Unleaded Gas Furnace

So the previous furnace adventure, where I replaced the return air intake with a much larger one, didn't fix the issue I had where the furnace kept going into over-temp fault. Since I bought some replacement molex pins, but didn't use them yet, I decided that today would be the day that I'd perform that experiment.

Here's the connector in question.

So, out comes the pin puller again.

I actually only replaced three of the pins, the two yellows for the over-temp circuit, and the orange for the inducer vacuum switch, as those were the only ones I'd observed malfunctioning.

And I'm happy to say that the job went smoothly.

So I buttoned it up, turned it on, and everything worked. Right?

Of course not, it went into overtemp again, almost immediately.

So why did it go into overtemp? Well, let's have a closer look at the other side of the furnace controller board, shall we?

I'm thinking those solder blobs look a little suspicious. Almost like these pins were soldered in with an insufficient quantity of solder; and lead-free solder at that, which is prone to fatigue cracking when exposed to mechanical stress, like heat cycling or vibrations. How delightful!

So, let's get out the soldering iron, melt our way through that conformal coating, and freshen things up with some eutectic leaded solder.

The burnt conformal coating makes these reflowed joints look a bit worse than they actually are, but trust me, they're very solid.

Here's a different view, shot directly with my iPhone instead of through the microscope.

The fatigue cracking of the original solder joints is very obvious to see here. In fact, the lighter green spot above the bottom-left pin is actually where the conformal coating has started peeling off of the board due to the pin wiggling around loose.

Now I have a little secret to tell: I'm not actually resoldering the board from my furnace, here. Instead, I'm resoldering the board that I pulled out of my furnace 4 years ago when I replaced it with a brand new one. Well, let's have a look at the new one, just to confirm that it has the same problem.

And that would be a yes, yes it does indeed.

Amusingly, this isn't the only spare furnace control board I have. Back in 2000 when I first replaced this board, about part way through the swap I glanced down beside the furnace and what did I see? A third furnace control board, no doubt left there by the previous owner after they swapped a new one in.

Thanks, ROHS.