Alternator Duff?

[redhot]

Is my alternator faulty?
When I start my engine (BMC 1.5) the charge light goes out and the rev counter works, albeit lazily.

However if I put a meter on the batteries, I show less than 12 volts, so surely this means my alternator isn't working, doesn't it?

Please help as I am stuck.
If it is faulty, where can I get one? I rang Thorneycroft and the quoted me £70 +Vat.

Thanks
Mike

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[ccscott49]

Check your belt tension first, if your rev counter is an alternator driven electric one, the lazy rev counter is a dead giveaway. Also check your meter, against another battery, say the car. The battery starts the engine? check it before you start and after. Then get back to us.

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[redhot]

ok, I have checked the belt tension, seems ok. The meter seems ok, so is that conclusive that my alternator is dead?

Thanks
Mike.

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[oldharry]

Check the voltage of the battery with every switched off - preferably when you first go aboard. It should be around 12.8 volts for a fully charged battery. 12.5 volts is just about acceptable. start the engine, run it at a fast tickover for 3 or 4 minutes, and check the voltage again at the battery terminals. It should now read about 14.2 volts; anything much below 13.5 volts or over 14.7 volts would indicate that the alternator is not working properly.

Before ripping it out and throwing money at it, check around for corroded contacts, particularly earth or negative returns. If these are corroded they will effectively prevent a good alternator from charging the battery. A simple way of testing for bad contacts is to connect the multimeter between the casing of the alternator and the negative battery terminal with the alternator running. If the reading is more than .5 volt you have a bad terminal. If you can do so safely, do the same between the alternator plus output, and the plus terminal of the battery.

Now if the wiring is good, and the voltage is still low at the battery with the alternator running, the alternator needs attention. Not necessarily replacement although thats the simple and expensive option. Get it off and take it to an automotive electrical specialist (Yellow Pages) and ask them to test it, which most will do for free as it only takes moments on a test bench. They will then tell you whether repair is viable.

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by oldharry on 14/08/2003 08:27 (server time).</FONT></P>

 

[Strathglass]

You will get an exchange one in your local motor factors. Or as sugested it may be repairable. Do you have a 'Lucas' near you?.

Iain

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[ccscott49]

I concur.

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[webcraft]

I;ve got 14.0 volts coming out of the back of the alternator when metered, but anything from 12.4 to 13.7 arriving at the battery. When it's very low (12.4V) the charge light does not go off. When it's medium (eg 13V) the light is dim, and at 13.5V plus it usually goes out.

I am guessing that this is a bad earth of variable badness.

Is this likely, or is there a more sinister explanation? (Batteries are both new).

- Nick

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[ccscott49]

Sound suspiciously like a bad connection, you can check the volt drop across each cable as descibed earlier in this thread, that should show you the problem area, then it's just a case of finding the bad connection.

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[oldharry]

Definitely dodgy wiring/connectors from what you describe. One quick way that might just reveal the problem area is to squirt each contact with WD40, then wobble it, while keeping the meter connected at the battery. You will see straight away when the voltage starts to vary as the connector moves. Unfortunately WD40 only provides a temporary solution to the problem, as when it dries out the chances are the fault will return.

Another quick test of the earth return is to connect the alternator casing directly back to the battery negative. This will immediately show if the fault is anywhere in the earth return path, which is much more dificult to track down. The simplest way to resolve an earth fault is to connect the jump wire permanently (use 20 - 30 amp cable for a permanent set up though), running from one of the alts fastening bolts. I have sorted several iffy charging circuits this way. However, be very careful that the starter does not also find your new earthing path easier going - it will burn the cable out unless you use proper starter wiring cable.

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[oldharry]

Definitely dodgy wiring/connectors from what you describe. One quick way that might just reveal the problem area is to squirt each contact with WD40, then wobble it, while keeping the meter connected at the battery. You will see straight away when the voltage starts to vary as the connector moves. Unfortunately WD40 only provides a temporary solution to the problem, as when it dries out the chances are the fault will return.

Another quick test of the earth return is to connect the alternator casing directly back to the battery negative. This will immediately show if the fault is anywhere in the earth return path, which is much more dificult to track down. The simplest way to resolve an earth fault is to connect the jump wire permanently (use 20 - 30 amp cable for a permanent set up though), running from one of the alts fastening bolts. I have sorted several iffy charging circuits this way. However, be very careful that the starter does not also find your new earthing path easier going - it will burn the cable out unless you use proper starter wiring cable.

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