Rough low Idle

[RonD]

Yes, ECU(PCM) pin 83 should how about 8volts engine running, or 12v key on engine off.

Wire from IAC valve to pin 83 OK??

Put a test light on the IACs 2 unplugged wires, turn on the key, test light should light up with full 12volts.
Start engine, should see test light getting dimmer and brighter if ECU is pulsing ground to close IAC Valve.

Yes, 1995 to 1997 should be OK, 4.0l Automatic, 2wd or 4wd won't matter, EGR does, so get what you have, i.e. w/EGR or wo/EGR

 

[ratdude747]

Yes, ECU(PCM) pin 83 should how about 8volts engine running, or 12v key on engine off.

Wire from IAC valve to pin 83 OK??

Put a test light on the IACs 2 unplugged wires, turn on the key, test light should light up with full 12volts.
Start engine, should see test light getting dimmer and brighter if ECU is pulsing ground to close IAC Valve.

Yes, 1995 to 1997 should be OK, 4.0l Automatic, 2wd or 4wd won't matter, EGR does, so get what you have, i.e. w/EGR or wo/EGR

Measuring from battery positive (ground switched circuit) 0V at pin 83 (pin in wire at plug) in both cases with the IAC Valve plugged in. Open circuit I'm getting 12.1V key on engine off. Since the valve measured as good (10 ohms), that means that something between the ECU end of the wire and the switching transistor in the ECU (electronic component that controls current) has high resistance (or there something blown in the ECU and the 12V is leakage through some sort of protection circuitry). Since the ecu connector looked to be in good shape and the contact for pin 83 looked nice and shiny, I pretty much have to conclude that the ECU has fried somewhere.

I did crack it open, didn't get too far (4 years of Electrical Engineering Technology School only gets one so far without proprietary documentation).


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Noted on the EGR. I might need to run to PAP sometime soon as ebay pricing is crappy right now (not to mention nobody mentions EGR or lack thereof).

 

[RonD]

Did you measure voltage at pin 83 with key on, it would only be a Ground with key on and ECU powered up.

Also measure pin 83 with key on and meter Grounded, should for sure see 12v if wiring is OK
(Battery)12v---ECU(PCM) Relay---------------IAC--------------pin 83------meter---ground

 

[ratdude747]

Did you measure voltage at pin 83 with key on, it would only be a Ground with key on and ECU powered up.

Also measure pin 83 with key on and meter Grounded, should for sure see 12v if wiring is OK
(Battery)12v---ECU(PCM) Relay---------------IAC--------------pin 83------meter---ground

That's not how mine is wired according to the schematic- I referenced mitchel ones from this thread. According to that (4.0 schematic 4/4 and 3/4) the IAC is between the PCM 12V supply (along with the MAF, Injectors, Etc.) and the PCM itself on pin 83. No relay. Thus, it's a ground side (NPN) switch, so the correct way to check is between 12V and the PCM conenction. I did verify that the 12V supply to the IAC is good.

Thus, since I'm getting a good ground on pin83 (at the PCM connector) only with no load, and with no apparent issue with the PCM connector itself, I reason that something in the driver (switching) transistor in the PCM has fried and not able to sink good current. The little bit that shows up with no load could be protection circuitry inside the PCM sinking trace amounts.

 

[RonD]

I would recheck your diagrams

PCM(ECU) relay sends the 12volts to ECU, fuel injectors and IAC valve

Pin 83 at ECU is the Ground for the IAC Valve

 

[ratdude747]

I would recheck your diagrams

PCM(ECU) relay sends the 12volts to ECU, fuel injectors and IAC valve

Pin 83 at ECU is the Ground for the IAC Valve

That's what I said? I thought you were saying that in the last part that the ECU controlled a relay that controlled the valve, which based on this post we can agree is NOT the case.

Since I'm getting good voltage (>12V) at the PCM relay connection to the valve with the valve connected (key on, engine on or off), I'm pretty sure the issue is NOT that connection, which is why I followed the ECU to valve (ground) side connection.

 

[RonD]

Yes, that part we agree on

The wire from IAC to pin 83 was what I was concerned with.

You tested pin 83 from battery 12v to pin 83 and got 0volts(key on?) which would indicate issue inside ECU, because that should be a ground with key on

I was suggesting testing pin 83 to Ground with key on, that should show 12v if wire from IAC to pin 83 is OK,
because the 12volts would be flowing thru IAC Valve(10ohms) to pin 83

 

[ratdude747]

Yes, that part we agree on

The wire from IAC to pin 83 was what I was concerned with.

You tested pin 83 from battery 12v to pin 83 and got 0volts(key on?) which would indicate issue inside ECU, because that should be a ground with key on

I was suggesting testing pin 83 to Ground with key on, that should show 12v if wire from IAC to pin 83 is OK,
because the 12volts would be flowing thru IAC Valve(10ohms) to pin 83

Come to think of it, I did check that. That's what I saw, 12V on pin 83 relative to ground.

 

[RonD]

Cool, then ECU IAC circuit is last place fault could be in

ECU has several Ground wires, not all are shared internally, so IAC circuit could have its own Ground pin

Pins 24, 25, 33, 51, 76, 77, 103 should all test as Grounds, this would be last test before replacing ECU

ECUs do fail, just rarely, there are literately MILLIONS of them in use and very few failures even at 20+ years old

 

[ratdude747]

Cool, then ECU IAC circuit is last place fault could be in

ECU has several Ground wires, not all are shared internally, so IAC circuit could have its own Ground pin

Pins 24, 25, 33, 51, 76, 77, 103 should all test as Grounds, this would be last test before replacing ECU

ECUs do fail, just rarely, there are literately MILLIONS of them in use and very few failures even at 20+ years old

Good catch. Will check this afternoon.

 

[ratdude747]

Question: Running by PAP this afternoon (Yeah it's easter, but they're open and I only have sundays off at work right now). Will an explorer ECU work? Or must it be from a ranger? Asking because 4.0L 1995-1997 motors and the like are easier to come by in explorers it seems from past luck.

While I'm at it, I'll nab an IAC as I suspect that if my ECU is fried a bad flyback/snubber diode in the IAC may be to blame (Not sure if ford put said diode in the PCM or in the IAC).

 

[RonD]

As long as it is a 4.0l OHV, 4.0l SOHC also came in 1997 model year Explorers
Most Explorers were automatics
EGR yea or nay as well

 

[ratdude747]

As long as it is a 4.0l OHV, 4.0l SOHC also came in 1997 model year Explorers
Most Explorers were automatics
EGR yea or nay as well

Cool. Stuck inside an escape at PAP waiting on a rainstorm to pass...

 

[ratdude747]

Here's where I am at:

ECU: all grounds good, swapping for a JY unit didn't help.

White (lt blu stripe) wire wasn't showing as good when backprobing. cutting the wire at both ends and shorting with a lenght of 20 gauge wire didn't help.

Cut the connector pigtail, used alligator clips and my battery to watch the IAC move. Or not move, as I here and feel the solenoif move but neither IAC valve acutally has the valve itself move. I've moved it to different spots within the motion of the valve (what little motion there seems to be), and same thing, clunk clunk no move.

Are both IAC's bad then? Time to buy another?

 

[ratdude747]

More data:

Reset ecu while doing diagnostics. Ran it at 1000 rpm for 10-15 minutes, ltft looking really good (-0.8%).

Broke the plunger on one of the iac valves, used it to confirm the iac as the only issue; said broken valve resulted in a 3000 rpm idle, which is what I expected.

My next move will be a new IAC. Are aftermarket ones any good? Or should I eBay a motorcraft one?