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).

There is no way the freon charge influences the electrical current drawn by the compressor clutch or fans to the extent it would blow a fuse.

Another piece of the puzzle: I tried swapping the red round A/C relay in the fuse box with the relay at the other end of the box (position G) and when I went for a drive the circuit (G) which is the fan for aux air booster behind the instrument cluster was powered on all the time even after I shut off the engine. pulling the relay was the way to stop the fan from spinnning. Hence that relay which was the original A/C relay in the fuse box was shorted on when the fuse got blown.

Or the current from the short in the blower or clutch arc welded the contacts together.

Of course I could be wrong answer from a pro :confused: (last answer from NickD)

perhaps you should check the coil resistance. What it should be I don't know.

If you have a front condensor with the fan check to see if the fan turns freely. Over time they become seized and because they are not on a fuse circuit (not unless a PO did the mod and install a fuse, wink, wink) the motor start drawing current until it starts melting the harness. Google or do a search for condensor fan fire Porsche.

Advice from JW on another post about the same subject

Well, the new relay came and after installing in the smuggler's box the fuse blew again! I try putting 12V across the two wires of the condenser fan near the battery and did not hear any whirring noise. Checking connectivity resistance across the two wires (brown and yellow/black) showed infinity. Next I looked at the evaporator fan under the smuggler box lid. Removing the square plastic cap (with the wire on top of the box), I felt the fan and it was warm to the touch.

Question: if the evaporator fan is toasted what is involved in replacing the fan portion only and not the evaporator coil underneath the fan?

Well since the evaporator fan isn't necessarily burned and costs over $250 to replace, I took out the motor for the front condenser fan. Sure enough as John Walker said, that motor was burnt (hence the acrid smell coming out the vent) where the brushes made contact with the shaft. The fan cage was hard to turn by hand and only freed up after lubrication. Ordered the replacement motor and should install after june 1st.

So with that fan disconnected will the evap fan run w/o blowing the fuse? Many folks add an in-line fuse to that front cond. fan.

If you review all the previous posts, J.W. never posted on this thread. J.W. was quoted on this thread.

Tigerrat, I haven't tried running the evaporator fan with the front condenser fan disconnected. I'm not sure if the evap fan will run if the fuse is replaced (ran out of fuses).

I suppose my problem is not related to the overfilling of the freon as some have objected to. Just a coincidental timing for the fuse to blow and the motor to burn.

i hate to tell you this. you cant tell if the system is full unless it is running, and you have air flow over both condensors.
get a box fan and place it on the rear glass blowing on the rear condensor. make sure the front fan is working. you may even want to put a fan up front, but not too close, just a little extra air flow.
with the AC on hi, doors closed, motor at aound 1500-2k, check pressures.

r134 or r12?

if you let freon out based on 45 psi static, that sounds low, i think it is usually around 60 with it off, but that still does not mean it is full.

you have a site glass on the dryer, check it with the system running, that is actually better than pressures.

porsche does not use a low pressure switch, unless they did on the newer ones.

i have seen the clutch make a car cut off and back on quickly. my volvo is doing it now, i had a jeep that did it and my moms volvo also did it, dont ask me why, but it does. i got so frustrated with mine on a trip, i bypassed the low cut switch, best thing i ever did, the ac blows colder and yes i know why it is there but i dont care.
i hate the ac system in that car and i am tired trying to get it to blow cold.
new compressor, cleaned the evap and condensor, flushed, vacuumed and it still blows in the 60's at best, now it blows 40 with the low cut bypassed.

T77911S, I agree with you, was only trying to diagnose why my fan and A/c system is blowing the fuse and doesn't come on anymore.