Help, I overcharged the A/C and blew the fuse
 

Stupid me,

I added some freon to my 1988 carrera coupe and it was fine. (gauge reads just near the bottom of filled zone slightly above the refill zone.) I then added another 12 oz. and when I next started the car the fuse blew and I smelled an acrid smell coming with some smoke out the ventilation vents. Checking the fuse #2 showed that fuse blew but there appears to be a short since the next replacement fuse also blew when I turned on the A/C control on the floor console.

Would someone please explain what happened and how to fix the problem. The static pressure of the A/C at the low side with the compressor not running now reads just at the border between filled and Alert (Blue and yellow zone of A/C charge gauge.)after I bled some of the refrigerant out through the fill port.:mad:

Does your system have a low/high pressure switch?

probably but I wouldn't know where to look. IS it in the smuggler's box?

It's likely in the a/c hoses near the compressor. Look for an electrical switch attached to one of the hoses, with wires running off of it.

Scott,
do you know if it needs replacing that the system has to be opened up (freon recycled)? Also there is a expansion valve which may be the problem?

If you have to replace the hi/lo switch, then yes, you'll have to reclaim the freon and open the system.

Is it the compressor that's blowing the fuse, or the fan blower?

I looked for a pressure switch in the wiring diagrams and didn't find one, only the temperature switch. Either way, unless the Porsche part functions radically different than what appears on other vehicles, I don't think it would cause the problem you are describing. To my mind, this switch failure is the only way to tie your charging the system to the scene of the crime. The Magnetic clutch on the pump doesn't care what the compressor is doing and neither does the fan. Personally, I'd pull some of the easy stuff first, like the power to the fan and see if you still pop fuses.

Now that I think about it, I suppose the other reason I doubt the presence of a pressure switch is that my pump runs even though the system is completely discharged.

I looked and did not find any switch on the compressor itself, just the wires to the clutch on the side of the compressor. So I concur that there is no hi/lo pressure switch in the 911 A/C. By the way I've located the relay in the smuggler's box for the A/C.

Vreference, how do I disconnect the fan power and how does the relay work. I assume the relay in the smugg box powers the evaporator fan and the relay (round red one in the fuse box) powers the compressor clutch and ventilation fan switch down on the floor console. Which fan are you suggesting depowering?

I'd say you should get a electrical diagram like from the Bentley and start isolating the problem. Figure out if it's the fan, compressor clutch or what blowing the fuses. You disconnect the clutch right near the compressor and the evap fan in the smugglers box. Pull back the trunk carpet to get to the front cond. fan.
My 88 did not come with a hi/lo cut-out switch. Is your system original?
I'd also get a set of gauges and a digital thermometer so you can figure out your possible over-charge issue. Ken

i dont think over charging the A/C has anything to do with you blowing a fuse. It sounds like you toasted your blower motor

My A/C is OEM not modified or updated. I did make the mistake of not shutting off the ac when I last turned off the motor.

I'm not sure I would consider that a mistake. It's certainly not what caused this. The blower that is the most likely suspect for why you had smoke blowing through your vents is in the smuggler's box under the carpet in the trunk. I had to take off the fiberboard HVAC cover as well to open mine. You can follow the wires that come out of the top of the blower a few inches to a plug. I have not looked at the diagram enough to say whether or not this will disable more parts of the system than just the blower so if this stops the dead-short it may not be conclusive but you can certainly go from there.

I had the same problem a little over a year ago after topping off the charge. I had the system evacuated an recharged with the correct amount of refridgerant and no more problem.

http://forums.pelicanparts.com/porsche-911-technical-forum/346678-new-c-problem-melting-fuses.html?highlight=melting+fuse

Well, I tried disconnecting all the plugs one at a time and trying to start the A/C and there was no actuation of the fans and clutch on the compressor. Also the fuse did not blow. It seems to indicate the fault lies in the console switch itself. Anyone have taken it out for replacement before and can give me a lead on howto access the switch? (A/C switch is the fan switch to the right of the temperature control switch in the floor pod in front of the gear shifter).

Mark,
your situation appears to be different in that your fuse doesn't blow and starts to melt. Mine doesn't start blowing (fan speed knob doesn't turn on the fans) after the old fuse blew (with new fuse in place).

Did someone once changed out the factory switches (which must be pricey with ones that are more common and less expensive and graphed the old knob fronts on to appear stock?

I turned ignition switch to on/accessory position and turning the A/C fan switch to 1 position I hear a click in the engine compartment but the fan doesn't come on.

Does the front cover pop off after the retaining rings on both switches are removed to see the wires behind?

I'm not sure I'm quite on board with your fan-switch idea just yet but I'm pretty sure the modules that go into the center console are retained by screws as well and If I'm not mistaken you can't just pop the A/C module out easily.

My glance at the wiring diagram this morning had me a little worried that pulling the fan motor connection might disable other parts of the A/C system; are you sure that's not throwing off you diagnotics? My car is too far apart to help you on my end I'm afraid.

Well, I've ordered a new relay for the smuggler's box and been bleeding off the freon (or condensing it back into the can by chilling the can down to dry ice temperature (-78 deg C). Next weekend I'll give it another go at tracing the fault that keeps the fan from coming on. The evaporator fan may just be blown during the time the fuse got tripped and the cabin vents gave off the acrid odor and smoke like freon/lubricant
discharge. The current theory for my problem is still overpressure in the system tripping some protective measure that prevents the compressor clutch to engage and the fans to circulate.

For what it's worth , the pressure gauge reads 45 psi off the low side inlet static (compresssor not running).