Improving Scorpio A/C Performance

Scorpio / Scorpio Cosworth Discussions - Questions, problem resolution, general talk, technical tips and modifications.
Ed Lijewski
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Improving Scorpio A/C Performance

Post by Ed Lijewski » Tue Mar 26, 2019 9:26 am

Interesting tech article on improving (Euro) Scorpio a/c performance:

http://www.fordscorpio.co.uk/acmod.htm


Home page re (Euro) Scorpio a/c system:

http://www.fordscorpio.co.uk/airconelectrical.htm

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Tue Mar 26, 2019 1:04 pm

I pioneered that mod back in '99 on the IMON list.
Below is that original post..

To be honest, it gives a little boost, but running the condenser fan
with the AC clutch works better.



"I think I might have found the ultimate cure for the Scorpio's
poor A/C performance. The following modification seems to do
the trick. However, I just did this today (8/1/99) and it has
not yet been "time tested" so proceed at your own risk.

In addition, this mod assumes that you have a perfectly
functioning A/C to begin with AND I recomended that you
have performed the "A/C electric fan on when A/C Clutch on"
modification that has been previously descibed in the archives.


THE THEORY:

The main controlling element of the Scorpio A/C is the de-ice
sensor located on the front passenger side of the evaporator
box. This sensor controls when the compressor is turned on
and off. It will turn on the A/C clutch when the vent temp
rises above 50 degrees and will turn it off when the vent
temperature falls to about 45 degrees. This data is based on
fellow list members Richard Johnson's informal A/C survey
as well as my own measurements after recently recharging with
R-12. Thanks Richard for your input.

Note that the above temperatures are measured at the VENT and
that the actual core temperature (where the sensor measures)
will usually be slightlty lower. As the name implies the sensor
is there to prevent the evaporator from icing up.

In my opinion, this range is set a little on the high side on the Scorpio
What I wanted to see is a range more like 38-43 degrees.

The de-ice sensor is a black box with an attached sensor probe
that projects into the evaporator core. It's a little tricky to
remove. You will have to get a small 90 degree phillips
driver to get the bottom screw out. Once you have the little
box loose you then can carefully remove the probe from the core.

I was hoping that I would be able to open up the little black
box and find some factory calibration screws but unfortunatly
the unit is potted shut.

It turns out that the probe is actually a negative temperature
coefficient thermistor. At about 80 degrees it measures about
10Kohms and at 32 degrees it measures about 35Kohms.

What we have to do is somehow fool the little black box into
thinking that the evaporator is warmer than it actually is by
lowering the effective resistance of the thermistor. We do this by
putting another resistor in PARALLEL with the thermistor.

For experimentation purpose I wired in a potentiometer in parallel
with the thermistor.


THE EXPERIMENT:

I made the wires long enough so that the potentiometer would reach
into the cabin. My initial calculation to shift the resitance by
about 10 percent implied that I needed about 1 megohm in parallel.
I ended up using a 500kohm potentiometer with a 470kohm resistor.
in series.

Driving around (in 90 degree weather) with the maximum resistance
setting, the vent temperature cycled around 39-40 degrees.
I lowered the resistance and I was able to achieve 36 degrees.
I did not want to go any lower although I could have.
Once you get below about 40 degrees the compressor rarely turns off.

I added a little more resistance (330K ohms) for a total of about
a 1.4 megohms and I was able to get the temps to cycle between
40-43 degrees which I felt little safer with.

The results were excellent, a 5 degree drop in vent temperature
does'nt sound like much, but it really made a noticable difference
in the percieved comfort.

THE CONSEQUENCES:

You have to exercise a little caution here. This is definitely not
a case where "more is better". Keep in mind that as evaporator
temps go down, condensor temps go up and compressor gets no rest!


THE MODIFICATION

After my little experiment I began to think a little about it and
decided that the safest way to do this would be to have a fixed
resistance wired to a switch mounted on the dash that would "cut in"
the parallel resistor. Sort of an "aftercooler" switch :)

Thus on very hot days (which we have been having a lot of lately
here in the NE) I could turn on the extra cooling. Once the cabin
temp comes down I could then turn it off and let it cycle at the
factory setting. I will also leave it in the off positon for the
rest of the year when it is not so hot out.

