ammeter

[ron wilson]

I have a 59 TR3A that has an electric engine fan.
When I turn the headlights on the ammeter reads approx 10 volts positive. If I turn on the fan by itself the meter also indicates about 10 volts positive. However when I turn them both on at the same time the ammeter goes to about 10 volts negative. Before I start messing with the charging system would like to know any thoughts about what the problem might be.

 

[piman]

Hello Ron,
are these readings with an idling engine?
If so drive the car and see what you get.
I would normally expect a discharge with a heavy load and at idle.
If it shows a discharge when you are running, firstly check the fanbelt, if that's OK, then you may need to check the brushes on the dynamo.
By the way the ammeter indicates amps, not volts.

Alec

 

[ron wilson]

thanks for the info. The ammeter reads minus 10 amps(thanks for the correction) both at idle and at speed (belt is ok) so will take a look at the brushes.

 

[Dave Russell]

Hi Ron,

The way it stands, the generator is only capable of putting out 10 amps. It could be the brushes as suggested above or there could be a problem with the control box.

Here is a pretty good tech article on testing the generator & control box. Although it is for a Mini, they are pretty much the same. Section 5A.4 covers generator & control box testing.
https://www.coopermania.it/0646-05A.pdf
D

 

[ron wilson]

Thanks for the article. Looks like good information.

 

[Geo Hahn]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by ron wilson:
I have a 59 TR3A that has an electric engine fan...<hr></blockquote>

Me too. I just went out and ran it with the headlights and fan to see what I got. I didn't drive it but did take it up to 2000 rpm since (as noted) idle speed is to low to give you a meaningful idea of charging rate.

With the lights on a 10 amp charging rate is about what I saw. Turning on the fan made little change in this.

I still have a stock fan and only use my electric in stopped traffic on hot days (manually switched) so I never really see the headlights/fan combination.

If you have only an electric fan then you may often be using both and (considering where you live) the wipers too. That should still be within the capabilities of the stock dynamo but if you get into an amp on your stereo, driving lights, tape deck, cold morning starts, etc then you might have to look at some alternatives (e.g. an alternator conversion).

 

[Dave Russell]

I realize that this is old hat to most people but it may help a few.

---- What the ammeter reading means, a brief explanation ---


1- The ammeter is connected between the generator/alternator & the battery. It measures how much current is going into or out of the battery. Charge or discharge.

2- The amount of charging current is determined by battery state of charge & generator output capability.

3- How various loads affect the ammeter reading depends on whether the particular load is connected to the generator side of the ammeter or to the battery side. - See 7 below.

4- If a 10 amp load were conected to the generator side of the meter, the meter would not directly see this 10 amps. Eg. If the battery was receiving a 10 amp charge & a 10 amp load were connected, the meter would still show 10 amps flowing into the battery & the additional 10 amp load would not show, provided that the generator was capable of putting out 20 amps.

5- If the generator was only capable of putting out 10 amps, it's output would go to the 10 amp load & the meter would show zero going to the battery. If a 20 amp load were connected to this same 10 amp generator, 10 amps would come from the generator & 10 amps from the battery. The meter would show a 10 amp discharge.

6- Most older Lucas type generators are capable of putting out around 28 amps if they are in good condition & the belt is not slipping.

7- There are a few situations where a load may be connected to the battery side of the ammeter. Usually an intermittant high current load. In this case the meter will read the total generator output, not just the battery charging current. For this example, the battery is receiving a 10 amp charge & a 10 amp load is connected. The meter will now read the total charging plus load current of 20 amps.

8- For either connection, a fully charged battery will show no charging current on the meter. For the first case (connection type) (4) a load of 10 amps will not change the meter reading. In the second case (connection type) the meter would show the 10 amp load as an additional 10 amp charge. Provided that the generator is capable of putting out the required current.
D

 

[Bugeye58]

Dave, perhaps old hat to many, but my guess is that it will help more than you think.
If you are lucky enough these days to get any type of gauge for the charging system at all, it will more than likely be a voltmeter, so there may be quite a few people unfamiliar with the operation of on ammeter.
Of course, if you're an old fossil such as myself.............

Jeff

 

[ron wilson]

Thanks Dave and Geo. I should now have enough info to solve the problem.