We had the air conditioner running the other day and the air was not cold. I could hear the compressor cycling on and off outside, so I knew it was trying to work. I went in to the HVAC closet and looked at the copper lines that connect to the compressor. One of them was cold, with ice building up on it. I've played this game before, so I'm pretty sure the problem is that we are low on freon. Also we had it filled up about three years ago and the HVAC guy told us we had a small leak and would need to refill it now and then. The system is designed to compress freon at the compressor outside. Taking "stuff", in this case freon, and squeezing it into a smaller space makes the temperature go up. It runs through the outside coil to transfer this heat into the air outside the house, then through the high-pressure line to an expansion valve and into a heat exchanger coil inside the house. It expands, and since it has a certain amount of heat spread out over a greater volume the temperature goes down. I have a "central air" unit integrated with my furnace so the furnace fan blows air over the cold coil (and the air cools down as heat transfers from the house air into the freon) and the cool air goes on through the duct work and cools the inside of the house. The freon goes back through the low pressure line to the compressor outside, where it is compressed again. When the freon level is low the system can't keep the freon at high pressure in the high-pressure line. It expands before it gets to the expansion valve, so it cools the copper pipe instead of the coil. I am not an HVAC professional so some of my descriptions might not match the industry terms, but the theory is correct. I know this stuff because I'm a mechanical engineer, because I've had my share of troubles with this HVAC system over the twelve years I've had it, and because I can put those two things together with a little common sense.

My lawnmower would not start, so this is what I did to troubleshoot & fix. Keep in mind that once the starter is cranking an engine needs three things to run: air, fuel, and spark. If you have those three things it will run (barring internal mechanical problems); if any are missing it won't start. And more than one thing can go wrong at once.

Problem: Won't start, nothing happens when I turn the key
1. Charge the battery

2. I connected a jumper cable to the positive battery terminal and touched the other end to the bolt on the starter where the starter cable is connected. The starter spun but the engine didn't "kick".

That told me the battery was charged and the starter worked, so I needed to look for a problem in the wiring, to get the starter spinning when I turned the key. I figured it was most likely the key switch or the starter solenoid.

3. I took out the key switch. This mower uses a generic switch held onto the dash panel. You spin a round plastic-covered nut off the front of the switch, then pull the switch out the back side of the dash panel. I pulled the connector off the back of the switch and looked at the contacts in the connector and on the back of the switch. The terminal that is connected to the battery (red wire in this case, IIRC) was pretty corroded so I took a fine file and removed a lot of the rust, so both sides of the connection were shiny.

(I could have stopped here, had I know what was wrong.)

I plugged the switch back into the harness and tried again - nothing.

Since this process took me four days I decided to put the battery charger back on to make sure it was topped off. The mower has been out in the rain all week with the wires hanging out, so that could have discharged the battery. When I put the charger on it the voltage read 7V, so the battery was once again part of the problem. After a few minutes the charger read "bAd bAt" meaning it won't charge & should be replaced.

4. I pulled my truck over and connected jumper cables between the batteries. I turned the key and PRESTO the engine cranked. It still didn't start, though.

5. I took off the air filter and sprayed some Berryman B-12 carburetor cleaner in the intake.

No start

6. I disconnected the spark plug cable and took out the spark plug (with a spark plug socket). The spark plug looked good and was very clean. I didn't gap it but it looked like it was around .050 inches. I probably should have closed it up about half way but I didn't.

I plugged the spark plug back into the spark plug cable and clipped a test lead from the negative lead (the ouside L-shaped lead on the end of the spark plug) to a spot on the engine that looked well-grounded. You don't strictly have to use the test lead, but it makes it easier, in my opinion.

I cranked the engine again and watched the spark plug. I could see a spark jump across the gap in the spark plug, so I eliminated that as a problem. I put the spark plug back in the engine. Be sure to put a little oil on the spark plug threads, especially with an aluminum engine block.

At this point I was pretty sure the engine had electricity to crank the starter and the spark plug was sparking. As long as the carburetor was connected to the engine it would have air (I guess the choke could have been closed because of a problem with the linkage) and the carburetor cleaner should have worked as fuel, so it should have started. Maybe I missed something...

I went back to the intake and sprayed more carburetor cleaner in it. This time I held the button down far enough to release some pressure out of the can, and it sprayed in past the throttle blades pretty well. I cranked the engine again and this time it fired! It started and it ran and kept running. If it had died in a second or two I would have looked at the fuel system for blockage, starting with the fuel filter, or possibly bad gas.

So in the end I had dirty connections at the key switch and a bad battery.

I also have three flats, one of which popped off the bead. I'll deal with that in the morning.

My last post on this: http://sharpcraft.com/content/node/102