IAC Wiring

[JerrySab]

Ok, related parts question. I went to JY to pull an IAC from an '89 2.9. it ohms out within range, the valve opens and closes when supplied power through mightymax battery. all good. BUT. I did notice, the leads are in fact polarity specific. whereas, on a newer 'motorcraft' IAC, it will open and close regardless of which lead is powered and which is grounded.
Does it even matter though?
Both have D imprinted on the plug, so internal diode, I'm inclined to go with the JY iac, but I would need to reverse how the wires go into the plug.

 

[Uncle Gump]

That bulletin I listed earlier specifies that if you connect it and it doesn't move the valve... reverse the wires in the connector.

 

[JerrySab]

That bulletin I listed earlier specifies that if you connect it and it doesn't move the valve... reverse the wires in the connector.

YEP. makes sense now.

 

[franklin2]

That bulletin I listed earlier specifies that if you connect it and it doesn't move the valve... reverse the wires in the connector.

Doesn't make sense to me. A sure way to smoke the diode is to put the power wires on backwards. The backward voltage spike is reverse polarity, so the diode looks like a dead short to the spike and snuffs it out. That is why if you look at the diode, it is placed backwards in the circuit as far as the 12vdc part is concerned. Reverse the DC polarity and now the diode is conducting like a dead short, smoke comes shortly after.

 

[RobbieD]

A sure way to smoke the diode is to put the power wires on backwards.

Well, you could do brownies instead, but these days you can get diode gummies.

 

[Uncle Gump]

Doesn't make sense to me. A sure way to smoke the diode is to put the power wires on backwards. The backward voltage spike is reverse polarity, so the diode looks like a dead short to the spike and snuffs it out. That is why if you look at the diode, it is placed backwards in the circuit as far as the 12vdc part is concerned. Reverse the DC polarity and now the diode is conducting like a dead short, smoke comes shortly after.

See post 4 here...

2.9L IAC questions

So I've done a little testing on my IAC. Key ON I could feel it "click" but it didn't appear to be doing its job. I pulled it and it wasn't plugged up solid but I spent a little time cleaning it and put it back on. Same thing... click but not working. So I pulled it back off and tried bench...

www.therangerstation.com

And maybe I'm wrong... but a diode allows current flow in one direction only. If it's reversed... zero current flow.

 

[JerrySab]

Doesn't make sense to me. A sure way to smoke the diode is to put the power wires on backwards. The backward voltage spike is reverse polarity, so the diode looks like a dead short to the spike and snuffs it out. That is why if you look at the diode, it is placed backwards in the circuit as far as the 12vdc part is concerned. Reverse the DC polarity and now the diode is conducting like a dead short, smoke comes shortly after.

No it’s just on the connector for IAC. Seems Super unnecessary to me. Bottom pin (lead? Tab? Sorry my vocabulary is lagging) on 89 is ground, whereas bottom pin on 88 is power. But plug is same shape.

 

[Curious Hound]

Doesn't make sense to me. A sure way to smoke the diode is to put the power wires on backwards. The backward voltage spike is reverse polarity, so the diode looks like a dead short to the spike and snuffs it out. That is why if you look at the diode, it is placed backwards in the circuit as far as the 12vdc part is concerned. Reverse the DC polarity and now the diode is conducting like a dead short, smoke comes shortly after.

Reverse polarity should not smoke a diode. Thats their whole function in life - to only conduct in one direction. When conducting, they should not smoke and when reverse biased, they should not smoke, unless they encounter voltages outside their design parameters. That diode should be designed to handle any voltages normally seen in an automotive power distribution system, either forward or reverse biased, without being destroyed.

 

[franklin2]

When a diode is forward biased, there has to be something to control the current through the diode. The current will go higher and higher till it burns out unless there is a designed component in the circuit to control the current once the diode starts conducting.

Possibly the ECM driver circuit that grounds the IAC coil is designed not to over current the diode if it's hooked up backwards? I doubt it.

You can see in the diagram below, the diode is in the circuit reversed bias, it does not conduct when there is 12v on the red wire. Turn the diode around backwards and the 12v will forward bias the diode any time the red wire has 12v on it. So it will look almost like a dead short to the red wire, only having .7 volts across the diode. We can assume the ECM terminal will be very close to ground potiential at times to fire the solenoid. Close to 12v across the diode will zap it if it's turned the opposite way it's shown in the diagram.

 

[Uncle Gump]

Pretty much all those control circuits contain internal protection to save itself from shorts to power and ground. My guess is it will also set a dtc. Maybe to early... but the newer stuff will.

Even the tech article says pretty much the same thing I said earlier in this thread....

 

[franklin2]

They indeed must have some sort of circuit protection in the ECM since they so nonchalantly tell you to swap the wires. You can see why the valve would not work if the wires were wrong, the diode would be forward biased and be like a short around the coil making it not work.