Advice please!

[LPGC]

As far as I can ascertain my petrol fuel trims haven't deviated to the point of being problematic (it starts promptly on petrol) and I don't want to lose that.
If utilisation of the grey wire(s) is helping this should I consider just tagging the AFR meter to one of the O2 sensor wires?

This:''The mixer system may hold lambda voltage above 0.5v above 4000rpm,'' is a feature of mine. If not the 4000 rpm threshold then a (configurable) TPS threshold. That is one of the things I want to monitor (and tweak) with the AFR meter.

Worth considering leaving the grey wires connected but connecting the LPG system in the 'normal' way without the grey wires might not affect starting etc and even if it did you could easily go back to having the grey wires connected. Same regards the 4000 rpm threshold, the petrol ECU might switch to open loop before 4000rpm anyway... But if it remains closed loop above 4000rpm, and if grey wires are not used, then since the LPG system intentionally holds mixture rich above 4000rpm the petrol trims for above 4000rpm will eventually go fully lean (which may be better than fully rich but unless you've seen what trims are already and confirmed they've gone fully lean above 4000rpm already the car could drive a bit flat above 4000rpm on petrol if grey wires are disconnected).

However you wire in the LPG system, if you wire in an AFR meter it will need to connect directly to lambda signal voltage - But usually an AFR meter (at least if it's wide band) will need it's own (wide band) lambda sensor. Your truck's current lambda sensor must be narrow band if the mixer system connects to it. Some AFR meters allow you to replace the vehicle's narrow band sensor with AFR meter kit's wide band sensor, the AFR kit can then output a narrow band equivalent voltage for the petrol ECU and Millenium etc to connect to (as if a narrow band probe is still fitted).

[Pinger]

It'll be branded Millennium and will have Millennium firmware but if you look at the circuit board, it's still an AEB175, so quite what the other pair of wires are doing is anyone's guess. Can you pull the boot on the plug back and see where the extra pair of wires are connected?

Moulded on the back of the ECU box it has TIPO: AEB 175, Leonardo, Regmatic, Millenium. Previously, I identified it by the Regmatic and Millenium stickers on its front.

As for the wiring. There's a small bundle (within their own conduit) that goes to the four pins (in square formation) next to the smaller fuel level gauge/dash switch plug that has four wires coloured purple, white, orange and blue.
The other purple wire is in the middle of the plug and sits beside the only grey wire. Presumably then that grey wire is branched into two somewhere between the AEB ECU and the GM ECU but without taking the loom apart I can't confirm this.

[Pinger]

However you wire in the LPG system, if you wire in an AFR meter it will need to connect directly to lambda signal voltage - But usually an AFR meter (at least if it's wide band) will need it's own (wide band) lambda sensor. Your truck's current lambda sensor must be narrow band if the mixer system connects to it. Some AFR meters allow you to replace the vehicle's narrow band sensor with AFR meter kit's wide band sensor, the AFR kit can then output a narrow band equivalent voltage for the petrol ECU and Millenium etc to connect to (as if a narrow band probe is still fitted).

My AFR meter is designed to run from the existing narrowband probe.

Worth considering leaving the grey wires connected but connecting the LPG system in the 'normal' way without the grey wires might not affect starting etc and even if it did you could easily go back to having the grey wires connected. Same regards the 4000 rpm threshold, the petrol ECU might switch to open loop before 4000rpm anyway... But if it remains closed loop above 4000rpm, and if grey wires are not used, then since the LPG system intentionally holds mixture rich above 4000rpm the petrol trims for above 4000rpm will eventually go fully lean (which may be better than fully rich but unless you've seen what trims are already and confirmed they've gone fully lean above 4000rpm already the car could drive a bit flat above 4000rpm on petrol if grey wires are disconnected).

I think I want to leave the grey wires connected - not least as far as I can ascertain it has been OK configured thus (more on that in a moment).
GM fuelling strategy AFAIK is to go open loop not at pre-set rpm but at threshold TPS value after a period of time. Thus, sufficiently out of kilter to make preserving the grey wire worthwhile?

The AEB has both purple wires (but only one grey) and I'm assuming it better to leave it like that.
The obvious place to connect the AFR meter then is to tag it to one of the purple wires coming from a Lambda sensor heading for the AEB ECU. But for the concern any voltage emanating from the meter will be detected by the AEB ECU.
If I (initially) get the truck running and connect the lap top to it such that I can see the AEB activity ie, Lambda and stepper activity, if I then bring the AFR wire to the purple wire any untoward effects will immediately show on the Lambda and stepper activity - yes? If nothing untoward I can hard wire it. Any reason not to?

If that works, I can periodically switch to petrol running with the AFR meter and see where the mixture is ie, fuel trims. If they are wayward, I can consider the alternative wiring arrangement (run the Lambdas direct to GM ECU, tag them for the AEB ECU, tag one of them for the AFR meter and delete the grey).

Does this make sense?

[LPGC]

The AFR meter can't show a very wide range of mixtures if uses a narrow band probe because narrow band probes can only measure a very narrow range of mixtures near stochiometric.

Whether you connect the grey wire or not shouldn't have any effect on how it runs on gas but can have an effect on how it runs on petrol when you've first switched back to petrol. You may think if it ain't broke on petrol don't change it.

The AFR meter obviously has to see raw lambda signal, which is the same signal the AEB175 has to see. The AFR meter and AEB175 shouldn't pull the voltage on the lambda signal wire (unless they go wrong) but some petrol ECU's do pull voltage on the lambda signal wire in certain conditions by design, this implies another potential positive for using the grey wire. For example, some (older) ECU's will pull their own lambda voltage input to 0.5v during open loop, fault conditions, or warm-up. It's easy to imagine the ECU designers thinking that they could program the ECU to ignore lambda readings during these conditions but that might take a bit of coding and the ECU might be a bit short of memory... Or they could just wire in a simple on/off output which when on (during open loop, warmup or fault conditions) connects 0.5V through (say) a 1K resistor to the ECU's own lambda signal input. Since the ECU is probably programmed to increase fuel trims for anything above 0.45v and decrease for anything below 0.55v holding voltage at 0.5v means the ECU doesn't change fuel trims. If we disconnect a lambda sensor from many vehicles that have 0>1v lambda sensors and measure the voltage on the ECU's lambda signal input wire there's around 0.4v, which might beg the question why not 0v.

Yes any untoward effects should be visible because you can use one reading to check the other. You could even use a digital multimeter or oscilloscope to check if connecting either/both AFR/AEB175 effects readings, all voltage readings should agree same as if you'd connected 10 digital multimeters / scopes and connecting/disconnecting such component shouldn't effect the reading.

Makes sense.

[Pinger]

Thanks LPGC. It will be done as described and we'll see if it works (or not).