After struggling through the unholy heat of summer rush hour, gritty with sweat and exhaust fumes, how delicious it is to enter the cave-like cool of one's air-conditioned dwelling and be soothed by the hum of this wondrous appliance. What misery to instead confront a boiler room-like home and a defiantly silent machine - especially if it's Friday night and company's coming.

In these energy-troubled times, some consider air conditioning a luxury; but to many, especially city dwellers enveloped by stagnant pollution, it's a necessity. So why not get the most cool for the least wattage possible by keeping your air conditioner in top form? The payoff is lower utility bills and staving off your units's untimely demise.

Tenants and condo owners hooked up to a central system need not trouble themselves with maintenance, but homeowners and renters wwith individual units have to take matters into their own hands.

There's nothing like that old ounce of prevention to assure your air conditioner's keeping its cool all summer. Naturally, the best time to give your unit a good going-over is before hot weather begins in earnest, but it's still not too late. Also, some parts of the procedure ought to be repeated during the season.

A professional should check your air conditioner's refrigerant charge, electrical connections and motor amperage. But this is not a yearly must if your equipment was properly installed and maintained by a reliable serviceman for a year or two after installation, and if it operates in a fairly dust-free setting. You can do the basic annual maintenance yourself.

Cleaning and changing the filter is probably the easiest task and prevents the costliest damage to your unit (a dirt-laden filter clogs the whole works and eventually burns out the machine). Foam, plastic and metal filters can be washed with clear water and dried. Paper and fiberglass filters should be replaced. Check the filter every six weeks, more often if your household includes many people (all the more lint), shedding pets and any activity that generates dust, like wood-working.

Also in line for periodic cleaning are the evaporator (inside) and condenser (outside) coils. This usually entails removing a window unit - not an impossible task (especially if yours is designed to slide out of its casing like a drawer). Just be sure that the unit is properly tilted when reinstalled (usually 1/8 to 1/4 inch lower on the outside) and that everything is screwed back tightly. A vacuum cleaner can be used to blow off dirt and lint. One authority claims that a vacuum only removes surface dirt and to really do the job, you must get an air compressor or take the unit to a gas station and use the air hose; 80 lbs. pressure is suggested. if you go this route, proceed with caution since the air hose could damage delicate motor fins. Another air-conditioning expert recommends dousing the coils with the garden hose for five or ten minutes, taking care to avoid the motor and control switches (try masking them off with tape).

Once or twice a season lubricate the fan motor(s); window units have one fan, central units have two. Oiling instructions should be printed on the motor, but to be absolutely certain check with your manufacturer or dealer. You don't want to over-oil: Excess oil will gum up the motor as well as drip and collect dust.

Eyeball the wiring to make sure nothing's worn or frayed, check fanbelt tension (as prescribed in your owner's manual), clear away any obstruction on either side of the unit and you're in business.

A well-maintained air conditioner should last nearly twice as long as one left to its own devices. "If everyone changed their filters and oiled their motors, two-thirds of the air-conditioning companies in town wouldn't be here," one local repairman observed.But despite dilligent loving care, something may go awry. Unless you know a thing or two about electrical work, it may be time to call in a pro. If you don't know what it means to discharge a capacitor and how to do it, you shouldn't fool with the inside of an air conditioner, even an unplugged one.

But don't dash to the phone in a mad panic. Take a minute to study your air conditioner. While you may not be able to fix it yourself, you're apt to get prompter - and more honest - service if you can say with some aplomb to a repairman, "The problem seems to be . . ."

If the air conditioner is grinding away but no cool comes out, you may be lucky. It may just be a dirty filter or condensor coil, simple enough to clean. Unfortunately, cleaning the filter and coils may be too little, too late if the compressor is hot. The compressor is the heart of an air conditioner and very costly to replace. The tipoff can be the presence of ice, indicating a build-up, or the acrid smell of burnt freon, its lifeblood. A unit that's running but not cooling may have lost its freon charge via a leak. Freon is an inert gas, so there's no danger. But it's expensive, and costs whatever the market will bear. The current going rate is $6.50 a pound for residential units. Air conditioners need certain amounts of freon like tires need air. Central air conditioners need 6 1/2 to 7 pounds of freon, window units generally about 1 1/2 pounds. Somes types of air conditioners may stop completely if the freon level drops drastically.

A good repairman should be able to find the leak's origin quickly with either a dye or les over the leak. The defective part, typically a joint, should be resoldered or replaced before the system is recharged with freon.

If the air conditioner is completely dead, check the fuse or circuit breaker. A dirty filter can cause the unit to use more energy than it should. If the circuit or fuse is not the problem, there may be a broken wire or loose connection somewhere along the line.

These are but a few of the ills air conditioners are heir to. Again, don't reach for the phone until you're checked the basics, like the plug and the thermostat. Honestly. You may save yourself a hefty service charge.

