IAC testing

My 57 Chevy with a 89' 350 TBI is having an idle issue. I believe I tracted it down to the IAC but need to confirm as it is a new IAC valve. The car doesn't increase idle at startup or the normal idle increases. It actually will not start unless I crack the throttle. I'm getting 52 ohms through A to B and C to D on the IAC. The wiring diagram for the specific motor showed the wiring different that the connector. I switched wires with no luck. I get 10.5 volts on both sides of the 4 pin connector. How do I tell which side is high side and which is low side? The IAC does go back and forth when I switch wires around. I've been goofig with this for a year and I need to fix this. I can push the pintle in and the car fires right up reaching through the window. After a few starts the pintle close and it's back to hard starts and low idle. I need to esting information and how to determine which wire goes to which pin on the IAC. I have the wiring diagram if needed.

EDIT: If someone is looking for procedure to test IAC, wiring and ECM here is the write up and a diagnostic PDF from GM.
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What did your wiring hrness come off of? What IAC do you have flat or round?

Did you upload you wiring diagram here?

EDIT: I have neve seen one push in so it may be a bad IAC. They spin in on a gear drive.

being able to push the pintle in isn't a bad sign, it's actually a recommendation when installing a new one, along with reseting the ECM.

Here is the wiring diagram:
http://eagle-mark.com/pictures/album...normal_Fig.jpg
The IAC connector is square 4 pin. I remember pulling the block stamping numbers at it is a 350 TBI for a 1989 G-10 van. The wiring harness is also from a G-10 based on the wiring diagram. The IAC screws in but I've seen were some wiggle in. The IAC seems to function in and out.

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Originally Posted by RobertISaar

being able to push the pintle in isn't a bad sign, it's actually a recommendation when installing a new one, along with reseting the ECM.

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Weird I can't move a couple I have here?

Blue wire white stripe to A on IAC connector.
Blue wire black stripe to B on IAC connector.
Green wire white stripe to C on IAC connector.
Green wire black stripe to D on IAC connector.

HTH

Thanks, I'll give it a shot tonight.

Tried the above wiring suggestion but it didn't work. Well, I should say it worked backwards. The pintle opened in diagnostic mode (A & B jumpered). I'm going to reverse the colors and see if that works.

What ECM do you have? What BCC do you have?

Wow....haven't had time to touch my car in forever.....the ECM is a 1227747. I don't remember the BCC. I need to finish my car and get it running correctly. Any help I can get will be appreciated. Finally getting back to fixing the problem

I've played with the ECM to IAC wiring a few times, and it doesn't seem to make a difference, which I know is odd, because you would think it would work backwards, but it doesn't for some reason.

I plan to revisit playing with the IAC in the not so distant future.

Well for some reason the BB chevy has 2 wirescrossed compared to SB chevy. But I never tried to figure out why...

Six, are you talking about reversing wires across the coils?
That makes sense and it seems that all wiring diagrams for my year and the years around mine are all different. Some have the wires reversed across the coil on one side and the same on the other side.
I have wired it like the stock wiring diagram has shown and the results have been no idle adjustment from the IAC. I reversed the blue and green wires across the coils and get a high idle which would indicate that the stepper motor is pulling in and working the opposite of what it needs to. When it was hooked up to a computer the stepper motor was within spec steps but the car just wasn't running like it should so either the scanner was lieing or my ecm is but I need to make sure my wiring is correct to verify.

Maybe this will help diagnose the IAC correctly.

Thanks Eagle. Some differences in my wiring diagram and the one you posted. The wiring diag. for my motor says that A&D are one coil and B&C are the other coil. So, Lt BLu-Blk goes to D and Lt Blu-Wht goes to A. Lt Grn-Wht goes to B and Lt Grn-Blk goes to C. Buttttttt....when looking at other connectors as well as the way my original connector was wired, Blu-Wht goes to A, Blu-Blk goes to B, Grn-wht goes to C and Grn-Blk goes to D........
I'm not sure if GM didn't screw something up or something else is wrong but here is what my plan is:

I'm going to check resistance between the terminals. Across the coil pins I should read 20+ ohms, across the coils I should get infinity ohms.

All I do know is that something is not telling the IAC to open during cranking. Any ideas as to what that would be?

The IAC doesn't usually move during cranking. It does however, get parked at key off, of the previous engine run cycle. This park setting is to help the engine during start on the next run cycle. After the ECM sees the right number of DRP, it then moves to close the IAC to a point where it will idle, in correlation to engine temp.

Mine only moves when I throttle it. It doesn't move after start up. That is sort of the problem. The car is running to rich and won't idle up at startup. I'm running out of things to test.

Is your chip stock?

Any error codes?

