DIY: How To Connect [almost] any temperature sensor to an Aeroforce Interceptor gauge

[toedrag]

Purpose
This explains how to connect an SU112 fluid temp sensor (or just about any other similar sensor) to the Aeroforce Interceptor's analog input to measure something like oil temperature. While Aeroforce does sell sensor kits, if their kit can't work for whatever reason in your application, as you'll read below, a custom circuit to marry the sensor up to the Interceptor may be necessary. I ran into this while working on my Oil Cooler (thread link).

If you plan to use an SU112 and you have a Interceptor CN-series, ver 1.7 already, you can probably skip down to "Step 4: Once you're happy with your R1 & R2 values, now you get to actually build the circuit and install it." If you're using a different sensor and/or you have a different Interceptor, best to start at the beginning and read the whole thing.

As an aside, I realize that this write-up isn't likely to be useful for most people; I expect there are only very few who might need to leverage this, but oh well. Here it is anyway.


Background/Theory of Operation
The Interceptor is a OBDII scan gauge, more info on the Aeroforce website, and it has 2 Analog inputs for connecting various types of sensors. I had been happily using an Interceptor for several months, mainly as an easy method to read the oil temperature from the ECM. As time passed, I found a need to add an actual oil temperature sensor; I say 'actual' b/c the G8 has a calculated oil temperature which has proven to be pretty inaccurate after adding a cooler. If you have doubts, read this post (link) and be enlightened. The 2 analog inputs require a signal that varies from 0 volts to 5 volts. The challenge is that most temp sensors are simply variable resistors; for example, at 70 deg F, a sensor's resistance might be 2800 ohms. At 300 deg F, a sensor's resistance might be 50 ohms. Directly connecting these to the Interceptor will not work. Remember Ohm's law, V=IR, from school? Well, here it is. To introduce a voltage where there is resistance, you need some current flowing through that resistance. Without any current flowing, the Interceptor would always see 0 volts at this input.

To make it easier for customers, Aeroforce rightly provides a "Fluid Temperature Sensor Kit". The kit consists of a temp sensor and a 5 volt regulator. You can read about the kit here here (pdf link). An important point here is that the sensor is an isolated ground type, which means that the sensor has two terminals, one ground and one signal. The casing & threads are not connected to either terminal. Contrast this with a common ground sensor, where there is only 1 terminal which is the signal; the casing & threads are intended to be grounded to the car's chassis ground once the sensor is installed in the hole. If you have a choice, use an isolated ground sensor since using a common ground sensor introduces some challenges that are not easily overcome (I won't address it here). The function of the 5V regulator is simply to take 12V from your car and convert it to a clean 5V supply. Then, it applies that 5V to one of the temp sensor's terminals. The other terminal on the temp sensor will have a variable voltage due to the changing resistance that the temp sensor introduces, and this is what gets connected to the analog input of the Interceptor. The end result is, for example, At 70 deg F, the Interceptor might see a signal of 0.5V, and at 300 deg F, it might see 4.5V. How the 5V regulator does this is through the magic of a basic circuit called a Voltage Divider (link) circuit.



In terms of the above picture and how it relates to the Interceptor, Vin = 5V. Z1 = the temp sensor's variable resistance, or there could also be another resistor in series with the temp sensor (this is required with the SU112). Z2 is another resistor, whose value is chosen so that the proper voltage is seen at the Vout point. Vout is then connected to the Interceptor's analog input. In the Aerofoce temp sensor kit, the Z2 resistor has already been chosen and soldered somewhere in the wiring of the kit.

Problem I'm solving
Aeroforce's kit offers either a 1/8 NPT or 3/8 NPT sensor. My car doesn't have either of those holes readily available; I had an M12 x 1.5 hole on a Mocal oil pan adapter to work with. There are threaded adapters that convert M12 x 1.5 to 1/8 NPT, but I couldn't use these due to: 1. vertical clearance for my chosen sensor location, and 2. The non-threaded portion of the temp sensor is too big for most adapters. This means I can't use the sensors provided with the Aeroforce kit. As it turns out, the GM coolant sensor 15369305 was also used as an oil temp sensor on various LS engines, which has a Duralast cross-reference, SU112, and is also M12 x 1.5.

The Solution
Simply stated, all that needs to be done is build a new voltage divider circuit by choosing new values for Z1 and Z2. I'll switch terms at this point and will use R1 and R2 instead of Z. Note that every sensor, even two of the same part number, might require different resistors for R1 & R2.

