What sort of preventve maintenance can I do my 140,000 mile AC system? Specifically, I was wondering if it makes sense to add oil or a can of refrigerant to the system. I figure I can get the cans and taps no problem, but am unsure about how much is enough/too much, etc.

Thanks,
Tom

The only preventive maintenance I can think of is keeping the condenser and evaporator (if you can get at it) free of debris. Organic matter can get wet and cause poultice corrosion of the evaporator requiring expensive repair, especially oak tree leaves or pine needles - both of which are very acidic. The front condenser should also be free of debris by flushing with water.

Unless you have a drop in performance (vent air not as cold as it used to be), the a/c system is sealed for life and should not be "maintained". When refrigerant needs to be topped off, it should be done with a set of a/c manifold gauges attached to high and low pressure ports to monitor system condition. Even some accomplished DIYers are hesitant to touch an a/c system. Yes, it is possible to put too much refrigerant or oil in the system!

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95 E320 Cabriolet, 159K

Kestas is offering some valuable wisdom here!

An a/c system is a great place to apply the old term "if it ain't broke, don't fix it.

Every time you break open the system, you risk introducing moisture. Moisture combined with refrigerant creates an acid that will eventually eat its way out of the system. I have seen systems on centrifuges, scintillation counters and other scientific instrumentation units that once serviced started giving frequent trouble.

When a system ceases to produce cool air it is then time to give it the attention it deserves. If you are lucky, your system will have a one ounce per year leak causing a need for recharge after 15 or 20 years. If you are not lucky you will have a "sure enough" leak sooner than that. At that time, you fix the leak, probably replace the filter drier, thoroughly evacuate and recharge. Until then "don't mess with it!"

This is the only subsystem of your car with lubricant in which you should NOT Change oil hot and change oil often.

Good luck,

Thanks for the help.
Does it make sense to check pressures periodically, or just wait until the compressor cuts off on a low pressure signal?

I personally still feel squeamish about attaching gauges to the system -- (what if the schreader valve decides to stick open?... or I get a bad connection?) Plus you bleed off a little refrigerant into the manifold lines with each check.

I vote for leaving the system alone until you notice a difference in a/c performance... like you said, when the compressor cuts out from the low pressure signal, or the vent temperatures start getting warmer. This system works best when left alone.

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95 E320 Cabriolet, 159K

Again, I totally agree with Kestas. There are plenty of subsystems on your car that can benefit from competent preventive maintenance, but as far as the a/c goes, DON'T MESS WITH IT until it is no longer cooling.

If it will give you some piece of mind, look at the sight glass every now and then.

Have a great day,

One thing I strongly recommend to car owners to protect the a/c system is to put 1/4" hardware cloth in front of the condenser to protect from rock damage. This is true only for cars that have an open grille. Debris can puncture the prone condenser tubes that are in front of the radiator resulting in expensive a/c repair (new condenser, dryer, vacuum, and recharge). Where I live there is a lot of road debris. Nearly every car has windshield damage. One only has to use their imagination to figure out what's going on at the level of the grille!

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95 E320 Cabriolet, 159K