This consists of 2 diodes from the alternator pos out put going to each battery positive. A diode is like a one way valve allowing current (voltage measured) from alternator to two different batteries but does not allow current from one battery to go to the other. Which also means no current from either battery back to the alternator. If this is not the case the alternator pops terminal should connect directly to the battery positive via an isolation switch. ol'will

 

[rogerowen]

Thanks William, You are absolutely right - my boat does have dual batteries and a diode system. Things electrical have never been my good point, but I'm learning. Thanks again.