TR3A Volt Meter (Magnolia Face)

[lindatr4]

On 6/19/2023 at 10:52 AM, RobH said:

 

This is odd. If the ammeter still registers a discharge  for the lights it must be working OK.  First thought was that maybe the current loop had become unsoldered and had shorted to the case but that cannot be if it still works - though a remote possibility is that it has somehow burnt off the short circuit and is working through a very iffy connection.   Current must be going through it to work the ignition.  Also the lighting pick-off is from the alternator end of the ammeter so that connection must be OK too. 

You obviously need to pull the thing out to examine the internals and the connections.   It sounds as though either the alternator connection is no more, or the alternator isn't generating now.  If it has been feeding into a short circuit the rectifier stack will not have been happy about that. 

The ammeter will look something like this inside.  That loop of thick wire carries the current and if the solder gives way it might touch the case, resulting in a short circuit. 

(As explained several times before, just because the alternator can supply 45A doesn't mean it does.  The charging current is likely no different than with a lesser one as it is determined by the state of charge of the battery.  The 30A ammeter won't really care as the peak current is only there for a very short while.)

 

 

 

 

Edited June 23, 2023 by lindatr4

[lindatr4]

Thank you to everyone for your sound and helpful advice. 

[lindatr4]

On 6/23/2023 at 12:23 PM, lindatr4 said:

 

Fitted a new ammeter and new Dynamator on order. I am tempted to protect the circuit with an inline fuse but need some advice, bearing in mind that the short in the last ammeter led to the demise of the previous alternator

[RobH]

An in-line fuse is a good idea but the right value is important.  To protect the Dynamator you need a fuse which will blow to protect the rectifier stack. It's too easy to get it the other way round with the rectifiers blowing to protect the fuse.  

If the generator you have chosen is a 45A one then it would seem sensible to use a 40A in-line fuse (which is the maximum value for the standard sized blade type).  A modern blade fuse of that rating can carry 40A continuously without blowing.  It will take a fault current of 80Amps to make it blow quickly (under 1 second) so provided the rectifiers can withstand that overload, it will protect the Dynamator.    

The next standard fuse size up is 50A which would need a fault current of 100A to blow so may not provide any real protection. 

 

 

 

 

[lindatr4]

28 minutes ago, RobH said:

An in-line fuse is a good idea but the right value is important.  To protect the Dynamator you need a fuse which will blow to protect the rectifier stack. It's too easy to get it the other way round with the rectifiers blowing to protect the fuse.  

If the generator you have chosen is a 45A one then it would seem sensible to use a 40A in-line fuse (which is the maximum value for the standard sized blade type).  A modern blade fuse of that rating can carry 40A continuously without blowing.  It will take a fault current of 80Amps to make it blow quickly (under 1 second) so provided the rectifiers can withstand that overload, it will protect the Dynamator.    

The next standard fuse size up is 50A which would need a fault current of 100A to blow so may not provide any real protection. 

 

 

 

 

The Dynamator is indeed rated at 45 amp. There was I thinking a lower fuse rating would protect the Dynamator better and blow long before a repeat of the last excursion. Really nervous about fitting another Dynamator at £££. 

Removing the + terminal (car is now negative earth) should I be getting 12v between the cable and terminal post? 

Edited June 27, 2023 by lindatr4

[RobH]

2 minutes ago, lindatr4 said:

There was I thinking a lower fuse rating would protect the Dynamator better and blow long before a repeat of the last excursion.

Yes it would.  If the car is standard with no extra high-current loads you might get away with a 30A fuse but even that will need 60A to blow quickly.   

6 minutes ago, lindatr4 said:

Removing the + terminal (car is now negative earth) should I be getting 12v between the cable and terminal post? 

Where exactly is this cable and post? Do you mean at the battery?    

[lindatr4]

1 minute ago, RobH said:

Yes it would.  If the car is standard with no extra high-current loads you might get away with a 30A fuse but even that will need 60A to blow quickly.   

Where exactly is this cable and post? Do you mean at the battery?    

Sorry, yes at the battery

[Ralph Whitaker]

31 minutes ago, RobH said:

An in-line fuse is a good idea but the right value is important.  To protect the Dynamator you need a fuse which will blow to protect the rectifier stack. It's too easy to get it the other way round with the rectifiers blowing to protect the fuse.  

If the generator you have chosen is a 45A one then it would seem sensible to use a 40A in-line fuse (which is the maximum value for the standard sized blade type).  A modern blade fuse of that rating can carry 40A continuously without blowing.  It will take a fault current of 80Amps to make it blow quickly (under 1 second) so provided the rectifiers can withstand that overload, it will protect the Dynamator.    

The next standard fuse size up is 50A which would need a fault current of 100A to blow so may not provide any real protection. 

