Volvo: Volvo 850 engine stops if AC compressor connected, volvo 850 glt, volvo technicians

Question
Thank you for your work on this site. We have a 1996 Volvo 850 GLT w/ ‘deluxe' HVAC system, ~75,000 miles.  A/C system was replaced in summer '03, when evaporator failed. In summer 2005, A/C compressor began cycling on/off every 45 sec or so and did not cool if outside temperature over 90 (we live in west TX, where summer temps are >95, probably 125 inside black 850). Volvo dealer TX found “OK” Freon level and good compression.

In Dec 05, at stop on freeway trip, car would start but die w/in 20 sec. Car towed to dealer in Oklahoma City, who diagnosed power coil failure, replaced power coil, and unplugged compressor so car would stay running. We drove car from OKC to west TX without incident; although engine was ‘roaring', no dash lights came on and temp gauge normal. Next day, car would not start. Local Volvo dealer said power coil never the problem, replaced AC compressor and confirmed that engine function OK. BUT – they have found that car will stay running only if AC compressor unplugged, apparently because of failure in electrical system. For past month, Volvo technician here and Volvo technicians at Volvo in NJ have pored over wiring diagrams, and technician here is conducting wire-by-wire search, with no success. Volvo national office suggested replacement of wiring harness, which dealer here did not recommend because cost of $2500-3000 is more than car is worth. Any suggestions about what might be going on would be welcome.  Thanks.


Answer
Hello! I have to say, I have never seen or heard of a fault like that. My first guess would have to be in the ECM (electronic control module). When the compressor turns on, the ECM make adjustments to the engine to compensate for the extra load. If the adjustments are not made the car will stall. The ECM may need to be re-programmed or replaced. A clogged throttle body can also cause the problem. A wiring harness short could also cause the problem, but that will require extensive wire by wire fault tracing. Does the car sometimes restart right away after is dies? What was the "roaring" sound? What dealer in NJ was it at? I know Volvo technicians all over NJ. I may be able to talk to someone about it. If you have any more info on this please let me know. The more info I have, the better I can help.

Jimmy