'97 855 GLT: Measurements and Teardown on a PTC

[jreed]

There have already been many good posts on the PTC ("Positive Temperature Coefficient") component in the positive crankcase ventilation system.
Ozark Lee: https://www.matthewsvolvosite.com/forums ... =1&t=27526
cn90: https://www.matthewsvolvosite.com/forums ... =1&t=53448
kickin_it: https://www.matthewsvolvosite.com/forums ... =1&t=60133

I always wondered what was inside the PTC valve and how it worked electrically, so I decided to take apart one of the ones I harvested from the junkyard a few months ago.

The PTC valve

Volvo part number 1367839

I removed the rubber bushing by prying it up and off. Then I used a hacksaw to slice down one side of the main PTC tube. While I was sawing the last bit of the side, when the blade dug into the bottom piece it popped off. The bottom piece is also copper and was soldered onto the copper tube.

After slicing through one side

After slicing through both sides

Opening up the halves and looking inside the copper walls of the PTC tube

On the side of the main copper tube is a small 2mm diameter hole that aligns with the external small vacuum nipple. I was surprised to see how small this hole is, but when I checked the inside diameter of the vacuum nipple, it was also 2mm, so they are matched.

The small vacuum hole in the side of the copper tube

Next I removed the copper wall from the side of the PTC with the electrical connector. I had to pry a bit with a small flat screwdriver and I heard a cracking sound as I pried it apart to reveal the PTC heater element.

PTC heater element

The cracking sound was from breaking the solder joints between the copper wall and the contact pin and PTC disc. I measured the resistance from the surface of the disc to the pin 2 in the connector and got ~20 Ohms, which was the resistance between the pins before I took it apart. This indicates that the resistive element in the PTC valve is the disc. The disc is soldered to the copper wall and conducts heat to the copper wall and thus to the gas passing through the PTC tube. It's interesting that the copper wall is itself part of the electrical circuit.

I cut apart more of the plastic case on the connector side to expose the edges of the PTC disc. During this process the disc cracked in half. I removed the loose chunk of the PTC disc to reveal part of the copper bond pad underneath.

Side view of PTC disc showing copper bond pad embedded in plastic of PTC case

The disc is about 12mm across and 2mm thick.

PTC disc is 12mm diameter and 2mm thick

I took a look at one of the chunks at higher magnification. It is a tan/gray color, almost translucent at the edges like a ceramic. The material can be snapped easily by hand and is brittle but not crumbly.

Based on reading up on the PTC effect on Wikipedia (quoted below, from http://en.wikipedia.org/wiki/Thermistor), the disc is most likely a ceramic called barium titanate.

From Wikipedia: "PTC: Most PTC thermistors are of the switching type, which means that their resistance rises suddenly at a certain critical temperature. The devices are made of a doped polycrystalline ceramic containing barium titanate (BaTiO3) and other compounds. The dielectric constant of this ferroelectric material varies with temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply."
I've read the above paragraph a few times and I must admit I'm going to need to study up quite a bit more to really understand what the mechanism of the PTC effect is.

In the meantime, to test out the switching effect, I measured the resistance of the PTC over temperature to see if the resistance really did increase sharply. This would help confirm that the PTC is the switching type, as compared to other types that change resistance linearly with temperature.

Set up to measure resistance over temperature

PTC 12 Measuring Resistance at Temperature.jpg (161.36 KiB) Viewed 9632 times

On the set up shown above, I'm measuring a second PTC that I harvested from the junkyard. I'm measuring 15.1 Ohms on the lower meter at 59F (15C) as shown on the upper meter, which is connected to a thermocouple pressed against the copper wall inside the PTC tube.
I cooled the PTC by putting it in a freezer and heated it with a hair dryer and recorded the resistance at various temperatures from about -15C (~ +5F) to about +105C (~ +220F).
I plotted up the data, shown below. At lower temperatures up to about 60C (140F) the PTC has a resistance of about 15-20 Ohms. But the resistance starts to rise dramatically at higher temperatures. At 80C (175F) for example the resistance has already increased to about 33 Ohms. This makes for a good self-limiting heater because the heater element itself will start to restrict current flow (and thus heating) as the element reaches the design temperature, which seems to be in the range of 60C to 70C. This built-in feedback mechanism will act to help keep the temperature inside the PTC relatively constant.

