Eliot Silverman

Eliot Silverman

Thursday, May 31, 2012

How your A/C System works

At ambient temperatures a/c refrigerant is a gas.  Years ago cars used r-12, commonly know as Freon.   They stopped using r-12 because it eroded the ozone layer in the atmosphere.  Freon has been replaced by r-22, also known as Suva. 
When you turn on the a/c system, through a relay, and sensors the compressor clutch “Locks up” and the a/c compressor turns.   Suva enters the compressor as a gas, and comes out as a high temperature liquid.  By compressing the gas into a liquid the temperature goes up to around 250 degrees Fahrenheit.  The liquid leaves the compressor, travels through an aluminum line and goes through a “Condenser.”   The condenser looks like, and acts exactly like a radiator.  Outside air goes through the condenser and cools the refrigerant inside the condenser.  The refrigerant exits the condenser as a liquid and close to ambient temperature.   
The cool liquid refrigerant travels through another aluminum pipe, through an expansion valve, into the evaporator.  The expansion valve allows the liquid to ‘expand’ and become a gas.   When the liquid refrigerant expands and becomes a gas, its temperature decreases to just above 32 degrees Fahrenheit. 
The evaporator is another radiator.  It looks like a radiator, but is much smaller, and it is generally located behind/near the glove box.  The refrigerant enters the evaporator at around 32 degrees Fahrenheit, cooling the evaporator.  When you turn the a/c on the fan blows air across the fins of the evaporator cooling this air; hence you get cool air.  The hot air blowing across the evaporator heats the refrigerant in the evaporator.  Radiators/evaporators are also known as heat exchangers.  The air blowing across the evaporator is cooled, and the refrigerant inside the evaporator is warmed; hence heat is exchanged.
Hot air contains a lot of moisture.  As the evaporator cools this moist air, water condenses out, and is drained under the car.  When the a/c is on, and you are standing still, you will see a small puddle of water under your car.
If the refrigerant was below 32 degrees the water would freeze onto the evaporator, restrict air flow through it, and you would get diminished cooling.
If the drain in the evaporator gets clogged, the condensed water drains inside the car, and the carpeting on the front passenger seat gets very wet.  This is generally fixed by using a coat hanger to unclog the drain.
The refrigerant leaves the evaporator as a warm gas.  Through another aluminum pipe the refrigerant travels to an accumulator/dryer.  A dryer is used since water combined with refrigerant creates hydrochloric acid. Obviously acid is undesirable to the system. 
Through the last aluminum tube the refrigerant (still a gas) leaves the accumulator/dryer and goes back to the a/c compressor completing the a/c cycle.
A/C Compressor:
The front of the compressor has a clutch, and the clutch is turned by a belt.  When the a/c is off, the clutch turns, but the a/c compressor doesn’t turn.  When the a/c is turned on, an electrical current goes to the clutch, locking it up to the compressor so that the compressor turns.

High Pressure Sensor
This sensor is located between the a/c condenser and the evaporator.  Here the refrigerant is a high pressure liquid.  If the pressure exceeds the manufacturer’s maximum allowable pressure, the senor “Opens” and stops the flow of electricity to the a/c clutch.  When this sensor “Opens” the a/c clutch Is de-energized and the a/c compressor stops turning; hence the pressure goes down.  This is a safety device which prevents the system from “blowing up.”

Low Pressure Sensor:
This sensor is located between the evaporator and the a/c compressor.  If the pressure is too low, it “Opens” and stops the flow of electricity to the a/c clutch.  When this happens the a/c compressor stops turning.  This is a safety device which prevents the compressor from turning when there is not enough refrigerant/oil in the system to properly lubricate the compressor.

Aluminum Lines:
Aluminum lines have one problem… Aluminum parts sometimes corrode together.  If they did not corrode together, life is good and then they easily come apart.  However; if the aluminum parts corrode together they cannot be normally separated, and I need to replace both parts. 

For example, I had a Honda with a hole in the aluminum line between the condenser and the compressor.  The aluminum line corroded together with the condenser.  When I removed the line from the condenser, there where no threads left in the condenser to use to reconnect a new aluminum line.  I need to replace the condenser.  In this case an inexpensive repair became a very expensive repair.  Unfortunately I cannot tell ahead of time if the parts have corroded together or not corroded together.

Your a/c system is unique in that it never needs maintenance.  It is a closes system that continually recycles the refrigerant and the oil.  Oil is used to lubricate the a/c compressor.  Without this oil the compressor would seize-up.
If you have any questions please e-mail them to me at eliotsauto@aol.com
My web site is http://www.eliotscompleteautorepair.com/

A/C System

A/C system
Your a/c system is self contained and does not need maintenance.  If the air comes out cold, you are done, and you don’t have to worry about this system.  If the air comes our cool, or warm there are two possible problems with the system.

