1pk wrote: ... Closer inspection indicates that both Resistors are Cracked and the one that is discoloured has been overheated by excessive current. I have little hope that just replacing it will correct the problem. Something else must be Shorted.
Is it possible that the resistors were damaged by overheating or is it possible to tell if overcurrent was the cause as he said?
The schematic for these chargers hasn't been traced yet, so we don't know anything for certain. However, it's reasonably likely that we can learn from experience with other EV chargers. We strongly suspect that at least one of these resistors, probably the one that was cracked, in a pre-charge resistor. In normal operation, it will be shorted by the input relay about a second after mains is applied. This resistor has a hard life, pulsing a lot of power over a short period of time. (Then it does nothing for 99.9% of the charge time). But to crack, it likely seen this high power for longer than the usual one second or less. This can happen if the input relay doesn't come on, or if its contacts are burned. In Elcon/TC chargers, a common failure mode is that a pair of the MOSFETs short out, putting a short on the DC bus, which causes the power supply to drop out, which then causes the input relay to drop out. Now the pre-charge resistor is supplying 120 V or 240 V into more or less a short circuit. It will see possibly 10x or 100x its nominal power rating, which causes it to overheat. It appears to be a wire-wound type, which will often act as a fuse, going open circuit. There are other fuses there, but the pre-charge resistor often prevents the current from reaching a level where the actual fuses will blow. 200 watts will blow a 5 W resistor to pieces, but it's well short of the current normally passed by the charger (up to 3.3 kW).
So the short answer is, unfortunately, that I agree with the repairer: something else has likely shorted. Hopefully Kenny will be able to trace at least the pre-charge part of the schematic soon, so we know more about what's going on. For example, we don't know why there are two large white resistors. My guess is that one of them is a bleed resistor, so it will have a high value (hundreds of kilo ohms), and the other will be the pre-charge resistor (tens of ohms).
It's not uncommon for high power ceramic resistors like these to discolor with age, so the other resistor could well be fine. But of course cracking is not normal.