I wonder if the driver in ECM can go bad without an error code... well you say it moves when throttled which it should. But when you turn the engine off it should open/park. Then after start go in, towards close, to control idle. At this point you may have to remove IAC or use your other one so you can see what it is doing. Then change wires to correct wiring diagram, then switch the 2 for big block and test.

Now back to Big Block wiring... only 2 wires are crossed... I don't know why... if it has to do with chip because ECM numbers were the same... or if it had to do with IAC which last time I looked was same part number... next time I have a 7747 on my test bench I will reserch this, but right now I have a 16197427 on there... if I have time I will see if they are same wiring and test...

Well here goes how the IAC is wired and works on 427 system on test bench.

Wiring is the same as I mentioned earlier.
Blue wire white stripe to A on IAC connector.
Blue wire black stripe to B on IAC connector.
Green wire white stripe to C on IAC connector.
Green wire black stripe to D on IAC connector.

May want to check your wiring at ECM to ensure correct wire color to pin number.

When key is turned off it moves all the way in, then extends a little. I beleive this is because there is a possibility of 199 IAC counts but most chips are set to park at 145.

When key on it does not move. When engine is started it moves in to controling idle position. When turn key off, back to park.

Read PDF posted earlier for GM Diagnostics of IAC.

EDIT: Forgot to add... Jumper A and B on ALDL port to enter diagnostic mode.

Use an OHM meter and check between A and B? Should be greater than 20 OHMS. Check OHMS between C and D should be greater than 20 OHMS? If not replace IAC.

Diagnostic procedure says to use test light and all termianls should light... but I do not have one other than 12 volt. They are refering to a test light that will light for any voltage. So I used a multi meter.

Testing wires from harness to IAC connector.
A should be about .5 volts.
B should be about 12 volts or batterey voltage
C should be about 12 volts or batterey voltage.
D should be about .5 volts.

If not check wires are connected to proper pin at ECM. Then check wire for break by connecting ohm meter to each end of wire for continuity. If all wires are OK and all wires are connected to proper pins on ECM? Replace ECM.

This would be exact GM test procedure from IAC diagnostics PDF I posted earlier.

HTH!

I think the 454 iac wiring has to do with the iac itself. ON the 4.3-5.7 tib's, the iac threads into the tbi and has a square shaped plug. The iac on my 454 tbi unit is held on with two screws and has a flat 4 wire connector, identical to the plug on the ignition module.
So I think the wires are all the same on the ecm/pcm side, and just two of them swapped on the 454 because of the diffrent iac. It must be internally diffrent.

Hmmmm! I think your right as I have an eary BB TBI with the biggest injectors and it has the IAC you describe... but years ago I had a harness that had the four square, regular 5.7L TBI IAC that had issues when diagnosed it had the wires were crossed, I think... man I must be getting old... insomnia hasn't helped lately... :mad1:

Yeah, I've done all those test multiple times. Ohms are good across AB and CD. Voltage is also good. My next step is usually reseting the IAC. Jumper AB to reset.
No codes present during reset. IAC pintel completely pushed out in trottle body. Remove battery cable to reset ECM.
Restart car, car does same thing as before. Fires and dies, crack throttle car starts, let off throttle car idles low, no increase in idle when cold, no increase in idle when brakes are applied, no increase in idle when steering wheel is turned.
Climb in engine bay while car started. Roll throttle IAC valve pulls in and back out to full closed. Garage fills with rich exhaust. Have to turn off before I die.

Current IAC wiring:
Pin A - blue white
Pin B - Blue black
Pin C - green white
Pin D - green black

TPS testing:
Voltage at closed throttle .51v
WOT throttle 4.0v

I may just buy another 7747 ECM and swap it to check.

I'll do the test one more time. If that doesn't work then I'll go buy a ECM locally for $50.
Josh

Well I'm glad someone is using the diagnoses I posted. Sorry your still not running right.

You know if your idle is above what it is supposed to be in chip. The IAC will close all the way! Hit throttle and it will open! That is what your IAC is doing correct? ECM is working and IAC is working!

You may have fixed the IAC but opened the throttle stop screw? Does linkage to pedal have a little play at idle? Not holding throttle open a little.

What is your idle RPM? If your throttle blades are open to far your IAC will not need to add air. Same for a vacuum leak! Quick way to see is plug you PCV valve this is just a controlled vacuum leak. Did idle come down, any IAC counts now on scantool or laptop?

Better yet what motor and trans (89 Chevy TBI)? Cam specs? Stock Chevy TBI system? Stock chip? What chip? Timing set to 0ish with tan wire black stripe disconnected? Check engine light comes on then off then on steady till car started? No error codes set? Just trying to see if all the ducks are in a row.

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You know if your idle is above what it is supposed to be in chip. The IAC will close all the way! Hit throttle and it will open! That is what your IAC is doing correct? ECM is working and IAC is working!

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Yep, that is what is happening.

