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MG Midget and Sprite Technical - Stuttering Fault

My Sprite has an electric fan - ex Rover 400 scrap yard model. Controlled by a Ford Fiesta thermost. It rarely comes on. But if it comes on when I am stationary, as when waiting at the traffic lights, the engine, purring at 900 rpm on tickover, will faulter. It drops the revs and then picks up again, but strugggles to recover to the steady purr until the fan goes off again.

Its more of an annoyance than anything - or rather there is that moment's worry that it will stall and refuse to start again. But it never does. Any idea what actually causes this and is there some alteration to the way the fan or the ignition is wired that might prevent it happening?
GuyW

Guy,
Your fan is consuming a lot of power so when it is running the alternator/dynamo has to produce more current. This extra load on the engine drops the revs. Mine does the same the revs drop by about 100 rpm when the fan cuts in, nothing to worry about, its perfectly normal.

Mike
M J Pearson

Is the fan supplied by a relay? It sounds from your description like it's pulling a lot of current.
Greybeard

I can check the fan current, but as the car has a 52Ah battery l don't understand how the coil can be significantly starved of power. The coil itself doesn't take much. Surely there's enough to go round?
GuyW

Guy, it not anything being starved of power, its just the alternator/dynamo having to work harder so putting more load on the engine.

Trev
T Mason

Oh, so the alternator puts more load on the engine when components are drawing current? I thought they just span round all the time the engine was running, putting out a continuous 14.8 volts.
GuyW

Guy

1st law of thermodynamics - you don't get owt for nowt.
Oggers

I've got an uprated alternator that was receipted as 65 amps tho' I'm sure it was advertised as 55 amps and when my electric fan cuts in its effect on the engine revs when the car is stationary via the alternator is very noticeable.

Any power gain from taking the engine fan off is lost when the electric fan cuts in but of course the electric fan isn't constantly on like the engine fan and engine power is very low at tick over.
Nigel Atkins

Yes Oggers. I hadn't forgotten that but assumed that the alternator did its thing continuously (drawing energy from the engine) whilst it was spinning, and that the electrics fed off the stored energy in the battery as and when required.

Not argueing the point though. Just stating my misunderstanding.

So, this means that the alternator becomes harder to turn when a current is being drawn through it to feed something, like the fan in this case?
GuyW

Guy,

The volts may be fairly constant, but the amps demanded and provided changes. If you have (or put) a volt meter in your car, you might be able to see if there is a problem somewhere. Do you have an electronic ignition? I think they may be more voltage sensitive, and so could more easily be starved for electricity.

Charley
C R Huff

Yes Charley, electronic ignition module from an MG Metro.

It just hesitates, drops the revs, recovers and continues as before but, as Nigel reports, at about 100rpm or so slower. With its 276 cam this is sufficient to initiate the "lumpy tickover" syndrome.

Its no great problem - its been this way for nearly 20 years now. But I wondered if there was perhaps some magic trickery to the wiring that would mitigate the effects. (other than turning up the tickover!)
GuyW

Guy
Is the power supply to the fan controller hooked into the ignition circuit near the ignition coil
If so the coil circuit could be getting electrical interfearance causing your problem
If it is like this. moving the power pickup away from the coil or fitting a relay with a diode in the ign side and direct 12v for the controller would probably fix it
I had a similar problem with my Elan
The fan cotroller was hooked direct to the ign. side of the coil and the tacho was all over the place and couldn't get a cosistant idle---Moved the power pickup point to the fuse box instead of the coil and all returned to normal
willy
William Revit

I don't think it's that, Willy. I have a relay box on the front splash panel by the radiator, with 5 relays for the lights and fan. Power supply to them is by twin 2.5mm cables, fused individually, coming from the starter solenoid post. This also supplies the input to the fan relay so it is all quite independent of any ignition circuit.

I think the explanation of the varying load imposed by the alternator must be it. And the effect is just more pronounced because of the 276 cam which doesn't like running at much below a 1000 rpm tickover.
GuyW

As with many things the timing can be a bit of a compromise across the rev range.

