So ignoring the gaskets for the time being, I've been investigating the heater plate, but forgot to take photos.
I've had the manifold back off & the 3 bolts off of the heater cover. I'm guessing that the whole heating element should come out at this point, but it's well & truly jammed in there. I decided to leave it where it was.
I measured the heater resistance at this point, red feed wire to an unpainted section of the manifold. It's was still about 0.5 ohm. So, still sounds like a short somewhere.
Next the giant C clip thingy came out, which allowed the plastic cover to come away, revealing 4 little discs. On the inside of the plastic cover are 4 springs, all linked together, which push down onto the 4 discs. These springs are connected to the red feed wire.
Thinking that one of these discs must have gone short circuit, I measured their resistance. Each sits at about 2 ohm. Going back to my school days (which was in Mansfield, so...) I think 4 resistors at 2 ohm, in parallel, would indeed give me the measured 0.5 ohm for the heater as a whole.
Since I know it gets hot, I'm guessing it must be ok. Right? So I spent a bit of quality time with Google & it sounds as though the resistance will increase as the discs get warm. Ok, back on the bus with it & testing time again.
With the manifold mounted, the resistance is around 0.5 ohm still, but measured to an earth on the bus. I powered the heater up with a length of wire again, via a 20 amp fuse. Same again, the fuse glued, but didn't blow. After a 1 or 2 seconds, it stopped glowing. I kept it all connected for around 30 seconds, then measure the resistance again, as quickly as possible. It was dropping very quickly, but I saw 10 ohms hold itself for a second, my meter couldn't really give me a reading before that, the resistance must have been dropping too quickly.
My take from this though, if it's drawing 20 amps to start with when the resistance is 0.5, it must be drawing lots less when at 10 ohms.
If...
I = V / R
I = 12 / 0.5
I = 24 amps (I bet the voltage drop brings this down, but by the time the engine starts & voltage increases, the resistance is up)
So, when running, plus lets say the alternator got carried away...
I = V / R
I = 15 / 10 (resistance could be even bigger than 10, but that's all could measure)
I = 1.5 amps
I wish I had a method for measuring the current through this heater, but I've nothing that size. 10 amps is the max I can measure & I would guess that's a struggle for the poor old multimeter.
Now the confusion. I've had this heater wired through a 15 amp fuse for years now, plus the heating element on the carb is on the same feed, so will have been using some current as well.
SO WHY DIDN'T THE FUSE BLOW!
My only thinking at the minute is that the relay has some sort of current limitation, but that seems really unlikely to me. It would just get turned into heat. There's no way it was the wiring, the cable I used would have sat at 50 amps I would think, let alone enough to pop a 15 amp fuse. Basically, I'm confused.
The current plan is to wire it up as it should be & see if the new 15 amp fuse pops. If it does, stick a 30 amp in there since in stock form there is no fuse at all.
![Confused :?](./images/smilies/icon_confused.gif)