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Throttle Position Sensor Issue

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#1
jspicer

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I have read many posts here that discuss tapping the OEM TPS output (green/blk wire on a NB2) signal for Aim and other data acquisition systems (w and w/o 1000ohm resistors). I have been constructing a DIY data acquisition system. All works (RPM, brake and oil pressure, temp, etc.). The last circuit I'm working on is the TPS.

 

I tapped the line and measured it with voltmeter to confirm range of output (about .6 to 3.7V). Next, I used an OBDII reader to see range from PCM. First surprise - it reads 11% idle to 76% full. Holy s!!! I must be down on power!! Swap out a spare and it's the same - tested by both hand at TPS and pedal. Sigh of relief upon determining it's normal and I keep going.

 

I attached the TPS signal to my system (see below for description of system). My system apparently changes the load and signal the PCM sees and the TPS range from the OBDII is now 5% to 58%.

 

Question: Does this impact on PCM TPS readings happen when connecting to Aim, Traqmate, etc.? I also read online that the PCM reads the differential of idle to full open, not absolute, so 5% to 58% could work as well as 11% to 76% if this is true. Any info on this? I would have driven the car to directly test this idea but live in a very well patrolled town.

 

 

System Description:

I used a STM32 "blue pill" microcontroller. It is interconnected with a 9 axis accelerometer/gyro/magnetometer chip. The STM32 interfaces with a bluetooth transmitter chip to send data to a phone or tablet for logging. GPS can come from the phone, an external bluetooth GPS or a dedicated GPS connected to the system. I have brake and oil pressure signals from dedicated sensors. I tapped the instrument cluster temp gauge wire for temp signal (this sensor/signal is independent of temp sensor for PCM and doesn't impact PCM). RPM signal is also from cluster. This is about $20 of chips from Amazon. The whole thing will cost about the same as an Aim brake pressure sensor when done.

 

This has been a good project while I'm being socially distant.

 

Thanks



#2
RWP80000

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I attached the TPS signal to my system (see below for description of system). My system apparently changes the load and signal the PCM sees and the TPS range from the OBDII is now 5% to 58%.

 

Question: Does this impact on PCM TPS readings happen when connecting to Aim, Traqmate, etc.? I also read online that the PCM reads the differential of idle to full open, not absolute, so 5% to 58% could work as well as 11% to 76% if this is true. Any info on this? I would have driven the car to directly test this idea but live in a very well patrolled town.

 

Thanks

 

The GCR section for the SM class is 9.1.7    You can find the sections for the Engine Control requirements in section 9/1.7.C.1.o. paragraphs 2., 5., 6., & 7.

 

Paragraph "o." 2. is in reference to the use of an aftermarket supplied replacement being carried by Mazda in their Competition Parts catalog with the paragraph specifically stating that the harness must be used in its entirety and may not be modified in any way.

 

Paragraph "o." 5. states that the ECU and harnesses must be as supplied by Mazda and that No circuit modifications are permitted but does allow damaged harness wires and connectors to be repaired by splicing OEM equivalent connectors and leads.

 

Paragraph "o." 6. addresses that all sensors related to engine operating parameters must be used and must be stock Mazda parts.  It also requires these sensors to be retained in original mounting brackets and locations and that the wiring harness leads may not be altered except as allowed in the section below, which is Paragraph "o." 7..  This paragraph addresses an issue related to know problems with the OEM cam position sensor/connector integrity and allows directly soldering wiring leads to the cam position sensor pins along with "potting" of these leads and substitution of an alternate three pin connector in place of the OEM harness cam positions sensor connector.

 

Based on the above, tapping into the engine harness cam position sensor wiring leads for a signal is not an allowable means of obtaining a signal for your data acquisition system.  Under Paragraph "o." 6. " Data Acquisition sensors may be added." This would mean that you could utilize an addition TPS sensor and bracket or a 'string pot' but the wiring for these would be independent of and separate from the Engine and Emissions electrical harnesses.

 

Rich Powers



#3
jspicer

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I figured out the solution. I included an op amp to isolate the signal from the measurement. Using an OBDII reader, the TPS signal now reads the same with and without the data system connected.

 

Rich - I've read multiple posts regarding tapping the TPS wire which appear to have come from Spec Miata racers in this forum and others reflecting your comments, i.e. harness modification is prohibited. I believe the rules are pretty straightforward. As a counterpoint, I believe "nearly everyone" taps the tachometer wire to get its signal. Ultimately, I'll worry about it when I start worrying about being impounded - i.e. after much improvement.

 

I’m playing a diy game with myself. I’ll probably buy an aim when it’s all done. I just like the build as much as the driving. It's mostly done and tested in the garage. I'll get it tested on the track when I can which may be a while in this environment. I'll post it after that. Probably more a lemons/champ type of solution for diyers.

 

John Spicer



#4
RWP80000

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I admire your inginuity and engineering accumen with this project and I am not trying to discourage your creativity.  There is no problem in tapping into the instrument cluster harness to obtain the tach signal or to get the temp sensor signal as this is not considered to be part of the Engine and Emission system harnesses.  It is also acceptable to obtain information from the OBDII connector.  This can be used to provide data to a data acquisition system as well. What the rules are set up to do is prevent manipulation of the Engine and Emission control systems through creative methods that interact by modifying the sensor outputs in order to change what data the ECU has for processing.

 

I know there are numerous competitors who are or have tapped into the engine harness for the TPS signal but as the rules are currently written it is not an approved method within the rules.







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