Volvo alternator/tacho

I

ians

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Evening All
Just need a bit of advice
Just had to replace a faulty battery master switch, which had switched off a couple of times while the engine was running.
fitted new one not the alternator warning light and buzzer is going off.
the system is charing but the tacho is dead.
read that if the master switch is shut off while the engine is running it can kill the alternator.
is this the case, I have checked all wiring etc.

any advice please

regards


ians
 
Barry

The alternator is charging, but does not show through the gauges.
I have checked all the wires and they seam ok.


ians
 
Disconnecting the battery from the alternator while it is running can blow the diodes. If it is still apparently charging OK all may not be lost but it would be advisable to check that you can still get the full normal current output. You may have blown some of the diodes but not all of them.

If you have any doubts about it get it checked by an automobile electrician.

I do not understand what you are saying about the warning light. If it is not behaving normally get the alternator checked.

I do not understand what you mean by "not showing through the gauges"

Do you have a voltmeter if so what's it reading, do you have an ammeter if so what's it reading.

How do you know it's charging?
 
Sorry Viv
Charging I tested at the batteries with a voltmeter, got 14.6 volts

There are volt meters on the instrument panel, these do not show any charge while the engine is running, ie sit at just approx 12 volts

There is a warning light on the ignition which is staying on aswell.

Looks like its a blown diode

thanks for your reply

ians
 
Sorry I cannot rationalise the observations. If the waning light is on it indicates that something is wrong.

14.6 volts is a bit on the high side. I dont understand why you can get 14.6 volts on a independent meter but only 12 on the panel meters (unless the battery in the independent one is on its way out)

Maybe the regulator has gone haywire. I definitely think you should get the alternator checked. If your tacho is fed from the alternator them get that output checked.
 
Double check the connections, checks for breaks in any wires you may have disturbed.

if you blow just one diode in a bridge rectifier the whole lot is shot and you get nothing from the alternator. As the diode feeds the field current from the alternator output across the warning light equalising the voltage and the light goes out, but if you've blown a diode no output, no field output then the light contiues to shine. but you say you are getting 14.6v at the battery, so therefore you are getting output and it indicates a problem with the supply line to the panel. Double check you are getting output.

14.6 v is high end of normal , try substituting the multimeter for the panel meter, and also test the panel meters with a known voltage if the panel meters check out and the multi shows 12v the problems downstream of the panel meters, just keep checking back to source.

Also check the source of the power / signal to the tacho it may be an independent failure.

Did your panel meters ever show higher than 12v previous to your present troubles?
 
[ QUOTE ]
if you blow just one diode in a bridge rectifier the whole lot is shot and you get nothing from the alternator

[/ QUOTE ] Sorry that's not true. Its a3phase generator and a full wave 3phase rectifier. You've got to lose all three phases before you get nothing!

Had it happen to an old Lucas ACR alternator. On that you could see all the diodes and some of then had physically broken. How I realised something was wrong is not relevant but it was still generating with several broken diodes.

There are different ways in which the field current is obtained too. Most have a separate trio of field diodes but it is possible for the field current to be supplied from the main rectifier. In that case the main output from the alternator passes through a blocking diode
 
Barry

I will go back and check all the wires again, yes the instrument volt meters did read higher before when the engine was running.
now they just read 12 volts.
I may swop the alternator from the other engine to see wether that works ok and that should tell me if its the alternator at fault

ians
 
I second Baza in that the multimeter may be over reading. This occurs with flat batteries in a digital meter. So check the ships battery with engine stopped or your car battery if they all show higher than they should put a new battery in multimeter.

If this is the problem then your alternator is not charging at all.
And yes I agree an alternator will give some current with some diodes blown that current can show 14 volts at the battery. But it would take a long time to recharge a flat battery. olewill
 
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