Nobody else seems to have this problem, so I'm open to opinions on what is going on.

I have a Neptune RTP with an MTX-L and TunerViewII regular, in a 98 LS with a GS-R swap. The MTX-L and TunerView are powered and grounded from the ECU power circuit (OBD2 A11/A10). I am using D10 as the wideband input with the resitor mod.

I have a sizeable offset (around -0.6 V) to correct the ECU signal with the AFR on my gauge. However when I accelerate the offset changes, and the reading on my D10 input leans out relative to the display on the gauge. I believe this corresponds to the offset becoming less negative.

Any guesses as to what is causing this? I have replaced the engine and transmission grounds. This behavior has been identical in two cars, my current 98 LS and my old 96 GS-R. The ONLY common parts are:

-MTX-L
-TunverView & controller
-Xenocron ECU w/ NeptuneRTP
-B18C1 and its harness

 

I forgot to add that other inputs don't seem to be affected -- they hold steady regardless of load.

 

It's most likely the tolerance on the resistor you used for the D10 circuit mod. Especially if it's from radio shack. I've never found D10 to be more accurate than D14 though even with good components.
Anyway you should be using an isolated ground for the wideband, and make sure the output is correctly programmed to the voltage tables you're using in Neptune.
But if you have an MTX and a Demon why wouldn't you just use the direct input?

 

I use the direct logging for tuning. It's just that it requires my laptop to be out and plugged in to both the MTX-L and Demon. For example it means I can't do automated onboard AFR logging.

I didn't replace the resistors with anything -- I just removed them.

The output is correct for the wideband, but I can switch grounds if that will help.

 

i wouldnt call .6 a sizeable offset but I run the same offset in mine but with PLX although it is linearly fixed and doesnt change with load.

 

Some of this might be redundant but check anyway.

Make sure in Preferences > Logging you've selected the WB input as D10 and selected the correct voltage table.
Additionally you should connect to LM Programmer and re-program the same values, just in case.

If the problem is signal noise in the ECU you can compare the analog voltage at the wire to the voltage in Neptune. If that's the case just use D14.

If the voltage at the wire conflicts with the MTX gauge (i.e. if 14.7 AFR on gauge does not equal 2.45v at wire) and it is programmed to default settings, then either your power and ground are noisy or the controller is bad.