How do I *repair* my hedgehog?

[CJH]

No, not this one:


This one. It's the heater that sits in the underside of the inlet manifold, often known as the 'hedgehog' I believe.


The above is the one that came with a manifold and carb that I bought, but having removed it from the manifold I've discovered it rattles. I think one of the heater plates or springs inside has broken. I've sourced a replacement, but I'd like to salvage this one. I found this image of what it looks like inside, and apparently the component parts are cheap to replace.



But I'm looking for bright ideas about removing the lock ring. It seems really tight, and I can't get anything behind the ends to pry it out. I could drill a small recess to get a screwdriver in there, but I'm wondering if there's a knack. Anyone done this?

[CJH]

I drilled a small indent into the aluminium housing at one end of the lock ring, which allowed me to get a small screwdriver behind it and pry the lock ting out. You can just about see the shiny indent in this photo.



So then the cause of the rattle became clear. All four discs are in pieces.



I've been googling around and have found some discussion on Russian and Polish forums - this one for instance. As far as I can tell from those discussions these are PTC disc thermistors - they heat up to a certain temperature and then their resistance rises rapidly which stops the current flow.

So the question for you sparkies out there (Ghost, OBG?) is how do I find the specs of the broken thermistors so that I can find some replacements? They're 21mm diameter, ~1.1mm thickness. The total resistance of the four of them in parallel is between 0.25 and 0.5 ohms at ambient temperature, but I don't know at what temperature they need to stop the current flowing. Measuring a working one in situ might be possible, but I'm not sure I could measure the operating temperature accurately.

There's a suggestion in that Russian forum that I could buy a packaged PTC, remove the outer cover and unsolder the legs. I've also seem some places that sell the bare discs but I'd like to know the specs to order.

[CJH]

I soldered some wires to the biggest bit of PTC heater that I've got left, and then made some measurements. I used the temperature probe that came with my multimeter. Not the easiest stuff to solder to, but plenty of flux seems to have done the trick.



On connecting a 12V battery the initial current shot up to over 6A, but quickly dropped down as the PTC element heated up. It reached a stable current of 0.37A when the disc got up to 350 degrees C. With four of these in parallel the limiting current will therefore be about 1.5A through the hedgehog, but the initial current will be over 25A*. I don't think the cable to it is rated that high, but presumably it's safe because the disc quickly increases it's temperature, and hence it's resistance. I understand that the power supply is controlled by a relay that turns it off when the coolant gets up to temperature.

All of this means that I'm looking for 4 replacement PTC heater disks, ~21mm x ~1.1mm, room temperature resistance = ~2 ohms, limiting temperature ('curie' temperature?) = 350 degrees C. I think the package size isn't crucial as long as it's close. No luck so far. Any ideas from you lot?

*Actually, I think the resistance is inversely proportional to the size of the disk (at least my measurements on the smaller fragments give higher resistance). So a smaller disk has higher resistance and therefore a lower current, but I think the Curie temperature is a function of the material, not the size. I think this means that a full-sized disk (not my broken fragment) will have lower resistance so the initial current will be even higher, as will the current at 350 degrees.

[CJH]

The replacement hedgehog that I bought has arrived, so I did some measurements just to see if it's working. On my kitchen floor in open air it reaches a steady state current of 10A - rather higher than the 4x0.37A I was expecting. With hindsight this is obviously because the hedgehog casting is a very effective radiator, so the PTC wafers never get to their maximum temperature and their limiting resistance. When the hedgehog is sitting in the manifold below the cold in-rushing air/fuel mix the hedgehog will likely stay even cooler and have an even lower resistance, meaning it'll draw even more than 10A. That's quite a big chunk of current. It's a good job this heater switches off once the water gets up to temperature.

Haha - I've just noticed a new addition to the swear filter. Ghost is now changed to Ghoolie.