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Megasquirt Idle Control


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As part of the Megasquirt setup i'm looking at options for (full time) closed loop idle control.

I've got one of those rubbery cruise control balloon things and a solenoid vaccum valve from my old LPG mixer so was wondering if i took a vacuum line from the plenum and through the solenoid valve to the cruise control balloon. Then used the idle output from MS to pulse the solenoid using PWM.

I know it would take trial and error to get the PWM settings right but i think it would allow upto 75% throttle opening using idle control, which would be good for steep climbs, hydraulic winch loads, and cold starts too :lol:

Anyone tried this?

(Plan B is a 2-wire Audi idle valve)

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MS can drive a stepper motor (hotwire idle control valve is a stepper), which would give you a bigger range of options. The solenoid may not enjoy PWM if it's not designed for it, they like to be open or closed and giving it a PWM signal will probably not result in a very accurate system, if it works at all. Some Fords have a proportional idle valve that is PWM controlled, look in the MegaSquirt documentation for info.

You shouldn't need the cruise control gubbins for idle control unless you want something more akin to an engine speed governor, stock idle valves should be able to flow enough air to make it tick over at ~2k on their own. You can also go for inherent idle stabilisation through the fuel and ignition maps which works well, Dave Haynes who did a lot of development work was even looking at forcing the timing so far back as to try and run the engine backwards to give massive engine braking on steep descents! :blink:

His latest thoughts on the matter are on his website: www.ep90.com, one day we'll get him and SimonR in a room together and they'll probably come out with a landy powered by cold fusion or somesuch :lol:

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The solenoid may not enjoy PWM if it's not designed for it, they like to be open or closed and giving it a PWM signal will probably not result in a very accurate system

Cheers for the compliment.

You know that the head position on your hard disk drive is controlled by a solenoid, fed with PWM to select which track it's looking at? It may not work well with MS - but it is possible to achieve exceptionally high accuracy and high speed by this method.

Just my 3.14 pence worth.


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Now I'm imagining if hard drives were made to LR engineering tolerances, we'd probably have as much as ten megabytes on the average hard drive :lol:

Feeding a solenoid with PWM isn't a bad idea but aren't some built to be open or closed, just as some DC motors don't like being speed controlled? Just thinking it might result in hot coils or poor performance compared to a designed-for-the-job PWM idle valve.

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I am using the bosch IAC valve (0280 140 505) thats mentioned on the MS forums/info. It can be found on may cars at scrapyards, think i got mine form a peugeot.

Its run PWM.

Theres 3 ways you can use it with MS - warm up only, closed loop only, or both.

I found absolutely no need to use it after warm up in closed loop, as the idle can be controlled fine with VE.

But it works a treat in warm-up mode.

I will post some pics.

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  • 1 month later...

Just to update, gave it a try the other day.

Vacuum solenoid itself was quite happy running PWM at 40hz or higher, got mildly warm.

Found that the delay between the valve closing and the vacuum balloon contracting (and revs increasing) was too great, ended up with revs oscillating up and down every few seconds getting higher each time. tried quite a few different min / closed values in PWM open loop control but gave up in the end.

Plan B was a 2nd hand Audi 2-wire valve, using settings from the forum on msextra.com worked for a couple of seconds regulating the idle then shot to fully open / 1600 rpm even though Idle DC meter in Megatune was dropping down to 0% there was still voltage going to the Idle Valve? disconnected the wires to the valve and revs dropped back down.

Tried various settings for a few hours then gave up (back to the manual adjustment screw i have in the bypass pipe)

Guess the next step is trying a Bosch/Peugoet valve.

Idle torque doesn't seem the same as the Weber 500, guess i need to do more work on the fuel/ignition maps.

HoSS - I'd be interested in the settings/screenshot of you're running for your PWM valve if you have them to hand?

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Around idle you should be running fairly rich (obviously no problem when the Weber was on there :lol: ) and you can stabilise the idle by sloping the map down a bit to the left/bottom of the idle area. That means that as the idle drops, it goes lean which makes it idle faster so it goes back up and goes rich. If you make the slope too great it will hunt / cycle round but if you get it right it will idle nicely. You can do a similar thing with ignition maps for a very stable idle.

This may be daft but are you sure with the 2-wire valve that you weren't mixing + and - and 100%/0% PWM values - for example the idle circuit in MegaSquirt grounds the idle valve to operate it unless you have had it modified. Also sometimes if you have set an output to be inverted it will mean that 0% PWM would actually be flat out. Another check is that your MS has an uprated (TIP121 or similar) idle driver transistor as the original (ZTX450 / 2N2222) cannot handle the current for PWM. See step 62 of the MegaManual for info.

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Thanks. Will try tweaking the maps.

When you say mixing + and - on the valve, do you mean the polarity? I did test the valve on a battery first and either polarity seemed to cause it to open so i guess polarity doesn't make any difference?

Figured MS was switching it by grounding the negative side, most of the outputs seem to work this way.

Am a bit unsure of the range of values in Megatune, are they 0 to 100 (percent) or 0 to 255 (byte) ?

I did try telling it that closed DC was greated than minimum DC which caused the IdleDC to increase to 255% gradually rather than decrease. No matter what i tried the valve pinged open at a certain DC regulated for a second then opened fully but wouldn't close at either 0%, 100% or 255%!. Beginning to wonder if the valve is sticking. I did also have the flow direction of the valve wrong initially (didn't look at the arrow :rolleyes: ) but did correct this.

My MS was built by extraefi.co.uk, and has the uprated transistor to drive idle valves directly


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  • 1 month later...

Revisited closed loop idle today.. Success!

I needed to put a diode (1N4001) across the valve (with the banded end to the +12v supply).

Found some MS closed loop settings on a VW forum and used that as a starter, did some tweaking and got:



Takes a wee second to respond when the engine labors but then it's away, I did try reducing fast recovery time and dashpot settings but that caused an oscillating idle.

Tested it on a bank in the garden, feet off pedals, before (with no idle valve) it would stall in low 1st. Now it goes up happily in Low 2nd B)

On the way down it shuts down the valve for a little extra engine braking.

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