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Test IAC

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Old 04-10-2017, 01:47 PM
Outlawsquirrel Outlawsquirrel is offline
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Test IAC

Hello. IAC has 2 wire plug, is one ground and one power and if so when should there be power? I thought it was powered up when the key is on?
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Old 04-10-2017, 01:48 PM
Outlawsquirrel Outlawsquirrel is offline
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OOOppppppps its an 85 f150 5.0 fi.
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Old 04-10-2017, 02:12 PM
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The IAC is pulsed.
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Old 04-10-2017, 02:34 PM
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Is there a way to check pulse, the reason i'm lookin at the iac is that with truck running i unplug it and no difference,also unplug electrical from map sensor makes no difference. truck used to run decent except for surging idle after warmup as time went on it burned richer and richer now it runs like crap, cutsout,no power. there is no scrader valve to check fuel pressure and the plug to hook scantool to is long gone before i got the truck although there are a couple wires laying there.
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Old 04-10-2017, 02:47 PM
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It actually isn't an Idle Air Control valve, but an Air Bypass Valve. This describes how it works: http://www.tomco-inc.com/Tech_Tips/ttt23.pdf
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Old 04-10-2017, 03:41 PM
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Bear with me sir and thank you for info so far. If i understand correctly the solenoid has power with key on and the ground controls the circuit?
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Old 04-10-2017, 04:04 PM
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Yes, as seen here:
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Old 04-10-2017, 04:16 PM
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Thank you sir, guess i'm to dense to understand, if i am understanding power for tab comes from eec relay but i still cant figure out if tab should have power when key is turned on or not, my interpretation is that it should and the ground is what pulses the power.
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Old 04-10-2017, 04:22 PM
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Quote:
Originally Posted by Outlawsquirrel View Post
Thank you sir, guess i'm to dense to understand, if i am understanding power for tab comes from eec relay but i still cant figure out if tab should have power when key is turned on or not, my interpretation is that it should and the ground is what pulses the power.
You are correct. And see the power spreads all over on those red wires? If you have several components that are not working, you could have a wiring problem with one of the red wires that feeds several things at once.

It also sounds like your throttle adjustment screw is too far in, the butterfly is too far open. It should barely idle when the IAC is unplugged. If it's not working at all, the engine should still barely idle.
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Old 04-10-2017, 04:28 PM
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You probably shouldn't call it TAB as that acronym has already been taken. Look on Page 60 in the upper right where it says "Thermactor Air Bypass Solenoid (TAB)". So, lets call it TABP like Tomco?

Anyway, look at Page 65 on the left side. The second paragraph says the EEC Power Relay supplies power to Circuit 361 in Start or Run. And, since 361 powers the TABP, it will have power with the key on, meaning Run.
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Old 04-10-2017, 04:32 PM
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Wiring problem makes sense, test lite grounded to battery and no power to tabp harness, would also explain why unplugging map dosent affect it. thank you very much sir, tomorrow i'll start checking wiring.
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Old 04-10-2017, 04:33 PM
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Don't miss that there's a fuselink (Z) supporting the TABP.
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Old 04-10-2017, 04:36 PM
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Ok thanks again i appreciate your patience.
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Old 04-11-2017, 11:06 AM
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I found that if i wiggle the probe from voltmeter around inside of harness pigtail to tabp i got intermittent voltage, i sprayed electrical cleaner inside a few times and now have steady voltage. i believe i have a bas tabp, i am checking it with an ohmmeter and if i am doing this correctly it apparently is bad. i have a cheap meter,when i set it to ohms no matter what range it show a number 1 on far left and when i touch probes together i get 0. Test from either prong on tabp to solenoid body and meter stays on 1, test across both tabs and it stays at 1?
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Old 04-11-2017, 11:13 AM
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Your Tabp is bad.
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