I'm a master mechanic too, and I taught Automotive Electrical for 9 years at a community college. That doesn't prove I always know what I'm doing. The reason I specify a "cheap" meter is because I got yelled at once by someone who said they came to this web site because they were broke, couldn't afford a mechanic, and how in the heck did I expect them to buy a hundred dollar meter to use once. The Harbor Freight Tools meters that go on sale for $2.99 work fine.
I've been in tv / vcr repair for over 35 years and my most expensive digital meter cost 40 bucks on sale. Got twelve of 'em so I can find at least one when I need it! Don't have enough time left in my life to waste on auto-ranging meters that have to bounce around deciding which range it wants. I'm smart enough to know which range to select.
I'm not sure what you're referring to about discharging the battery unless you do that to give a place for current to go during the load test. That isn't part of any normal charging system test. In fact, no charging system test is accurate unless the battery is fully charged. Professional load testers use a carbon pile variable resistor to provide the current path up to well over 400 amps. A battery can't sustain a charge rate of much more than 20 amps without overheating. Regardless of your methods, any charging system test is not valid if the voltage is below 13.75 volts. You could compare that to a hydraulic pump. It must be good if it can deliver 10 gallons per minute, but what good is that if it can't build more than 20 pounds of pressure? Your generator might be able to deliver 80 amps but that is worthless if it can't get higher than battery voltage to charge the battery. If you're using a manually-adjusted load tester, you have to watch that system voltage never drops below 13.75 volts. If you can't get it higher than 11.8 volts, you're running off the battery, not the generator. Maybe tractors are different but if they're running a 12 volt lead-acid battery, the charging system will have the same requirements as on a car.
As for the switch assembly, the older mechanically ones that were perfectly reliable had a combination of vacuum and electrical switches. Each push button opened a set of vacuum ports to run the mode doors, and an electrical switch to turn on the fan motor. The electrical part could fail in any push button without affecting the others. Some used a series of plastic slides behind the switches that opened various ports and turned on the switches. On some, using the recirculate mode bypassed one fan resistor in the assembly to increase speed when you were running already-cooled air through again. In normal mode the condenser had a bigger heat load with the humid, hotter air coming in and they slowed the fan down to give it time to condense that humidity.
When we started going to computer controls we ran into all kinds of unusual problems. You can look as far back as the early '80s Cadillacs and Corvettes to see all the problems computers caused. It isn't always obvious either if the car uses an HVAC computer. Look at the miserable '96 Caravan with a pair of simple turn-knobs. There's a whole computer module behind them. You haven't experienced frustration until you tried to recalibrate one of those after disconnecting the battery. The same two knobs are used on the older Dodge trucks but instead of a computer they use a simple cable and electrical switches. Those never cause a problem.
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Tuesday, June 19th, 2012 AT 11:02 PM