A/C & Fan Issues

Two issues with a (recently purchased) 1999 Prizm.

1. The a/c system has an apparent freon leak. "Apparent" in that it blows cold air when freon is pumped in, but within a couple of days it goes back to hot air. A mechanic put in some freon with that green dye but he couldn't find a leak with his u/v light. He said to drive it around a few days and he would try again.

2. The radiator fans go on immediately when the ignition is turned to "on", even before the engine goes on, and with the engine cold. The fans are on at all times that the power is on. I have another Prizm without this problem so I swapped the fan relays and the main engine relays between the two cars, and nothing changed. Mechanic thinks it might be the a/c "dual pressure switch", but he wants to swap these between the two cars to be sure before he orders one.

The mechanic also thinks it's possible that the two problems are related.

(This mechanic does not strike me as the most knowlegable one alive, but I'm trying him out because he's conveniently a block and a half from my house and because one neighbor gave him a good review.)

Does anyone have any thoughts based on the above? Can these two issues be related? Is the a/c switch the likeliest culprit? Any suggestions?

With the A/C on the fan should run at all times with the key on. If you are low on refrigerant then you have a leak. The green dye shows up a lot better with a UV light and yellow glasses. Rubber hoses with crimped connections at the ends are likely sources. Look at places where the hard line might rub. Check the compressor body. If all else fails you might have a bad evaporator.

With the A/C on the fan should run at all times with the key on.Understood. The fan is on at all times even without the a/c on.

I came across a mechanic on the net who had this problem and it turned out to be the a/c switch, but that doesn't mean this is the case in all such situations.

BTW, is there anything wrong with letting the fan run all the time? I would imagine people who live in hot areas do this anyway, since they run the a/c whenever they drive, so I would have to think the system must be engineered to allow that. (The mechanic poster cited above claimed that the fan motor would burn out if it ran too much.)

if you are low on refrigerant then you have a leak. The green dye shows up a lot better with a UV light and yellow glasses.This guy used the green light but I don't recall if he had yellow glasses. He showed me some freon with the green light and w/o glasses and I was able to see it, but perhaps that's because he showed me a lot of it.Rubber hoses with crimped connections at the ends are likely sources. Look at places where the hard line might rub. Check the compressor body. If all else fails you might have a bad evaporator.Thanks a lot.

Another possibility for the fan always running is there is a break in the circuit to the coolant temperature fan switch, normally mounted along the bottom of the radiator. The switch is normally closed when cold, when it is disconnected the fan(s) should turn on. A wire could be broken, the connnector contacts may be corroded, or the switch itself may be bad.

If the evaporator was leaking, you would be able to smell the refrigerant in the air. You can confirm it my shining the UV light onto the water that drips out of the drain tube from the A/C box under the car. I don't know if the dye used is water soluble, so the puddle may have a faint glow, or it may fluoresce as tiny blobs of oil on top or the edges.

Another common leak area are the high and low pressure connection valves covered by the service caps. If you hear a light hiss when you unscrew them, the valve is leaking. You can hold your finger over the connection and feel for building pressure.

Finally, leaks through the compressor shaft are hard to spot since it is obscured by the pulley and clutch assembly. You might fish a long piece of paper towel edgewise behind it (engine off) on the underside of the compressor's shaft and shine the light on the towel's edge. Any dye will be absorbed by the towel.

Hope this helps!

Thanks a lot. This is all very helpful.If the evaporator was leaking, you would be able to smell the refrigerant in the air. You can confirm it my shining the UV light onto the water that drips out of the drain tube from the A/C box under the car. I don't know if the dye used is water soluble, so the puddle may have a faint glow, or it may fluoresce as tiny blobs of oil on top or the edges.On a related note, I initially freaked out at the thought that it might be the evaporator, since these apparently tend to require disassembly of the entire dashboard and are beyond the ability of DIYers and cost a thousand dollars at the mechanic. But I looked at my trusty Haynes book for this car specifically, and they say that it only requires removal of the glove compartment (plus some doorsill trim, since that covers one of the screws to the glove compartment). I can get a new evaporator for about $80, so it now looks like no big deal. Could Haynes be wrong?

The other hard part is draining the system first. They say this needs to be done by a professional, with special equipment. Do regular mechanics commonly do this as a service (IOW, when they're not doing any other work on the car) & how much should I expect to pay for this?

I was also wondering if I even need to do this altogether, since they may be no freon left in the system altogether. Is this likely, and how would I know?

Another common leak area are the high and low pressure connection valves covered by the service caps. If you hear a light hiss when you unscrew them, the valve is leaking. You can hold your finger over the connection and feel for building pressure.Would leaks continue to show in this manner after the freon all leaks out?

Hope this helps!It does. Thanks again.

Quote:
"But I looked at my trusty Haynes book for this car specifically, and they say that it only requires removal of the glove compartment (plus some doorsill trim, since that covers one of the screws to the glove compartment). I can get a new evaporator for about $80, so it now looks like no big deal. Could Haynes be wrong?"

It's usually more involved than what Haynes says...and the main problem is usually accessing the screws you have to remove to get all the covers off and the tools you have on hand to reach them. I would allow a good weekend to get this replaced...it may be fast and it may not. I'd also arrange for alternate transportation in case you have to go to the store for something.

Quote:
"The other hard part is draining the system first. They say this needs to be done by a professional, with special equipment. Do regular mechanics commonly do this as a service (IOW, when they're not doing any other work on the car) & how much should I expect to pay for this?"

If the refrigerant hasn't all leaked out, then you should have it recovered at a shop...don't know what the cost is, but probably based on their labor rate. If it has all or almost all leaked out, then you're good to go.

Quote:
"I was also wondering if I even need to do this altogether, since they may be no freon left in the system altogether. Is this likely, and how would I know?"

The sooner you can fix the leak the better, since an open system allows moisture from the air in. There is a dessicant in the cylindrical drier that absorbs the moisture, but can be saturated if the system is open. If you don't have gauges, you can do a quick check for pressure by quickly depressing the tire-like valve in the low side service port (smaller cap) with a small, long screwdriver. If more than a light, brief hiss of refrigerant comes out, it's still got some in there and able to keep air from going back in through the leak.

Either way, the system will have to be vacuum pumped out before you recharge to remove the air and any moisture that gets in when you replace the evaporator. It's also suggested that the drier be replaced if it is the OEM part. The pump is an expensive piece of equipment, so usually only had by repair shops...again typical labor rates probably apply plus the cost of refrigerant. If you have a A/C gauge manifold set and can plan ahead, I have in the past borrowed one from the parts store ($200 refundable deposit and have to "order" it from their warehouse). Since you are new at this, I'd suggest going the shop route, but Haynes does have a decent troubleshooting guide for A/C repair if you want to learn more.

Quote:
"Would leaks continue to show (at the service ports) in this manner after the freon all leaks out?"

No, if there's little or no pressure, the ports won't leak. And FYI, seeing the green dye in the low side port isn't always an indicator of a leak since that's how it went in. If there is still good system pressure, air blow it clean, cap it, run the A/C, and recheck for dye trace under the cap. Dye in the high side port means a valve leak there.