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Steam Cleaner. Re-Birth of a Karcher MPDS

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awemawson:
When I moved to the farm here eight years ago I needed a stand alone pressure washer - ie independent of electricity or even piped water as at that time I hadn't put services everywhere and yet I was churning up loads of mud with my JCB 3CX. It was of course putting in those needed services  :bang:

Casting around I came across a military version of a Karcher Steam Cleaner. Powered by a diesel engine, and burning diesel to heat its water, it could suck up its water supply from a pond or stream and seemed ideal, if a little large.

When it arrived it turned out that it was an 'MPDS' or Multi-Purpose-Decontamination-System. Intended for field deployment on the German Plains as the Russian hoards swept over using battlefield nuclear weapons, this would hopefully clean up tanks and vehicles as well as personnel.

I managed to find the military instructors guides on the web which I attach to this post

Superb beast driven by a Farymann 6.5 hp diesel engine, one of which I already had on a larger vibrating 'wacker plate' so I knew the engine.

awemawson:
Over the years it's given me good service, with the pressure washer bit never letting me down. However the steam side became more and more un-reliable - not firing up on demand. At various times I'd done bits of investigation but never managed to bottom the fault(s) so resolved to have a 'good go' at it.

Problem was that the control box is tightly packed, has numerous relays, and although I have a circuit diagram, relays weren't numbered so it all got a bit confusing. I'd had my suspicions that the 'flame failure' circuit was at the heart of the problem, but it's control box was buried deeply in the main control box, and anyway couldn't easily be tested unless in-situ and connected to it's various peripherals.

I decided to bite the bullet, pull it out, and configure a test rig for it so that I could test it on the bench.

awemawson:
Pulling the Flame Failure unit out was a bit of a hassle involving taking much of the surrounding controls and indicators off first, so  a very methodical approach was needed if wires weren't to be misplaced.

Eventually it came out

awemawson:
The first thing I did was a careful visual examination which revealed no obvious issues, then I went through all the components to ensure spares were still available. They were, albeit that that i/c (which is a window discriminator) was being offered on eBay at £75 !!!!

So what does it need to work? Well it wakes up by being supplied with 12v DC. Then it monitors a Cadmium Sulphide photo-cell that is looking into the combustion chamber. It needs to see no light at this stage which is high resistance of over 1 Megohm. If this is the case it turns on the relays controlling the fuel and ignition. Then in the next ten seconds it needs to 'see' light via the CDS cell, represented by a low resistance of about 400 ohms. If this happens then the ignition is cut off, the fuel left on, and it sits in that state until either the flame goes out or the 12v is taken away.

It proved impossible to easily remove the original CDS cell to use it in the test rig as it was hard wired, so I measured it's characteristics and used resistors to emulate it's two states with a toggle switch to select flame or no flame. 12v torch bulbs were wired as the  three outputs (Ignition, Fuel and Fault)

awemawson:
The mounting of the CDS cell was rather Heath Robinson - it must have been replaced at some time and was a different diameter to the housing, also the housing had to be spaced off the boiler by a pile of washers to allow for various flangy bits. So I re-engineered it all, turning a ptfe collar for the CDS cell and specific spacers to replace the washer pile - sorry no pictures.

Well the test rig worked splendidly. I could not only prove that it went through it's various sequences, but also that it was able to take a reasonable degree of 'tap testing' and voltage variations.

On the face of it slightly disappointing that it wasn't the fault, but in fact actually re-assuring that now I knew positively that it was working and could be eliminated from the diagnosis.

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