Hi all,

During a 3 hr interstate drive in the 90F, humid midwest yesterday, my A/C evaporator started freezing up for the first time in my 07 MS's life. I didn't immediately know that this was the problem, but some searching on FitFreak produced some very useful threads (here and here) [in short, ice forms and prevents air from flowing through the vents with any power; turning the A/C off for a few minutes and either letting it sit or running it on just the fan thaws out the ice until it freezes up again a few minutes later].

What I don't know is if:

a) this was an isolated, freak case of high humidity, high temps...one that won't likely repeat itself

--or--

b) some part (e.g. the thermistor) actually failed

I should note that I've driven on hot, humid days with the A/C running for hours at a time and have never had this problem. It happened yesterday after about 1.5 hrs of driving.

Also, I replaced my cabin air filter exactly one year ago (but since I was out of the country for the last 9 months, it sat in the garage, un-driven, and I've only driven it 2,500 miles since).

A post in one of the above threads suggested it might be the thermistor (part #18 here). If this is the case, I seem to be in luck since the part only costs $10.

On that note: has anyone replaced one of these him/herself? Or is it, as user polaski was suggesting, a complex tear-down of the whole compressor?

Or...could something other than the thermistor have gone wrong?

Thanks in advance,

EDIT: I also found this post suggesting that the thermistor doesn't actually go bad but is simply designed this way (I am aware of Polaski's clever work-around). Steve244 wrote that replacing the thermistor requires removing the evaporator completely. Has anyone verified this? Thanks again.

 

Having sat for that long, it may have leaked some refrigerant. I'd have it serviced. If it's low on refrigerant this can result in icing, paradoxically. Something to do with "working too hard." At the same time this check is made, they'll validate the expansion valve's function.

If everything checks out then yes, it probably is the temperature sensor that needs replacing. Not sure on the 07s. 09s and up the shop manual calls for removal of the evaporator to get at it.

 

Lo charge
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Thanks so much for the helpful responses.

Would low refrigerant have presented a problem (e.g. icing up) on previous long drives? Now that I'm back and driving it again, I've had to make several 200+ mile trips and have run the a/c without problems. To be sure, it was never quite as hot or humid as yesterday (when the problem first presented itself), but the difference wasn't that great, either. Then again, maybe low refrigerant only causes freezing when, as you say, Steve244, it's working really hard?

Thanks so much,

 

Might've been a fluke. If it continues to cool and doesn't ice up frequently I think I'd ignore it.

 

I've had the same issue on and for years. My thermistor was replaced about three years ago. Problem came back last year. Last fall my dealer evacuated and recharged my system. This didn't help. They have ordered another thermistor and are waiting for me to bring my car in.

 

I have had my A/C freeze up even before it had over 2000 miles on it... I removed the cabin filter last summer, which was the hottest one on record, and haven't had a freeze up since... It was actually pretty comfortable when the ambient temperature was 116 F degrees. I have since used lightly oiled K&N filter foam in place of the pleated paper filter. I have had one person that was insistent that no car with a properly operating A/C is going to freeze up but every car I ever spent any time in during the summer here has frozen up if there wasn't enough air flowing through it... Old Caddies and Lincolns that had trunk mounted A/Cs that would frost up but not so much that they wouldn't keep a case of Lone Star or Pearl Long Necks sitting on the back seat nice and cold... It's pretty sad that cars made 58 years ago had much better heaters and A/C units than any of the cars being built today.

 

I really appreciate all of this input. You all have convinced me that it may have just been a fluke and that the most sensible decision at this point is to see if it ever happens again before spending too much time thinking about it.

And spreadhead, your comments that neither a recharge nor a new thermistor fixed the problem speak volumes. Thanks for sharing that frustrating experience.

Thanks, all.