 

 

 

 

Hi Rob, I have an alternator fitted, and have put an 40amp maxi fuse in the main feed from the battery to all electrics in case of a short. I was thinking of all the battery power being run through the ammeter and a similar scenario to that above.  However my understanding is that an alternator should not be run if the battery is disconnected as that will blow the diodes etc. Is this correct?  In which case if the fuse blows it will disconnect the alternator anyway. I thought of wiring the alternator directly to the battery so that in the event of a short whilst driving the alternator would be saved, but then that would mean the ammeter would only measure a discharge, in which case a change to a voltmeter would make more sense.

Ralph

[RobH]

1 minute ago, Ralph Whitaker said:

  However my understanding is that an alternator should not be run if the battery is disconnected as that will blow the diodes etc. Is this correct? 

It might have been true in the very early days of alternators Ralph,  when the voltage reference was from the battery and diodes were limited in their voltage ratings -  but not now with machine-sensing regulators and modern diodes. . 

[RobH]

42 minutes ago, lindatr4 said:

Sorry, yes at the battery

There should be a reading of around 12.7V  from the battery terminal to earth, if the battery is fully charged.  I don't believe there should be a reading between the battery terminal and the cable if everything is switched off, as that would mean something was connected to provide an earth path and would be draining current from the battery all the time.  If you do get a reading perhaps you have a clock or radio which is always powered? 

Edited June 27, 2023 by RobH

[stuart]

One thing to bear in mind is those Dynamators dont tend to last long unless well protected from any heat as they have a shrouded case like a dynamo but with alternator internals that need a good amount of cooling.

Stuart.

[lindatr4]

2 minutes ago, stuart said:

One thing to bear in mind is those Dynamators dont tend to last long unless well protected from any heat as they have a shrouded case like a dynamo but with alternator internals that need a good amount of cooling.

Stuart.

A very valid point! Alternator terminals very close to exhaust down-pipe. Hopefully now rectified by fitting exhaust wrap.

[stuart]

Just now, lindatr4 said:

A very valid point! Alternator terminals very close to exhaust down-pipe. Hopefully now rectified by fitting exhaust wrap.

I would be also fitting a heat shield as well just to be sure.

Stuart.

[lindatr4]

20 minutes ago, RobH said:

There should be a reading of around 12.7V  from the battery terminal to earth, if the battery is fully charged.  I don't believe there should be a reading between the battery terminal and the cable if everything is switched off, as that would mean something was connected to provide an earth path and would be draining current from the battery all the time.  Perhaps you have a clock or radio which is always powered? 

I do have an aftermarket clock fitted but isolated for the time being. Getting a reading if I alternatively disconnect both terminals and connect multimeter across.

[RobH]

9 minutes ago, lindatr4 said:

Getting a reading if I alternatively disconnect both terminals and connect multimeter across.

Sorry, but again that isn't quite clear. What is it you are trying to measure?

[lindatr4]

1 minute ago, RobH said:

Sorry, but again that isn't quite clear. What is it you are trying to measure?

I'm trying to measure for any parasitic drain on the battery. I am getting a reading of around 12v between the battery terminal and the removed cable. No matter which cable I remove.

[RobH]

I'm not sure measuring voltage is going to tell you anything. Try measuring current instead.  Leave the battery earth wire connected, set the meter to (say) the 100mA DC range and connect it between the  + terminal and the power cable. 

[lindatr4]

20 minutes ago, RobH said:

I'm not sure measuring voltage is going to tell you anything. Try measuring current instead.  Leave the battery earth wire connected, set the meter to (say) the 100mA DC range and connect it between the  + terminal and the power cable. 

Assuming that I have made the correct multimeter setting (no 100mA setting) I have a reading of 1.3

[RobH]

So there is a 1.3 milliamp drain which means something is connected but it is only a very small current.   Do you still have the alternator connected ? If so can you unplug the leads from it and see if the reading changes? 

[lindatr4]

1 minute ago, RobH said:

So there is a 1.3 milliamp drain which means something is connected but it is only a very small current.   Do you still have the alternator connected ? If so can you unplug the leads from it and see if the reading changes? 

Alternator is out and awaiting a new replacement

[RobH]

Well that is very odd.  It's such a low current that it can't be any of the normal circuits.  Have you modified any of the wiring e.g. fitted a USB socket or anything like that?

[lindatr4]

25 minutes ago, RobH said:

Well that is very odd.  It's such a low current that it can't be any of the normal circuits.  Have you modified any of the wiring e.g. fitted a USB socket or anything like that?

I have fitted an auxillary plug socket and a few other extras but they are all isolated from any supply so it's a bit of a mystery where this small current is going!

Edited June 27, 2023 by lindatr4

[RobH]

The thing that concerns me is that you apparently have had a short circuit when things got rather hot.  It is possible this current is flowing through some carbonised insulation, which is a weak point which may fail later in a more comprehensive way. 

[lindatr4]

8 minutes ago, RobH said:

The thing that concerns me is that you apparently have had a short circuit when things got rather hot.  It is possible this current is flowing through some carbonised insulation, which is a weak point which may fail later in a more comprehensive way. 

Thank you RobH. You have given me much to consider and think about. I am grateful for your patience and knowledgeable advice.