Ammeter wiring and operation

[RobH]

There have been a couple of recent threads ( and a number of old ones) discussing ammeter problems and it appears there is a variety in the way wiring has been changed on some cars which makes the ammeter read oddly.  This post is just to clarify how things are supposed to be.

The ammeter is a useful instrument which can tell the user a lot about how the car electrics are functioning and what the state of the battery is.  It is intended to show what current is flowing into and out of the battery and it can only do that properly if the wiring is correct. It ought to be connected like this:

As the battery charges, current flows through the ammeter:

Other than for operating the starter, the only time the battery supplies current is when the engine is not running and  the generator (dynamo or alternator) is not working.  Apart from a few minutes after starting, while the battery is recharging, the ammeter should always show zero current if all is well. A discharge is only shown if the generator is not working or is unable to supply sufficient current and power is being drawn from the battery.

When the engine is running above idle speed, all the electrical power for the car comes from the generator .   It follows that all connections to the car circuits, other than the starter, should be connected directly to the generator output.    

If loads are connected directly to the battery instead, then the current to them from the generator will flow through the ammeter and is indicated as a continuous  ‘charge’.  This is incorrect but extra circuits for new devices are often connected that way by owners who do not understand this, because it is convenient to make the connection there.

When installing an alternator in place of the dynamo, sometimes an incorrect connection is made direct from the alternator to the battery.  In this case all currents drawn during normal running will show as a discharge on the ammeter and battery charging current will not be indicated.

What an ammeter can tell you:

When the ignition is turned on a small irregular discharge is shown during cranking. This shows the ignition and points are working.

When the engine starts there will be an initial high current charge indicated. This should reduce fairly quickly and the charge will then continue to drop at a slower rate over a few minutes until it settles at zero.  That shows the generator is providing a sufficient charge and that the battery is  working properly.

If the current reduces very rapidly to zero the generator may not be working properly or the battery may be losing capacity.

If a charge continues to be shown after some time it  may be the generator is producing too high a voltage or the battery is not charging properly.

If no charge is seen immediately after starting then there is something wrong with the generator or the fan belt that drives it.

 

When driving, the ammeter should show zero current all the time. If a discharge is indicated there is something wrong with the generator or the fan belt.   

( It is possible when driving at night and using lights, wipers, cooling fan etc simultaneously , that a car with a dynamo may show a small discharge. This is because the dynamo output is limited and may not be able to supply enough power for all the demand, though it may also be an indication that not all is well with the dynamo.)

 

With the engine not running, when turning on individual loads -e.g. brake lights - the current drawn will be shown as a discharge on the ammeter, indicating that the circuit is working correctly. (If LED bulbs  have been fitted the current will be very low of course and may just be a small twitch on the needle.)

 

 

 

Edited September 17, 2023 by RobH

[RogerH]

Hi Mods,

can this be pinned in the GENERAL section.

 

Roger

[Phil Kirk]

Rob, Thank you for taking the time to post this. Very helpful and easy to understand. I have printed off to keep with my manuals 

Cheers Phil

[Ian Vincent]

To the above I would add that when something like an electric fan kicks in and the engine is idling, the ammeter needle may twitch momentarily into discharge even with an alternator but as soon as the revs rise the needle should return to zero.  At least this is what mine does with a 40amp Nippon Denso alternator that is correctly wired.

Rgds Ian

[RobH]

If it does that Ian then your fan must be very power-hungry on start up and the alternator is incapable of supplying the peak current demanded.   It isn't a problem though.

 

 

Edited September 17, 2023 by RobH

[Andy Moltu]

4 hours ago, RogerH said:

Hi Mods,

can this be pinned in the GENERAL section.

 

Roger

Consider it done.

[Andy Moltu]

Of course if you have a volt meter the location of the take off for the equipment won't affect the reading.

[Ian Vincent]

41 minutes ago, RobH said:

If it does that Ian then your fan must be very power-hungry on start up and the alternator is incapable of supplying the peak current demanded.   It isn't a problem though.

 

 

I possibly run with the fan belt a smidgen on the loose side to avoid knackering the bearings on the water pump or the alternator.

Rgds Ian

[Z320]

Rob,

to write this reference post is great,

because this indeed has been „problem“ and confusion to some owners.

Thank you very much.

Do you allow me to use your text and illustrations for an article for our German TR IG club magazine, please?

Cheers, Marco 

[RobH]

Of course Marco - go ahead. 

[Waldi]

Excellent explanation Rob!

Similar to Marco’s request, can I post a translation in the Dutch TRCH club magazine?

Cheers, 
Waldi

 

[RobH]

Of course you may Waldi.  Marco found the diagrams were too low resolution for publication so I will PM you with better copies. 

