What's this then?

[Gilbertd]

Car is a '96 GEMS 4.6 P38 Range Rover fitted with an OMVL Millennium single point system. While tidying the wiring I found a small black plastic box tucked down between the ECU housing and the battery. It looks like it had a label on it once but all it is now is a piece of blank paper. The wires on it are connected as follows:

Red connected to the ignition switched supply (from the feed to the petrol injectors)
Black I assume goes to ground as it disappears into the LPG system loom
Blue is connected to the blue wire on the LPG system so is powered only when running on gas
Yellow and brown wires intercept the ignition retard line between the engine and gearbox ECUs. This uses a 12V PWM signal to retard the ignition timing to reduce torque during gearchanges.

So what the hell is it doing and why? The car drives no differently on petrol or gas and gearchanges are fine on both fuels, no different to my car with a similar LPG system but without this mystery box.

[CNG]

Later version has a different case, so wouldn't swear, not seen any early one for years, but it looks not unlike an early 'AEB Joker' advance box, but you reckon this retards the ignition?

[Gilbertd]

The line it intercepts is used to retard the ignition (see below), whether it is possible to advance it over that same line I've no idea. Without doing back to back checks with and without it connected while monitoring the ignition advance, I can't think of any other way of checking it. As I've now moved it to inside the ECU housing and soldered and heat shrinked all the connections, that isn't going to be a simple job.

From the Auto gearbox section of the electrical troubleshooting manual:

Interface to Petrol Engine Control Module (ECM)
(Z132)
The Auto Gear Box Control Unit (Z255) takes the
engine torque, engine speed and throttle angle
signals from the ECM, calculates which gear is
required, outputs to the solenoid valves and sends a
torque reduction signal back to the ECM, thereby
retarding the ignition point on each of the cylinders
and so giving a reduction in engine power during
gear changes that result in a smooth change.

[Tinleytech]

I think that is a fix made by Autogas Developments in Shropshire to reduce the chance of backfire on gear change which can be a problem with a mixer on P38

[Gilbertd]

Thanks, sounds like I can take it off then. I'll have the lid off it first though to see what is in it.