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  1. #26

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    Confused to a certain degree does this mean some have 3 wires going to the harness? I have to look at mine but will cut out the diode and maybe this will fix my strange idle
    Ben
    67 camaro convertable project
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  2. #27

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    No because power still needs to be supplied to the IAC, all that is being stopped is any communication between the diodes.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  3. #28
    FEP Power Member Ourobos's Avatar
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    ^ my thoughts exactly. Once I opened up my harness and saw how the dioode was in there, it was a no brainer which to connect back.
    1986 CHP SSP Coupe

  4. #29
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    Okay, I just want to back up a little back after re-examing all of the posts. I think I may have been misunderstanding the initial post. It sounds as though you have an upgraded IAC that already has the diode built in AND you were using an older style harness that had an inline diode. From what I am gathering, the inline diode was battling with the built in IAC diode so you simply inactivated the inline harness diode to let the built in IAC diode do its thing unopposed. On my bone stock 1986 GT, the only diode that is there is the external inline one. My question now is, do I really have to do anything with this external diode if I am running the stock 1986 IAC? It would seem that the answer would be no. From what I gather, the TSB may have been to address IAC problems if someone upgraded to the newer IAC but left the other diode in place also. In short, the mod you described only needs to be done if the IAC is a newer replacement one and that this really doesn't apply to original stock 1986 IAC, which doesn't have the diode built in.
    Last edited by HAD85B4; 06-14-2012 at 01:12 PM.

  5. #30

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    You got it....

    stock IAC=leave harness diode ALONE

    newer model IAC AND 86 (possibly early 87)=eliminate harness diode

    Again this only applies to my car which is the 86 GT harness V8 after I had exhausted all the high/surging idle checklist possiblities.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  6. #31
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    From what I have read, the newer IAC's with the built in diodes started around 1990 model year. Do some of the 1986 and 1987 cars have these from the factory or are they replacements that may have been done for idle problems?

  7. #32

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    My Ford tech buddy and I came to the conclusion that Ford probably saw money savings as they moved into the new EFI era and streamlined stuff such as this later on during the learing process. Hence now all that is available is the IAC's with the diode built in as it wouldnt be cost effective to build each part that is the same except for one thing internally.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  8. #33

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    is there any way to tell if the iac has the diode built in?
    00 GT Terminnated
    86 GT Vert 9L
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  9. #34
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    Quote Originally Posted by kcobra00 View Post
    is there any way to tell if the iac has the diode built in?
    Yes. A diode is like check-valve for electricity.. it allows flow in one direction only. Use an ohmmeter to check the resistance of IAC valve. If it has a diode in it, you should get a reading with the leads hooked up one way (no idea what normal spec is - probably like 5-10 ohms) and a very high reading or OL (out of limit) or open circuit with the test leads switched. If you get the same reading in both directions, there is no diode.
    Last edited by tripice351; 06-15-2012 at 01:52 AM.
    Marc aka tripice351. 84.5 GT-350 2.3T.. and now, 1982 GL coupe.

  10. #35

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    Wow this was great info thanks. I did inspect mine and it had no diode (harness) and my harness came off of an 87 I will have to check the IAC to make sure it is correct
    Ben
    67 camaro convertable project
    86 gt project (for sale)
    90 porsche 911 (sold)
    09 toyota tacoma
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    mustang parts for sale http://vb.foureyedpride.com/showthre...42#post1406542

  11. #36

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    Quote Originally Posted by tripice351 View Post
    Yes. A diode is like check-valve for electricity.. it allows flow in one direction only. Use an ohmmeter to check the resistance of IAC valve. If it has a diode in it, you should get a reading with the leads hooked up one way (no idea what normal spec is - probably like 5-10 ohms) and a very high reading or OL (out of limit) or open circuit with the test leads switched. If you get the same reading in both directions, there is no diode.
    thanks! that's what i needed to know
    00 GT Terminnated
    86 GT Vert 9L
    00 SVT contour

    Past cars
    92 coupe 2.3t conversion, 85 SVO 1C, 87 turbo coupe 1C, 88 turbo coupe 1C,98 svt contour black

  12. #37
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    Quote Originally Posted by tripice351 View Post
    Yes. A diode is like check-valve for electricity.. it allows flow in one direction only. Use an ohmmeter to check the resistance of IAC valve. If it has a diode in it, you should get a reading with the leads hooked up one way (no idea what normal spec is - probably like 5-10 ohms) and a very high reading or OL (out of limit) or open circuit with the test leads switched. If you get the same reading in both directions, there is no diode.
    This site is awesome for information but I'm really going to have to get a dictionary that translates stuff like this into English...

  13. #38
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    I just tried this and it didn't work? I pulled the pins out of the connector, switched them to opposite sides, plugged it back in, and now the car wont start?

    Could this have blown one of the diodes? Any Ideas on what to do now?
    Last edited by 86 GT Vert; 06-15-2012 at 05:11 PM.
    1986 GT Convertible Daily Driver.