Why Doesn’t My A/C Work?
First possibility: The system may not have enough a/c refrigerant to enable a/c compressor to engage.  Under the best of circumstances an a/c system slowly loose its refrigerant.  It takes somewhere around 7 years for a perfectly good a/c system to leak enough refrigerant such that the compressor will not start when you try to get cold air.  The oil which lubricates the a/c compressor floats in the refrigerant.  As the refrigerant leaks out, the oil leaks out.  If the compressor engaged when there was inadequate amount of refrigerant, the compressor would not be properly lubricated, and it would seize-up.  To prevent this from happening, a pressure sensor prevents the a/c compressor from running when you are low on refrigerant.  

On a side note, to keep the compressor lubricated in the winter, the computer turns the compressor on and off. 

Also, when you turn on the “defrost,” the a/c system automatically turns on.  The air first goes through the a/c coils in order to “dry-out” the air, and then the air is heated by the heating coils so that it can evaporate the condensation on the windshield.

Second possibility:  The system is full of refrigerant, but due to an electrical problem, the a/c compressor does not turn on.   For example, you could have a bad button (on the dash), bad relay, bad fuse, bad wire, or a bad sensor. 

How Do I Diagnose a “Non-Operating A/C System?
I start by connecting my A/C gauges to your car.  If the lower pressure gauge indicates good pressure (oddly enough good pressure in ‘psi’ is equal to room temperature) then the car is full of refrigerant.  If I read good pressure but the a/c compressor does not engage, I know you have an electrical problem.

If the gauges indicate you don’t have any refrigerant, or a very low quantity of refrigerant then I add refrigerant and yellow dyed oil.  If the compressor engages, then I know the electrical system is good, and the system did not work because of “Low refrigerant.”  After I fill the system, I look for a refrigerant leak.  Looking through tinted yellow glasses I can see refrigerant leaking from your system.  Unfortunately, this only allows me to see “Big” leaks. 

A small leak takes 3 days to two weeks for the system to loose enough refrigerant such that the a/c compressor no longer engages.  Within days of the A/C no longer working, you need to bring your car back here so I can use my geeky yellow glasses to find the oil leaked.  The oil leaked at the same spot where the refrigerant leaked. Once I see which part leaked, I can give you a price to replace that part.

Why Do I Have to Refill MY A/C System Once a Year?
You may have a seal around the a/c compressor which shrinks when the temperature gets cold (winter) and allows the refrigerant to leak out.  When it is warm outside (summer) the seal expands and no longer allows the refrigerant to leak.  You fix this by replacing the a/c compressor, or you refill the system every summer.

Why Shouldn’t I Refill the A/C System in the Winter?
If you wait weeks before bringing your car back, the yellow dye with disappear and I will not know form where the refrigerant leaked.  It is for this reason that I hesitate refilling the a/c system in the late fall, winter, or early spring.  If it refill the system, and you don’t use it for a month, the refrigerant may have leaked out and you didn’t know it since you did not use it.  Months go by before you realize the a/c leaked, and by then I cannot see any trace of the yellow oil.

Thursday, May 10, 2012

How Do I Know If I Need a Brake Inspection or a Brake Job??

Brakes turn the energy of motion into heat.  In other words, when you want to stop the friction between the brake materials, and the brake rotors/drums (more on this later) creates heat.  The faster you go the more heat the brakes create for you to stop.
There are two types of brake systems, shoes and pads.  Shoes are the old system, and they are rapidly being replaced with pads.  Many years ago the government mandated that all cars have front brake pads.  Most of the cars I work on have  4 wheel brake pads. 

Brake shoes:
When you press the brake pedal you push brake fluid from the master cylinder through the brake lines into the wheel cylinders.  The pressure of the fluid in the wheel cylinders pushes two plungers outward which force the brake shoes to make contact with the inside of the brake drum.  The pressure/friction of the shoes on the drum causes your car to stop.   One of the biggest problems with this system is that the shoes don’t always make 100% contact with the brake drum.  When you stop braking springs retract the shoes and remove them from touching the brake drum. 
Brake Shoes and Drums

Brake Drums:
These rarely need replacing.  The brake shoes rub on the inside of the brake drum.  When the brakes are inspected, we measure the inside diameter of the drum.  If the diameter is too large (there is a federal limit which is different for every model) then we need to replace the drum.  If the drum is not round, you will feel the brake pedal pulsate, and we need to resurface the brake drum.  If the inside surface is flat, we do not need to resurface it when re replace the brake shoes.

