Building a better Oil Pressure and Temperature Gauge

[Hohn]

I'm not sure where else to put this, so forgive me if this goes somewhere else.

Some of you know I'm active on a prominent oil discussion site and quite passionate about engine lubrication. This is why I always was frustrated that I couldn't see what the oil pressure or temperature was on my 10th generation Accord (with K20c4 engine).

I started looking into ways to get oil pressure or temperature info. But I found myself stuck with a set of challenges and desires that seemingly nothing could meet:

• Honda puts nothing on the OBD, and there's no oil pressure or temperature sender anywhere in the OEM sensing. A basic pressure switch is all you get.
• I keep cars a LONG time and try to keep them nice, so I don't want any hole drilling or any kind of interior butchery. Whatever oil pressure or temperature setup I have, I has to be perfectly reversible and leave no trace whatsoever.
• The factory oil pressure switch port is BSPT into the aluminum block and VERY risky to remove and reinstall and there's almost no space around it to tee into anyway. I am going to interface somewhere other than the pressure switch.
• I need the display to be both high resolution (i.e. integer units) and yet responsive. Analog displays don't give good resolution, especially with the tiny 90 degree sweep of most cheap electric gauges.
• Most sending units and sensors are poor quality and quite cheap, even in "premium" gauges. I'd like to have sensors that are more industrial and not so much cheap consumer. Better precision, better robustness, faster response (for pressure).

Since there was no off-the shelf option in my LLM-assisted research, I decided to try to make my own. In theory, it's not so hard-- a cheap little Raspberry Pi PICO for $4 as the microprocessor. It takes in the signals from the sensors and put the information on a small display that you can mount. And why not make the display touch screen so you can do things like swap units, reset memory values, or adjust the color or brightness?

The search for good sensors landed me on two that work very well for my purposes.

For pressure, I'm using a Dwyer Omega PX119a. While they have a few variants, I chose a simple 4-20ma current output for a 0-150psi pressure range (about perfect for oil pressure, don't you think?). It's fast , precise, reasonably compact and not too expensive. It also has a 1/8 NPT male interface that is very friendly for integration.

PX119 specs:


For Temperature, I went with a PT1000 RTD from Evo Sensors.
The RTD has good precision (better than a thermocouple) while having sufficient response time and ease of integration. Plus, the EVO product is stainless and has high temperature insulation and such-- it's suitable for under hood environment without special shielding and so forth.

The Raspberry Pi Pico (hereafter just "Pico") needs some intermediaries to work well with the sensors. In the case of the RTD, this is super easy because Adafruit offers a small daughterboard designed specifically to interface with RTDs. The Max 31865 is a great little board and reasonably priced.

The pressure sensor needed a bit more for integration because of the 4-20ma current output. I chose that for its resistance to noise and interference in the mobile application where a straight voltage output might struggle. So the current signal comes back to the main controller board where a shunt resistor network converts that to a voltage value the Pico can handle and understand. I settled on using a nominal 250 ohm shunt to turn the 4-20ma value into 1-5V, but do it inside the controller. Fairly standard practice with these things from what I gather.

However, the Pico can only handle up to 3.3V on the ADC input that will read the pressure. So had to add a voltage divider that would scale it to 3.3V max. I got pretty close to perfect by sorting a bunch of resistors to get the actual values needed.

Anyway, after a month of struggling with PCB layout, cooking a Pico and needing to order a replacement, and many hours of software debugging, I've got my working concept done with bench testing.

 

[Hohn]

So how am I going to get this into the car?

I didn't want a really bulky thing on my dash, so I broke the system into two modules. The display will be as compact as I can make it, mounted in a compact printed housing that lets it perch atop my existing CarPlay screen like a webcam. It's a 2.8" TFT touch screen. This was the smallest screen that I felt gave good legibility.

That housing is wired with Belden 9540 10-conductor cable through a cable gland to a CPU module that mounts under the dash. This module houses the Pico and its main controller board with the daughter boards, power supply, etc. It's just a small plastic Hammond box. The Belden cable terminates at the CPU housing in a GX-16 -10pin aviation style connector (like a micro-XLR sort of, but 10 pins).

The "cpu" Hammond box has a bundled pigtail out of it that makes the connections it needs from the input side: 12V in from the OBD pin 16, ground at OBD pin 5, the RTD temperature sensor leads (hot and ground), and the pressure sensor signal return.

I bought an OBD splitter so I can get power and ground there without messing with fuse taps or anything. My entire setup draws only 300ma, so we're not talking about enough current draw to cause issues on the OBD.

Finally, we have to trace our leads through the firewall grommet and to the engine. The RTD is just a simple NPT male as is the pressure sensor.

I'll be using an oil filter spacer with NPT ports in it to supply the sensing locations. The temperature probe is mounted directly on the sensor in the path of flowing oil. The pressure sensor will end up being remotely mounted with a length of hose to damp the signal a bit and package easier. Oil pressure is deadhead and doesn't need to be bathed in oil.

So that's the overall plan. We're somewhere around 40% done.

 

[jmarkwolf]

I understand your motive very well.

Years ago, 1983 IIRC, my Pontiac J-2000 had an oil pressure guage and I became quite accustomed to scanning that guage frequently.

My next car (1987 Pontiac Grand Am) didn't have it, and it bugged me.

I built an oil pressure guage from a pressure sending unit, an analog-to-digital chip, and 7-segment display, and a circuit board. It didn't do temp but a guage on the stock panel showed me that. It fit perfectly into a little cubby in the dash. It lived in that car the whole time I had it, until my 1993 Pontiac Grand Prix.

I don't know if you can still get such parts anymore, but I think I still have that board somewhere. If I can find it, I'll post pics here just for shits and giggles.

 

[Hohn]

There are thankfully a lot of options on the market now.

The best option I could find that is closest to what I’m trying to do is honestly the RaceTech (formerly SPA Technique) dual digital gauges. They are famously accurate, have selectable colors in the lighting and give you both temperature and pressure with high resolution (and good precision) in a single 2 1/16”/52mm gauge.

But I still didn’t like the idea of a bulky gauge pod mounted somewhere. Nor of transducer with “trust me, bro” as the spec sheet.

So I’m DiYing it.