davew wrote:The aircon switch is a none starter anyway, you're after sustained high tickover not a 1 second pulse, which is all the aircon/drive wire gives you when it changes state.
I wonder if the 1500 RPM limit is caused because you don't have enough pulses ? If you're tickover is at 750 then the ECU will think it's getting less than 200 RPM and move the IAC to it's cranking position. ISTR the normal cranking rate for the engine is around 140 RPM and it wouldn't expect the RPM to drop below around 600 in normal operation. My guess is that it's gone into some sort of "safe" get you home mode on the IAC due to an apparently out of range input. Probably moving the IAC to a predetermined safe position.
Does the closed loop idle control still work ?
Further experimentation (all done purely on LPG) as suggested, has revealed some interesting points..
1. Removed the stepper lead when engine is off and then started up with normal connection on the flyback lead causes it to idle at about 3200 rpm. This is due to the ECU winding the stepper motor right out when the ignition is turned off (normal operation), but shows what 'right out' will do.
2. Reconnected the stepper and then tried the '1/4' connection again and as before it idled at 1500rpm (actually there is a slight creep to about 1800rpm as the vapouriser etc warm up).
3. Tried the closed loop experiment with the 1/4 lead in place (braked down to about actual 800rpm) and it appeared to work in that dipping the clutch resulted in revs rising to about 3200 (sound familiar?), but then the revs didn't fall again - just stayed at 3200. i.e. Closed loop now inoperable?
4. Turned ignition off and then tried again but idle was still 3200rpm.
5. Returned to normal wiring and idle was normal 750rpm, but when returned to '1/4' wiring idle was up at 3200rpm again.
6. Disconnected the battery to reset any fault-codes and tried again - idle was once again 1500rpm with the '1/4' lead in place.
Conclusion? Hmm... Not too sure. It does what I want, but it looks to me like it is only just 'getting away with it' and that if I load the engine enough to slow the idle speed, it will then generate a fault condition (it will think that the engine is misfiring, presumably) which will require re-setting. Since I have QR battery connectors and a cheap radio, resetting a fault condition is no problem but is obviously something I would like to avoid.
With regard to why it's running at 1500-1800rpm, it does sound likely that the air bypass valve is set to some sort of intermediate 'cranking' position.
Anyway - for now I think that I will put a relay in the signal lead so that either normal or '1/4' signals can be selected, and take the power for the air system from the same switch when I get a round tuit.
Sorry if this is boring to most people, but I love messing around like this and trying to work things out - I always have...