Evaporator Capillary Thermo Tube question

[Koizumi]

I replaced the evaporator's thermostat capillary tube (attached to a new switch) a few years back.

During one of my random OCD cleaning and inspection episodes yesterday, I realized that the non-insulated part of the tube is like 8" in length. Seems too long!

How much of the tube should be inserted into the evaporator? I understand that it should be inserted at an angle as well.

Also, should the remaining part of the non-insulated part of the tube that's sticking out be wrapped up in some sort of insulation? There is a good few inches that don't fit into the evaporator that are exposed.

Thanks!

[Koizumi]

Anybody have some tips?

[JRSIII]

I wouldn't worry about it. Mine is the same way. The evaporator is only about 3" thick so you can only stick it in so far. Maybe kuehl will chime in, as he's the expert on this stuff.

[86 911 Targa]

Originally Posted by Koizumi

I replaced the evaporator's thermostat capillary tube (attached to a new switch) a few years back.

During one of my random OCD cleaning and inspection episodes yesterday, I realized that the non-insulated part of the tube is like 8" in length. Seems too long!

How much of the tube should be inserted into the evaporator? I understand that it should be inserted at an angle as well.

Also, should the remaining part of the non-insulated part of the tube that's sticking out be wrapped up in some sort of insulation? There is a good few inches that don't fit into the evaporator that are exposed.

Thanks!

To prevent the tube from bottoming out/damage,

on our '86, the tube is about 1/2" from the bottom of the evaporator housing.



I did re-use the insulation from a previous capillary tube. (See black tape).

Gerry

[Koizumi]

Fantastic help, gentlemen! I'll attempt to leave it 1/2" from the bottom. It's hard to feel it touch the bottom, so it might be about a bit of luck. I guess it doesn't hurt to insulate the portion that's unsheathed that remains outside the box (I figured it might help the probe get better readings).

Thanks again

[HaroldMHedge]

The capillary tube fits inside the guide tube (copper, I think) 901-573-903-00 which angles into the factory evaporator and sets the depth. If using an aftermarket serpentine evaporator then it goes straight into the evaporator each supplier provides some guidance on where to place it. The top of the guide tube is even with the outside of the evaporator housing.

Guide Tube (photo from competitor website)


As other have mentioned the exposed capillary tube is covered in a white plastic insulation. If everything is not covered, I would not be worried.

[kuehl]

In reality you could use a thermostat capillary sensor tube (the aluminum tubing from the thermostat to the evaporator) without any insulation. A simple test you can do to affirm this is:
1) Get yourself a glass or container of cracked ice, or freeze a common 12 oz water bottle, remove the cap (it might pop off due to expansion of the water to ice), drill a 18" hole into the frozen water about 3" down.
2) Extract the aluminum sensor tube from the evaporator core; it should be inside a brass protective sleeve, leave the sleeve in the evaporator core.
3) Vehicle parked on a flat surface, E-brake on. Turn on your ignition key to the accessory position, turn the evaporator blower motor to any of its 3 fan speeds, turn the thermostat knob from its off (fully CCW) position to CW about half way. At this point, if you have a front condenser its blower motor should be running (thermostat turns the motor on and off as well as compressor clutch).
4) Insert the aluminum sensor tube into the ice. In less than 10 seconds the front condenser motor should turn off. Remove the aluminum sensor tube from the ice and wrap it in the palm of your hand and in 10 seconds or less the front condenser blower motor should turn back on.
You can play with the position of the thermostat knob to determine where 33-32F is on your dial. Make a small white dot on your console's fascia where the knob is pointed when you are at 33-32F. This is good to know in cases where you have issues with the evaporator core freezing, just dial back CCW a few degrees to prevent freezing in humid weather

The factory used a thin wall plastic tubing that tends to crack with age, much of it for protecting the aluminum tube from abrasion as it winds through the center console and and into the smuggler's box area in the front trunk. So, best to re-use the old whitish colored protective sleeving or tubing or replace it.

The reason there is about 8" give or take of aluminum tube without a protective sleeve at its end is so you can bend the tube to get things to where they have to be. The aluminum tube length is about 48", a bit longer than needed, however you can coil the excess in the smuggler's box area next the evaporator and secure the excess to the side of the evaporator box by using a long tie wrap, wrapped around the 3" nominal OD black air outlet tube. You want to be sure your steering shaft does not smack the tubing.

As Harold mentioned, there are different requirements for depth of insertion and angle depending upon if the the evaporator coil is a stock tube and fin design or a serpentine design.

