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MG MGA - Generator Testing

I was testing the generator/voltage regulator this weekend and I had a question.

The old owner commented that the car drained the battery when operating at night with the headlights.
I have been following the Workshop Manual instructions and those of Barneys site to check the operation of the generator.
Basically you join together both terminals and you measure the voltage between these terminals and ground. The reading should be approx 14v.
I have been testing it with a digital voltmeter and the readings I get are fluctuating a lot within one or two second.
I gets like 6v, then 0, then 12v, then 0, 9v, then 0, 19v then 0, etc (all this within a period of 1 or 2 seconds).... the current never stabilizes even if revving the engine a bit, and keeps moving.
Although the current ofter does go above the 14v required, I am not sure if the direct current is stable enough to charge the battery or is it too erratic?
I am guessing that my voltmeter is too sensitive and the generator output is cyclic by nature...

My question is; do you you think the generator is faulty (no constant DC output) or is this kind of reading normal when measuring it with a very sensitive digital voltmeter?

I have gone and finished the tests with the voltage regulator and didn't find anything abnormal.

If the car is generally draining the battery only when the headlights are on, what do you think is generally the issue?

REgards,

Gonzalo
G Ramos

Read the bit on how the regulator works with square wave modulation. It switches field current on and off at high frequency to yield a desired "average" voltage to the field coils. Without the battery connected the output voltage may change and dither too quickly for some digital volt meters to follow. Solution is to use an analog volt meter (moving needle type).

Alternate is to only check voltage with the digital meter when the battery is connected. I set mine around 15.5 volts at the dynamo output terminal (2500 rpm engine speed). It will be as much as one volt lower at the main battery cable on the starter switch. You do well to have close to 14 volts at the battery terminals. When you switch on headlights and accessories, almost all of the dynamo output goes to powering devices in the car with very little power left to charge the battery. In that situation voltage at the battery may be more like 13 volts. Voltages in general will be lower with low engine speed.
Barney Gaylord

Thanks for the response.
However I was testing the Generator output with the Battery connected.
I agree too I may have to purchase an analogue voltmeter.
I shall continue my investigation and also measure the voltage across the starter switch.

Thanks Barney!
G Ramos

Using these tests is only necessary for final fault finding.

Just to determine if you have a problem is very easy, even with a digital meter although as Barney points out an analogue meter is far better.

Simply start the engine and place the wires of the meter across the battery terminals (this can be as stated on the battery lead at the starter switch and earth) With the engine running at 2000RPM a reading of at least 14 volts should be on the meter. over 15.5 and I think it would be over charging.

If you do not have between 13.5 and 15.5 volts then you have a charging problem and therefore you will need to trouble shoot the problem as decribed on various websites including the specific MGA one of Barney's

The 2 main problems I have found are either worn out bushes in the dynamo (generator) or dirty points in the regulator.

If I understand Barney correctly I adjust my regulator to give out 14.5 volts across the battery which I think was Barney's technique?
Robert (Bob) Midget Turbo

A digital multimeter is not a good instrument to use for testing the generator with or without the battery connected. A digital instrument displays a value that has been sampled and then samples the input again. During the sampling period the reading remains on the last sample. Once the current sampling time expires, the reading is updated. This not only causes the the readings to jump around, if the generator is being tested by tying the field and output terminals together, you don't get a smooth rise shown on the multimeter and could very easily allow the output voltage to exceed safe limits without knowing it while the meter is taking the next sample. For this kind of a reading, even a cheap analog meter is far better than a digital meter. Cheers - Dave
David DuBois

Right. Not that I have solved my carburetor problems I went back to the Generator testing. To be honest I am a bit confused as to what the symptoms suggest;

I have charged the battery fully and it is now reading like 12v aprox.

This is what I found doing the various tests;

1- Voltage across starter terminal and ground is the same as the battery, and does not change with RPMs. In fact if you increase the RPMs the voltage goes slightly down (like -1v). Is something drawing current?

2- I connect the dynamo terminals together and measure the output of the dynamo and earth. Voltage moves steadily up and goes up to 50v if you rev the engine a bit! So looks like the generator is working.

3- Keeping the terminals link in, I measure the the yellow wire in the 'D' terminal of the regulator and it gives the same output as the generator. Yes, there is continuity I guess. By they in this set-up the ignition light got very bright and then burnt out.

4- Keeping the terminals link in, I measure the yellow/green wire in the 'F' terminal and I get an output of 12v which goes up to about 14v when I rev the engine.

I am going to study the diagrams and instructions in Barney's site to see if I can understand better what is going on. My feeling is that the regulator is not set right.
The ignition light also stays on while driving.

Any suggestions welcome.
G Ramos

Gonzalo -
DO NOT rev the engine up with the test link in place - that 50V will kill the generator! 20 V is absolute max!
Your control box is not working correctly, so concentrate on the tests and settings for that.

FRM
Fletcher R Millmore

Only 12.0V at the battery?

Sounds like the battery is shot or their are some bad connections in the main cables (cables to clamps, clamps to terminals, cable to earth point etc.)
Neil McGurk

Gonzalo,
I am a bit curious...didn't you write that a new alternator was included with your car? If so, why not just install it and save yourself a lot of aggravation?

Cheers,
Gerry
G T Foster

I got the Generator problem sorted. It took me like 4h of messing around, changing cables, and testing with the analogue voltmeter.

