Piston Slap: Bullish on Ford's Electronic Automatic Temperature Control?

Ryan writes:

Hi Sajeev,

I have a problem with the air conditioning not functioning properly on my ’89 Ford Taurus LX equipped with Electronic Automatic Temperature Control (EATC).

When I start the car on a warm day the AC can blow icy cold air immediately, but the temperature gradually rises to ambient temperature by the time I reach the freeway, and is bad enough that I turn the EATC off. When I turn on the EATC by pressing AUTO, it often feels like it’s blowing maximum heat. I have noticed that when this happens the AC compressor clutch is not cycling on and off. During the winter, the AC compressor clutch clicks on and off. The problem seems to be intermittent and only occurs during warmer months. Also, during startup, you can hear an audible “whirring” noise in the dash, and that started to sound abnormal about the same time the AC stopped working. By “abnormal” I mean it lasts much longer than it used to. I believe the noise I’m hearing is the temperature blend door being moved so the EATC knows the position of the blend door.

The pressure in the AC system is normal, and I have tried a functional test of the EATC, which showed no fault codes by lighting up all of the LCD segments (above).

I have tried swapping a lot of items, thinking there might be a problem with a temp sensor out of calibration, or a problem with the EATC controller, but nothing I have swapped caused the AC compressor to click on. I did not change the blend door actuator or the relay module above the radiator. This problem only seems to occur during the summer, which makes me think the relay is the problem.

I have recently acquired another relay module surrounded by an entire 1988 Mercury Sable LS wagon that could be swapped in as a test, which I assume is a good part since the Sable LS only has 95,000 miles, and the Taurus LX has over 337,000 miles. Any ideas for what the fault could be?

Sajeev answers:

Thank the Heavens: someone else loves the first-gen Taurus/Sable! I’m still waiting for my dream 1986-88 Sable LS sedan with dark brown velour and digital gauges to surface in near mint condition…or the almost cocaine-themed “White Knight” Edition:

I weep for anyone who watches this Miami Vice inspired TV spot and doesn’t turn into a raging Lincoln-Mercury fanboi. But, once again, I digress…

You have a comprehensive understanding of the ETAC system, and your shop manual (unpublished photos) informed you of no faults, so I reckon your A/C clutch is worn and can’t engage at higher engine loads. Original clutches won’t last this long, and remanufactured A/C compressors (with rebuilt clutches) for this era of Ford are a mixed bag in my experience.

The failure under freeway driving is what put it all together for me.

Aside from one failed 1985 ETAC head unit in 1992, my time with Ford Electronic Automatic Temperature Control systems from 1972 to 1995 has been trouble free. For decades! The rest of the HVAC system is subject to wear and tear, but knock on wood, those electronics are rather solid. And the Taurus/Sable’s bespoke head unit is the coolest of the bunch.

Slap in a new A/C clutch or compressor/clutch assembly and report back!

[All Images: OP]

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.