Automotive Air Conditioning Information Forum (Archives)

Minx on Sat October 23, 2010 3:35 PM

Year: 1997
Make: Mercury
Model: Sable
Engine Size: V6
Refrigerant Type: 134

Toward the end of the summer, when evening temps get low enough to turn the car's cold/hot dial away from COLDEST just a bit, I've noticed I can no longer moderate the temperature using the dial.

The vent temps don't change (very cold). The compressor cycles after the temp drops, as expected

But I can't raise the vent temps with the dial.

Think this is a problem with the dial? I'd like to know before opening up the dash.

NickD on Sat October 23, 2010 4:40 PM

Recall Ford having blend door hinge problems in this year, well, not Ford's problem, but yours. But can also be linkage problems as well, some were self adjusting simply from rotating the temperature dial from minimum to maximum and back again, could be that catch is broken. Really have to crawl under the dash and look, mechanism is simple at coldest temperature, door is closed, at maximum heat, its fully opened.

Assume your heater hoses are hot, both inlet and outlet. Some have a plugged core.

GM Tech on Sat October 23, 2010 5:21 PM

Sables and their brothers- the Taurus are famous for heater cores pluggin up--and even the water pump's fin corroding (wearing down) to the point they are no longer able to transfer fluid.---a good heater core flushing usually fixes the heater no heat problem- but be leary of the entire system needing flushed, and/or a new water pump.

If you can hear the temp door slam shut cold to hot and hot to cold--(if cable driven) then your temp door is not the issue....

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Minx on Sat October 23, 2010 9:20 PM

Guys, what I'm really confused about these answers is, I'm only talking about AC performance.

Am I mistaken to think that when I am running the AC and I inch the cold/hot dial down from the coldest setting, I should get less-cool air from the vents and slower cycling (more clutch disengagement) at the compressor?

To my thinking, the heater core shouldn't come into play until the dial gets halfway to hot.

TRB on Sat October 23, 2010 10:10 PM

A lot of vehicles use the heater to control the cold air passing through it, I.E. Cold evap core - Hot heater core - air out the vents. It's easier to add hot water to the heater core than to try and regulate the evap core temp for a desired vent temp.

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NickD on Sat October 23, 2010 11:22 PM

Quote
Am I mistaken to think that when I am running the AC and I inch the cold/hot dial down from the coldest setting, I should get less-cool air from the vents and slower cycling (more clutch disengagement) at the compressor?

Regarding the compressor operation, yes you are mistaken, your mode switch in the AC or defrost positions applies 12 volts to what is called a cycling switch that monitors system pressures inside of the AC unit, if the pressure gets too low, it switches of the compressor, when the pressure gets high, it switches the compressor back on again.

What determines the cycling time more than anything is the outside temperature, the cooler it gets, the more your compressor will cycle. This has nothing to do with your temperature dial, that just operates a door to allow more or less heat after the evaporator the cools the air first. You can actually get some pretty hot air out of your vents even with the AC on as the heater core is far more powerful than the AC evaporator.

Your evaporator can not get any colder than 33*F, but your heater core gets up to 195*F and stays there so you still can get 162*F air temperatures from your vents which is quite hot.

NickD on Sun October 24, 2010 5:40 AM

You do get those days, when its a bit warm inside your vehicle, but gets too cold with the AC on. Some of us frugal guys will freeze first before adding heat because we know we are paying for that cold air. Does conserve a bit to reduce your blower speed to the lowest setting, but when the wife is in the car, have no choice but to add some heat.

Does seem a bit crazy to pay for cold air then have to reheat it a bit, but that is the way it is. Vehicles with automatic climate control work the same way to maintain a preset temperature, at first they reduce the blower speed, if that doesn't do it, open the blend door to increase the temperature. That blend is opened and closed by a motor controlled by the temperature inside the vehicle.

So when its too warm with the AC off, but too cold with the AC on, you add a little bit of heat to get it where you want it to be. But that temperature control knob has absolutely nothing to do with controlling the on time of the AC compressor even like a cheap home window unit. AC controls itself, trying to keep the evaporator between 33 and 38*F, and on cooler days, that is too cold.

Actually with these newer vehicles, really doesn't make that make that much difference in fuel economy with the AC either on or off, have a couple of vehicles equipped with instant mpg. Worse fuel economy is driving in a city, one car I have drops from 31 to 16 mpg, and why not, spend better than half your time sitting behind a red light. On the highway, hilly roads cut the fuel economy, takes a lot more gas to climb a hill then what you save coasting down a hill. May even have to use your brakes going down a hill as a cop is sitting there waiting to give you a 300 buck ticket if your car goes above 55 mph. Another factor is driving in a head wind, can really cut your fuel economy if driving into a 40 mph head wind.

So what does our government do to conserve on fuel? Reduce the speed limit on interstates, but what can you expect when you have idiots running this country! City driving is worse and that is where 90% of the driving is done. I have gotten as low as 7 mpg on the same vehicle capable of getting 41 mpg on the open interstates when creeping along on the Northwest Expressway in Chicago. That's because they are merging six lanes of traffic into three lanes, a constant daily jam. And while the gas tax has risen by a factor of eleven times, feel that money is ending up in someones pocket rather than updating the roads with drastic increases in the population.

Minx on Sun October 24, 2010 4:35 PM

Quote
Originally posted by: NickD
So what does our government do to conserve on fuel? Reduce the speed limit on interstates, but what can you expect when you have idiots running this country! City driving is worse and that is where 90% of the driving is done. I have gotten as low as 7 mpg on the same vehicle capable of getting 41 mpg on the open interstates when creeping along on the Northwest Expressway in Chicago. That's because they are merging six lanes of traffic into three lanes, a constant daily jam. And while the gas tax has risen by a factor of eleven times, feel that money is ending up in someones pocket rather than updating the roads with drastic increases in the population.

I agree, Nick. And/or they pass CAFE standards that force carmakers to build thousands of vehicles nobody wants (read: small) at a loss so they can sell the ones people really want. And eventually, the cost destroys the companies. As we just saw.

I've always believed there was much more fuel efficiency to be gained by "smarting up" traffic lights for the least obstruction. But cities have no incentive to do that.

Thanks, everyone, for smarting me up on this subject. I'll be heading under the dash to see what's going on, and checking the heater core flow.

NickD on Mon October 25, 2010 9:37 AM

Problem with a plugged heater core, an aluminum head engine with a cast iron block and the cooling system dependent on that heater core for proper cooling if that severe engine damage may occur with a plugged heater core. Would definitely make sure both heater hoses are hot assuring proper circulation.

Yet another problem is refilling the system since they started putting heater core hoses higher than the radiator cap. Can get air pockets up there that is just as bad as a plugged heater core. I get around this by blocking up the front end of the car so the radiator cap is at its highest point.

You don't mention if you have the 3 or the 3.8L, never changed the water pump on the 3.0L, but did on the 3.8. Let me put it this way, was a challenge.

mk378 on Mon October 25, 2010 10:46 AM

First check that the radiator is full. With most cars, the first symptom of being low on coolant is that the heater will stop working. Then warm up the engine and feel the heater hoses near the firewall. They should both be hot. If not, there's no flow through the heater core.