I decided to perform this task on my car as I've got a d-jetro power fc and I didn't like that the air intake temperature sensor on the GTR was registering 15+ degrees of heat soak, changed so slowly and just generally provided no real tuning benefit.
I believe the reason for the temperature sensor being so poor is that it is actually a water temperature sensor that is being used to measure the temperature of air. This, in combination with being so close to to the radiator and head on high power engines leads to heat soak and very strange scenarios occurring when motorway driving.
I started doing some research and came across a very useful post on the SAU forums where a member had decided to use a GM IAT and performed the calculations so that with the correct resistors in parallel, it could actually be used with the PowerFC. Other ECUs shouldn't really need to do this because they should be able to change the resistance/voltage curve but I still recommend changing the air intake sensor.
Another reason for this guide is that since the member on SAU made this post a few years ago and now seems un-contactable, there should be a point of contact for this modification and a clear guide, with pictures as to how it is set up.
Problems I had with my old sensor
- Leaving the car afterwards allowed everything to heat up. The standard IAT got to a point whereby it was measuring the temperature of the plenum not the air inside it.
- Gross inaccuracy of the actual air temperature at all times
- Very slow to update
Credits to the person who came up with this: -
Rb26 Air Temp Sensor Upgrade For Power Fc - Tutorials / DIY / FAQ - SAU Community
and most importantly, the spread sheet: -
This is a really cheap mod to do and cost me no more than £100. You will need a re-tune as the tuner will have performed the tune on the old temperature sensor. The new temperature sensor should allow the tuner to give you a more accurate summer/winter tune as the temperatures will be much more realistic and should update very much faster.
The tuner should now be able to reliably add advance to your ignition timing based on the air temperature and retard it when it gets too hot. This should be the same for fuelling; it will actually read properly now.
The resistors I chose are 0.1% tolerance so should be quite accurate. This means that their margin for error will be within 0.1%. They also have a very good temperature coefficient so they should be good in even the hottest engine bays.
You need to get fairly decent heat shrink; I actually got some from the place I work. They gave me the wires to use too which are rated to go in aeroplanes, but that's just over the top. As long as the heat shrink and wires don't melt at engine bay temperatures, you're fine.
14k ohm resistor - RC55Y 14K 0.1% - WELWYN - RESISTOR, 0.25W 0.1% 14K | CPC UK
0.2k ohm resistor - RC55Y 200R 0.1% - WELWYN - RESISTOR, 0.25W 0.1% 200R | CPC UK
heat shrink - HEATSHRINK HEA 3:1 BLK 20MM 1.22M; Shrink Ratio:3:1; Shrink Tubing Boot Material:PO (Polyolefin); Colour:Black; External Length Height:1.22m; Fully Shrunk Diameter:6mm; Material:Polyolefin; Shrink Temperature:120°C; Unshrunk Diameter:20mm - HSBRS 206
Intake temperature Sensor - Amazon.com: GM Open Element IAT Sensor with 6" Pigtail by DIYAutoTune: Automotive
Shielded Electrical wire - RRW-V-105 PK 4 - PRO POWER - WIRE, 0.19MM, PURPLE, PK 4 | CPC UK
and some black insulating tape!
Multimeter for debugging
Aluminium welding (No way am I going to do this, I paid for it to be done, it cost £60)
I'd buy a few of the resistors as they're so cheap and use a multimeter to make sure that the resistances are as close to the values as possible. You're looking for 14.2k ohms but 14.19 or 14.21 isn't going to make a major difference. I couldn't find any 14.2k with the temperature requirements but this isn't a big problem, we can put two resistors in series
to create the desired reistance. i.e. 14.0 + 0.2 = 14.2. Parallel would not be correct so be sure not to put them like that.
Now, before I get started on the steps, I had no idea really about how to solder resistors together nor how to solder. This is another reason I recommend getting a few resistors. You can have a play around for such a small cost but I'll summarise what to do and where to go for advice.
Step 1 - Parts, Labour, Procurement
Purchase all of the parts and ensure you have all of the tools or people who will do some of the work for you. For example, I did the soldering, wiring and such but did not do the aluminium welding, thus you'll need to find an engineering site that does this. It's easy, have a look at the yellow pages or search online.
Before you do anything, make sure that the IAT is not faulty using your multi meter. The values in the spreadsheet represent what you should be seeing. If this is not the case, it may be that the sensor is faulty and now would not be a good time to leave a gaping hole in your intake system while you send back the IAT to be replaced.
If you want to do the welding yourself, you should already know how to do aluminium welding at a start. I asked a company called headline to do the work for me. They welded a tapered bolt to the pipe that connects to the air intake plenum. See below: -
Note: I took the piping off and gave this to the company to do the welding; it took them an afternoon to do which was pretty quick. Splendid job; looks great and positions the sensor nicely inside.
I run a water meth injection kit on my car and having the sensor just above it is perfect. I can see the change in air intake temperature immediately! This helps a lot with the tuning for part throttle on the d-jetro which is a bit of a pain. There is a correlation between the temperature of the intake plenum and the boost as the water meth is injected progressively
Step 2 - Wiring the two resistors in series with each other
It is very important to wire the resistors in series with each other so that you create the resistance of 14.2k ohms. I drew the following diagram to help clarify the difference between series and parallel: -
Soldering resistors together should be done by wrapping the resistor legs around each other: -
Step 3 - Disconnecting the wiring harness
I chose to leave the factory sensor in the plenum as I would otherwise have an air leak. It is a little fiddly but you should be able to get the wires from the harness out of the pigtail connector so that you have two ends to work with. You'll want to remove the little plugs off the end and potentially expose some fresh wire.
At this point you should have a wire coming from the harness and the sensor not connected to anything. If you were to start the car now, there would be a sensor failure, of course. My wire from the harness was quite long and had some excess slack, I used a cable tie to route it around underneath the plenum and away from the hot radiator. See the image below: -
Step 4 - Wiring the total 14.2k into the circuit in parallel
The connection to the sensor is a little bit weird; I actually messed up at first by not realising that I had to push the wires through the pig tail connector like this: -
[ actual sensor that goes into the plennum ] ---> thread wire from this side ----> [ connector/pigtail ] -----> to the harness.
Now, immediately this means that it's a bit complicated because we don't want to cut the harness which is probably a little brittle after all these years. The best action, I found, was to actually wire from the sensor side and solder onto the existing plenum.
Although we wired the two resistors together in series, the overall resistance needs to be added in parallel. The following circuit diagram shows how this should be set up: -
It doesn't matter whether you dpn't know which is the anode or the cathode; as long as the two wires that are left from step 3 connect to the sensor with the resistors that are built in series to create 14.2k are placed in parallel, you'll be fine.
The final look should be like this: -
I added another plastic tubing over my heat shrink, as you can see in the above image. I was a little concerned that the resistors could get slightly hot but they haven't had any issues so far and from what I keep getting told; the resistors are quite durable things!
Make sure to use the multimeter to test the resistance of the resistors that you've soldered. Too much heat from the soldering iron will damage the resistor so don't take too long to apply the flux.
Make sure that the wires are actually as they are in the final circuit diagram and you shouldn't really have any problems. The maths was already done for you.