Here are the high level steps. Each is explained in more detail below:
1. Measure Resistance vs Temp of the new sensor and enter the data into Excel. You don't need to graph this data, but I did because I'm weird like that.
2. In an adjacent column of your spreadsheet, enter the voltage divider equation.
3. Pick a value for R1 & R2 so that it results in a calculated temperature that is as close as possible to what the Interceptor is expecting.
4. Once you're happy with your R1 & R2 values, now you get to actually build the circuit and install it.


Step 1: Measure Resistance vs Temperature of the SU112 (or other sensor) in a controlled environment. Do this at your own risk. Do this at your own risk. Do this at your own risk. Here's what I used:

• Ohm meter
• Stove
• Some Olive Oil (because Olive oil has a smoke/boil point of 400+ deg F). Use enough to submerge the non-threaded part of the probe. I think I used about a 1/2 cup in a small pot.
• Digital thermometer that goes up to 300F+. Harbor Freight has a good one, Item 95382. The infrared temperature guns don't yield good results, fyi.

Drop the thermometer & sensor into the oil and connect your ohm meter to the sensor. Turn on your stove. Do this at your own risk. I used a medium setting so that the oil doesn't heat up too quickly. You really don't want 400 deg F oil boiling and splattering everywhere...be careful and don't burn yourself, someone else, or catch your house on fire with hot oil. Do this at your own risk. Turn off the stove when it the oil reaches 300F. With the stove off, the oil will cool off gradually. As it cools, record the temperature and corresponding resistance. If you can do it major points, like 300, 290, 280, 270, 260...all the way down to 100F or even lower, your results will be better later on. When I did it, it took about 30 min for the oil to go from 320 to 100.

Here's a sample of the graphed Resistance vs Temp data. I tested two different sensors, the SU112 and a VDO 323-092. Again, you don't need to graph these, but I went ahead and did it. You can see they have much different behavior.


Step 2: Put the voltage divider equation into Excel, choosing somewhat arbitrary values for R1 & R2. Assume a 5V input (aka Vin). The equation is Vout = (Vin*R2)/(R2 + R1 + Rsensor). If you decide to use a Vin source different from 5V, make sure to adjust the formula accordingly.

Step 3: For the Interceptor to display the proper Temperature at a given Voltage level, there is an equation that is programmed into the Interceptor. This equation is the same for all units of the same type & firmware version. Enter the Aeroforce-provided equation for the Interceptor fluid sensing function. FYI, there is a different equation for the Air temp sensing function, and I think the Air temp equation is displayed in one of the User Guides that Aeroforce provides. Anyway, the fluid sensing equation for my Interceptor (CN Series, ver 1.7 firmware) is:
Displayed Temperature = -0.5221x^6 + 8.2248x^5 - 50.401x^4 + 156.18x^3 - 261.15x^2 + 273.2x - 51.702, where x = voltage at the analog input

If in doubt, email Aeroforce Tech support and ask them for the fluid temp equation for your device if it's not a CN series ver 1.7.

Now, use the result from the voltage divider equation as 'x' in the Interceptor equation. Compare the 'Displayed Temperature' value from the Interceptor equation against the actual temperature that you had measured earlier with the olive oil on the stove. For example: let's say at at 200 F, the resistance is 204 ohms. With the voltage divider, this results in a voltage of 3.47 V. When 3.47 is plugged into the above equation as 'x', the result is 196 deg F, which is 4 deg F less than the actual temp of 200 deg F. What this example means is that the Interceptor will display 196F even though the actual temp is 200F, so it's not perfect. If only Aeroforce let you program in your own 6th order polynomial equation...

Once you have the comparison between the Displayed & Actual temperature for 100 to 300F, you can now play with the R1 & R2 values to see what combination results in the smallest delta between the Displayed and Actual temp. Depending on the sensor, you might not be able to get a good match across the entire range, say 30 to 300F. The important range is really between 200F and 300F; if the temp is less accurate below 200F, it's not really a big deal. If it's not accurate over 300F, that's okay too b/c if your oil is over 300F, you've got worse problems to think about.

