95 88 Roayle 3.8l High Idle, no IAC voltage. - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-02-2015, 10:33 AM   #1
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Default 95 88 Roayle 3.8l High Idle, no IAC voltage.

Morning guys,

Not a stranger to forums, or asking vehicle specific questions. I find the experts, and hopefully solve the problem.

I do charity work on a wide variety of elderly folks cars here in Vegas that have no business going into garages where a kid half my age wants to take them to the bank.

Have a 95 Oldsmobile Royale 88 3.8l with a very high idle, not supercharged. New IAC on engine, checked entire vehicle for vacuum leaks. Cleaned throttle body, MAF sensor port, IAC ports, bypass, PCV, and throttle plates-both sides. No avail, still high idle, about 2000, then higher when warmed up.

Checked voltage at connector for IAC, no voltage. Checked continuity from connector to wiring buss, passenger side, engine firewall, under cover. Continuity good on all four wire, blue/blk stripe, blue/white stripe, green/black stripe, green/white stripe, i.e. ABCD all good to the buss.

Key on, voltage coming out of most buss ports (at firewall connector) about 9.38v. Key off. Disconnect battery, reconnect buss wiring connector from engine loom and tighten 8mm, reconnect battery. Key on, and darn if all the voltage that was just validated at all four wire'* circuit connections disappears. I plug in the loom, and turn the key and the voltage just isn't there. Other components, MAF, TPS, all getting specific voltages. TPS validated,and functioning within spec.

When key is in the ignition, in "on" position, alarm bell dings continuously until engine has been started or key is removed.

So basically I have an engine that idles very high, an IAC that gets no voltage so it can't control idle, and an elderly lady and her caretaker cannot trust the vehicle to get them to and from. Engine runs great on freeway, or light to light as long as you don't mind riding the brake. No high temps, cooling system snug as a bug.

Cannot read codes, as my OBD II prompts error due to OBD I / OBD II confusion.

I know there is a logical explanation here. Is something else locking out the IAC circuit power ?. Is the PCM problematic on this year vehicle ?.

Thanks for the read, have a good day all, and thank you !.

A. (Las Vegas)
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Old 02-02-2015, 12:15 PM   #2
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you have to have a 1.5 scanner to read the codes. the diagram says there are four wires that go pcm to iac and thats it. if you verified them and no corrosion or loose pins on the pcm plug it seems likely the pcm is bad. its fairly common on obd 1.5'* it seems for pcm failure of different types.
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Old 02-02-2015, 04:40 PM   #3
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Default Thanks for the input !

I have chased a lot of wires in my day, and this set didn't take long to locate or validate once I got under the cover. I did continuity from connector at IAC to buss connector at firewall as I thought maybe there might be an intermediate connector along the way that might need a refresh. No such luck. So this model is one of those peculiar PCM setups, eh ?.

So the next option might be to locate a 1.5 reader, or simply bite the bullet, and acquire a $150 PCM and install it. These people I do this work for can afford a little....I save them the rest.

God Bless on the input. As for the PCM related issues, I have marveled at how many different problems root from this PCM fault of design. Interesting to say the least.
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Old 02-02-2015, 08:01 PM   #4
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Your not going to find BATT voltage at the IAC.....

Per my 1995 FSM...



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Old 02-02-2015, 11:36 PM   #5
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Thank you Mike !.
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