Brake Pads:
When you brake you push brake fluid from the brake master cylinder through brake lines into the caliper.  In the caliper, the brake fluid pushes a piston outward forcing the ‘inside’ brake pad onto the rotor.  This is a litter difficult to explain, so please bear with me… “For every force there is an equal and opposite force.”  When the brake pad pushes against the rotor, the caliper is forced back, causing the ‘outside’ brake pad to make contact with the outside surface of the brake rotor.  This is called a sliding caliper.  Ideally, both pads exert the same force on the brake rotor.   A few manufacturers don’t use sliding calipers.  These manufacturers use a brake caliper with pistons on the inside and on the outside of the brake rotor.

Brake Caliper with One Piston

Brake caliper with two sets of pistons.  One set for the inside pad, and one set for the outside pad

 If a sliding caliper cannot slide, one pad is substantially more worn than the other pad, and you need a new caliper.  Another problem with brake calipers is that the piston does not retract when you stop braking.  This is also clear by noticing one brake pad being substantially more worn than the other brake pad.  When this happens, you need to replace the brake caliper.

Since it is important that the front left brakes and the front right brakes work exert the same forces, when one caliper is bad, it is generally recommended that both calipers be replaced. 

Brake pads are simpler than brake shoes, and they work significantly better.   Because brake pads are significantly better than brake shoes, the government mandates front brake pads. 

Most, if not all cars made in the last 10 years use front and rear brake pads.  The parking brake only engages the rear brakes, so manufacturers tried to incorporate a parking brake mechanism inside the rear calipers.  This system never worked well so they abandoned that system.  We now see a hybrid shoes/pads in the rear.  When you press the brakes, the rear brake calipers work exactly like the front brake calipers.  When you engage the parking brakes, you actuate a small set of shoes which are located inside the hub of the rear brake rotors. 

You can see the parking brake shoes since that
The brake rotor has been removed.

Brake Rotors:
The brake pads rub on the outside surfaces of the brake rotor.  Over time the brake rotor thins due to this friction.  There is a federal minimum thickness listed for all brake rotors.  This minimum thickness is different for each car model.  When brake pads are replaced the brake rotor needs to be flat.  This can be accomplished by resurfacing the brake rotor or by replacing the brake rotor.  When re resurface the brake rotor we use a brake lathe which thins the brake rotor and leaves a smooth finish.   Ten or more years ago, the brake rotors were thick enough such that we could resurface the rotors two or three times before they needed to be replaced.

I’d say, for the last 10 years, new brake rotors are rarely resurfaced since their thickness are slightly above the federal minimum.  The good news is that new brake rotors are significantly less expensive than they were 10 years ago.  In many cases the new brake rotors cost less than the cost of resurfacing the brake rotors.  Many times I have replaced the brake rotors, not because they were too thin, but because the new brake rotors were cheaper, or the same cost, as resurfacing them.

If the brake rotor ‘warps’ you will get a pulsation in the brake pedal. To eliminate the pulsation you either have the brake rotor resurfaced, or you have the rotors replaced.  This is felt in the brake pedal NOT in the steering wheel.  On a side note, if the steering wheel ‘pulsates’ it tends to indicate bad tires or an out-of-alignment problem.
This is a typical brake rotor

Brake Master Cylinder:
When You press the brake pedal you force brake fluid from the brake master cylinder to the four wheels. 

For safety purposes, the brake master cylinder is actually two master cylinders in one unit.  One part pushes brake fluid to the front brakes, and the other part pushes brake fluid to the rear brakes.  For example, if the brake line to the rear brakes leaks, you loose all rear braking, but you still have front brakes, and visa versa. 

Normally when you stop the brake pedal goes down to a point and stops moving.  Even with more pushing, the pedal does not go any close to the floor.  That is normal and good.  If you have a situation when you are at a stop and the brakes pedal slowly goes to the floor, you probably have a bad master cylinder or a brake fluid leak. 

Proportioning valves:
Without these valves, when you would begin to stop the front of the car would tend to “Dip.”  With these valves, the rear brakes engage a fraction of a second before the front brakes preventing the front end from dipping.  I have never had to replace one of these because they were bad.  I have replaced them because I could not remove the brake lines from them without doing damage.
     Pictorial -Notice the two chambers               A typical brake master cylinder

Brake Hoses and Brake Lines:
Brake lines are steel, and brake hoses are rubber.  The steel lines, after many years sometimes leak due to rust.  The rubber in the brake hoses rarely go bad, but when they do, the rubber cracks and break. If either the hose or line breaks it becomes obvious because the brake light comes on, your brake pedal goes to the floor, and if you wait long enough, the car will not stop as quickly normal.  This diminished braking is very obvious. 
         Brake hoses                                                 Brake line

If you have any questions about your brakes call me at 773 935 2400