September 12, 200520 yr I'm not sure which forum to put this in, so I'll go with the trusty old-gen forum... I am trying to figure out how to build a device that controls the idle speed of my subaru according to the electrical system voltage. The idea is to prevent a discharge condition while idling with several large electrical loads (lights, heater, cooling fans, etc) turned on. The actual controlling of the idle speed could by done in three ways that I can think of: Some sort of manual control, like a cruise control servo unit. Tricking the ECM into thinking that the A/C is on so it kicks up the idle. Adding a second Idle air control valve or somehow manipulating the existing one. In principle, the sensing of the system's state of charge is simple, but I'm not sure how to do that. Basically, you would need a device that opens a switch at 13 volts and closes it at 14 or so. Would an external voltage regulator or some other type of device be able to do this? Any input would be greatly appreciated.
September 12, 200520 yr Well, i will not go into the wisdom of doing this, so... Start out using a precision voltage regulator, typically these are around 2volts, and use a voltage divider network so that your target voltage is just above 2 volts. Feed a comparator circuit (op-amp configured as a voltage comparator is usually easiest) with one input being the precision voltage reference output and the other being the voltage from your voltage divider network. This will give you an on/off signal that you can then buffer or amplify to drive whatever you need.
September 12, 200520 yr Author Well, i will not go into the wisdom of doing this, so... QUOTE] Not to sound combative or anything, but are you saying there is a reason that this is a bad idea? (I'm just throwing the concept out there and I am open to any criticism.)
September 12, 200520 yr Not to sound combative or anything, but are you saying there is a reason that this is a bad idea? (I'm just throwing the concept out there and I am open to any criticism.) Nope, just worried about the Law Of Unintended Consequences. If the alternator or battery fails, you just need to make sure that the idle actuator can't put the engine into a runaway acceleration scenario. I assume that the IAC method would not really allow this to happen, and the "cruise control" method would hopefully have limited actuation ability.
September 12, 200520 yr Author Okay, thanks. That would definitely be a problem if it could control engine RPM infinitely!
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