I could do some measurements on that pump for you if you want to post it to me @ChristopherMo? 

 

Your understanding seems very good.

 

And a "thank you" to @retiredbri for measuring his motor resistance.

Edited 4 October, 20196 yr by Wino

I've returned it today to the dealer. And I've said that I will not let them have the car for diagnostics until they can tell me beforehand in detail how they propose to diagnose the fault. I really don't want them just to mess about and then give the car back "fixed". Sorry if that sounds cynical🙂.

 

3 hours ago, ChristopherMo said:

I measured the steady-state current when the inflator was running (free of a tyre) and it was 7-8A (which is about right according to the rating plate).

 Was referring to the current draw of the circuit without the inflator connected.

 

3 hours ago, ChristopherMo said:

I don't think you are correct about fuse rating and headroom.

I am correct, was referring to normal operating not start-up. You are correct though in that Fuses have some tolerance for surge current. The manufactures states a max current draw of 15amps which is load dependent but this figure often does not include the initial surge current which normally should not blow the fuse, as you have pointed out but that will depend on conditions (such as the first point above). I suppose the point here is that you may be looking at the symptom and not the cause. After all a different inflator also blows the fuse.

 

4 hours ago, ChristopherMo said:

 Lower battery voltage would cause lower current (V=iR) 

The equation is correct but wrong application, the equation is W = V x I, an electronic item consumes power, if the voltage is lower, the current will need to be higher to achieve the required Watts (power).

 

A friendly forum member with electronics expertise, bench power supply and an Oscilloscope would be able to answer all the above questions, including start-up current but the chances of two different inflators faulty is unlikely and why I would be looking towards the car.

 

At the end of the day, just trying to help!

I appreciate that you're trying to help and I am grateful.  You are not thinking correctly about power. It is true that for constant power the current would have to increase as voltage falls. But this is not what happens, unless the device in question can actively manage current. Consider a good old fashioned tungsten filament headlamp bulb. As battery voltage falls (perhaps you've been cranking the engine on a winter's morning) the light dims. It dims because it has a resistance that it cannot actively vary (although the resistance varies dramatically depending upon the filament temperature, but that's another story). So although it might be nominally a 100W lamp, without adequate voltage, current must fall (i=V/R) and consequently the power it is actually consuming falls even more dramatically as P=V*V/R.  The motor in the inflator similarly cannot actively manage its resistance, although its resistance appears lowest when it is stalled, so it draws max current. Normally, the motor accelerates up to speed in a short time, so the apparent resistance increases until the steady state current is achieved. In the case of these two inflators, the stalled current is high enough and long enough to blow a 20A fuse. This points to defective design in the inflator (or defective execution and I happened to get two duds in a row). Another possibility is that I have a bad batch of fuses, of course. Otherwise it seems wholly unlikely that it is the car, as only two things would bring this about. The first is other device in the circuit already drawing current and so the fuse sees a high current; this is not the case because the current measured through the fuse prior to connection of the inflator is exactly zero. The other is extremely high voltage from the battery I haven't actually measured that but somehow I know it's not that! I know it seems unlikely that two-in-a-row would be faulty but I can't imagine that this device is the pinnacle of VAG's automotive achievement.

 

Hi all

From what I remember from college days (just after the introduced the abacus!!), an electric motor has a "static" resistance (the copper windings and brushes) and it measured on a ohm meter PLUS AND IN SERIES WITH a dynamic impedance (because it has copper windings on an iron stator/armature) called an inductance. 

As the rotates, it generates a "back-emf" (that tries to balance out the supplied voltage). It can therefore be considered as a complex non linear resistance.

So, the pump rotates at a no load speed where the "back-EMF" balances the 12V input and the current is measured (called the no-load current).

When it is connected to the tyre, the speed drops, the back-emf drops, the dynamic impedance drops and the current increases. The tyre inflates and heat is generated due to the electrical resistance (like an electric fire) and because the pump is doing work (like the end of a bike pump when you blow up the bike tyre). 

 

retiredbri

  @retiredbri explains the resistance is dynamic so to speak, a tungsten bulbs resistance is constant.

 

Anyway, good luck, I hope you get to the bottom of it! 

Measure the resistance of a filament headlamp bulb cold and then calculate the 'on' wattage assuming the resistance is constant...

