Piston Slap: So Hot, Yet So Cold!

by Sajeev Mehta

Published: November 24th, 2015

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.

Sajeev Mehta

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4 of 56 comments

Redmondjp on Nov 24, 2015

I had the same issue in my 1997 Civic. It was an intermittent (only when hot) in the A/C compressor clutch coil. It was a major bear to find! Only when I tapped into the power wires right at the clutch coil, with a tiny light bulb up where I could see it at the cowl, was I able to figure out what was going on.

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- Scoutdude on Nov 24, 2015
  
  Bingo this is the real culprit based on the symptoms. The temp causes the coil to open, current flow stops. Sometimes it cools back down and starts working again in a short period other times it takes an almost full cool down. It is more common to happen in stop and go traffic as under hood temps rise.
  
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Ryoku75 on Nov 24, 2015

Hondas never have problems and go to 300xxx without even oil changes, you just got a lemon in this 16 year olds opinion!

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- JohnTaurus on Nov 25, 2015
  
  Lol! Funny because it sounds exactly like what some clueless kid would say. My neighbor's 10 year old thinks Toyotas are unbreakable because his friends dad has hit several deer (supposedly, I happen to think hes a drunk and is blaming his alchohol-induced wrecks on deer) in his truck, but it still runs. Trying to explain that body damage doesnt affect mechanical reliability (unless of course the radiator or other critical component is damaged as well) to him is like trying to explain how WiFi or Bluetooth works to an 89 year old. "Son, in my day, if you had a blue tooth, you tied it to a door knob and jerked it out!!!"
  
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