Automotive/a/c on a 2002 vw bug

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Expert: - 7/28/2007


Question
The problem i have is i have hot air coming out of the vents.the a/c works,the compressor kicks in the pressure is at 45 and i can feel the vent cold  behind the radio by the fire wall but hot air is coming out of the driver and passenger vents.I tried disconnecting the battery and turning the thermostat to cold and hot along with turning on the defroster and rotating all the vents.

Answer
As indicated in my notes, I do not have (nor do I need for my regular requirements) reams of service information, so I cannot address the specific details of the 2002 VW.  What I can do is suggest a general area of investigation, based on the design philosophy used on many (but not all) automotive A/C systems.

Having said that, it is common for the A/C evapourator and the heater heat exchanger to both occupy the same plenum and share the same fan.  In most operating modes, only one or the other will be active.  In defog or defrost mode, both can be active.  This latter mode is to take advantage of the drying effect the A/C evapourator has on the incoming air, which is then reheated to apply to a foggy windshield.

Your problem therefore might be a control or actuator issue which has caused the heater core to be enabled in modes for which it should not be enabled.  This could happen due to mechanical (linkage) problems, or due to a nonfunctioning electrical or vacuum-operated valve actuator.  If the system is mechanical, you might find a disconnected or broken linkage.  If it is vacuum-actuated, you may find an open vacuum hose (pulled away from its fitting on a valve assembly or at the actuator itself) or a leaky actuator.

To note: some systems are "two deck" systems, comprising a "hot deck" and a "cold deck" - this is quite common in commercial building systems.   This allows for a blended mix of hot and cold air to be distributed to different outlets, depending on the temperature and airflow needs at the specific outlets.  It is a good system for independent control of different rooms in a building.  Older Fords used a variation of this type of system.  While great for temperature control, it does call for compressor action most if not all the time, and would not be considered energy conservative in an automotive context, unless managed by a supervisory control system that makes good use of outside air at ambient temperature whenever possible - at Honeywell in the seventies, we called this "zero energy band" operation.

Hope the first paragraphs above provide a start, if not a route to an answer.  

Good luck  ...  EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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