Being a car guy myself, I take notice in episodes where he is dealing with them. Pumping power steering fluid into the brake master cylinder resivoir in The Enemy Within really would not have worked, but let's say it did

, then the eggs in the radiator in Bushmaster, the pen spring in Hellfire, making his own spark plug electrode fom a nail in Ceasefire, robbing the distrbutor cap in the Endangered, etc. etc. all say he knew cars up and down. I'd say he most certainly had the knowhow to fix his own car, but as Dinoman pointed out he left Harry's car for a shop, which seems strange to me for this reason: Mac's car knowledge from his younger years would have put that car more in his realm of expertise than a newer one. Anyway, he certainlycould have at least done the routine maintenance on his car. As with all car guys though, if he had the money and a shop he felt he could trust, he might pass on getting dirty and leave it to someone else.
Now, I know the real question was about appliances, but I'll try to tie that in by saying I think appliances are sorta the same way. The stuff he grew up being able to repair, serviceable stuff, was being phased out in the 80's with circuit boards and sealed motors. He probably just bought high quality appliances and when they broke, he replaced them unless it was a simple fix. I definitely wouldn't hesitate to buy a new digital alarm clock if mine quit, it's not like I'm an electrical engineer who knows what I am looking at when I see a circuit board.
I guess what I am getting at is that Mac was a jack of all trades and master of only a few. He would tinker with things and get lucky most of the time, but if it was something out of his scope he had no problem deferring to someone else who was an expert.
On a side note, my how the times have changed when our grandparents bought an appliance, they'd keep it as long as they could and if it broke, they had the now almost non-existent appliance repair shop that would breathe life into it and get another 5 years out of a toaster. Now, the economics of it are that the mass produced cheap chinese/far eastern goods are cheaper than a repair bill would be. And Mac was sandwiched right in there growing up through both eras.
"It's a real ugly thing to see in a dog. It's even worse in a human being." --The Golden Triangle
"I'd like a 16-inch pizza, run it through the garden, hold the chovies." -- The Eraser