I know this is an old thread but I just fixed a similar problem on my Max-E-Therm heater.
jlgoode, you were very close to getting to the root of the issue before you removed the harness. Pool Clown was pointing you to the low voltage side of the system because the fuse only protects the 24VAC circuit which includes the control board, the pressure and flow switches, sensors, and the wiring that goes to them. Pool Clown is correct when he says only a short on 24VAC side of the circuit will cause the fuse to blow. In your case, you were probably wired up correctly on the 120V side until you removed the harness. Somehow in reassembling it, you thought you lost the 120V at L2 but as pool clown said, that is normal for the 120V configuration. When you measure between L1 and L2 you would get 120V but you dropped off before you got to that point. It's understandable that at some point it's not worth your time to continue trouble shooting. (Maybe you did and just didn't follow-up).
For anyone else reading this thread all the way though to the "dead end" above, I'll tell you how I solved the blown fuse issue. Maybe it will help.
I was able to find a short to ground in the wiring connecting the thermistor sensor on the left side of the water manifold. You may wonder how there could possibly be a path to ground at that point. It was a self induced problem related to my method for keeping rats out of the heater. My problem shows that Pool Clown is correct when he says only a short to ground on the 24VAC circuit will cause the fuse to blow. Once I fixed the short circuit (and replaced the fuse) all was good again.
Background on how the short was self induced in my case.
Two to three years ago I had two issues with rats chewing through the wires. I repaired it both times by reconnecting or patching in new segments of wire. The 2nd time I did the repair, I had to come up with a way to stop the rats from getting into the heater. I found out they sell kits that cage off the entry points but I decided to try my own idea. I bought some stainless steel scouring pads and stuffed them into all the openings around the water manifold at the front of the heater as well as stuffing some around the exhaust vent where it comes through the plastic (fiberglass?) cover at the back of the heater.
I knew the stainless steel was conductive so I did my best to tape off any exposed contacts for the switches and sensors at the front of the water manifold and down the sides where the various switches and sensors are connected to the wiring. It worked fine for almost 2 years until last week when the heater wouldn't come on. This time it was different. There was no indication of life on the heater. No lights or error codes. It was dead.
I opened the heater to troubleshoot and was happy to see it wasn't a rat problem again. The stainless steel scouring pads worked to keep the rats out and were still in place and in good condition. I checked power to L1/L2 and all was good. Eventually I got to the point of checking the 24VAC side and saw the fuse was blown. I couldn't find an AGC 1.25A replacement near me so I picked up some 1.0A fuses (along with some 2.0A just in case but I don't recommend using a 2.0A fuse). I assumed there was a short somewhere and casually checked the wiring but didn't see any issues. So used a 1A fuse and turned on the pool. The heater briefly came to life (lights on the heater came on) but as soon as the pool panel closed the 24VAC circuit, the fuse blew. So I got daring and installed a 2A fuse. I thought it might give me more time to find the source of the short circuit. It actually did help, This time, when I powered up, just before the fuse blew, I saw a flash of light coming from the left side of the manifold which still had the plastic side covers in place (and stuffed with the steel scouring pads). I removed the cover and steel pads but I couldn't find any exposed wire. I ran the test again with the cover and steel pads removed (using a 1A fuse) and all was good. The heater came on, ignited and started to heat the pool. I re-taped all the wiring in the suspect area and reassemble everything (including stuffing the stainless steel scouring pads back into place because it worked well to keep the rats out). All is working again.
A short circuit could also occur when a wire's insulation is damaged and touches a grounded metal surface along its path. It can happen from either being chewed by a rat or from vibration over time when it passes to tightly against a sharp grounded metal edge. It's not what happened in my case but it can happen so check for that too. Hope this helps.