When the tickover revs on mine dip from the fan cutting in they don't normally drop and recover they remain lower and lumpy but you have to also consider the fan cuts in because the engine is hotter and as you're stationary the fan is getting air through the rad but you've not got the rest of the air movement from the car moving (at say 35mph).
Nigel Atkins

On my work truck the same happens when the A/C is turned on i see about a 100 rpm drop on the tach and its a little jaring untill you get used to it

But its as you said, theres a large load on the system that the alt has to account for....meaning the alt is doing its job as intended

Id imagine yiu could rewire the fan on the rad to run independanyly off rhe battrey so as not to drop the voltage to the rest of the car, but if its running okay... why bother over a 100rpm drop, several hundred rpm, id be concerned

Just try not to use a power converter hooked up to a microwave oven at the sametime as the rad fan comes on....haha


.
1 Paper

Prop. Voltage ain't the issue here. It's the mechanical reaction load imposed on the engine by the alternator in response to the extra load imposed on the alternator by the fan. As someone else remarked you don't get anything for nothing. Ultimately all the power is derived from the engine and in this case at a time when engine power output is low.

Think of it this way...

If the battery voltage is 12v and the alternator is chucking out 14+ volts, then power is going into the battery, not coming out of it.

I'm afraid your theory is a bit flawed. Sorry mate.
Greybeard

As I had my fan wired directly to the battery I can confirm that it makes no odds, as said the alternator would still have to supply the battery for the fan (please ignore any technically incorrect terminology that I might have used - as usual).
Nigel Atkins

Prop, its the same thing with your air con, although it is drawing power the main reason revs drop is because the engine has to drive the compressor. Its the reason fuel consumption increases by about 10% when you use the air con.

Trev

T Mason

Electric and power generation is all black witches brew magic to me...im just happy when it works
1 Paper

With AC there is usually an electronic clutch which engages the compressor when the AC is switched on, increasing the load on the engine.

What l accept, but don't understand with an alternator is, it is already spinning and adding it's load to the engine before the electric fan comes on. The fan comes on and is amply fed from the battery. Indeed l have it wired so if the thermostat is still calling, it continues to run after the engine has stopped. So why and how does the alternator become harder to spin when the fan comes on?
GuyW

Could you test if it is load on the alternator rather than electrical starvation by unplugging the alt and see what happens when the fan cuts in?

Charley
C R Huff

Yes I could try that Charley, but I would need confirmation from those who know, that running the alternator like that with the output cable disconnected isn't going to inflict some mortal damage to it!
GuyW

Why not just slacken the fan belt so that it doesn't drive the alternator. It won't be driving the water pump either but you only need to do it for a short period of time.
Martin

Just pop the belt off if you want to try the experiment Guy. If you can get it past your crossmember of course.

But your battery doesn't really enter into it. Realistically it's only there to start the engine, after which the engine driven generator (whether an alternator or a dynamo) runs the show.

Current being drawn by the fan cutting in at crawling speed has to be supplied from somewhere so it gets it from the alternator. The alternator in turn gets it's energy from the engine and the load is a significant percentage of the power being generated by the engine in those conditions. Hence the drop in revs you describe.

This becomes clearer when you think what happens if your alternator goes bad. Its output voltage drops below what is required to maintain battery charge, so the battery takes over the supply task, but only until it has discharged to the point that it can no longer support the loads and the lights go out. And the ignition dies. Etcetera.
As this happens very quickly it demonstrates that in normal running it's the alternator doing the work, not the battery.

(Oops - crossed with Martin).
Greybeard

Greybeard,

I wouldn't agree about the battery discharging quickly as in my experience a decent battery can supply the electrics for a couple of hours after alternator/dynamo failure and that is with the headlights on. My own experience with the electric fan cutting in is that it made little difference to the idle speed and that was with an alternator fitted with a dynamo pulley so it ran slower to prolong the alternator life which I found short when doing a lot of motorway miles at times.
David Billington

The alternator continually monitors the battery's voltage to determine how much power is needed.

When an electrical load like the fan draws current from the battery it causes the battery's voltage to drop, which causes the alternator to increase its power output to compensate. The extra power draw puts more load on the engine, slowing it down. The fan's 5Amp* current puts enough load on the engine to drop it by about 100rpm.

The stutter will be from the fan's startup current. Though the fan may only draw 5 amps when it's running, it will draw 20A briefly while starting. Your alternator is reacting to the 20A draw and outputting full power for a brief moment.

Full power? Yup. An alternator's ability to generate power depends on its speed. The faster an alternator is spinning, the more power it can produce. A 16ACR is only a 35A alternator at 3000rpm (engine speed) or more. At 900rpm it's only capable of outputting 15A. When your fan draws 20A the alternator can't keep up, but tries as hard as it can.

You can mitigate the effect by reducing the voltage drop at the battery, and by increasing the alternator's output at idle.

-Increase the thickness of the wires between the fan and the battery. Bigger is better.
-increase your idle speed. Alternator output rises rapidly at lower speeds- that 16ACR can output 20A at 1000rpm.

