Piston Slap: So Hot, Yet So Cold!

Nick writes:

Hello Sajeev,

(I really wanted to put the “n” in there.)

I have a ’97 Prelude that will sometimes cycle on and off its air conditioning when it’s unbearable hot outside (June-September here in Phoenix). Air will come out nice and cold, then it will get real warm suddenly for about 30 seconds before getting cold again. It only happens when it’s extremely hot outside and I’ve been driving for awhile. It works fine the majority of the time. What do you think?

Sajeev writes:

Your letter (Sanjeev aside) points to a mostly healthy HVAC system. No leaks, no lack/overabundance of refrigerant, just a problem with…wait for it…your A/C system freezing itself in the hottest weather. That freezing fire thang ain’t so moronic anymore, no?

I learned this as a college student taking the last fast sweeper (in 100+ degree weather) before reaching campus. With copious body roll (from a 1988 Cougar riding on blown Monroe shocks) came a dousing of ice cold water on my feet from the evaporator core: kinda terrible on those lazy days when I wore sandals to class!

I am so confident in this diagnosis, yet I cannot find a thermal cut-off switch ( something like #6 on this link) for your specific application. I talked to my go-to Honda guru, TTAC commentator psychoboy, and the evaporator core has a thermostat in lieu of the switch, which accomplishes the same thing.

The evaporator thermostat is there to prevent icing. It’s set up to turn off the compressor clutch if the evap gets below 37° F. The test for the thermostat is really simple: looking at the plug with the clip at the top, put 12V to pin three (rightmost) and ground pin two (center). Hook a test light to pin one (left most) and pin three (the 12V).

Place thermostat in ice water. The light should go off at 36-39° F or lower, come on at temps higher. So, the light is on until you dunk it in the water and it adjusts to lower temp. Pull it back out and the light comes back on.

And there you have it.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.