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Voltage question?

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bgrushe Avatar
bgrushe Brent Grushecky
West Newton, PA, USA   USA
1977 MG MGB "None"
My 1977 mgb is showing 12.3 volts at the battery but at the ignition coil it is only showing 6 volts. The car started several days ago and I believe it showed around 9 volts at the coil. Let the car set for a few days and now I am at the first sentence and Tried to start the car but it acts as if the battery is draining when I start. Battery now measures 12.3 volts and six at coil. I'm sure there is some connection some where that is lose but where would be the first place to start looking.

Thanks,
Brent G.

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James74 James D
beach, VA, USA   USA
1978 MG MGB "The Bee"
Best to start at the battery If the battery is questionable take it out and have it load test at you local auto parts store as 12.3 volts could still be a bad battery. Also clean the terminals and out with a wire brush. If they look like they are ready to go replace them too. Once you get the battery back in and the terminals are clean and tight start the engine. Measure the voltage at the battery you should be getting about 13.40-13.80v if the alternator is working never seen a higher read with a Lucas alternator unless its going south.

Side notes: make sure your battery light lights up when you turn the ignition on. If it is not working the alternator will not charge, Also note the light should go out relatively quickly once the engine is started.

James



78..Bee, Leyland A/C (removed), Saturn 95 amp Alternator, Wire Wheels, Lumenition ignition, Weber 32/36, Four speed.


bgrushe Avatar
bgrushe Brent Grushecky
West Newton, PA, USA   USA
1977 MG MGB "None"
I put oil in dash pot and car fired right up. I guess I know of an O ring that may need replaced.

Thanks for your help.
Brent G.

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BobWhiles Avatar
BobWhiles Bob Whiles
Redlands, CA, USA   USA
Brent,

1) A little unclear here - "Tried to start the car but it acts as if the battery is draining when I start"

Are you saying you have a slow cranking? - slow cranking start? not slow cranking no start?

2) Something to think about?
Are you measuring static voltage or under a load (while cranking)?

There are two readings
For year and model I think 9 volts is what you would expect at the coil and 12 at cranking.

3) Don't be misled by the 12 volts at the battery - when you go to crank the engine what reading do you have at the battery?

A battery can have 12 volts - power the lights , sound the horn but not have enough amps ("left"winking smiley to crank or start the engine.

Might be time to
charge the battery and rerun your tests
charge the battery and test the battery
test the charging system

After you have this information then it's time to go looking for 'connection' issues after you know you have a healthy charging system

It's the reading you get under load (cranking) - If you assume that your battery is in good health just because it says 12 volts you could be wasting you time searching for another cause..

Yes - there could be other connection Issues


Bob



--------------
63' AH Sprite - Project waiting it's turn.
67; Morris Minor - Full Restoration in Progress
68' MGB GT - Driver
72' E Type 2+2 - Driver
86' XJ6 VDP - Driver

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
I suspect you are having a measurement problem. The later ignition coils have a resistor in circuit when the ignition is in the "run" position that drops the voltage to the coil. The problem is, when the points (or transistor) are open, there is no voltage drop and you get a full 12v at the coil. When the points (or transistor) are closed, you get about a 6v drop. So, the voltage will vary when the engine is stopped depending on what position the engine was in when it stopped. When the engine is running the voltage would be difficult to measure, partly because the points (transistor) are opening and closing and also there is a back EMF voltage from the coil itself. In short, measuring with the engine on will not be helpful.

I'm also not clear on whether the engine starts or not.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

BobWhiles Avatar
BobWhiles Bob Whiles
Redlands, CA, USA   USA
In reply to # 2431526 by ingoldsb I suspect you are having a measurement problem. The later ignition coils have a resistor in circuit when the ignition is in the "run" position that drops the voltage to the coil. The problem is, when the points (or transistor) are open, there is no voltage drop and you get a full 12v at the coil. When the points (or transistor) are closed, you get about a 6v drop. So, the voltage will vary when the engine is stopped depending on what position the engine was in when it stopped. When the engine is running the voltage would be difficult to measure, partly because the points (transistor) are opening and closing and also there is a back EMF voltage from the coil itself. In short, measuring with the engine on will not be helpful.

I'm also not clear on whether the engine starts or not.


I think the point is the measurement during cranking (not running) - which is a proper test to make.. The concern is starting the car or am I missing something? Which I would not start here taking these measurements if this is the case.

You have KOEO (Key on Engine Off) voltage should be about 6-9 volts (resister inline) (I have heard two specs for rubber bumber cars )
When you crank it, should jump to 12 volts
KOER should return to (6-9) volts

I don't recall ever having a problem measuring voltage at the coil with the engine running...