Alternator output graph attached. Note that "alternator speed" is double engine speed (thanks to the pulleys) so 2000rpm on the graph is 1000rpm engine speed.

*A reasonable figure for your average radiator fan. YMMV.



Growler

Okay David and Growler. I'll put me hands up and admit my explanation was over-simplified, but I think the principle idea of the alt doing the donkey work not the batt is sound enough.

Good call on the cable size. And the graph shows exactly why I opted to fit an 18ACR in place of the dead 16ACR. I too have an electric fan.
Greybeard

When the fan switches on initially it will draw more current, to get up to speed, than when it finally gets up to speed. This initial current surge is supplied by the alternator which, in turn, loads the engine, slowing it down. The alternator supplies the current, rather than the battery, because it produces a higher voltage which means it is more "attractive" to any load imposed on it.

If you removed the alternator, the fan would run off the battery and would have no effect on the engine.
Bill Bretherton

Yes I think I appreciate all of that. It does seem to me that the battery versus the alternator issue is simply 2 different inputs into the circuits; one is chemically sourced power and one electromagnetic but both provide the input for the fan depending on whether the car is running at the time or not. If the car is run without drawing power for starter motor, lights, fan, wipers, heater etc, the battery is capable of supplying the coil and running the car for several days, without input from alternator or dynamo. I know, as I have run it this way! The coil / ignition actually needs very little power on its own to work.

The stuttering issue is probably really to do with the way the engine runs at lower revs with the 276 cam. The 10% or so drop in revs when the fan comes on is sufficient to cause the cam speed to fall to the point where it goes chugga, chugga, chug instead of purr, purr, pur. Not a great problem, but it would be nicer if it didn't!

I don't think the feed wires to the fan are an issue either. The relay is nextto the rad and is fed by two, 2.5mm cables. Each capable of taking around 28 Amps, so potentially 60 Amps without significant heating/ loss. Maybe the wires between the relay and the thermostat switch and then to the fancould be increased though.

I still don't understand how the alternator physically slows the engine though. It will be turning at a fixed rate, generating power. This may not be enough for the fan, especially for the surge as it initially kicks in, but as the alternator output is speed dependant and the engine slows, this cannot help. The fan must draw this power needed from the (ample)battery output.

The alternator is basically an iron core wih its coils, spinning in a magnetic field. How does this get harder to spin when the fan a couple of feet away starts up? Surely the thing is already spinning and generating anyway, and other than the engine revs dropping slightly, it doesn't suddenly change speed to either increase its output, ot to add ectra load to the engine.
GuyW

Guy, yes, it's spinning and generating but little current is being drawn. When the current demand increases e.g. by fan, headlight etc. then the magnetic field within the alternator (between stator and field) increases because magnetic force is proportional to current. This results in greater effort needed to turn the alternator which is supplied by the engine. If you converted the engine output to kW equivalent you could determine the additional power in kW supplied by the alt. Initially there will be a much higher surge value as the fan starts, which loads the engine.
Bill Bretherton

Ah, l was viewing the alternator as a fixed physical device with one part spinning on its bearings - so a) how would it "know" that the fan had come in and it needed to increase its output, and
b) why would it suddenly become harder to turn.

So the answer, skipped or assumed up till Bill's comment, is that the magnetic field increases as the current drawn goes up. And the increased strength of this magnetic field makes the armature harder to spin, increasing the load on the engine.
GuyW

I still don't quite understand why, with a well charged 52ah battery ' on tap' the fan doesn't just draw on this available power source. Why is the increased current drawn at that start up moment via, the alternator rather than from the battery?
GuyW

Guy
What about fitting one of those little 3port vac solenoids wired into your fan supply circuit so that when the fan comes on the ported vac switch can put manifold vac onto your dizzy to lift the idle speed a bit
I'd imagine your fan is only coming on at standstill or low speed so the extra advance shouldn't cause any other bother
willy
William Revit

The alternator isn't just an iron core spinning in a set of coils, It's a core made of coils spinning in another set of coils. Put a little bit of power into the spinning coils to turn them into an electromagnet, use the outer coils to harvest that magnetism plus kinetic energy.

The more power you put into the spinning coils, the more magnetic they are, and the harder they fight against the outer coils (and the more power the outer coils make). The alternator's regulator decides how much power to apply to the spinning coils, and it decides based on the voltage at the battery.

When your fan takes power from the battery the battery's voltage drops, the regulator sees this and automatically compensates.

The only way to make the fan draw power exclusively from the battery is to disconnect the alternator. If the alternator's connected, it'll try to make more power (which is its job).

