T/S IAC Circuit... - Team Camaro Tech
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post #1 of 10 (permalink) Old Jan 14th, 04, 12:44 AM Thread Starter
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Working on an 85 Z/28 with an 89 Corvette 5.7 TPI. Engine doesn't idle steadly at any temp. No codes set, but 12 does show up at ALDL.

I have set the ECM into diag mode, unplugged the IAC harness, returned ECM back to normal and started engine. Idle is at 2200 rpm. Problem there I think. I measure the throttle opening and its .010". Base timing is at 6* BTDC.

I'm thinking its the ECM because its not controlling the IAC correctly. I measure with a DVM into the harness back to the ECM and A-B is 11.5 megaohms both directions, C-D is 5.45 megaohms both directions. Also, on C-D, there is 2.5 volts with IGN is off. The IAC itself measures 54 ohms on both paths/coils.

There are no vacuum leaks, and the engine is responsive other than idle. I select DEF or A/C, and it does take awhile to idle, its not instantenous.

Any thoughts?.....Thanks !!

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post #2 of 10 (permalink) Old Jan 14th, 04, 06:00 AM Thread Starter
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Just bringing this to the front page.......
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post #3 of 10 (permalink) Old Jan 15th, 04, 03:22 AM
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Everett,
Code 12 is normal it just means the ECM has the ability to diagnose itself.

Is this your car, was running OK and this problem developed? Or just something your working on and are not sure if it was right since the engine swap?

Are you using a Tech I scanner where you can look at IAC counts on data display? If you do make sure your on road test mode NOT diagnostic (10K) mode because the engine will default to a fixed high idle. See if the IAC counts change on data display under different engine load conditions. If the counts change your wiring is OK, if not possible broken wire. Could be just a weak IAC, slow to change (it's just an electric motor & pintle) or carbon build up in the passages or pintle seat.

I doubt the ECM is bad if it's running otherwise OK. Check the connectors & pins at the ECM to see if there is any corrosion (green growth - seen that many times).

Are you setting the timing with the distributor by pass connector unplugged? Are you setting it for the Z or the Corvette engine? I would be leaning towards the PROM on the ECM (if the IAC is OK) since this is controling the fuel/timing curves. A new GM PROM is about $40.00 if you wanted to try one for a 89' Corvette with whatever transmission the Z has in it. Possible injector size differences between 85' & 89'.

With the IAC you will have to drive the car around a little for the ECM to "learn" the various engine load conditions in order to idle correctly after unplugging with running or installing a new one.

Just some ideas to think about, keep us posted.

Drew<br />69' X-11 Coupe
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post #4 of 10 (permalink) Old Jan 15th, 04, 05:54 AM Thread Starter
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Thanks for the comeback, Drew. I agree with everything you've said. Timing done in bypass, no road test in diagnostic mode, etc.

Problem has always been there since son-in-law bought car, 1 year ago. I have no scanner(s), just the MAC card, P/N ECM-1. Even son-in-law's shop didn't find anyhing with their scanners, except the count appeared to be high.

I'm leaning towards the ECM because of the ohmmeter readings for the control circuit of the IAC. It has two coils and the circuits for both should be the same. And the count is intermittently high, son-in-law told after his shop looked at it, b/c the ECM is trying to correct the idle speed, but, IAC is slow acting due to lack of control, hence, the two different ohm readings between Coil A & Coil B.

The engine does idle, say after selecting DEF or A/C, headlights on, etc., but, after 3-4 minutes.

The control circuit for the coil is a push-pull amplifier. I would think an ohmmeter should read the same values in both directions, I'm just assuming. One cannot find electrical schematics for GM's ECM controllers.
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post #5 of 10 (permalink) Old Jan 15th, 04, 08:30 AM
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Quote:
Originally posted by Everett#2390:
One cannot find electrical schematics for GM's ECM controllers. [/QB]
Your right, they consider it confidential information!

It would be nice if you could try a used ECM from the boneyard or a known good one from a friends car before you commited on a reman unit to see if that would correct the problem.

Don't forget the PROM, I've seen dramatic changes by changing them out and you may want the 89' Corvette one in that Z. I had a 95' S Blazer (early built) that had 5 updated changes on the PROM and when I installed the newest version around 2002 it "woke it up" noticably. The GM ECM's won't come with a new PROM in them.

How's the weather at the Beach? I use to work at Oceana and lived in Moyock five years ago. Still have friends up there and relatives in Kill Devil Hills. Thought I could out run the cold by moving 600 miles South but it still gets cold here Guess I'll have to keep moving South, at least no snow yet! LOL I was in Key West on 1/4/04 and it was 75* & beautiful but mega $$$. [img]graemlins/beers.gif[/img]

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post #6 of 10 (permalink) Old Jan 15th, 04, 02:09 PM
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Hey dood, you sound familiar. Didn't I just reply to this post in another forum....Hmmmm.

Dave
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post #7 of 10 (permalink) Old Jan 15th, 04, 02:10 PM
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Quote:
Originally posted by dnult:
Hey dood, you sound familiar. Didn't I just reply to this topic in another forum....Hmmmm.

Dave
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68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #8 of 10 (permalink) Old Jan 16th, 04, 01:10 AM Thread Starter
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Yes, Dave you did. I moved the question to Electrical to get another viewing. Remember from Business class, the phrase: "Location, location, location?"

Yes, it does have Hypertech prom in it and a CalMap. It is an 89 ECM. They changed from electric carb, 85, to 89 Corvette TPI. I still do have the stock chip. Dealerships here get $75 USD for downloading a 89 ct chip. I know, I'm paying for the technology & engineering and don't do custom, read basxxxd, er, hybrid, set-up like this one.

Drew, weather here is cold, 25*, clear sky and breezy for the moment. Maybe some precip later. Rain on Sunday w/40*. I live two miles east of Oceana. I love the sound of jets. But, I'm awaiting the arrival of 80 Super Hornets this summer. I may have to wear flight deck ear muffs while doing the lawn. Ha!

Thanks guys for your responses, go see orench in P. Rico, have him put your cold drinks on my tab, see Juan.
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post #9 of 10 (permalink) Old Jan 16th, 04, 11:14 AM
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You can buy the equipment to program your own Proms for $75. You'd be surprised how minimal the changes that Hypertech makes are. It would plumb pi$$ you off.

Dave
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post #10 of 10 (permalink) Old Jan 17th, 04, 02:13 PM
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On GM marine EFI units there is an IAC functionabillity test that's real simple. Ensuring the connectons and wire harnesses are in good order with a DVOM (okay, ohm meter / continuity tester) is step one which you have performed.
Step two is taking a 12V powered test light, with the wire end on a good ground, probe all 4 terminals, one at a time all should flash... some faster than others... if they do not flash the problem is with the ECM..

I'm not 100% sure this is correct in this earlier automotive application, but it'd be easy to check and compare to a known good car IAC circuit

John...
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