IAC problem (Page 1/1)
larry mimbs APR 10, 04:54 PM
88 base notchback 2.5l IAC does not work. New IAC, wires check good to ecm with ohmmeter, throttle body passages completely clean, Snap-on scanner shoes IAC signal fluctuating like one would expect. Idle not changing. Idles dead slow (about 600 rpm) and dies frequently when stopping in traffic. Does the IAC operate at 5v. or 12v.? Is it safe to check ecm output at IAC connector with test light? Instructions with new IAC said drive over 35 mph for some time to calibrate, but some sources seem to have a bit of different info about that. Any info appreciated.
theogre APR 10, 07:24 PM
Many ECM parts are 5V including TPS but I think IAC is 12V.

ECM scanner can see ECM is sending X but ECM hardware says not happening.
One Typical fail is 87-88 Duke ECM not turn on injection because Driver(s) on the ECM board have problems.
Just 1 crack solder joint is all it takes to "Kill" a ECM.
All four IAC lines are driven by one 12V chip and maybe is "dead" for same reason or dead outright.

Meter or generic light might help or not because IAC is a Stepper motor and maybe one or both coils are off when tested. Worse is likely easy to fry the chip if you short the lines. For this reasons... Can get a Noid light for IAC sim to injector Noids but have 2 lights to check each coil feed.
88 TBI is 700 type and noids will match the connectors for each part so doesn't damage them.

IAC reset is drive over 35mph or car parked and throttle above 2000rpm for some seconds when ECM is scanned. Watch IAC/idle reset yes/no when you push the gas.

When you get another ECM save the PROM and move it to new one.
See my Cave, ECM Heat (and rest of section) because Heat doesn't help either even when ECM is replace.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

larry mimbs APR 10, 08:11 PM
I did a little more testing. When the scanner puts the system in diagnostic mode, there is voltage on one terminal only at the IAC connection (green/wht, I think). Test light brightness would seem to be 12v. Seems like driver problems in ecm. Scanner shows ecm varying IAC signal, but no actual output. (I think!)
theogre APR 10, 11:17 PM
Again... IAC is a stepper motor.
Look up that to find how it works.
larry mimbs APR 11, 10:47 AM
Thanks. I understand the stepper motor operation, but is it correct that in diagnostic mode the ecm will power up all four terminals at the IAC?
theogre APR 11, 10:14 PM

quote
Originally posted by larry mimbs:
Thanks. I understand the stepper motor operation, but is it correct that in diagnostic mode the ecm will power up all four terminals at the IAC?

No.
Control chip will Never turn on 12v to all 4 pins because it can't. No stepper chip would allow this.
Chip itself may turn on only 2 wires max at a time when turning the motor. the rest of the ECM may not even do that. Which wires depend on direction.
Only 1 wire needs power to Hold the motor at one position.

Meter should see + OR − 12v for each coil when moving.
The rest of time you might only 1 w/ power or none.
larry mimbs APR 12, 12:18 PM
Thanks again for good info . Can any 87-88 ecm be used with transfer of original prom?