Automotive Air Conditioning Information Forum

Wife's cars, I drive 440K mile junk. The '04 is a garage queen with less than 90K+ on it, the '11 has just over 200K. Neither car has had so much as the AC service port caps off. Both exhibit the same symptom: Good cold air for a couple hours then the not so cold and the air gets a bit stuffy. Shut the AC off for a while and you're good to go again. Noticed there is a good size puddle of evap water under both cars after a couple hours on the road. We're in the dog days of summer with high humidity (NC) so I'm wasn't surprised to see icing of the evaporator so I hadn't checked pressures yet.

Got to thinking that both cars should have evap temp sensors? It's a low motivation Sunday today and it's hot and humid so, I did a quick internet search (instead of dragging out the gauge set) and it seems yup, they do have evap temp sensors. In the course of that search saw something about the Honda evap sensor output over time not matching the factory graph for temp and output in ohms. Evidently the sensors are a pain to reach so some have been measuring output and adding an inline resistor so the computer will shut off the compressor before the evaporator freezes. Is this a common Honda problem and common fix if the pressures are in line?

ScottNC,
If these were my Hondas, I would first get the refrigerant charge correct. The evap thermistor is placed in an experimentally determined spot downstream or shoved in the fins of the evaporator. There is almost never a "perfect, optimum spot" so those Honda guys might have been chosen a spot that is not great, but kinda works, can be assembled, wiring can sort of fit, but the core is going to freeze at low charge. At a correct charge the thermistor might be in a properly refrigerant fed location of the evap, but at low charge it might be in a starved/superheated area of the evap.

It's counter-intuitive I know, but low charge can lead to freeze-ups for this reason. Residential A/C guys know that very well, happens all the time in those things.

It might also be a thermistor with resistance that has changed as you suggest, your logic is also sound there. But R-134a WILL ALWAYS LEAK OUT over time, both of those cars are probably due for a recharge.

When the A/C is working well, are all of the vents the same nice, cold temperature, or are 1 or 2 of them warmer?

DetroitAC wrote: ↑Mon Aug 23, 2021 8:23 am If these were my Hondas, I would first get the refrigerant charge correct.

Absolutely agree.

I'd also make sure the condensate drain line was clean/open.

Thanks for the replies. Having in the past experienced a low charge causing evap freeze-up on household AC I should have realized what was up. Have been waiting for a late-in-the-day slot to open in the schedule of a friends auto repair shop. He has the equipment to discharge and measure the existing refrigerant. All I have is a vacuum pump, gauge set and misc. AC specific tools.

Very good. Be sure to let us know the outcome.