ClaytonBridges

Hello! Im fairly new to the megasquirt game, and I'm a little stuck on this Variable TPS signal. I have a 1993, entirely stock minus the wide band an innovate MTX-L, intake, IAT and MS2PNP. I swapped over a BMW variable TPS and its fluctuating its values a lot. Ive been researching the subject for hours now, and still have a few questions. Ive opened up the harness on the passenger side, and have all the wires splayed out. My readings on the TPS only vary from about .1 to .2% at the moment.

A. I've heard that people twist the wires together, and this can mitigate the EMI. is this truly effective? I've tried to twist them together, but when I go further up into the harness I reach a point where the ground (light green/black) fade into the injector harness, and CAS plug. How could I possibly twist the two wires together. which wires am I supposed to twist together if those are wrong?

B. I don't fully understand how to shield a wire. I get how it works, but how is it supposed to be grounded? would I simply splice the shielded wire into the signal wire between the ECU and TPS itself? Ive heard of home made aluminum foil shields, but still unsure how thats physically grounded.

I hope I don't come off as completely ignorant, just unsure of what to do. For the record twisting all 3 of the TPS wires and physically moving them a little bit has helped a small amount that is noticeable on the TPS on the dash, which I guess is progress.

ClaytonBridges

Okay so I got shielded wires. I got a wire that has two that runs through the shield, and the shield is foil. I ran the green/white (signal) wire through it, and the return path/ground (black/green) through it. still have the noise. I even tried to run the red signal voltage wire through it, still bounces between either -0.1 and 0.1 or 0.1 and 0.2, is this as good as I'm going to get? or am I doing something wrong? where exactly is the shield wire supposed to be grounded and is the opposite end of the shield wire supposed to connect to the other wires that run through it?

for the record I've tried running the black/green wire with the green/white wire, AS WELL AS the green/white with the red, and tried grounding on both ends (I can't guarantee they're good grounds however)

Im really frustrated at this point,

Schroedinger

Hopefully somebody with some actual MS experience drops by to help you out, but until then I'll try to help you think this through.

AFAIK the VTPS is just a potentiometer (variable resistor) that is used as a voltage divider on a 5VDC voltage from the ECU. Simple DC circuit.

Shielded signal wires are usually used in AC circuits to eliminate radiofrequency interference (RFI). This is usually low-level, high-frequency interference that becomes an issue when you amplify the signal. You have no such issue here.

It seems likely to me that your inconsistent readings have more to do with the mechanics. Is there any play between the throttle shaft and the TPS hole?

All that being said, it seems like the BMW people saw fit to use a shield over the TPS signal wire. I'm sure they have their reasons.
https://www.miataturbo.net/megasquir...pnp-faq-12239/
Most likely the entire signal run from the sensor to the ECU is shielded in a BMW, which will not be the case in your Miata. Which makes the idea of shielding in your Miata kinda pointless.

If you are going to use shielded wire, use coaxial with a braided shield. You can get good wire from places that sell parts for guitar amplifiers. Not that crappy foil shield, it's impossible to work with. The idea of the shield is to shunt the RFI to ground. Terminate ONLY one end of the shield to ground, simple enough. If you terminated both ends, you probably created a ground loop which could also be causing you issues.

DNMakinson

iTrader: (3)

1) Have you calibrated the TPS at 0 and 100%? If not, that could be part of the issue. The MS could be using the wrong ADC per percent.

2) Assuming the BMW TPS is a standard 3-pin pot, the wire should be 3 core shielded cable with none of the working signals riding on the shield. Ground the shield at the MS only. I'm not saying you should be using a shielded cable, only saying how it should be IF you choose to.

3) That level of imperfection would not concern me. You will get 0.5% variability between throttle closings anyway.

EDIT: 2000 wiring diagram shows shielding only on O2 and knock sensors.

ClaytonBridges

yes its been calibrated. I basically calibrate it with every change I make. yes it is 3 pin. By 3 core you mean 3 conductors inside the shield? Also when you say grounded do you mean onto one of the blk/lt green wires? or just that side of things physically.
I have also been informed by a few other people that 0.1% - 0.2% or -0.1% - 0.1% isnt really a big deal, and I may be jumping at shadows.

ClaytonBridges

but it seems like you guys are saying, even if I had it hooked up correclty, more than likely this is not a noise issue?

ClaytonBridges

EDIT: 2000 wiring diagram shows shielding only on O2 and knock sensors.[/QUOTE]

true but the stock TPS is pretty much a switch so I wouldnt think it needs to be shielded in that state. the engineers at mazda probably didnt assume anyone would put a BMW variable tps on it

Schroedinger

That is my feeling, yes. At least not the type of noise that would be addressed with shielded wire.

How are your soldering skills? Any chance you have a cold solder joint?

ClaytonBridges

I mean the only time ive soldered on this is connecting the shielded wire. My soldering isn't super great.
I did have to swap over a new plug and pigtail, I connected it with butt terminals, should I redo that with solder?

DNMakinson

iTrader: (3)

true but the stock TPS is pretty much a switch so I wouldnt think it needs to be shielded in that state. the engineers at mazda probably didnt assume anyone would put a BMW variable tps on it[/QUOTE]

3-core means 3 wires inside the shield.

On NB's and possibly later NA's, the TPS is a 3-wire Potentiometer.

Yes, we are proposing that you are worried about a non-issue.

Yes, to the ECU ground. Could be the same place as the 3rd lead going to the sensor, the one that is "ground".