Hoping someone can help me...
1991 with the 4.0, 150k ish miles. Ive owned for 5 years and it has always idled real low when cold (4-500 RPM). I finally decided to fix the issue, so far Ive replaced ECT, MAF sensor, and IAC valve. I have 5 volts at ECT plug and 12 at IAC plug. Applying battery voltage (with test leads) to IAC it DOES move, but will not move when plugged into the harness. May or may not be related, but I cant get any codes to appear even if I unplug a sensor with engine running. Thanks in advance.

 

With key on engine off use volt meter on unplugged 2 wire connector for IAC Valve, should see above 11 volts, if not then computer is not able to ground it, that is its control

Open up the computer and have a look at its circuit board, look for any obvious damage, of course, and there should be 2 or 3 blue capacitors, they can fail and leak causing many odd issues
Should look similar to this inside: Ford EEC-IV

And the disconnecting and reconnecting the computer's 60 pin connector CLEANS those contacts
Check/clean any Grounds you see around the computer and in the engine bay
The above link shows the wires/pins on the connector that should show as good grounds

So does the CEL(check engine light) come on with key on?
It should

 

I do have more than 11v at the plug. 11.6 last time I checked. Check engine light DOES come on with key on. I pulled the computer apart. There’s one spot that looks like it got hot. Is that a capacitor of some sort? Do you know what it does?

I will check all the grounds in the connector

 

No, that's a diode and it looks OK , my 1994 has similar diode that has the same "hot mark" near it

Looks OK in there
I would skip that as the issue for now
Plug it back and see if that help, re-connection and clean pins

 

I checked the grounds from the ECM connector to battery ground and one pin (16) is broken somewhere. I will try track that down after work today.

 

The ECM checks out fine. The grounds are all good as well. I was looking at the plug backwards when I was suspecting pin 16 being an issue. I know the circuit runs through the injectors and TPS maybe…..is it possible my issue is elsewhere?
Plugged the ECM back in and no change.

 

Not sure what that means
Injectors get 12volts with key on from the EEC(PCM) Relay, as does the PCM(computer)
Computer grounds each injector to open it

The computer has a 5volt power supply(pin 26) that powers the sensors, like TPS, ECT, and MAF

Plug back in the IAC Valve and test voltage on the 2 wires with key on, should be the same 11volts

 

As soon as I plug the connector in I lose all voltage. Down to 16mA. Can I run new wires from ECM plug directly to the IAC plug?

 

Do this test first, to see if the 12volt wire is bad or the Computer ground wire

Connector plugged in, key on engine off
Test IAC red wire but to battery Negative, if voltage is gone then bad 12v wire

Then test "other" wire to battery Positive, should be 11v or so, but if Red wire tested OK and low volts on this computer ground then yes bad wire, or bad ECM, pin 21 is a direct wire to IAC Valve so yes it could be replaced, or tested and ECM connector

 

I was able to move the IAC by grounding the pin 21 at the ECM connector to the frame (with connector unplugged). I ordered an ECM and installed it yesterday. Fixed my problem. Thank you for the help.