With a LHD stock tube and fin factory evaporator core the insertion angle is approximately
60 (acute) or 120 (obtuse) degrees toward the windshield, and approximately 4" downward into the evaporator box top (same length as the brass protective sleeve).
With later design evaporators, say a serpentine design, insertion is straight down between the serpentine tubes, and depending upon the brand
of coil, about 4" down.

When you are working with or around the thermostat's aluminum capillary sensor tube you want to be careful not to damage it, for example bending it sharply, else you could break the tube and the refrigerant gas inside the tube will escape and render the thermostat kaput. The thermostat, unit with knob in your center console has a flexible metal bellow inside, this attached to the aluminum tube that is sealed a its end. As temperatures rise the gas inside expands and contracts, causing the bellow inside to make and break the electrical contacts... which turn on the front condenser fan relay and the compressor clutch. Depending where you have your thermostat knob turned determines when the AC system (compressor and condenser fan) turns on and off.

We (Griffiths) are closed Aug 11th through the 21st.

Stay Kuehl

[Koizumi]

Thank you for the terrific info! Awesome!

Cheers

[kuehl]

Becks

[Mervo]

I just bought a new Porsche-supplied AC switch with its associated tube. Aside from the minor challenge of getting the switch unit in the available area in the console, I have been wondering about the best route for the tube?

In most of the descriptions it runs a circuitous route back and through the smuggler's box to the evaporator. On my RHD 88 Carrera the evaporator box is right under the passengers floor board near the switch.

Why can't the tube go directly from the switch to the evaporator core? I know I am missing something here ..

[kuehl]

The end of the aluminum sensing tube (don't break the tube) is inserted into the evaporator core on both LH and RH drive models.

However, on the RHD, since the box is flipped around (blower side is one the back) you have a choice to insert it on either side, the cockpit side is easier.

RHD early Turbo AC overhaul

https://tm2ds3p93zs2zbag83par8d1-wpengine.netdna-ssl.com/wp-content/uploads/1974930evporatorbox.jpg

[Mervo]

Thanks for that! That image still seems to show the sensor tube coming from behind (to the front). So, just to be sure, I can run the tube (in some protective plastic or similar tubing to protect it) directly from the temp switch in the console to the evaporator inside the cabin, rather than down through the smugglers box and back?

[kuehl]

Yup

[Mervo]

Many thanks. Seems the RHD installation is easier.

[kuehl]

Here is just 1 example of how the thermostat aluminum sensing tube was routed in a RHD; as you can see the evap box is at the bottom of the LH side seating position. I'm not saying this is the perfect path to take, it is simply one example. The red-colored line is the path aluminum tube and the yellow circle is where it was inserted into the core, on a diagonal between the copper tubes and inside the brass protective sleeve.

The purpose of the aluminum sensing tube is to turn the thermostat on and off based upon the temperature of the evaporator core and the position of the thermostat knob on its switch, AND to prevent the core's surface from icing up with moisture.

Ideally, you want the insert the end of the aluminum sensing tube in a section of the evaporator core that reflects the 'average' temperature of the core. This is a bit challenging because sections of core can freeze up earlier or later than others depending upon air flow through the core. You could attempt to see where the core areas start to ice up early by running the system with thermostat's aluminum sensing tube temporarily withdrawn or out of the evaporator core.

Since the length of the aluminum tube, from the thermostat switch assembly to its sealed end, is quite long, you are going to have 'extra' length you need to coil and tuck away. Best to put this extra length or bundle towards the backside of the box where it won't pop out or get crushed.

When you determine where you are inserting the aluminum sensing tube into the cockpit facing side of the evap core, I'd try to keep it flush as much as practical, without putting a sharp bend in the tube, against the face of the core.

The evaporator core shown in the picture is an OEM style 'tube and fin'. The aluminum sensing tube should be inserted on a diagonal between the copper tubes. If you are using 'serpentine' evaporator you need insert the aluminum tube straight into the core between the serpentine tubes. Insertion depth is approximately 3" or 76 mm into the core. If you insert the aluminum sensing tube too much further through the core so that it's flopping in the breeze the thermostat will not be seeing the temperature of the core, instead it will be seeing air temperature which not the intention of the device.

[88911coupe]

IF the sensor tube is broken or cracked the compressor will just run non stop, correct?

[kuehl]

Originally Posted by 88911coupe

IF the sensor tube is broken or cracked the compressor will just run non stop, correct?

Yes. Inside the thermostat switch is a metal bellow. Brazed to the bellow is the aluminum temperature sensing tube. The end of the sensing tube, at the evaporator side, is pinched closes. Inside the tube and bellow is refrigerant gas. When the tube breaks from sharp bends, mishandling 'events', etc, the gas escapes and the entire thermostat assembly must be replaced.