Note that this way I have an "out" if things get out of control,
Like if the engine overheats, or the compressor start making noises,
or the evaporator starts icing up etc... Which I hope does'nt happen.

There may be a better way to do this. Perhaps an aftermarket "universal"
adjustable de-ice sensor exist. If you know of one, post it on the list.


MORE DETAILS:

I will post this modification on my web page (with pictures) so please
do not email me off list until I get it up.

http://web.starlinx.com/dhedin/

Click to the Scorpio pages, then projects

If I make any ongoing changes/correction I will post them on the list.
Thats all for now.

Dean
One cool '88 Scorpio "

Ed Lijewski
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Re: Improving Scorpio A/C Performance

Post by Ed Lijewski » Wed Mar 27, 2019 7:20 am

"To be honest, it gives a little boost, but running the condenser fan
with the AC clutch works better."

I assume the frog-eyes as are the base Scorpios referenced in the link would have the single speed condenser fan (since late '89 Merkur Scorps had it). Starting with that, the mod to control deicing compressor shut off cycling to decrease air temps from the center vents particularly makes sense, as supported by the recorded temp data.

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Wed Mar 27, 2019 1:03 pm

One of the issues with the Scorpio in the Summer is when stuck is stagnant traffic, the Scorpio A/C sucks.
Once you are moving it is OK.

So - on my current Scorpio I've done the mod to run the fan whenever the compressor is running.

However I find you can't do this reliably with the stock fan, it draws something like 20 amps. I have since swapped an aftermarket fan from ebay that draws about 6-7 amps. It does not move as much air as the stock fan, but it moves plenty enough. I really like things this way.

I have not done the the "de-ice sensor fake out" on my present Scorpio. The main reason is that I have converted to R-134 which already puts some additional strain on the A/C system components. (which the fan mod also helps address)

If I was still running R-12 I probably might play with the de-ice sensor.

Take heed from my IMON post "Keep in mind that as evaporator temps go down, condensor temps go up and compressor gets no rest! "

You really need to be careful if you fake out the de-ice sensor.

Ed Lijewski
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Re: Improving Scorpio A/C Performance

Post by Ed Lijewski » Wed Mar 27, 2019 5:51 pm

The Ford.Scorpio article makes it clear that bypassing the deicing switch is principally used to bring colder air through the center cabin vents faster. Once the cabin temp has cooled down the bypassing is turned off and the deicing function can be modulated or returned to normal. It's very likely that passengers would want the a/c to be "turned down" not long after cabin temp reached say ~70*. That might take say ~10 minutes, more on really hot and humid days but not opened ended continuous max cooling (think women in shorts, blousy blouses, and sandals).

So the compressor will get "rests". And the compressor itself is robust. And on the hottest and most humid days if system pressure builds upward beyond set limits the high pressure switch in the trinary switch will cut power to the compressor.

If you want to conserve compressor duty cycle life over cabin passenger comfort it's your choice. Most, the clear majority, would opt for great, effective a/c cooling and humidity reduction.

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Thu Mar 28, 2019 6:41 pm

All I can say is if you decide to do this mod - don't go lower than ~38 at the vent.
If you do it. Do it so that you can "switch out" the 1.4 megohms in parallel with the thermistor from the driver's position.
That way if anything goes wrong you are instantly back to stock.

Are you running R-12 Ed?
Last edited by brokencase on Thu Mar 28, 2019 7:01 pm, edited 1 time in total.

Ed Lijewski
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Re: Improving Scorpio A/C Performance

Post by Ed Lijewski » Thu Mar 28, 2019 6:59 pm

Icing of the evaporator isn't anything to worry about. As the Ford.Scorpio article points out, indeed,at length (have you read it all?), not one of several a/c techs queried by the author about that said that it can't cause harm to any system component. I can also confirm from personal experience that it causes no harm. If the evaporator ices up blower effectiveness decreases proportionately which will catch the notice of the car's operator who then only needs to shut off the a/c switch while leaving the blower on which will cause any ice to melt.