Resistance of PTC at various temperatures, showing "switching" effect at ~80C

When I applied 13V to the PTC units starting at room temperature, I got an initial current of about 1A which gradually tapered down to a final current of 0.3A and a final temperature of 65C (~150F). Both PTCs (the one I tore down and the one I measured over various temperatures) performed about the same.

Well, just wanted to share a quick write up of the measurements and teardown on the PTC... I didn't know that there was a chunk of barium titanate (or something similar) inside the PTC valve that was the actual heater element. Always something new to learn, even from an old car!

[draser]

Pretty accurate description of how PTC components work, graph clearly showing the exponential variation of resistance with temp.

[MrPc]

Very interesting, and nice write up!

So the actual electrical function then is just to keep warm to prevent oil vapor condensation from forming sludge and blocking the openings to the intake manifold or accordion tube?

If this is the case, then is it just the carefully chosen and balanced sizes of the different openings in the PTC which controls how the PVC system functions? Please check my understanding:

At idle, the intake manifold has vacuum, there's pressure in the crankcase and the accordion tube is at about atmospheric pressure. Vapors come out of the crank case, out of the accordion tube, and get sucked into the intake manifold through the 2mm port. The size of the large port to the accordion tube has to be restrictive enough to ensure that the intake vacuum draws crankcase gases through the PVC tubes from the collector instead of drawing fresh air from the accordion tube, in order for the glove to get sucked in at idle.

At boost, the intake manifold has pressure, there's more pressure in the crankcase, and low pressure in the accordion tube since it's at the intake to the turbo. Vapors come out of the crankcase, and pressurized intake air comes out through the 2mm port, and both are drawn through the port to the accordion tube. So the 2mm port has to be restrictive enough to prevent intake boost pressures from overpowering the flow of crankcase gases and pressurizing the crankcase. And the large port has to be unrestricted enough to allow gases be drawn into the accordion tube, to keep the balloon sucked in.

At points in between, the relative contributions of the intake port vs the accordion tube port are balanced by their port sizes in the PTC, so that gases are always drawn from the crankcase, either by intake manifold vacuum or low pressure at the turbo intake.

So, is that how it works? The electrical function of the PTC serves only to help prevent oil vapor condensation from changing the carefully chosen port diameters and altering the balance of pressures/vacuums which control the flow crankcase gases? Have I got it? Am I missing anything?

Thanks for the great write up, Jason!

Paul

[mecheng]

Wow fascinating stuff @Jreed and nice explanation @Mr Pc; I've always wondering the same.
Science is the only TRUTH!

I've never removed it, I cleaned it in-situ by dousing it with carb cleaner; your cutaway shows it is important to clean the port as well.

[abscate]

Nice. The positive temp coefficient stuff is a clever way of keeping something at fixed temp without feedback. My guess is the electrical part of this is really reliable and the only mode of failure is clogging.

Marked for inclusion into the VRD and duly 'gearbox' marked as same!!

[UncleAl]

Jason did a splendid job for us. One important fact that I got out of this is that the crankcase vapors get vented back into the turbo intake tube only via a ~ 5 mm hole in the copper nose of the PTC. That's all the escape route that the PCV vapors have, so that's how little vapor is being passed even at high RPMs.

[Sveedy]

The sludge build up on that 2mm port seems problematic to me...I see a lot of over engineering here, but that's probably my simple mind at work. And I'm probably a bit overly sensitive right now, what with the heater hose / firewall connection, the internal o-ring on a water pump that is powered by the timing belt instead of the serpentine belt, and this little PTC gem.