Hello,

I have been told that the RX8's TPS can be flaky. I decided to check on mine. In looking at the lines of data I could monitor on my OBDII reader, I came across several throttle and TPS-related entries. To be honest, I have little understanding of what these lines mean, so I was hoping somebody could decipher them for me.

I have attached four pictures of my OBDII reader's screen. The engine was shut off, with the key in the 'on' position. The 'Idle' pictures were taken with my foot off the accelerator pedal, and the 'WOT' pictures were taken with the pedal fully depressed.

I have to admit I'm alarmed that, at full pedal pressure, the only line that reads 100% is 'THROT CMD.' My guess is that's the pedal sensor, but I could be wrong.

If anybody could help me understand what I'm seeing here, that would be great. Thanks.

 

Without being an expert, I can offer a suggestion on what it might be. An expert might reject or confirm this.

Aboslute TPS is likely the raw reading from the sensor. And it makes sense that off the pedal and fully on the pedal it isn't giving 0 or 100 readings, as you don't want to chance that the pedal will be lower than 0 or higher than 100. Same way thermometers usually have absolute mins and maxes well outside of what is reasonably possible for the environment.

Rel TPS might be "real" tps? Of the throttle itself, not the pedal, and the actual range of motion that the throttle body is at for air flow. 100% "open" might not be 100% of the value that the throttle sensor can be. Much as above.

Throttle CMD is probably Throttle Command, and is what the ECU is actually using for the maps and what you are telling it to do. At idle, it shouldn't drop to 0, since you need a bit of airflow, but at WOT 100% makes sense for what you want from it.

TPS B is probably a backup check, or fall back sensor in case it detects failure from the primary.

The ACC ones I am not sure of. The values are too high at idle, that much throttle out of gear would be pegging redline. But they match the TPS values above under WOT.

 

REL is 'relative' TPS. 'Real' is awfully ambiguous. Exactly what it's relative to, I'm not sure. That's why I asked for information.

I am pretty sure that THROT CMD is the pedal sensor.

Thank you for...guessing, I guess.

 

No problem

It will probably take some work to figure out unless someone that has hacked open the ECU (or seen it's innards), will chime in with the info. With a fully electronic throttle, it makes sense that there isn't a single value for it.

 

[sarcasm]Install new coils and an upgraded starter - this fixes everything ... [/sarcasm]

 

Nobody needs to hack open the ECU. That wouldn't tell you anything.

I just need to know what the values represent. I'm convinced that's not arcane information or protected in any way. I just need somebody who, you know, knows what they're talking about.

 

From SAE J1979 revision 2:



Here is a system diagram of a Subaru drive-by-wire system. It's probably very similar to what's in the Rx-8 because they all use the same suppliers (chiefly Denso for stuff like this).

 

Accelerator Pedal Position sensors and Throttle Position Sensors have a dual design with inverse signals for built-in redundancy.





Note that these are Ford diagrams. I can't confirm that they 100% directly apply to Mazda stuff (which is probably sourced from Denso). There are two families of APP designs: Sliding potentiometer and contactless (inductive):





I suspect that the Rx-8 uses a regular old sliding potentiometer, similar to what's used on old Honda and Rx-7 TPS's. I doubt there's anything wrong with your TPS or APP. Interpreting the signals is just a little tricky because of the design.

 

Now that is what I'm talking about. Thank you very much for the great info. I'll stop worrying about it.

 

I have your exact CP9180 scanner at home to use on cars that I don't have specialized software for. It's just reading standardized OBD II PIDS from SAE J1979.

For all intents and purposes, the commanded throttle ("THROTL COMD") is how far your throttle is open. The absolute and relative TPS readings values look all funny because of

1) the dual TPS and APP sensors, which is why there are multiple readings

2) the redundant inverse signals... they're not supposed to read the same voltage

3) the specific way that SAE J1979 requires "absolute" and "relative" values to be calculated

4) the fact that usually a throttle plate (electronic or old cable operated) is never 100% closed or 100% open. There is usually some kind of stopper in the system:

 

I have an '04 Rx8 6 speed over the last few weeks I have noticed a sufficient powerloss.
I have come to find out that my 8 is in need of a new Throttle Position Sensor.

With a tune-up and new spark plugs, it's feeling ok, but I'm not too happy with the results. The part from the dealer is way too expensive on my college student budget.

Is there anyone that knows if I can buy an aftermarket performance TPS (if one exists)? If so please notify me asap, my baby is in dire need of a new one.

Thanks.
MV

 

it is integral to the throttle body, all or nothing deal, you are likely wrong, there's no value or savings in guessing wrong and switching out good parts

 

I guess you're right. It looks like I just have to start saving up my pennies this summer to afford the new TPS.

Is there even an aftermarket part available?

 

A used throttle body from carpart.com is probably going to be your best bet.

BC.

 

So there is a 'learned' idle TPS value...I wonder when does the car learns it. One of the things about my 8 is that there is a little 'white space' in the beginning of the pedal that you have to go through before the ECU knows you are on the pedal. Along with the hesitation at low rpm makes the car a little hard to upshift holding the same speed.

I get used to it after a few days, but I have to relearn this every time I switch back from driving my Protege5. In the Protege5 the car would respond with my foot lightly touching the pedal.

 

Aesir, I moved your post here: https://www.rx8club.com/new-member-f...cement-251065/
Please don't post completely off topic. If you can't find a thread that fits, start a new thread. You are restricted to starting threads only in the new member area for a bit, but that's ok, you can still post troubleshooting threads there. Follow the link to find your previous post in it's own thread.