In my case, I ended up with R1 = 12 ohms and R2 = 490 ohms for the SU112, which creates a 3 deg variance over the range of 200 to 290 F. Note that, depending on the behavior of the sensor you're trying to marry up to the Interceptor, R1 might be Zero. As a comparison, I went ahead and figured out the resistance values for two other sensors I had on-hand. I don't recommend using either of these to connect to the Interceptor; each has challenging caveats. This is just to illustrate that R1 & R2 can vary depending on the sensor:

SU112 (This is the sensor I'm using. If you use the SU112 too, you can probably just use these resistance values):

• R1 = 12
• R2 = 490

VDO 323-092:

• R1 = 8
• R2 = 205

GM oil level/temp sensor (GM p/n 12603781):

• R1 = 30
• R2 = 622

The graph of the SU112 voltage divider output compared to the Interceptor equation looks like the following. This tells you how accurate the sensor will be. In my case, it's not accurate below 200F, but that's okay...I care about 200 to 300.



I put together an Excel template for this to make it a bit easier to determine R1 & R2 if you have a different sensor or equation, but the forum won't let me attach an xls or xlsx file. If anyone wants this template, let me know.

Step 4: Once you're happy with your R1 & R2 values, now you get to actually build the circuit and install it. You have a couple of choices, you can either buy the Aeroforce regulator and modify it or build your own from scratch. It's really easy to build your own as long as you have basic soldering & circuit skills. This is what I did. Here's the parts list I used, around $20 for everything I think:

• 5V regulator, p/n 7805 from Radioshack
• 0.33uF capacitor
• 0.1uF capacitor
• 2 micro trimmers/aka potentiometer, 1k ohm, such as Radioshack 271-342 (in my pictures, I used a smaller trimmer for R2, but I wish I'd used the p/n I'm listing here for both instead of just 1 of them)
• Project Enclosure box 3x2x1 inches, Radioshack 270-1801
• Circuit board, aka perf board, Radioshack 276-1396
• Misc wires, connectors
• Littlefuse's 'Add a Circuit'

The capacitors (caps) are there to filter out noise from the power supply, both on the 12V input and 5V output. Make sure to keep the larger cap on the 12V side. The two trimmers represent R1 & R2; they're easier to use than discrete resistors in a situation like this because the resistance is adjustable using a small screwdriver.

The project box has 5 connections:

• 12V In
• Ground
• Temp signal Input from temp sensor
• 5V Out to temp sensor
• Temp signal Out to Interceptor

I built the circuit based on the following schematic. The capacitors, C1 & C2, are based on reference circuits from the datasheet for a 7805 regulator. The values don't have to be exact, but you do want the Input capacitor, C1, to be larger than the Output capacitor, C2. The trimmers are R1 & R2.


Here is a picture of my finished circuit & the enclosure. You'll count 2 caps on the input side of the 7805 b/c my local Radioshack didn't have a single 0.33uF cap, so I used a pair of 0.22uF caps:


Next, same box, zoomed out a bit, plus the Add-a-circuit and Aeroforce harness. I've used bullet connectors to make install/removal easier. Inputs use male ends and outputs use female ends:


At this point in the install, I removed the Insulator Panel, which is the panel directly above the pedals. To do that, follow these simple instructions, courtesy of CrazyPaul, http://www.grrrr8.net/crazypaul/IPIP.pdf. The only reason to do this is just for routing the one wire to the Interceptor, which I keep on the steering column with a strap-mount gauge pod during track days and remove the rest of the time.

To pick up chassis ground in the car, a convenient place to do it is on a support brace above the fuse panel. The brace is grounded, NOT the painted screw. Just loosen the nut, place your ground wire, and tighten the nut. The single black wire in the picture below is connected to this point. Since the brace has a hole much wider than the screw, don't tightly wrap the ground wire around the screw. Make a nice big loop of wire to ensure it makes good contact with the brace, not the screw.


Connect the various wires to the box.


Choose your preferred location for the Add-a-circuit. I chose a 2A fuse for the outside mirrors (F15). You might want to test the box at this point if you haven't already. Hook it up to the Interceptor, fire up the engine and see if the readings make sense.

Once you're ready to finalize the install, slide the box in the space on the other side of the support brace. Tuck your wires out of the way, and mind the clutch pedal and the actuator mechanism if you have one. Add a ziptie.


Put the Insulator Panel back on and viola. Note that everything is still relatively accessible even with the Insulator Panel installed:


When I'm not using the Interceptor, I just take out the fuse for the little black box; no sense in keeping it powered up if it's not going anywhere. I reinstall the fuse during my track prep.

[Desy182]

Thanks for the info. Good review!