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What is the white film on my thermal protector sensor?

MiataPete on Fri May 24, 2013 5:00 PM User is offline

Year: 2001
Make: Mazda
Model: Miata
Engine Size: 1.8L
Refrigerant Type: 134a
Country of Origin: United States

I'm rebuilding my AC compressor, a Denso TV12C. On the top of the compressor, in-line to the compressor clutch, I have a thermal protector sensor/switch. This sensor switches off power to the clutch if the compressor overheats. It is an aluminum bodied sensor that sits in a recess in a small cast aluminum housing, that is bolted to the top of the compressor.

When I took the sensor out, I found it and the recess that it fits into covered in a thin white substance ... kind of like a dried plastic tape. When I scrape it, it comes off, sometimes in flakes and sometimes as dust. What is it? Is it a dried lube? A plastic? Why is it there?

I don't think it is corrosion ... the sensor is stamped aluminum, the housing is cast aluminum.

When putting this back together do I need to add a lube of some sort? Wrap the sensor? ????

94RX-7 on Fri May 24, 2013 5:07 PM User is offline

It is probably thermal compound to make sure the switch has good "thermal continuity" to the vane housing it protects.

I'm curious....what was wrong with your compressor? The TV14C on my RX-7 has a leaking shaft seal. I just bought a new unit instead of trying to source the parts to rebuild it.

James89dx on Fri May 24, 2013 5:10 PM User is offline

I see aluminum valve covers/intake manifolds/other engine parts develop that white filmy oxidation/corrosion type tuff quite often. Are you sure it's not just that?

ice-n-tropics on Fri May 24, 2013 7:08 PM User is offline

Glad you asked,
Manufacturers place wet RTV silicone into the cast mounting recess immediately before inserting the bi metal temp sensor protection switch. The purpose of the RTV is to eliminate air at the interface. This standardizes the heat transfer and eliminates moisture condensation & road splash. Water causes corrosion which causes heat X reduction & the possibility of freezing.
Freezing dents the bottom of the sensor and changes the switch temp calibration. This is more important in cold climates such as Canada.
When replacing the sensor thoroughly clean the comp cavity to avoid denting the new sensor and use the recommended RTV, if possible. Honda used the correct RTV in Canada recalls.
Be very sure to fasten the sensor in place before the RTV hardens or the sensor can will dent and change the setting. Garden variety GE silicon should be almost as good.

Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy. How To Air Condition Your Hot Rod

MiataPete on Fri May 24, 2013 7:14 PM User is offline

To make a very long story short ... the problem with my system was me.

I don't really think there was ever anything wrong with my system. My wife complained one day that my AC wasn't very cold ... I reacted to that, and shouldn't have ... she's always overheated. I overcharged the system, and probably blew a seal. I heard it go, with a massive "WOOSH", and immediately turned it off.

Upon complete disassembly of the system, I have found NO particles anywhere in the system, everything is clean, and I haven't even found the "blown seal" or o-ring. Everything looks perfect.

I just want to make sure I do a good job putting it all back together.

My compressor, a TV12C vane-type, DID show some black coating on the compressor shaft, where the seal rubs ... this may have been where it blew.

My guess was exactly the same as yours ... some thermal continuity compound. But, what would I use?

I could have bought a rebuilt unit (I actually did). But, when I found out that I could rebuild the compressor myself for almost nothing ... and, since I was pretty sure that it was in perfect condition, I decided to do the rebuild myself (I returned the rebuilt compressor to Pep Boys unopened, for a full refund).

MiataPete on Fri May 24, 2013 7:19 PM User is offline

WOW ... these forums are incredible! How did anybody work on cars before this?

Thank you VERY much ice-n-tropics!!!! That makes perfect sense.


MiataPete on Fri May 24, 2013 7:24 PM User is offline

It could be corrosion, but the metals are quite similar. I'm just not sure.

If the sensor was stamped steel, with the aluminum housing, then yes, you would think corrosion for sure.

The answer from ice-n-tropics seems right ... it must be thin residue from a dried white silicone ... some of it still has plastic qualities.


94RX-7 on Sat May 25, 2013 8:47 AM User is offline

Originally posted by: MiataPete
I heard it go, with a massive "WOOSH", and immediately turned it off.

It sounds like the high pressure relief valve vented some refrigerant just like it is supposed to.

BTW, I'm told you're not supposed to touch the black ceramic part of the compressor shaft seal, and if you do it is ruined.

mk378 on Sat May 25, 2013 11:04 AM User is offline

Look for a relief valve in the back of the compressor.

The shaft seal is usually on the low pressure side and wouldn't blow out due to overcharging. Instead they wear out and start to leak slowly.

A new relief valve is considered part of a proper rebuild because they don't always re-close properly after opening.

MiataPete on Sat May 25, 2013 5:23 PM User is offline

THANK YOU, THANK YOU, THANK YOU, THANK YOU ... I never looked at that valve, or even really knew it was there. But, I bet you are 100% correct. That valve must have vented the excess pressure, before any damage occurred.

When I heard the "WOOSH", I immediately turned the AC off, and stopped. I got out, opened the hood, and very carefully examined the engine compartment for fluid traces, that would tell me what had blown. I could find nothing. When I later returned home, I again studied the engine compartment ... very carefully. Again finding nothing.

But, that valve is at the bottom back of the compressor housing. If it vented, the release would be straight down. The compressor is low on the driver side of the car, below the power steering pump ... you can barely even see the compressor, let alone anything underneath it. If it vented down, any trace oil would have been on the cross-member under the engine, or on the side of the block or oil pan ... all of these parts already carry a light oil film, so I would never notice any oil from the venting compressor.

I probably could have simply gone to an AC shop, had the system evacuated and recharged, and I would have been good as new.

I just removed the valve from the rear housing ... it looks fine ... I can't blow thru it. Can it be tested? Or, should I just buy a new one?

Thanks again,

MiataPete on Sat May 25, 2013 6:13 PM User is offline


I was thinking about what I just previously wrote ... ie "I could have simply gone to an AC shop ...".

This "WOOSH" occurred 18 months ago ... I had forgotten ... I DID go to a respected auto repair shop here in Scottsdale. I left the car with them for diagnosis.

They phoned with this message ... "Good news is we CAN fix it. Bad news is, 1) it will cost $3,000, and 2) we can't get a part we need for 30 days". The car has been very reliable for me, so I figured, OK I'll let them do it. But, in the intervening month I did research, and found out that I could do the same job for almost nothing ... a complete rebuild of the system for a small fraction of that $3,000 quote.

Now I wonder, why did they ASSUME I needed a new compressor and condenser?

I'm glad I'm doing the work myself. I'm OK with carefully rebuilding the whole system, and buying a few expensive parts from Mazda. It's a good learning experience, all knowledge has value, and I can be assured that no short cuts are being taken.

But, wow, I've lost some respect for that shop.

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