The fan doesn't know that there are two different power sources- it just draws what it wants when it wants it. The stutter is a consequence of your chemical power source (the battery) having a limited power output and slow response, and your mechanical power source (the alternator) being designed to compensate for the battery's shortcomings. It's working as well as it can.

If you wanted to completely eliminate the stutter and rev drop you'd need a third power source dedicated to running the fan without disrupting the rest of the system. A small battery or supercapacitor with a smart control system should do it. And no, I'm not being snide, the stereo in one of my vehicles is run by a boosting power supply because I was irritated at the way it would cut out while cranking the engine.

On a side note, *never* disconnect the alternator or the battery while the alternator's spinning. All that kinetic energy has to go somewhere, and if it isn't absorbed by the battery as charge current it'll turn into excess voltage and fry things that aren't designed to withstand more than 14V.

Growler

Guy,the battery has a terminal voltage of about 12.5v whereas a spinning alternator has a terminal voltage of 14v or so. It has to be higher than the battery in order to charge it (just like a battery charger).

Of course, the alternator output is clamped to the battery but, in an "equivalent" circuit, having two voltage generating sources in parallel, the higher voltage source will drive the current (within the limit of its current capacity) around the circuit so long as it remains the higher voltage source.

So even though the battery is capable of far greater cutrrent, the alternator will still be supplying any load current.
Bill Bretherton

Thanks Growler. I did understand that the alternator is more than just a coil spinning in a magnetic field. I was just seeking to be brief in that description as the relevant part was that the magnetic field strength increases due to the higher current. And this is what makes it harder to turn and therefore demand more of the engine.

This, to me, was the missing bit of the puzzle that was maybe so obvious to others no one thought to explain. Until Bill that is! ( must be your background Bill, you spotted the knowledge gap! ).
GuyW

I had asked several times why the alternator became harder to turn when it was already spinning and didn't increase in speed. I probably embedded the question in too much other description for it to be clear.

I could increase the tickover, and this does help. But to eliminate the camshaft derived lumpiness it needs to go up to 1500rpm to provide enough margines for the impact of the fan derived rev drop. Too much when it runs smoothly enough at 1000 rpm before the fan cuts in.

Or l could investigate this idea of a capacitor in the fan circuit. Sounds like a big one would be needed though. Or maybe the elegant idea of Willy's of some form of vacuum valve on the dizzy?

Or do what l have done for 20 or so years and just blip the throttle and raise the revs a bit until it settled down or the fan switches off again !

Good discussion though! :-)

GuyW

Guy, I suppose it's the problem of running an electric fan with a conventional engine lacking an ecu. A modern car's ecu would immediately sense the increased electrical load and adjust the fuel injection accordingly. Of course, you could convert to injection with ecu control......

I'll get my coat then.......
Bill Bretherton

Guy

The regulator within the alternator senses more cuurent draw is required to power the load - such as the fan - or anything else come to that. If there is a load, a current starts to flow, which in turn creates a magnetic field in the reverse direction of current flow. This hinders the rotor's movement within the alternator, so more mechanical energy - from the engine - is required to maintain the rpm needed to provide the increase in current draw.

Normally during driving - and if the fan is energized - this energy loss is masked by the engine working harder than otherwise required. You don't notice it, but it is there nevertheless. Idle is set when the fan is not energized, so if the fan is energized at idle, the rotor's reistance increases. Assuming you do not press the accelerator to increase the amount of mechanical energy required, the rpm will decrease instead - as the load on the engine is increased and you are not pressing the accelerator to compensate - incidentally by roughly an equivalent amount of energy required to overcome the resistance of the rotor.

You don't get owt for nowt......
Oggers

Oggers, that makes a good summary of what has been said.

I did understand, the no "owt for nowt was never in doubt!
GuyW

Guy,
instead of blipping the throttle you could hold the throttle steady at a fast(er) idle which would also help the water pump which would help with cooling which would held reduce the amount of time the fan is running. It might also help, perhaps only a little, the engine bay heat build up that in warmer situations can lead to fuel evaporation that can cause the idle to go lumpy . . .

IIRC my fan is 8 amps running/draw(?).
Nigel Atkins

Guy
Is it you that has that fancy lithium jump starter battery pack
Could you use that as the powe source for your fan relay and trickle feed a top up for when is not driving the fan

timmyk

No idea what my fan draws. When I said blip the throttle what you describe is pretty well what I meant. I blip it to catch and steady the faltering and then hold the revs at a slightly increased rate. TBH, it doesn't happen that often that the fan comes on at all. It usually needs the car to be stopped for several minutes before the fan starts up. Unless there is a long queue of stop start traffic a normal wait at traffic lights isn't long enough to waken the fan. Probably more indicative of our northern summer weather than of a particularly efficient cooling system!

