740 A/C system: low fan speed colder than high fan speed

I just converted my 1987 740 A/C system to R134. As of yet I have no
evidence of leaks or of any system failure. As it stands, here are the
quirks I am trying to understand.

1) This system is definitely relative to the outside ambient
temperature. The system cools approximately 35-45 degrees F less than
the outside temperature. When the temp gauge in my A/C vent reads
110-115F when I get into the car, it goes to 60-70F after a few
minutes.

However, if the fan speed is on number 1 (low), the temp gauge reads
about 48 degrees F.
On number 2 it goes to about 58F
On number 3 it goes to about 65F
On number 4 it goes to about 72F

Of course the actual number depends on the outside air it's cooling,
but the air is getting less cool as the fan speed increases.

To test this I also did the same thing in my 2002 Nissan Xterra. It did
not blow much colder, but it was a constant 52 degrees on all speeds.

Is there a specific part that would be symptomatic of not being able to
cool at all fan speeds? I am guessing the evaporator??

Thanks!
Jamie

 

The only other thing I can think of is that I partially removed the
blower motor, then reinstalled it. I did not reseal it.

It is a bit noisy, but still has some life. I don't know if it could be
sucking warm air in through where the old seal was.

 

The faster the airflow over the cooling coils allows for less of a
temperature drop (less time against the coils to cool). Thus at the lower
speed you will have a lower temperature than at a higher speed. It sounds
like the system is operating normally.

Allen

 

Converted R12 systems never work as good as they used to , as the condensor,
evap, OT, and compressor are all sized for R12. You get about 80-85% of the
performance when using R134a, which you seem to be getting.

Getting the best possible airflow through a clean and un-obstructed
condensor is the key to getting max cooling from it.

R134a charge is between 70 and 85% of R12, the exact amount for best cooling
is only determinable with H and L pressure gauages and acouple of
thermometers.

Tim..

 

OK, I made some progress on this:

1 - A few days ago I pulled the A/C controls away from the dash because
the slider knob was disconnected from the lever and the vents were open
in the HOT position. When I slid the control lever over by hand (under
the controls), I started getting cool air for the first time.

Since then it's been doing as I described. So, I pull the controls
completely off today thinking the slider knob was blocking the slider
arm from going all the way into COLD mode. I disassembled the controls
and re-attached the slider knob onto the control arm. I reassembled
everything and at about 95F in the shade at idle I was getting about
62F on fan speed 2 and 72 on fan speed 3 and 4. My aux fan is
disconnected so I put a blower fan on the front and it made no
difference.

2. Here's what made a difference. at about 90F a short while later I
went for a drive down a shady road. The fan blew a constant 52-54F on
all speeds. It was even blowing cold on high. Outside temperature and
sunshine do make a difference. As does cooler wind across the coils.

So, I plan to tint the windows and buy a thermoswitch for the aux fan.
The wires were all corroded so I repaired and reconnected everything. I
just need a place to plug it in - either the hose or the radiator tank.
I have no idea which is better. The hose has no thermoswitch and the
tank has a hole with a plug in it, but I don't know what part I need.

Thanks,
Jamie

 

Correction:
At first it must have been 100F around 7:00pm, because I just checked
Weather.com and at right now at 8:04pm it says it has cooled down to
97F.

Thank goodness for low humidity.

JB

 

I agree with all that, except the guages. The pros told me it is impossible
to determine the charge of a 134a system by pressure, but I had to try it
myself. They were right! My only attempt to charge a 134a system by guage
was a miserable failure, although I was practiced at charging R-12 systems
by ear (compressor cycling), by thermometer, by sight glass or by guage.
Huh.

Definitely start with the condensor, Jamie. Your symptoms point to either
insufficient charge or the condensor is not working well. Okay... guages
would tell you the difference in that respect, but listening to the
compressor clutch will do that too. If the compressor is cycling quickly the
charge really is too low. If it is staying on the condensor is choked. Wash
it out with water spray and check that it is getting lots of air blown on
it. If for any reason you don't have the aux fan running when the A/C is on,
focus on that.

Mike

 

Thanks Mike,
It's been a hard read. I've been test driving late evenings, but the
temp's been 97-100F. On a straight highway in the shade I'm blowing a
steady 52-54F.

My aux fan isn't working. I traced all the wiring, but no power to the
fan. As for the compressor, I haven't watched it long, but it isn't
cycling on and off like it's low. It seems like it is staying on -- but
I haven't timed it. How long should the cycles be for a compressor in a
normal system?

Condensor -- I removed it, poured A/C flush in both tubes and tried to
work it in. Then I took an air hose and blasted it out and the air
flowed well. I just don't think I really got much flush in there. I
used a funnel, but it's not like it free flowed. The air flowed very
well. though.

Other than some bent fins on the compressor from the aux fan breaking
off and spinning on it, I really have no evidence of damage.

So, the question is -- if it's 100F outside and I'm driving down the
road, is 52F considered normal? That's about a 50 degree cooling.

JB

 

Regarding the above, I did not test for power in the lines. I need to
do that. I think maybe the fan fried, if it broke loose and jammed
against the condensor. Just a guess -- it turns easily though.

I may rewire a new fan. I could take the wire from the ignition, run it
to a relay, then run out to a new fan. My question would be should I
power it off the battery or by a fused line from the compressor power?

 

That sounds pretty good to me for a converted system. You've pretty much
narrowed it down: the condensor isn't able to throw off the heat collected
from the evaporator. That is consistent with the compressor staying on the
whole time, which often also occurs with a healthy system. The A/C has to
have the interior pretty well cooled down or the outside temperature has to
be fairly cool before the compressor will cycle.

Supposedly, the big limitation in 134a conversions is in the condensor. The
more air you can get flowing past that thing, especially when the vehicle
isn't moving, the better you can expect the A/C to work.

Mike

 

Agreed. I never took the time to learn how a car A/C works until
recently. Once you learn what happens at each stage, it is very
interesting and somewhat simple. I just wish it were a bit less
expensive.

;-)

JB

 

I disagree- charging via the gauges in conjunction with thermometer is
second nature after abit of practice,
although its far easier and quicker to at least have a rough idea of the
weight of gas you have put in.

Yes, the condensor fan must be working, you cant expect it to work without
adequate condensor cooling...

Tim..

 

Sounds pretty reasonable to me, particularly if the aux fan is not
running and you have some bent fins.