 

[BlueTR3A-5EKT]

Thank you Rob.  I too will print out and add to my garage service and technical info

[Mk2 Chopper]

I wired my TR6 fuel pump via a relay direct from the battery, and of course this meant the ammeter needle wasn't at zero, once I understood what you've explained above I was able to change the wiring to allow the ammeter to work properly again. 

Thanks for the post, I'm sure it will help a lot of people. 

Gareth

[Richardtr3a]

Thank you very much Rob H for such a clear post and instructions. My ammeter does not show the correct reading. I think that this may be because my regulator was modernised incorrectly by me. I am looking for a solid state blade terminal RB106 to work with my dynamo C39 and the car is positive earth. I am staying with the points and quality plugs and the car is running well. Cold starting is a 30 second hand prime and then OK all day. 

I am sure that someone on this forum has a clear link to a quality regulator.

Thanks Richard & B

[RobH]

Part no 2, halfway down the page. 

https://www.dynamoregulatorconversions.com/reproduction-rb106-and-rf95-regulators.php

[Lebro]

+1 I had one from them on my 1935 Hillman. Fit & forget.

Seen your pm, this is the answer.

Bob

[Richardtr3a]

Thanks very much for such a fast reply. I have sent Classic Dynamo and Regulator Conversions an enquiry and will wait until they reply. Is there any TR owner who has one of their RB106 replacements ?

If there are some users on here please let me know.

Thanks Richard & B

[Richardtr3a]

Just when I thought that I had an answer, buy a new quality regulator.    I met an electrician this afternoon. We had a short discussion about my ammeter always showing negative. He explained that I had reversed the polarity. I am not sure how I could do that on a 12v system ??

I will be out in the garage tomorrow. It currently looks as if there are two wires connected to the ammeter. They are both substantial and the connections on the ammeter are not identified by numbers etc. One cable is plain brown and the other is brown with a white stripe.

I do not have a wiring diagram and if I change the two wires over will it then show positive charge when I am driving instead of negative.  I have fitted a solid state DVR3P made by dynamo regulators.com . It has a label showing 12v and 16 amps. They are not in business now making car regulators and only make motor cycle units.

I am hoping that there are some members on the forum who can explain. 

Thanks Richard & B

[RobH]

 Yes swapping the wires will reverse the indication but that is probably NOT your problem.  

When does your ammeter show a discharge and how big is the reading?  

Is it there all the time?

Does the reading go to zero when you switch the ignition off? 

Does it change if you switch the lights on? 

 

If your ammeter is showing a discharge all the time while driving it probably means the dynamo is not working.  

The ammeter should only indicate a charge immediately after starting the engine, and this reading should rapidly reduce to zero after a couple of minutes, as the battery re-charges.  At all other times  when driving, the ammeter should read zero .  The only time it should show a discharge is if the engine is not running and you switch something on. 

 

 

Edited October 9, 2023 by RobH

[Z320]

Hi Richard,

could you please tell us what you did before the issue appeared the first time, please?

Some photos from the engine, battery and regulator could also be helpful.

Your TR3 is really still positive or by accident negative earth?

My idea: your battery is fitted the wrong way round by accident

and have one or more electric devices directly connected to the battery.

If you have a radio fitted it should not work.

Ciao, Marco 
 

Edited October 10, 2023 by Z320

[Richardtr3a]

Thank you for the answers.Early today I had the bonnet open while the car was on the drive. The engineer , who was servicing the oil fired boiler in the house,  was interested. He offered to check the wiring with a more modern meter than I have.  He checked the positive earth connection for the battery which was correct. We disconnected the ammeter and then made the two connections clean and reconnected the ammeter. He ran checks on the battery, the dynamo and the fuses. He was very fast and in less than 10 minutes he finished. My car had a clean bill of health and the ammeter was working normally. After he had left the house and the car,  I put the ammeter back in the dashboard and it was a bit tricky to fit the brass hand nut that locates the fixing strap. But the ground was dry and the sun was out and the ammeter was fitted

How can this be possible after weeks of cleaning and checking ?? This morning I was going to buy a new regulator and may end up having to do that over the winter. But at the moment all is well.

Richard & B

[RobH]

A good result Richard. From what you describe I suspect that your problem was a bad connection in the output feed from the dynamo, or one of the other connections on the control box. 

 

 

Edited October 10, 2023 by RobH

[lynchpin]

It is also very important that the regulator box is correctly set up as this will affect the readings on the ammeter, my mechanic brother set mine when we refitted the dynamo after being rebuilt, it now does as previously mentioned and goes to high discharge momentarily on start up and quickly recovers, setting it up requires putting a piece of cardboard between some contacts, I am sure some of you now how to do this, it really made a difference to how the ammeter works when set up correctly..

  Phil..

Edited November 2, 2023 by lynchpin

[Z320]

I know what you mean,

but I’m sure this confuses a hobbyist more than it helps him