Electrical 1987 - IAC valve (Diode & Ohms)

R82148V

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May 26, 2020
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Hello everyone.

Just a weird question about the IAC valve, diode and Ohms and not too sure how to explain it.

I have a 87 GT vert. There is a diode in-line going to the IAC.. I've read that all remanufactured IAC valves today now come with a diode built in.
I've read that you are not suppose to have both in-line and IAC valve diodes working at the same time or you will have surging - hunting - hanging idle.
To find out if your IAC has it built in, there should be a "D" on the plastic connector on the IAC or use a DVM on ohms and probe the IAC tabs. One direction should read 7-13ohms and the other way should be infinity.
Everywhere I read, everyone says:
IAC should have 7-13ohms, if not then replace.
IAC should have a "D" on the plastic connector to have a built in diode
86-88's IACs came with non built-in diodes. The diodes are the wiring in front of the plug
89-93 IACs came with built-in diodes.
If your IAC valve has a built in diode, you need to remove the diode that is in-line right before the plug.

sounds straightforward.

What is so confusing me is, I've tested the IAC that was on the car. There was a "3D" on the IAC connector so it should have the diode built in but when I check the ohms, it reads 5.3 ohms both ways!
so I purchased a new one the other day from Rock Auto - brand Standard motor products. After receiving it, I looked for the "D". This one didn't have any markings and also ohm'd out to 5.3 both ways.
so I purchased the best possible part from Rock Auto - brand Motorcraft CX1824 knowing this is original Ford. I received it today and there is a "4D" on it but again when I ohm it, it's 5.3 both ways.
I even contacted Standard motor products customer service to see if they are actually built with the diode even though there isn't a "D" on it and they confirmed yes there is a diode.

I'm completely stumped here why they are not 7-13 ohms and don't want to remove the inline Diode if these now (3) IACs don't have them built in.
Funny thing is all 1986 to 1993 IAC valves, today, all are the same PART#'s
So that tells me an IAC for a 1987 and one for a 1991 must all come with a diode built in since 1989 and up do not have a diode in-line before the IAC plug

What do you all think?
Thank you for your time!!
 
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Mustang5L5

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If you look at the wiring for the 87 and 92 model years, you’ll see the diodes accomplish the same thing.

They are fly back diodes tasked with preventing an energy spike from the closing solenoid to travel back to the ECU. The diode directs the current back in a loop though the coil where the energy dissipates by resistance.

The later valves with the internal diode are marked with a D

1987-1988
D319AEEF-4DCE-4977-9085-45E00BB55677.jpeg


89+
F858537C-53A6-444B-8800-5BEECF203982.jpeg



Due to this wiring the later valves with the diode can be installed in earlier cars (provided the diodes run in the same direction, which isn't an issue with factory wiring) , but early valves that lack the diode should not be used in the 89+ year cars that lack the diode in the injection harness (located about 6" away from the IAC plug.

As for having two diodes in parallel, it should not affect anything unless you reverse the wiring and have the two diodes going opposite each other. If you are really concerned about two diodes, you can clip the diode in the harness out and just rely on the internal diode.

There is actually a TSB that mentioned this

TSB 89-240-4 covers this as follows:

Quote:
ISSUE: A rough idle created when the air bypass valve (idle air control) is replaced may be caused by a revision made to the valve's wiring harness. Vehicles built before the 1989 model year had a diode in the air bypass valve's wiring harness. The new replacement valves have a "D" marking on the plastic portion of the solenoid, above the connector cap, Figure 1. They have a diode in the air bypass valve. On the air bypass valve used on vehicles built before the 1989 model year, the positive and negative leads are not important to the valve's operation. However, the polarity on the new replacement air bypass valve is important because, if the wiring harness is reversed, the valve will not work.

ACTION: Reverse the wires in the air bypass valve connector and retest the valve. Refer to the EFI Engine Application Chart for engines using these air bypass valves.
 
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R82148V

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Wow this is good stuff! Both diagrams help to visualize the direction as well.
I went ahead and removed the diode from the IAC wiring harness out of precaution due to the diode already being in the IAC valve.
I'm about to perform a base idle set and will let you all know the outcome.

I also relocated my TFI ignition module to the passenger side apron. Built my own heat sink out of an old projection TV, used some Artic Silver heat paste and build my own wiring harness extension while shielding and one sided ground.
I was going to use the kits you can find off the internet, but something about building your own feels much more gratifying.

TFI HEAT SINK.jpg

TFI WIRE HARNESS EXTEND.jpg

Engine.jpg
 
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jrichker

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Hello everyone.

Just a weird question about the IAC valve, diode and Ohms and not too sure how to explain it.