There is no science or magic about below 38* F.

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Thu Mar 28, 2019 7:06 pm

Well - I've done the mod Ed. 20 years ago. I know what it is about.

I wouldn't worry about the icing as I would be about the head pressures and strain on the compressor.
Especially if you are running R-134.

I would also not do the mod unless you have done the other mod where the electric fan turns on with the a/c clutch.

Ed Lijewski
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Re: Improving Scorpio A/C Performance

Post by Ed Lijewski » Thu Mar 28, 2019 7:16 pm

Screenshot_2019-03-28-19-02-21.png
Screenshot_2019-03-28-19-02-21.png (1.2 MiB) Viewed 1879 times
Everything involves a tradeoff. Everything.

When it's hot as hell and humid as Manaus, some of us choose beautiful dry cold air from our a/c systems. At any potential cost. And we know the high pressure cut-off switch is in place to do its job if called on.

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Thu Mar 28, 2019 11:25 pm

Are you running R-12 or R-134?

Ed Lijewski
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Re: Improving Scorpio A/C Performance

Post by Ed Lijewski » Fri Mar 29, 2019 7:11 am

I'd never run R-134; it was always a second level (at best) choice without major refitting and expense.

R-12.

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

dimswits
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Re: Improving Scorpio A/C Performance

Post by dimswits » Tue Apr 02, 2019 11:38 am

The only thing holding R134a back (assuming you've changed the dryer and expansion valve for compatible items) in a stock scorpio is the condenser. The stock serpentine condenser has the phase change and heat rejection happening at only one single spot. That worked fine for R12, but 134a requires a parallel flow condenser for best performance.

Getting one in there can be easy or hard, depending on the parts and tools you have on hand.
Dimitri in NYC
88 Scorpio 2.5 Turbo
89 Scorpio 4.6 DOHC
02 E46 330Ci
17 MKC

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Thu Apr 04, 2019 1:32 pm

I've been running R-134 for the past 3 years and it performs fine. Running the electric fan with the compressor helps. I get vent temps at or below 40F. I'd rather have the cheap and easy availibility of R-134 instead of chasing down $25 cans of R-12.

The only issue I find is that it tends to leak out of the sytem a little bit over the Winter and I have to top it off in the Spring. Either I have a leak somewhere (which I doubt) or it is just migrating through the "non-barrier" hoses.

Never changed expansion valve or dryer. Probably should have but I don't care - it works fine.

Ed Lijewski
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Re: Improving Scorpio A/C Performance

Post by Ed Lijewski » Thu Apr 04, 2019 6:32 pm

Compressor shaft seal is possible leak point.

YMMV
Descartes: "Cogito Ergo Sum"
Lijewski: "Sum Ergo Drive-O. Mucho!

brokencase
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Re: Improving Scorpio A/C Performance

Post by brokencase » Thu Apr 04, 2019 9:08 pm

I was driving home today on my normal commute. I took a snap shot below. I'm getting plenty cold - its around 36-40F at the vent.
But of course it was only 70F outside...

...and this got me thinking further about the de-ice sensor mod....and why I don't think it is worth the trouble.

For argument let us say it is 95F outside and you accidentally parked your Raven interior-ed Scorpio in the bright sun for many hours.
Under these conditions the car interior can really get hot....like 120F. Everything inside heats up to 120F.

Now you start on your way and you turn on the A/C, set it to recirculate, temp to 0, and full blast.

Under these initial conditions you are sucking in warm interior air into the evaporator - it cools it the best it can but your not going to get
anywhere near the 32F core temp. Its fighting all those hot heat reservoirs in the cabin, the heated dash, the hot leather seats etc...

What maters is not the ultimate low temp the system gets down to, but how fast it can pull the cabin temperature down.
That is where the "condensor fan on when compressor is on" modification makes a big difference. The overall heat transfer of the system is improved. Especially if you get stuck in idle traffic. In my case I can actually see the engine temp drop a few degrees when I turn on the A/C since it not only cools the condenser but also blows over the radiator.

Now other items can make a difference also, like running R-12, etc... But I don't think fooling the de-ice sensor is going to help that much on the hot days that matter.

Image

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