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

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.

Jamie wrote:

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.

1. 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

Jamie wrote:

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

Jamie wrote:

"Tim (remove obvious)" wrote in message news: snipped-for-privacy@bt.com...

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 questi> "Tim (remove obvious)" wrote in

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?

Jamie wrote:

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 Michael Pardee wrote:

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.