  14. #39

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    Which IAC do you have? When I unplugged my IAC to rule it out it wouldnt run. Id say you have the original style IAC that doesnt have the diode thus you need to put it back. Just remember this is the last resort after exhausting all other poor idle contributors.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  15. #40
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    Quote Originally Posted by 86 GT Vert View Post
    I just tried this and it didn't work? I pulled the pins out of the connector, switched them to opposite sides, plugged it back in, and now the car wont start?

    Could this have blown one of the diodes? Any Ideas on what to do now?
    Yikes. As I noted above after re-reading the original post, the fix he outlined are for cars that are equipped with an external harness diode AND an upgraded IAC that has the built in diode. Does your car have a stock setup? If so, chances are your IAC doesnt have the built in diode and you will probably have to put things back.

  16. #41

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    +1.....I know when I was narrowing to the IAC as the suspect area like I said when I went with it unplugged my car would not run but I have far from stock so Im sure that was part of it.

    I will add another bit of help on leaky seals, oil in the intake, etc. So I did some reading and was ready to set up inline filter catchers to stop oil getting sucked in the intake. I have the baffles removed for my stud rockers for clearance. I changed out to stock Ford PCV for the one for the 70 Boss 302 which has much lower vacuum. My vacuum was on 5-7 when I measured it which is WAY lower than the stock PCV is meant for thus, tons of oil was getting pulled because it was never closing. So having the incorrect PCV for your specific engine is essentially a controlled vacuum leak. Just thought I would add that from recent discovery before this one.
    Last edited by QisJob1; 06-15-2012 at 10:17 PM.
    85 GT Vert' (CFI converted to EFI), 306 TFS 170's, Systemax II, Lunati cam, 8.8 w/3.73, Ford Racing T-5, AFM Powerpipe
    317 rwhp 304 rwtq

  17. #42
    FEP Super Member 86asccoupe20's Avatar
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    I have the diode. I believe I have a later iac but need to check the ohms.Maybe tomorrow.
    Stan
    1986 Ford Mustang Saleen #80 Gray/gray
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  18. #43

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    Quote Originally Posted by QisJob1 View Post
    So my 85 CFI was converted to SEFI using an 86 GT harness years ago. Slowly the dreaded hanging and surging idle has gotten worse with all the upgrades even after Mass Air conversion.

    I tried everything....vacuum lines all replaced, gaskets all replaced, charcoal cannister. Reset idle, checked codes....on and on and on. And beer...lots of beers to offset hair pulling aggravation.

    Then today my Ford tech longtime friend and I were in the garage and I told him I just didnt get it and I just didnt get it. He said, hey ya know what...that diode in the IAC wiring harness could be the problem. He said there was a TSB years ago for that, the harness diode fights the IAC's because all the updated/new IAC's have the diode built in. The TSB says to swap/reverse the wiring in the connector to the IAC. We never did that in all the years it was present....guess what, firing up smooth as a baby's bottom!!!

    Most if not all the IAC's now have the IAC built in....if you have the diode either remove it or switch the wiring into the connector to deactivate the harness diode and see what happens. Oh man I think Im just gonna go out to the garage and crack open a few and watch it idle for an hour!!! Hope it helps at least one other on here.....

    So after countless attempts at fixing the hunting/surging idle on my 86 I found this thread. Removed the diode (had already replaced IAC) and now my car idles like it should. I went the route of snipping it out and soldering the wires back together, hopefully my sketchy idle problems are finally gone.

  19. #44
    FEP Senior Member dtmilsap's Avatar
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    Where about is diode located. Have idling problem. Iac/act is located in cold air intake.

    Sent from my SAMSUNG-SM-G900A using Tapatalk

  20. #45

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    I'm curious about this myself...
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
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  21. #46

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    Quote Originally Posted by dtmilsap View Post
    Where about is diode located. Have idling problem. Iac/act is located in cold air intake.

    Sent from my SAMSUNG-SM-G900A using Tapatalk
    it wired into the connector coming off of the idle air controller.
    Name:  iac diode.jpg
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    mine was in the wiring harness (not my engine but the best picture i could find)

    Name:  sr-throttlebody-8693-install-06.jpg
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    Last edited by erockk; 10-05-2016 at 10:18 PM.

  22. #47
    FEP Senior Member dtmilsap's Avatar
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    After removing cover this what I found. installed new idle air control about one month ago. Should I cut red and reconnect them and just remove yellow.

    Sent from my SAMSUNG-SM-G900A using Tapatalk

  23. #48
    FEP Senior Member dtmilsap's Avatar
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    Found this diagram today

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  24. #49

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    Interesting! So, in theory since my harness is 1990MY, it shouldn't have that.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  25. #50
    FEP Super Member 84StangSVT's Avatar
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    Quote Originally Posted by dtmilsap View Post
    Should I cut red and reconnect them and just remove yellow.

    Sent from my SAMSUNG-SM-G900A using Tapatalk
    That would be correct. Remove the yellow wire and fix the spliced area so it does not ground and then remove the red wires and connect them together.
    Brock
    1984 Mustang LX Convertible 3.8L V-6/Auto (SOLD)
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