90 No Idle !
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Joined: Mar 2005
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90 No Idle !
Well, my idle problem resurfaced. Out of the blue, the car doesn't idle-
short term fix: I put a nut on the throttle stop screw to crack open the throttle. Then it idles fine, if a tad fast (900 rpm). Car acts OK through rev range.
codes: H74 for a dimmer switch circuit. I think this happened last time too.
tried: idle reset procedure
only clue: the dimmer circuit 1952 and the IAC circuit 452 appear to share the grey connector on the CCM. But the dimmer works fine now while the idle still won't.
short term fix: I put a nut on the throttle stop screw to crack open the throttle. Then it idles fine, if a tad fast (900 rpm). Car acts OK through rev range.
codes: H74 for a dimmer switch circuit. I think this happened last time too.
tried: idle reset procedure
only clue: the dimmer circuit 1952 and the IAC circuit 452 appear to share the grey connector on the CCM. But the dimmer works fine now while the idle still won't.
Melting Slicks
Joined: Aug 1999
Posts: 2,240
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From: Baltimore, MD USA
Originally Posted by ls5454
codes: H74 for a dimmer switch circuit. I think this happened last time too.
only clue: the dimmer circuit 1952 and the IAC circuit 452 appear to share the grey connector on the CCM. But the dimmer works fine now while the idle still won't.
The CCM controls the following LED circuits.
1)Fog Lamp LED
2)Rear Defogger LED
3)Radio Control Head Control Button LEDs
When the CCM is powered up, Circuit 1953/1955 is pulled low. After 20 milliseconds the CCM pulls the line high. After another 20 milliseconds the CCM pulls the line low again. The CCM monitors these low to high transitions and if it finds that all samples were low (transitions not made) Code 74 is set.
The IAC (idle air control) is controlled by the ECM and has no connection to the CCM. It has 4 wires.
The wires are on circuit 441, 442, 443 and 444. The wires go directly to the ECM.
Most likely you need to remove the throttle body, clean the idle passages on the bottom of the throttle body,
remove the IAC and clean its bore and pintle.
What manual/schematics are you using?
Last edited by Hooked on Vettes; Mar 24, 2005 at 01:47 PM.
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Joined: Mar 2005
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You're right. I was looking at the TPS circuits (too long to explain that one), not the IAC, and 1952 goes to the LCD dimmer. (1990 GM manuals)
Oddly enough, I'm still thinking the code 74 set last time this happened.
I'll give it a try.
Thanks.
Oddly enough, I'm still thinking the code 74 set last time this happened.
I'll give it a try.
Thanks.