Fantastic! Human creativity directed at solving a real problem. For some reason, this prompted the thought that it is slightly odd that the automotive industry has not eliminated the need for spare tyres and gloop and inflators and space reserved in the car. After all, we don't carry spares for other parts of the drivetrain. We just expect it to work - and largely it does - as a consequence of a century of R&D. I wonder why the road wheels did not benefit from this evolution?

9 hours ago, ChristopherMo said:

I wonder why the road wheels did not benefit from this evolution?

 

Tweel, coming soon? 😉

https://www.michelintweel.com/

 

Thanks AG Falco

 

 

Forgive me for being too lazy to re-read the thread, but have we tried these inflators in  another car under the same conditions?

1 hour ago, AGFalco said:

Tweel, coming soon? 😉

 

Should just go full solid section tyres like an industrial forklift or Bobcat skidsteer. Once the tread wears down, just keep re-cutting new treads with a tread cutter until your right down to the rim. Just think of how many miles you would get out of those babies and not a puncture in site. Not likely to be much worse for grip than eco tyres either🤢

Hey and if you need winter tyres, no problem! Just get out the screwdriver and drive in some self tappers, instant studded tyres, honky😉

Edited 5 October, 20196 yr by Gmac983

2 minutes ago, SuperbTWM said:

Forgive me for being too lazy to re-read the thread, but have we tried these inflators in  another car under the same conditions?

 

Didn't notice anyone say so. I did ask the question a while back. 

The dealership says that they have, although they haven't managed to play back to me what they actually did. I have a suspicion that they will have merely plugged them into the car and turned on, then reported success. This works fine in my car, too. It is turning on after connecting to a (well inflated) tyre that blows the fuse. I say again that I cannot conceive of any likely condition of the car that would bring about this behaviour (save the ones I have already identified). The dealer is supposed to be considering how to resolve this, but I have little confidence, I'm sorry to say.

 

On 21/09/2019 at 16:45, ChristopherMo said:

Does anyone know where there is a table of fuse ratings for the Superb, just in case the issue is an incorrect fuse rating?

Has your dealer even confirmed to you that the 20A fuse value is correct, according to their info?

 

It should be possible to look up the fuse values on erWin, but it would cost you some dosh, and that shouldn't be necessary.

Edited 9 October, 20196 yr by Wino

I haven't asked the dealer this question. I have seen photos of various Superb fuse assemblies and fuse 40 always appears to be 20A by colour. It did strike me as very odd that VAG regards this information as commercially-sensitive and charges a fee even to find out!

Well, it's been a long time but if anyone is still interested, here's another couple of chapters -

 

A few weeks ago, I needed to call into the workshop about something unrelated to this inflator issue, but I couldn't resist asking them to take a look. Along the way, the workshop's poor understanding of electrical engineering or lack of familiarity with the circuit design of this car (or both) was revealed by some wrong-headed questions. The eventual verdict was that the inflator was fine and it did not blow the fuse. I said that I was doubtful about that and I said that I would try it myself on their forecourt. The fuse immediately blew. Noone from the workshop offered to do anything. Service reception said I would need to book the car in for a day so they could trace the fault. I reluctantly agreed, replaced the fuse myself from my own stock (!) and left.

 

Took the car in on Friday. At the end of the day, they told me without a blush that they had determined that the inflator was in fact faulty and they have replaced it under warranty. They could not explain why it had not been faulty in their judgement on any of the previous opportunities that they had had to assess it. They thought that both the faulty inflators that I had been given must have been from a bad batch (although I had noted the manufacturing dates on each of them at the time and they were months apart!). They had replaced a defective rear wiper blade "as a gesture" to make up for my inconvenience. TBF, they had also given the car a very thorough valeting at no cost to me.

 

I know that I should not accept that this is the end of the story, because, of course, I still have not tested the newest inflator. No doubt, I'll get to it in the coming days. I'll report back with the outcome.

Dont forget to remove the paperclip they will have put in place of the fuse before testing

 

I think we know what the next installment will be.............

They must have checked that the replacement didn't blow the fuse....

 

 

 

 

 

 

 

 

 

 

 

 

 

...right?

I have had exactly the same issue. I shall use this as evidence for the dealer. Thanks. Went through a whole bag of cheap fuses from eBay. Also, the 12v socket is on the same fuse so I can't charge my phone either!

Cheap fuses from Ebay . . . . . . . . . . 

 

Was it a reputable seller or did they come from China?

Reputable. And regardless, it popped the factory fitted spare the very first time I used it.