Toastybrainmatter

To get things started, this is a 1991 miata with a DIYPNP. I am retrofitting a VTPS and I am having trouble tracking down this problem. Tuner studio indicates throttle at 99.9% constantly if the VTPS is plugged in at all, I thought the VTPS was broken so I got a new one and it is the same case, so now I am really thinking something electrical is going on.

I have jumped TPS SIG to 4L, and VREF to 1N according to https://www.diyautotune.com/support/...3-mazda-miata/
This is the VTPS, it's a BMW type - https://speedyefi.com/product/vtps-a...-vtps-for-na6/

I started doing some janky testing by cutting the retrofit harness included in the kit and touching wires together to see if ANYTHING would happen.


I have made the wires i'm dealing with easier to see in this diagram

Touching the Light green-white wire to either black light green or red the result is 99.9%
Touching red to black light green is VERY BAD and results in everything freaking out in tuner studio

Grounding red to the engine does the same very bad thing
grounding black light green to the engine does nothing
grounding light green white to the engine MAKES THE THROTTLE POSITION GO TO 0%
The only way to make the position go to 0 is either by touching light green white with my bare hand or grounding it to metal in the engine bay.


Voltages measure (honestly, idk if I was measuring it correctly, I had one lead on the wire and the other lead just touching the engine for a ground)
light green white - 0
black light green - about 11.60
red - just under 5

The only other notable thing to say is with the VTPS unplugged, the position indicates about 50%
And I did check the VTPS with an ohm meter, the resistance does change when moving the inside part.

I pulled open my ECU to double check everything was wired correctly and it all appears correct.
I really am lost, I can't figure out what is wrong
Not sure if I am extremely overthinking this or not, but I would really appreciate some insight.

on a small note, I have noticed my tach likes to jump sometimes and I believe it was happening when the car was stock, I cleaned every ground I could find, replaced the coil pack, replaced the ignitor, and watched the cam signal in tuner studio appear normally (so it shouldn't be a CAS?). So it could be possible I have something going on somewhere...

Thanks for reading this giant post <3

Martin Y

Well the first thing that strikes me is the drawing you've annotated doesn't match what you've got, since it shows the black/light green wire grounded near the TPS and somehow you have about 12V on that wire. But the drawing must be in error anyway as it shows the grounded wire connected to the output pin of the TPS so it could never produce an output. What you should have on those 3 wires is a 5V reference (VREF from 1N) a ground reference and an output going back to 4L which swings between 5V and 0V as you move the TPS. Instead it looks like maybe the output is swinging between 5V and 12V because somehow the ground wire has 12V on it.

Toastybrainmatter

The near 12V in a wire that is supposed to be a ground got me sketched out too, that's why I wasn't sure if I even measured it correctly. There is 5V coming from 1N which is the red wire so we should be good there, nothing comes from 4L because a signal should be going to that wire from the tps, so it leads me to think something is up with the ground.

Toastybrainmatter

WELL WELL WELL...
I decided to follow the wiring harness to see where it goes, and I noticed the slightest bit of damage on this loom, it was pretty much not visible unless you pulled the whole loom up.
As you can see, the light green black wire is completely disconnected, it was likely touching one of these other messed up wires to give me such an insane reading.

I am in awe that everything in this car has functioned normally considering this corrosion, it looks like it's been like this for a while. My only question is HOW did this happen lol, I have had this car for about 3 years.
This part of the loom is under the evap canister if anybody seeing this was wondering.

Martin Y

Ah. That looks like it might explain the weird symptoms.

Mouse, I should think.