I have a 87 GT vert. There is a diode in-line going to the IAC.. I've read that all remanufactured IAC valves today now come with a diode built in.
I've read that you are not suppose to have both in-line and IAC valve diodes working at the same time or you will have surging - hunting - hanging idle.
To find out if your IAC has it built in, there should be a "D" on the plastic connector on the IAC or use a DVM on ohms and probe the IAC tabs. One direction should read 7-13ohms and the other way should be infinity.
Everywhere I read, everyone says:
IAC should have 7-13ohms, if not then replace.
IAC should have a "D" on the plastic connector to have a built in diode
86-88's IACs came with non built-in diodes. The diodes are the wiring in front of the plug
89-93 IACs came with built-in diodes.
If your IAC valve has a built in diode, you need to remove the diode that is in-line right before the plug.

sounds straightforward.

What is so confusing me is, I've tested the IAC that was on the car. There was a "3D" on the IAC connector so it should have the diode built in but when I check the ohms, it reads 5.3 ohms both ways!
so I purchased a new one the other day from Rock Auto - brand Standard motor products. After receiving it, I looked for the "D". This one didn't have any markings and also ohm'd out to 5.3 both ways.
so I purchased the best possible part from Rock Auto - brand Motorcraft CX1824 knowing this is original Ford. I received it today and there is a "4D" on it but again when I ohm it, it's 5.3 both ways.
I even contacted Standard motor products customer service to see if they are actually built with the diode even though there isn't a "D" on it and they confirmed yes there is a diode.

I'm completely stumped here why they are not 7-13 ohms and don't want to remove the inline Diode if these now (3) IACs don't have them built in.
Funny thing is all 1986 to 1993 IAC valves, today, all are the same PART#'s
So that tells me an IAC for a 1987 and one for a 1991 must all come with a diode built in since 1989 and up do not have a diode in-line before the IAC plug

What do you all think?
Thank you for your time!!
Since the diode has greater resistance than the solenoid coil, you will never see it when you measure the resistance with a Multimeter or DVM. The meter will see the lowest resistance (the solenoid coil) and return that value.

Using a diode as a dampener in automotive circuits:

Theory of operation:

The diode acts as a surge absorber or one way gate. Anytime you turn the power off an inductive load (coil, motor, solenoid, etc.) the collapse of the magnetic field creates a voltage spike of opposite polarity to the voltage applied. The positive wire sees a negative spike and the negative wire sees a positive spike. The diode is wired backwards so that it does not conduct during normal forward operation. When the magnetic field collapses, the diode sees the reverse polarity spike and clamps it. This prevents the spike from possibly damaging the rest of the electronics in the car.

When used to clamp spikes in automotive circuits, connect the lead that comes from the end of the diode that has the band on it to the positive side of the circuit. Connect it the other way and you will make smoke.

Diode markings

attachments\586744\


A properly functioning diode will measure very high resistance or open circuit in one direction. Reverse the meter leads and it will measure a lower resistance.

attachments\50089\


Use 1N4005, 1N4006 or 1N4007 for the replacement diode

WWW.digikey.com. www.mouser.com or www.newark.com are good sources for capacitors, resistors and just about any other quality electronic part. Avoid Radio Shack unless it is a temporary repair or emergency situation. Almost all of their parts are less than top quality stuff and a lot of it is just plain junk.
 
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R82148V

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UPDATE

I attempted to start it this weekend and it kicked right over! Before, when I had both diodes in, on a cold start it would jump to 2000 rpm for a few seconds and then slowly come down.
Now it doesnt do that. Cold start idle is something around 1200 now and then comes down.

The Ignition module relocation worked like a dream! Stayed nice and cool to the touch. Its in line with the cooling fan so i'm sure that helps some too.

The car always idles at 1000 rpm. I set the base idle and TPS which brought the idle down to somewhere around 800 or 900..

I do have a question though. When the engine was running and I disconnected the IAC, idle dropped down to probably 700 range. I turned the Throttle plate screw out to get it down to 650 but the screw adjustment didn't seem to have any effect. I could turn it all the way out and it would still idle around 700.. I dont get it. I thought it was supposed to stall if the the screw was backed out too much???


Thank you everyone.
 

Mustang5L5

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Verify that the throttle blade can freely move and close all the way. Might need a cleaning if there is gunk inside that is preventing it from shutting.

Another possibility is that you have a vac leak somewhere that is supplying enough air to prevent the car from stalling.
 
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R82148V

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I have changed everything under the hood that pertains to vac. Gaskets, hoses, lines, you name it. Doesn't mean there isn't something leaking though.

So when the IAC is disconnected and if you turn the throttle plate screw out too much, the engine should stall.?