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You may have fixed the IAC but opened the throttle stop screw? Does linkage to pedal have a little play at idle? Not holding throttle open a little.

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After trying all the test, car idled real low. So I did adjust throttle stop. The last shop I had diagnos it removed the cap and adjusted it.( A side note, the butterflies at idle are completely closed.) I made sure to take out the slack in my pedal when I installed the Lokar pedal. I also felt the throttle stop needed adjusted because it was not even touching.

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What is your idle RPM? If your throttle blades are open to far your IAC will not need to add air. Same for a vacuum leak! Quick way to see is plug you PCV valve this is just a controlled vacuum leak. Did idle come down, any IAC counts now on scantool or laptop?

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I don't know what my RPM is, no tach yet. It feels low until I adjusted the throttle stop. Came up a little but still low. Didn't want to open the throttle all the way with the screw...lol. My PCV valve isn't plugged into anything. I ran over the intake plenum and broke it. I have tried to close up the butterfly's with my hand with no change in idle. I will say this, that tbi unit whistles like crazy. Sounds like a supercharger.
No scantool or ADLD. tried building one but failed. Had it put on a scanner and IAC steps where correct at idle....who knows, they guy unhooked my O2 sensor to reset the timing cause he thought that was the bypass.....geeze.

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Better yet what motor and trans (89 Chevy TBI)? Cam specs? Stock Chevy TBI system? Stock chip? What chip? Timing set to 0ish with tan wire black stripe disconnected? Check engine light comes on then off then on steady till car started? No error codes set? Just trying to see if all the ducks are in a row.

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Motor is a 1989 Chevy G10 350 motor and a 2004r trans (not sure of year). Everything is stock, from front of motor to wiring harness to ECM. I tied my custom wiring harness into the stock harness (powers and grounds). I actually just reset the timing again with bypass wire disconnected. Set to 0. Check engine light is weird, I've had it come on at starting and sometimes it doesn't. While driving the car home from a show the light was off then it came on when I got almost home....almost blinded me where I have it placed. It could be throwing soft codes. No error codes produced by jumpering AB.

Question, any way to trick the ECM to think they motor is cold without the CTS hooked up. I want to unhook the CTS and make it think the motor is cold to see if idle changes to verify cts operation.

If your light came on last night there should now be a code set even if it went off. Check before other stuff below.

I don't think this is your IAC issue, but, it needs to be looked at. You need a small gap in throttle plates to bore. It's called a Min Air Adjustment. All EFI cars have it. TBI is more important because with closed blades the fuel can not enter the intake properly. It will throw everything off!

So your timing is also not set correct.... can cause idle to be above RPM set in ECM and end up with no IAC counts. If you have IAC counts? Then IAC is controlling idle, you may think its low but some cars are 550 which everyone thinks is low.

OK you said you set timing with disconnect, good. Now do the Min Air Adjustment, when you do this it will reset IAC, then check TPS voltage (it changes when you move throttle plates) to be .54 ish or up to 1 volt they say but I know you can get it close to .54 not below.

Setting timing will cause a code. Remove power from ECM or disconnect batt for a minute.

Check engine light should come on and blink once with key on (don't start) then stay on till engine is running.
Motor has to have PVC. It is a calculated vacuum leak and will increase idle!

Not sure how to trick CTS to think it's cold... disconnecting causes a error code. But when they go bad they read -40C and add choke like crazy. No code...

I think if you go through this it will run properly, can't raise idle by the screw cause it messes things up for ECM.

Whistling is the air going through IAC. Will go away when reved up and IAC opens. If you want higher idle disconnect IAC when key off, then start, should go up, exact opisite when doing min air adjustment as it forces IAC closed, then disconnest and set idle to 100 RPM lower then Idle desired setting on chip.

So came home and got started. I had to run a bypass from my CS130 alt to the starter first but that didn't take long.
Next I set min. air based on instructions from another site (basically jumper AB to get IAC fully closed, unplug IAC, start car, adjust throttle stop screw to get it close to 450rpm, turn car off, reconnect IAC and restart.)
After min air set, car started alot better. Still didn't think it adjusted idle up for a cold start but I could have not adjust min air out enough, no tach. I'll adjust it more closely when I get a tach installed.
Next I check TPS. .58V at normal position. Smooth voltage increase to 4.2v as throttle was opened.
Nexted checked CTS. It's about 60 degrees here and I got 600 ohms. That seems correct, was testing at 2000.

I can now reach in the window and start the car right up, something I could not do before. I believe the throttle plates being closed was the problem. I still want to make sure things are staying put before I get excited.

Clear codes and start it in the morning! :jfj:

Will do when I get back home after work. Light wasn't on last time I started it but might as well do it. Thanks for all the help. I think it fixed it finally after almost 2 years. We will see.

:ttwwop:

Very good/great diagnostic thread on the iacv!!