Duo valve capacitor value? (cheap fix)

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I've traced the heating problem on my W208 to a single capacitor (I think it's a capacitor), but I'm having real problems identifying the value.

It's a blue ceramic disc type and the markings on it are:

07
K20 RU (the R is reversed and joined to the U)
______
9946

My guess is it's a 20 Pf with a +/- 10% tolerance(K), but I could be way off. :dk:

Any help is greatly appreciated, then I'll be off to Maplins to hand over my .29p, fix my duo valve, post up the pics of the fix, and feel very pleased with myself for having saved a stack of cash.

Also if anyone has replaced these components before, did you use a krone tool to fit the cap, or just solder it on?

Cheers

(apologies if this in the wrong section)
 
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SpanishBattery
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Images

Here's an image if it helps?



(first image upload so I hope this works)



(photography isn't my strong point... you'd think I'd never heard of macro)
 

johnsco

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Are you sure it's a capacitor ?
It looks as if it could be a VDR (voltage-dependent resistor).
These are commonly used for interference suppression, where DC switching into inductive loads takes place.
(Which clearly is the case here)
 
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SpanishBattery
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I'm not sure at all. It's a guess purely based on articles I've read on mbclub.

A vdr seems to make more sense in the circuit.
I still can't relate the markings to the vdr I need.
Looks like maplins is out the question now.

I'll keep looking. Thanks for your help, I've probably spent all night looking for the wrong thing.
 

Dieselman

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Looks like a 20k microfarad polyester capacitor to me.

The U symbol is micro.

Take it into Maplin who will have them in stock.
 

johnsco

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I've re-edited post #4 with a bit more information.
Maplins should stock sonmething like this.
If not, they'll certainly be able to get you one.

The idea is that when you break the circuit to an inductive load, a very-high breaking voltage is generated and can make quite a big spark.
This burns your contacts and can cause interference.
If the switching is from a transistor or other semiconductor circuit, the "back-voltage" can damage the control circuitry.

The VDR limits the back-emf and suppresses these undesired effects.
 

johnsco

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Dieselman may well be correct.
A capacitor can achieve the same objective.
Some circuits employ both techniques.

" 20k micro-farad "
Not really 20 thousand micro-farads !! ??
 

Dieselman

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If the switching is from a transistor or other semiconductor circuit, the "back-voltage" can damage the control circuitry.

The VDR limits the back-emf and suppresses these undesired effects.
Unless the cap/varistor is short circuit, which it doesn't look like it is as they generally blow apart once the dielectric is breached, I don't see how it can be stopping the solenoid operating, because as you say, it's there to stop back EMF.

With the component removed from the circuit the solenoid should still work.
 
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SpanishBattery
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yeah I see that now. Can't believe I missed it. Heading back out to the car to check solenoid and double check which point I actually get continuity to.

Thanks again guys
 
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SpanishBattery
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You guys certainly know your stuff.
Shame I don't... I've been a bit of a numpty!
Just rechecked and found there's no continuity across the ends of the copper wire on the left solenoid (there is on the right one which works).
I must've been half asleep when I first checked the valve.

The solenoid doesn't look as straightforward to fix. Nothing to lose, so I'll poke about a bit more.
Thanks again for all your help
 

johnsco

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There should be electrical continuity between both ends of the winding of each coil.
I would expect them to have similar resistance.
The most likely cause is a broken wire close to one end of the coil.
If you can remove it (IF ??), examine the coil for breaks in the wire.
If you find a break, then unwind the coil by one turn and re-solder the wire.

It will work with one turn less.

PS ... This also works for the coil in MB keys, which tend to fail in this way.
 

jefrs

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Thoughts, for all the reasons above, it is probably a MOV
And then -
If it were a capacitor then it is constructed like a ceramic, but looks epoxy dipped, which is a lot of trouble to go to and means they would have fitted a Mylar=polyester as better and cheaper, which it don't look like.
If you would test with an ohmmeter then a capacitor will be open circuit and a MOV will have some resistance, assuming they are in good working order (the other one works?)
Some capacitors have similar makings where K20 or 20k could mean 0.020µF = 20nF but more common would be "223" (22k).
 
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SpanishBattery
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The coil wasn't too difficult to slide off the contact pins. Replacing is easy too.

Bad news is that the break wasn't at an end, which I would've expected. The break is in on the side of the winding that comes from the middle, if that makes sense.
The outer pin and winding has continuity wherever I test, apart from the other end of the cable.
No external signs of a break with tape removed.
The plastic bobbin doesn't come apart, so I don't see any way to get to the internal side of the winding.
I think I'll have to bite the bullet and get hold of a used one on this occasion.
 

jefrs

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You /can/ unravel, repair and re-wind solenoids and other such coils but it is usually cheaper by less effort to replace.
 

johnsco

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You may as well replace it with a second-hand coil.
I had one recently in wrecked E300TD.
Sadly it recently went to the great nacker's yard in the sky.
It may or may not be the same coil.

OR

You could buy my coil winding machine and stock of wire that will shortly be going on e-bay.
 
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SpanishBattery
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That sounds like a lot of work. Thanks anyway.

£22 spent on ebay, and a valve, along with an additional pump thrown in, is on its way.

I'll swap the coils over when it gets here and let you know how I get on.

Cheers everyone.
 
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SpanishBattery
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Swapped the coils, connector and both metal covers all in one go, as the replacement was of a slightly different design.
All working perfectly now.
This car just keeps on getting better.
 

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