Oil Pressure and Temperature (Part 3)

Hapy cinco-de-Mayo. My last post ended with some testing of the Porsche combination gauge with the modern VDO sensors. I'll pick up where I left off.

Where Were We?

Porsche combo gauge

After I wrote all of those posts about the TDI install, I realized that the little UltraGauge does not show my oil temperature or pressure. I can see the coolant temperature, though. When we drive up a long grade, we can see the coolant temp climb, and I wondered if the oil temperature was leading that charge upward, dumping heat into the oil:coolant temperature exchanger. So, I started collecting sensors to compliment the gauges I had lying around from back when I had the original air-cooled engine and thought that having an effectively oil-cooled engine without a gauge about oil was just silly. That's especially true when that engine was 5 meters away. In my usual fashion, I can't just run wires from sensors up to the dash and stick a couple gauges in back of my windscreen and call it good. Oh no. That would just be too easy. Instead, I got this notion that a combination gauge from a 70's-era Porsche could fit into the 3rd hole in a VW bus dashpod. I was right. But, does it work with the new VDO sensors? Why, yes it does.

Simple Initial Testing
I did some very simple tests of the sensor-to-gauge circuit in my last post. All I did was demonstrate that the gauge would respond to 12V, and if I had a sensor hooked up, the gauge would lift slightly off the bottom post. This is no casual "who cares" moment, in my opinion. Most important, this indicated to me that the big circular thing that I had shipped halfway around the world had operational gauges. But, would they respond correctly to real-world input from modern VDO sensors?

Temperature

water still steaming, needle at 9:00

Testing the viability of the oil temperature circuit was fairly easy, once I had the circuits set up again. While Boo loves me, I still had to tear down the tests for dinner. So, things needed to be set up again. This time, I included a small glass cup and a meat thermometer. Into the cup I set the sensor, wrapped with a grounding wire with a sender wire on the end. With the circuits powered by a big auto battery, I poured boiling water into the cup. The gauge moved from pointing down to 9 o'clock (correct for water boiling-point) which should be around 210*F. I thought I could confirm the test with the meat thermometer, but instead I found that the meat thermometer was not correct. How fun! In subsequent tests, I discovered that the probe on the meat thermometer needed far more surface area to reach the correct temperature reading. So, rather than triangulate on the proof that 9 o'clock means 210*F, I learned that my meat thermometer reads cool unless at least an inch of it is inside the target. Good to know for future baking. Anyway, this test passed.

Pressure

decoding the no-number gauge

Testing the pressure was not as fulfilling as testing temperature. For temp, we know the boiling point of water so if the gauge doesn't rise to halfway, then we would know that the sensor is on a different resistance gradient than the gauge. That would have been a real bummer. For pressure, I needed to depend on my Mity-Vac. The Mity-Vac has already proven its worth of the years as I have tried to bleed brakes and pull fuel through lines after running the tank empty (I know; bad owner). It also has a pressure setting though. With the circuits wired up, I set to it. I keep a short stretch of fuel line in my Mity-Vac kit, and that fuel line is the perfect size for the threads of the pressure sensor. I threaded one end onto the sensor and put a basic auto-adapter from the Mity-Vac kit in the other end. To that, I connected one of the short clear lines and then the Mity-Vac. This set up allowed me to send a pressure signal to the sensor. I jammed the grounding wire between the end of the fuel line and the sensor housing on the threads (establishing a viable ground) using that friction to hold the wire in place.

around 15 psi and around 1 BAR

Once everything was powered, I simply applied pressure with the Mity-Vac and watched the needle rise off the post. Translating "Druck Pressure" to PSI is a brief exercise in understanding German. Druck is effectively barometric or BAR and 1 bar = 14.7 pounds per square inch. So, to confirm the gauge is operating correctly, I needed to get "Druck" = 1 when I had just shy of 15 pound of pressure on the Mity-Vac. That succeeded: the needle on the gauge climbed to 1 and then slowly dropped as the pressure was slowly released, matching the needle drop on the Mity-Vac. I would have liked to have proven the mid-range of the gauge, but the Mity-Vac and my cobbled-together plumbing couldn't handle more than 15 pounds of pressure. I thought about getting my air compressor involved, but concluded that the test was already successful.

Conclusions
So, what did this tell us? That sensors bought today will correctly align with a gauge pulled from an old Porsche. We demonstrated earlier that the gauge would fit in the spare or "third" hole. I think we have a winner and I'm beside myself with excitement about it. I will clean everything up, replace some bulbs, run a wire bundle, and road-test everything, etc. when I have some time outside with Hapy. You can see in the pictures on this post taken during the tests that the gauge now sits nicely centered in the hole. I think it looks fantastic, but it's my bus, so its my choice. Knowing that I will have visibility into the lubrication system, and can start analyzing whether the oil temps are leading the temperature increases on long climbs is pretty exciting.. for me anyway. If my theory is proven, I can start looking into improving oil temperature management so climbing hills or mountains eventually have a much reduced impact on engine temperature. First, we need to see the data.

Thanks, as always, for following along. More next time-