Adendum:
Yes Tim, I do have that lithium jump start. What you suggest would need something like a contact relay switch built into the fan wiring so that when the fan was off it connected the auxiliary battery into a charging circuit, but isolated it as soon as the thermostat signalled for the fan to start up. Nice idea but I am not exactly sure I really want to bother!
GuyW

Nigel

If you run the engine faster at idle, you increase the heat load on the cooling system as you are converting more chemical/mechanical energy into heat. Water pump would not help with cooling at idle - it would just increase the flow rate of the hotter water around the cooling circuit. The fan would actually have to stay on longer at idle to remove the excess heat generated in the cooling system.

Guy

Yes, hadn't read previous posts. Perhaps one solution is to employ some sort of centrifugal clutch pulley on the alternator shaft, engaging only when rpm is above idle - might drain your battery though if idling too long...that blasted 1st law again!
Oggers

Oggers, I thought you were summarising the varied collection of earlier posts! Same conclusion.

The left-field ideas are beginning to emerge now! How about devising a system that switched the engine off when the car stops, and then restart when you put your foot down on the clutch ready to engage gear and move off. Now that would be a neat idea wouldn't it. It would need a decent battery and a reliably starting engine though.
GuyW

Guy,
I believe that that is how the Toyota Prius works (without the clutch pedal) so, unfortunately not a new idea but the clutch version would be nice.
Martin

Oggers,
the faster idle is relative to the too slow idle, increasing the idle could get you to a sweeter spot in the overall cooling system/alternator/water-pump/fan/engine-bay-temp efficiency, the increased rate also goes to help overcome the alternator pull(not the correct word) at those lower revs.

Try it, drop your idle to too slow, look at the temp gauge needle and see if/where it moves when you increase the idle by foot on pedal in the situation Guy (and I) was referring to.

Same as with fuel evaporation issue, putting more petrol through has cooler petrol at greater volume so less heat in the petrol overall in that location - these may not be the very best solutions but are a practical solution for yet another compromise on ancient technologies like our fossil fueled classics - and not that great for MPG either. :)

Martin,
what Guy was referring to was modern fossil fuel cars Stop/Start (or is it Start/Stop).
Nigel Atkins

Nigel,

The Prius is a modern fossil fuel car which happens to have an electric assist. It still uses gasoline. Like all other hybrids.
Martin

Martin,
sorry I was rushing to miss the edit guillotine, I should have put something meaning non-hybrid/electric/other, strange things words, over here the hybrids use petrol. :)

This is what I meant, in someone else's words - http://en.wikipedia.org/wiki/Start-stop_system
Nigel Atkins

Guy...it would also probably need a new crankshaft far quicker than normal due to the bearings running dry for far longer due to the more frequent stop/start.
Oggers

I was being ironic regarding the re-invention of stop-off feature. ;-)
GuyW

They make electric water pumps for cars... make thst fit into your equation

Prop
1 Paper

Guy
It would be interesting to put full manifold vac. onto the dizzy with the fan operating to see if that gives enough idle speed bump up
If it does a little 3 port vac. solenoid wired to the fan would be the answer
and easy to do-----
willy
William Revit

I've got it! Solar panel strapped to boot lid. Not sure about when it's too dull overhead or raining (in Cumbria....). But then, surely you only take the Sprite out when it's sunny?
Bill Bretherton

Nice idea Bill, and would work as the fan only comes on on the 3 days of sunshine in the year. And only then if one of them coincides with the summer solstice.
GuyW

You might be able to get your solar panel for free as I see on the news that they are going to start putting them on social housing starting with Ealing but with most of them going up north. Maybe you could claim that your Spridget is a pensioner.

Trev
T Mason

Trev

I refuse to subsidise the running of Mr W's Spridget...
Oggers

But, but - it's homeless. It lives in the street, outside in all weathers. Not even a cardboard box. :-(
GuyW

Cardboard box! ...bl**dy luxury! --- you knew it had to be said...
Oggers

be careful, if it becomes obvious that you're unable to look after it properly it could be taken from you and put into care with a foster home.... (me for instance!)
David Smith

Or even worse the council might pick it up with the recycling.

Trev
T Mason

This is getting to close to reality for comfort ! ;-(
GuyW

This thread was discussed between 30/08/2017 and 03/09/2017

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