When you do call for help, be forewarned: This is peak season for air-conditioning firms and they can afford to be choosy. Unless you're a regular customer or have a service contract, you're likely to have to wait a week or more for salvation. If lack of cool air creates a medical emergency, say for someone with asthma or a heart problem, have your doctor phone. Too many servicemen have become hardened by phony "emergency" pleas.

Rather than play roulette with the Yellow Pages, try to use a company recommended by someone you know. While not foolproof, word-of-mouth is still pretty reliable. Or get a reference from the manufacturer or dealer.

Once you've narrowed your choice, there are some ways to ascertain whether you are dealing with a reputable firm. Here are a few tips suggested by the Washington Consumer Checkbook: COMPLAINT RESPONSIVENESS

Are there many complaints against the firm? How well do they respond to complaints? Check with the Better Business Bureau of Greater Washington (393-8020) for a firm's complaint track record. CUSTOMER SURVEY

For an unbiased sample, ask the firm for references who fit some strict criterion - for instance, customers who live in your neighborhood or who had work done in 1976. LICENSING

A license assures that a firm is at least capable of doing your work, although only one person per firm needs to be licensed. Most air-conditioning firms are licensed for plumbing and electrical work in all local jurisdictions. A specific heating and air-conditioning license is not required except in the District and Fairfax and Prince George's counties. To check on a firm's license contact the District of Columbia Housing Regulations Division (724-4417), Prince George's County Electrical Inspection Department (779-3850) and Fairfax County Mechanical Inspections (691-3361).

Other items to check ar how flexible are a company's hours, whether they schedule appointments (so you don't have to wait around all day), do they warranty their work, and of course, how much do they charge. Labor rates vary widely, although you can expect an initial charge of $25 to $30. Some firms have a minimum charge, some include a travel charge and some have both. Whether a firm charges by the hour, half-hour or quarter-hour can make a big difference. Be wary of unusually low labor rates - you may make up for it with highpriced parts and materials.

Once the repairman of your choice is face to face with the recalcitrant appliance, he should be able to diagnose the problem quickly. You should naturally then be told what's wrong, what he proposes to do, how much it will cost and approximately how long it's going to take. Get a second opinion for any job a repairman tells you will cost $200 or more.

For your own protection, make it clear that you want back all replaced parts (this is required by law, but only if you ask) as well as a performance report. This describes what's been done, the temperature inside and outside your home afterward, the equipment's general condition and the warranty on the work.

Be skeptical of a repairman who tries to sell you a new unit or a service contract: He may be working for a commission. Service contracts are annual agreements for a firm to service and repair your air conditioner for a set cost (often more than $100). Although a service contract entitles you to priority treatment, it's rather expensive insurance and not absolutely necessary unless a member of your household is medically dependent on air conditioning.

Window units are the lepers of the industry - no one wants to touch them. "The road to hell is paved with window air conditioners," one repairman said. "Getting parts for them is a nightmare. They're made to be thrown away and parts become obsolete quickly."

Although the phone book abounds with air-conditioning firms, most handle only central air conditioning. To find one that services window units look under "Air Conditioning - Room Units." Here are a handful of firms that will come to a lone window unit's aid: ATLANTIC HEATING & AIR CONDITIONING, 4701 Baltimore Ave., Hyattsville. 927-9204. 8 to 4:30 Monday through Friday. Handles all windows units if you bring them to the shop. There's often someone there Saturdays, but call first. Labor is $15 an hour. BETTER APPLIANCE SERVICE, 10503 Wheatly St., Kensington. 949-2500. 8 to 5 Monday through Friday. Makes house calls for window units and tries to accommodate emergencies. Minimum is $24.50. BISHOP EQUIPMENT CO., INC., 2825 Dorr Ave., Fairfax. 573-1500. 8 to 4 Monday through Saturday. Services window units all over the area. There's currently a 10- to 14-day wait, but they'll try to squeeze you in if they're in your neighborhood. Minimum $17.50, or $30 an hour plus parts. ROD MILLER, INC., 2616 Garfield Ave., Silver Spring. 587-1517. 8 to 5 seven days a week. If you live in Montgomery County, the District or parts of Prince George's County, they'll service your window unit, but there's a seven- to 10-day wait. You pay $30 the first hour, $7.50 every additional quarter-hour plus a percentage for overtime, if any. ROBINSON'S AIR-CONDITIONING & HEATING, INC., 5850 Wicomico Ave., Rockville. 871-8902.9 to 5:30 Monday to Saturday. Goes all over Montgomery County to repair window units. There's a wait of approximately three days, but they do give priority to medical emergencies, warrantly work and service contracts. There's a 24-hour answering service. Minimum $24.50 for the first half-hour on the job, $15 for each additional half-hour. SEARS, Service centers in D.C. and Prince George's County 296-5100; Montgomery County 840-8800; Northern Virginia 321-8200. 8:30 to 4:30 Monday through Saturday. Services any size unit as long as it's a Sears unit; schedules a week ahead and tries to handle emergency calls. Basic $11.95 plus parts and labor.