I'll definitely check to see if the blade closes all the way, tonight. The blade does have a small factory hole in it so I don't know if that has something to do with the engine never idling too low where it would stall.
If I turn the blade screw far out, the blade then sticks when you stab it. Almost as if the blade it stuck.
 

7991LXnSHO

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What TB is in there that has a factory hole?
Usually, I read about that as a work around for more idle air with a malfunctioning IAC and a big cam. I can pull the stock TB from the box to verify, but others can tell if I have a good or poor memory.
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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Kearney, NE
Since the diode has greater resistance than the solenoid coil, you will never see it when you measure the resistance with a Multimeter or DVM. The meter will see the lowest resistance (the solenoid coil) and return that value.

Using a diode as a dampener in automotive circuits:

Theory of operation:

The diode acts as a surge absorber or one way gate. Anytime you turn the power off an inductive load (coil, motor, solenoid, etc.) the collapse of the magnetic field creates a voltage spike of opposite polarity to the voltage applied. The positive wire sees a negative spike and the negative wire sees a positive spike. The diode is wired backwards so that it does not conduct during normal forward operation. When the magnetic field collapses, the diode sees the reverse polarity spike and clamps it. This prevents the spike from possibly damaging the rest of the electronics in the car.

When used to clamp spikes in automotive circuits, connect the lead that comes from the end of the diode that has the band on it to the positive side of the circuit. Connect it the other way and you will make smoke.

Diode markings

attachments\586744\


A properly functioning diode will measure very high resistance or open circuit in one direction. Reverse the meter leads and it will measure a lower resistance.

attachments\50089\


Use 1N4005, 1N4006 or 1N4007 for the replacement diode

WWW.digikey.com. www.mouser.com or www.newark.com are good sources for capacitors, resistors and just about any other quality electronic part. Avoid Radio Shack unless it is a temporary repair or emergency situation. Almost all of their parts are less than top quality stuff and a lot of it is just plain junk.
J. Do you still have a local Radio Shack? Amazon is the closest I can get now for cheap electronics. Thanks for posting about where to get the good stuff.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
37,338
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Massachusetts
What TB is in there that has a factory hole?
Usually, I read about that as a work around for more idle air with a malfunctioning IAC and a big cam. I can pull the stock TB from the box to verify, but others can tell if I have a good or poor memory.

Pretty sure my stocker and my Explorer 65mm have the hole in them. I'd have to look later
 
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R82148V

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From what I can tell, it's just a stock TB. Speed density 1987 vert. It doesn't look like anyone drilled the hole. I've read that some come with the hole and some don't.
When I unplug the IAC, the idle definitely drops but never stalls. It actually idle smooth. I just cant get the idle down to that 650 mark or even get it to stall. I can try to plug up the TB plate hole with my finger and see what that does.

Looks like this.

1637611877426.png
 

R82148V

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I came across something in another post about possible air leaking with the EGR valve. Between the diaphragm and plunger. To spray between there and if its leaking, the idle should slow down (a lot). This is one part I have not ever replaced. I've cleaned it a ton of time and replaced the gasket but never the valve itself. Fingers crossed this is the Vac leak we seek!
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
37,338
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224
Massachusetts
From what I can tell, it's just a stock TB. Speed density 1987 vert. It doesn't look like anyone drilled the hole. I've read that some come with the hole and some don't.
When I unplug the IAC, the idle definitely drops but never stalls. It actually idle smooth. I just cant get the idle down to that 650 mark or even get it to stall. I can try to plug up the TB plate hole with my finger and see what that does.

Looks like this.

1637611877426.png

That’s how my Stock and explorer 65mm tb look with the hole
 
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R82148V

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I looked at my BBk/Edelbrock tb has a hole in the center that appears to be blocked by the shaft :shrug:
So strange how some TB's come with the hole and some don't for the same engine. :chinI would guess and assume w/out the hold would be better for stock to allow the IAC to fully do its part, but then again if you think about it,the hole is probably there as a fail safe if the IAC fails.. At least it would run using that hole which would be technically metered fail safe air. I dunno. I'm just thinking out loud and out side the box.
 

R82148V

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After changing all vac hoses, gaskets and looked for possible cracks, I did some research of other possible non thought of vac locations. I came up with some possible vac leak locations causing high base idle when the IAC is unplugged.

1. Leaking EGR valve between the diaphragm and plunger. Need to test this theory with the engine running and spraying some brake cleaner in between the diaphragm and plunger looking for idle fluctuations.

2. Cruise Control Servo.. possible servo diaphragm disintegrating or leaking vac hose. Your CC can still work if your diaphragm is on its way out with leakage.
Will disconnect the vac line at the tree and cap it for testing.

3. Brake booster Check Valve and Rubber Gasket. possible leak

I'll keep everyone updated and I hope this might help someone.