I also believe that all point systems have resisters in the circuits "somewhere" - voltage may vary depending on the design - without a resister the points would burn up "early"

I think the question is valid about the measurement BUT the real question is what is the problem you are having?
I assumed from what I read is that you car is hard to start "acts like the battery is draining" I interpreted "slow cranking"

I personally would not start at the ignition system - I would take measurement on the ignition system if I had a Crank No start..

Here the place to start is the Battery and determine if it's healthy - and remember that with a 12 volt battery the 12 volts is not the point -

1) check the battery -
If the battery has been correctly tested (load test) after it has a full charged
Then you are on to checking voltage at different points and then to Ignition timing
2) still slow to crank - ignition timing to be sure it's not too advanced - time it statically or retard the timing for giggles and try to crank.

I think starters either work or don't work so I don't think they would result in the "battery draining" situation
But my experience with starters is just that - that either work or don't (symtoms can be no crank)
Bob



B



--------------
63' AH Sprite - Project waiting it's turn.
67; Morris Minor - Full Restoration in Progress
68' MGB GT - Driver
72' E Type 2+2 - Driver
86' XJ6 VDP - Driver

RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Rubber bumper cars used 6 volt coils that had a boost to 9 volts, when the car was being started. RAY

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BobWhiles Avatar
BobWhiles Bob Whiles
Redlands, CA, USA   USA
In reply to # 2431600 by RAY 67 TOURER Rubber bumper cars used 6 volt coils that had a boost to 9 volts, when the car was being started. RAY

Hi Ray

I don't own a RB MG so I am in the dark (ohh but I own British cars so could be in the dark smiling smiley )

But I looked at the wiring diagrams and don't see a resister inline from the starter to the coil on any model year.
I see the resister for the "running" side of the circuit

I have been more than wrong once in any given day smiling smiley and have miss-read my share of circuits,, But it looks to me that the coil would get a full 12v at cranking without the resister -

So here is my question - How are they dropping the voltage from 12v down? Is there a resister in the wire that is not shown OR do the wiring diagrams not detail that the wire itself is a "resister wire" - Could that be it??

But as we all know the concept of "specs" and British Manuals is kind of in the shadows sometimes.

Either way the truth is the voltage to the coil is increased during cranking - I guess your mileage may vary --
And at 9 volts cranking that would make just as much sense to me so I am not arguing but I am curions .. . . . .

Bob



--------------
63' AH Sprite - Project waiting it's turn.
67; Morris Minor - Full Restoration in Progress
68' MGB GT - Driver
72' E Type 2+2 - Driver
86' XJ6 VDP - Driver

RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
The resistor, on rubber bumper cars, is indeed in the wiring harness. RAY

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ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
Quote: I think the point is the measurement during cranking (not running) - which is a proper test to make.. The concern is starting the car or am I missing something? Which I would not start here taking these measurements if this is the case.

You have KOEO (Key on Engine Off) voltage should be about 6-9 volts (resister inline) (I have heard two specs for rubber bumber cars )
When you crank it, should jump to 12 volts
KOER should return to (6-9) volts

I don't recall ever having a problem measuring voltage at the coil with the engine running...

I would recommend doing the test with the engine not cranking, but in a position that the points (transistor) are closed. That way the current is steady and you can get a stable reading - it should be somewhere around 6 volts.

There is an additional problem of measuring when *cranking* - during cranking the resistor is bypassed, so you will be measuring a full battery voltage at the coil.

As for testing with the engine running ... In the Olde Days, the moving coil meters would roughly average a pulsing voltage. So, I suspect that taking a reading off the coil (whose voltage would vary between 6v and battery voltage (~12v) every time the points opened and closed) was probably not a problem. I would imagine it would average to something close to 9v.

Modern digital meters "sample" the voltage and hold it for display. Of course, a pulsing voltage can really confuse some meters and you never know what they're going to read.

I think I've got this right - someone will no doubt correct me if I'm wrong! winking smiley



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

BobWhiles Avatar
BobWhiles Bob Whiles
Redlands, CA, USA   USA
Terry you make good points ( no pun inteneded smiling smiley )

There still is something missing here and I can't put my finger on it. I will have to check my library and get back as I am unable to test live since I don't have any points distributors in any of my cars. I use an analog meter when testing a coil not a digital meter (although I have many of both)

So not to challenge you but to test myself I will research further.

Bob



--------------
63' AH Sprite - Project waiting it's turn.
67; Morris Minor - Full Restoration in Progress
68' MGB GT - Driver
72' E Type 2+2 - Driver
86' XJ6 VDP - Driver

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
I just renewed my measuring device see picture for just €2,50 P&P included. It shows 0,2-0,3 Volts higher indication than the same, older unit.
After a drive my battery now reads 12,8Volts in rest unloaded and once driving of course moves to 13,4+ Volts (varies to 13,8V in this range) depending on the charging condition of my VARTA


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