The Robinair AC Machine is one of the most popular units for car air conditioning service.

©DenLorsTools.com Summary: In this auto repair article we discuss the reasons why a car’s air conditioning system may leak water inside the vehicle. Sometimes the fix is simple and other times the repair can be much more labor intensive. As an automotive technician that’s worked over 25 years in Florida, I’ve dealt with this problem many times on different models. If leaks aren’t fixed quickly, the carpet and padding underneath can become a breeding ground for mildew and harmful bacteria. People with certain allergies can be affected the most.

There are several reasons why a car’s AC can leak or drip water into the passenger compartment. First it’s important to understand that water dripping from under the car when the A/C is on is normal. It’s only when the water drips on the inside of the car there’s an issue. Varying amounts of water dripping (condensation) under the car is to be expected, depending on the level of humidity in the air. The higher the humidity, the higher the volume of water can be seen dripping under the car from the evaporator drain tube. I’ve actually had customers that complained because they thought that too much water was dripping under the car after an AC repair. On several occasions customers would say “It never had that much water draining out before.” It’s funny how some car owners pay closer attention to certain things after a repair has been made. After explaining that it’s a normal for condensation (water) to accumulate on the evaporator fins, drip to the bottom of the case and run out the drain tube; only then they understand that there’s not something wrong.

One of the most common causes of water dripping inside the car is because of a clogged drain. If the evaporator drain becomes restricted, the water backs up in the HVAC (Heating, Ventilation & Air Conditioning) case. The case is designed to allow water to travel to the bottom and out the drain tube. The case is not designed to be completely water tight in the event the drain becomes clogged. Sometimes if excess water remains in the case, some of it can blow out of the AC vents. In extreme cases the water can damage the blower motor and/or short out the blower resistor. Usually the blockage is caused by debris in the case, like leaves or dirt. Many times the evaporator drain can be cleared by blowing compressed air into the drain from the firewall side. A metal coat hanger can also be used to stick into the drain tube to loosen the debris and allow it to exit with the water.

Another common cause of water entering the passenger compartment can be from a faulty seal which is located between the HVAC case and the firewall. The water can drain out of the tube (unrestricted) – then run back to the firewall and leak into the passenger compartment, underneath the carpet. This is actually quite common with vehicles that don’t use a rubber “L” shaped hose on the plastic drain tube. Chrysler dealers (especially when vehicles are still under warranty) will sometimes install a simple rubber o-ring onto the plastic drain. When the water starts to run back on the underside of the tube towards the firewall it meets the o-ring. When water touches the o-ring, that causes it to drip downward instead of traveling the full distance to the firewall and entering the passenger compartment past the faulty seal.

The Chrysler Sebring pictured at the beginning of this article has a separate drain tube that attaches to the firewall which prevents the o-ring fix from being used. The problem is that the drain tube attaches to the firewall and NOT the drain on the case. If the seal is bad, the water can leak inside the car regardless of the firewall mounted hose. In cases like that (the Sebring and also Stratus) – the evaporator case has to be removed and the seal replaced or resealed. On every evaporator I replaced (any make/model) – I would always apply “Right Stuff” sealer to the bottom portion of the seal, to prevent a water leak after I’ve had the case out. Having to remove the case to repair the leak is obviously the worst case scenario, but sometimes it can’t be avoided.

Another cause of water leaking into the passenger compartment can be the HVAC case seams simply leaking. On the Chevrolet Cavalier and Cobalt the HVAC case uses a groove and tongue system that has no sealer from the factory. If the case becomes slightly distorted, it can allow water to blow past this seam and leak onto the carpet near the drain. The evaporator case has welded plastic rivets, that holds the housing together; if the case is removed, the welds have to be drilled out to separate it. When the case is apart, a sealer can be used like silicone or Right Stuff (my preference) in the seams. When putting the housing pieces back together screws are installed in holes that are provided. It is possible that the seams near the drain can be sealed externally with a sealer on the Cavalier or Cobalt. It is necessary for the sealer to work, that the area it is being applied to is completely dry. It’s also necessary to allow plenty of time for it to dry before running the AC.

Question: I have a Mitsubishi that leaks water on the driver’s side and wets the carpet. Could this be condensation from the air conditioner? If so how can I repair the leak?

Answer: The evaporator drain is on the driver’s side on some Mitsubishi models. If the drain seal is bad the water can run back inside, usually underneath the carpet. On the Mitsubishi Endeavor, the drain cannot be seen from the outside of the vehicle. I have been successful with the Endeavor and other Mitsubishi models, resealing around the drain without removing the case. 1. I removed the gas pedal and pulled the carpet back. 2. Removed the bad seal from around the drain with a pick. 3. Dried the area thoroughly with compressed air. 4. Wore latex gloves and applied Right Stuff, forcing it around the drain and through the firewall. – A bit messy, but definitely worth a shot before removing the entire dash and case.

Question: Can I fix the drain with the use of silicone instead of Right Stuff?

Answer: Probably. However I prefer Right Stuff because it sticks to surfaces better and dries harder, more like rubber when it is fully cured.

Question: I have a 2006 Dodge Caravan, should I try adding a hose to the drain or would the o-ring fix work for my van?

Answer: The plastic drain, will not accept an “L” shaped rubber hose, due to the slot that is cut out in the bottom part of the plastic tube (which is part of the case). The o-ring fix for your van should work well to prevent AC condensate from leaking back into the vehicle under the carpet. According to Chrysler TSB 24-001-06, this should work on all Dodge Caravans and Chrysler Town and Country vans from the year 2001-2006. The dealer/factory/OEM part number for the o-ring is 68001994AA, although any o-ring that fits over the plastic drain tube should work.

The tips in this article are very basic and NOT intended to be a complete guide for servicing your AC system. Please refer to a repair manual for further instructions.

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