As usual, the only way I found to fix these kind of things is to understand exactly how I should work first and then mess around with it. So I spent some time reading the manual and tracing the wiring diagrams. ;0

I ended up changing the regulator (I had a new spare one-although the old generator was probably working too) and have to set it up as the factory setting was not right either. The contacts needed cleaning as they were not making good contact.
I found it was useful to measure the resistance across the D and F terminals while I did the adjustments so I can see the regulator working.

Anyway I took it for a ride after that and it looks good. The ignition lamp now turns off.


Gerry, you are right to be curious...I will soon probably install the alternator, but I was a personal challenge to get that working and drive it a bit in the original condition before I start making modifications. I guess then you guys have gone many times through the performance vs. originality debate before....
G Ramos

Gonzalo -
Good man, that's how you do it.
Tip; Points of any kind -ignition, fuel pump, regulators - oxidise when not in use, and commonly need to be cleaned when put back in service. Same goes for other types of electrical contacts, like light bulbs in their sockets.

FRM
Fletcher R Millmore

Gonzalo, The generator in a MGA is adaquate for keeping the battery charged when it is operating properly. I have never seen the need to change over to an alternator unless you are adding something that requires a lot of electircal power. It seems a lot of work for little gain to me. But, lots of people do it, so there must be a good reason for it. I will stick with the generator until convinced otherwise.
Ed Bell

Gonzalo,

Good for you. By the time you get your car sorted out, you’ll be the one giving us information!
David Werblow

Gonzalo,
I admire that you took the effort to learn how it works. The best way to have confidence in these old cars is understanding how everything works. That said, an alternator in more reliable than a generator and eliminates the need for the regulator. One less thing to worry about on the road. The beauty is, it's very easy to reinstall the generator if desired. But hey, either way is just fine.
Cheers,
GTF
G T Foster

As I commented earlier, I think I correctly set the generator to charge properly (I think anyway). The other day I drove with the headlights and it drained slightly the battery so went in an tried to re-calibrate the regulator.
The 'regulator' is set to give like 16.3v (where it stabilizes) which is what it says in the manual.
However I am not able to set properly the cut-off as my generator is ALWAYS giving at least 15v at idle (800rpms) so the cut-off is immediately operated as soon as I start the engine.
Am I missing something?

The reason I am asking is because today I had a look at the battery and there were a couple of drops of acid on the to surface of the battery around the positive terminal, and I was wondering if this could have something to do with improper charging (i.e. overcharging) or just that the battery is getting old (prod date oct'05).
What is your experience in this?


G Ramos

Definitively a regulator issue.
Acid droplets onto battery are indicationg an overcharging behaviour
Wipe them off and check again after a while.
Take care, these regulators are thermally compensated at adjustement stage, so its mandatory to carry on adjustements quickly in order to avoid discrepancies. I join you the AUTO BOOKS method for adustements and values. Alternators don't eliminate voltage regulators, these devices are internally built in alternators.


Guy RENOU

And page 1 now


Guy RENOU

Gonzalo

You can see the entire workshop manual here

http://clancy.ch/Workshop_Manual.html
dominic clancy

Dominic,


Pretty interesting documentation. I did it mine by bookmarking the link and I had been amazed by the epoch drawings illustrating factory procedures!
Where in switzerland are you living in ?
Guy RENOU

Ok, thank. I will check again. Indeed when I did adjust it since it took me a while to get it right probably the heating effect had kicked in...
Confirms what I thought about the acid droplets. I hope this is not dangerous...risk of battery explosion?
G Ramos

Guy,

You were right, I readjusted the voltage regulator cos it was putting out 17.5v (although when I set it I was sure it was more like 16.4v).
I think is pretty accurate now, and also the cut-off.

Guy, where are you from in France? I used to live in Toulouse before moving to the us with my wife (who is from Agen).

G Ramos

Hi Gonzalo,

I am happy to have been useful, good new. From my own, I ses no risk for battery explosion iprovided you leave car in a well ventilated area. About 16V is more accurate in terms of battery longevity.
I am living in Grenoble neighbourghood, just in front of snowy alpes. I too know Toulouse where I went occasionnally for my job and Agen ( les Pruneaux )and Bordeaux my in law family site.
Unfortunatly, I am not yet MGA owner.
Regards.
Guy RENOU

One other thing that sometimes causes battery drain when high electrical load is applied is a loose fan belt. The belt should not be excessivly tight or it can cause premature failure of the water pump and/or generator bearings, but it should not let the generator pulley slip when under heavy load.
Ed Bell

Guy, I'm in Zurich, but frequently come to Geneve and Lausanne.

Maybe you would like to join the mad guys driving through Switzerland on their way from China to the UK. We meet in Chur on 3rd July, and will drive over a few passes on the way to Lausanne and Geneva then onwards in the direction of le Mans. I can reserve you a hotel room if you like. Chur is not (yet) a problem on the rooms, but Interlaken has only one room left in our hotel. mail me directly if you would like to join in.

Dominic
dominic clancy

I am supposed to be in the US for 3years. Whenever I come back to France the plan is to take the MGA back...

Guy, good luck with the MGA hunting.

G Ramos

Thanks gents, It s fun to discuss MG with "overseas" fellows. Dominic sorry not to promess but, I am unavailable by July beginning, duty offsite with madam.
Many thanks for the proposal. One further time?
And enjoy.
Guy RENOU

This thread was discussed between 22/02/2010 and 08/03/2010

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