Fried Alternator?

[SimonP85]

The auto electrician said its not worth repairing, as did Woodleigh Power (suggested above) as by the time you've got the parts and the labour to replace it you could buy a new unit, which is understandable.

It is tempting to leave it and see, but what I don't want is to discover its not charging when we're trying to come back into Portsmouth Harbour entrance!

I didn't try loading the battery while it was charging but the voltage was certainly rising and stayed at a higher level when the engine was turned on.

I think the safe bet will just be to get a new unit (and possibly diode splitter too) rather than risking things packing up mid Solent!

 

[Playtime]

I think the safe bet will just be to get a new unit (and possibly diode splitter too) rather than risking things packing up mid Solent!

I agree. Then you could take the old one apart at your leisure and maybe even get the bits to fix it and have a proper working spare.

Regarding the diode splitter, this is easy to test with a multimeter (MM) using the resistance scale (ohms). Just disconnect the 3 wires and test from the input to each output terminal, firstly with the negative MM lead on the input and then with the positive MM lead on the input.

The reading to both outputs should be low resistance one way round and high resistance the other. If you are not getting this result then it is definitely suspect and probably blown. At that point you might reconsider replacing it with a VSR.

 

[Como]

Using the normal resistance setting on a digital multi-meter (DMM) to test the diodes will not work but most DMMs have a position with a diode symbol that will. The voltage on this setting is higher than the other ranges to overcome the voltage drop in the diode.

 

[Playtime]

Using the normal resistance setting on a digital multi-meter (DMM) to test the diodes will not work but most DMMs have a position with a diode symbol that will. The voltage on this setting is higher than the other ranges to overcome the voltage drop in the diode.

Agreed - if the meter has a dedicated diode test facility, then this should be used.

However, a simple resistance test can also work. At least these people seem to think so.

 

[halcyon]

I didn't try loading the battery while it was charging but the voltage was certainly rising and stayed at a higher level when the engine was turned on.
QUOTE]

If you had charged the battery on mains, the voltage should have gone striaght up to regulation voltage. Slowly rising, or 13 volt is normally a sign of one diode gone.

Checking a diode with resistance, you will see around 300/500 ohm one way, and infinite resistance the other. It will not be a small difference, but 100's one way, million's the other way.

Brian

 

[VicS]

Analogue multimeters can be used on their resistance ranges to test diodes but not all digital meters can be used on all their ranges. Some will work on the higher ranges, some not at all.

Diode test range is therefore the range to use as already said.

 

[halcyon]

Analogue multimeters can be used on their resistance ranges to test diodes but not all digital meters can be used on all their ranges. Some will work on the higher ranges, some not at all.

Diode test range is therefore the range to use as already said.

Used resistance on a digital multimeter for years, in the old days we did not have a diode range. The only problem was to remember the probes are reversed, that is conducting is black probe to red probe, not red to black as with a diode test.

Brian

 

[VicS]

Used resistance on a digital multimeter for years,

Some will work some wont, Brian

IIRC my old digital AVO was OK but my little cheapy has to be used on the diode test range.

Also I think they differ in polarity on the leads but you dont have to worry about that. You just have to get infinity one way and a much lower reading the other.

 

[Playtime]

Used resistance on a digital multimeter for years, in the old days we did not have a diode range.

Brian

Yep - same here, hence my previous post. I still haven't got a diode test range.