how can you test the idle air sensor
how can you test the idle air sensor my car doese not idle good. The live scan showed nothing from the iac sensor. I unpluged it and it did not put a code up. Is there a way to test it and see if it is bad???
IAC is a small stepper motor. There are four drive phases
that are chopped by the PCM. If you put a voltmeter from
any one of them to ground, at idle, you ought to see a
voltage in the midrange (couple of volts). If you had a
PCM swapped in from a non-IAC vehicle like an electric
throttle platform, you might have no drivers on the PCM
and see only millivolts of electrical noise, with the IAC
plugged in (use needles to pierce the insulation and
probe the signals, or poke at PCM red pins 76-79).
IAC will probably not show a code, you can unplug it
and the plunger will just sit wherever it was at.
If it had an effect it would just be idle speed.
Look at your idle trims (STIT, LTIT and if they are
drifting way out and you are not on target for idle
RPM, that implicates the IAC though an air leak
(vacuum leak) of substantial size can also overrun
the IAC's authority (can't flow less than zero).
Similarly a ported TB where the IAC port has been
epoxied, may choke flow below what the IAC and
tables "think" should occur, making the idle speed
either unstable (saturated, no more flow for more
IAC steps) or low.
If you have messed with the TB such that the TPS
voltage reads high, or the TPS is showing more than
0%, there is a chance that the PCM thinks you are
not at idle (closed throttle) and is not trying to bump
the stepper motor. This can be a catch-22 for idle
tuning, car won't run unless throttle is jacked open
but can't learn to run on its own until throttle is
properly closed.
If you think the IAC is fried internally you can measure
winding resistances on the two coils. Coil A is blu/wht
and blu/blk, Coil B is grn/wht and grn/blk. Should look
like some reasonable low ohms (tens to hundreds I'd guess
but have no specific info). Measure with the IAC
unplugged.
that are chopped by the PCM. If you put a voltmeter from
any one of them to ground, at idle, you ought to see a
voltage in the midrange (couple of volts). If you had a
PCM swapped in from a non-IAC vehicle like an electric
throttle platform, you might have no drivers on the PCM
and see only millivolts of electrical noise, with the IAC
plugged in (use needles to pierce the insulation and
probe the signals, or poke at PCM red pins 76-79).
IAC will probably not show a code, you can unplug it
and the plunger will just sit wherever it was at.
If it had an effect it would just be idle speed.
Look at your idle trims (STIT, LTIT and if they are
drifting way out and you are not on target for idle
RPM, that implicates the IAC though an air leak
(vacuum leak) of substantial size can also overrun
the IAC's authority (can't flow less than zero).
Similarly a ported TB where the IAC port has been
epoxied, may choke flow below what the IAC and
tables "think" should occur, making the idle speed
either unstable (saturated, no more flow for more
IAC steps) or low.
If you have messed with the TB such that the TPS
voltage reads high, or the TPS is showing more than
0%, there is a chance that the PCM thinks you are
not at idle (closed throttle) and is not trying to bump
the stepper motor. This can be a catch-22 for idle
tuning, car won't run unless throttle is jacked open
but can't learn to run on its own until throttle is
properly closed.
If you think the IAC is fried internally you can measure
winding resistances on the two coils. Coil A is blu/wht
and blu/blk, Coil B is grn/wht and grn/blk. Should look
like some reasonable low ohms (tens to hundreds I'd guess
but have no specific info). Measure with the IAC
unplugged.
You can use the tech 2 and use the special functions to control the idle and see the readings. I don't think any of the cheaper non GM scan tools will let you do it. You can also take it off and trying spraying some brake clean through it and tapping on it to see if it gives you any results but that'll only help if its carboned up.
