Stumbling Idle ... need assistance

[MrShorty]

Due to internal circuitry of PCM, a left/front HO2S signal
short to power could produce a Code 542 or 543.

Again, assuming your '94 is similar enough to a '95 (I expect it is in this case, but could be wrong), then you're looking for a short to power to the signal wire for the left HO2S. The pinpoint test actually specifies doing all the testing at the O2 sensor connector.

 

[Mic_]

Again, assuming your '94 is similar enough to a '95 (I expect it is in this case, but could be wrong), then you're looking for a short to power to the signal wire for the left HO2S. The pinpoint test actually specifies doing all the testing at the O2 sensor connector.

I've attached the circuit I was refering to...I don't know if it will help.

I will try to check out the O2 sensor(s) wiring.

 

[MrShorty]

Because they are all basic 4 wire O2 sensors, I fully expect the external wiring to be the same. I still can't say anything about the internal PCM wiring being the same, but I wouldn't be surprised if it is. Your wiring diagram is as good as anything I would have as far as pin assignments and such. I wish I had ready access to pinpoint tests for a '94, but I'd have to go to the library like anyone else would. You've either got to get a hold of pinpoint tests for a '94, or move forward on the assumption that a '94 is the same as a '95. If you move forward assuming '95 is the same, then you'll be looking for a short to power to the left HO2S signal wire.

 

[shamaal]

What an interesting problem!

My ramblings may be of value,

At first look, there is no relationship between the fuel pump relay and the O2 sensors. They do not use the same power or common points. If I understand the Mr.Shorty message correctly, a HO2S short to power affects this circuit.

The 359 line appears to be a sense line generated by the PCM-46. I guess the theory is that a HO2S sense element shorted to the +12 HO2S heater, raises the sense line and internally throws off the reading at PCM-8. I'd sure like to see the PCM schematic.

If so, my hats off to the tech that ran that one down!

If the HO2S fuse is pulled, and the HO2S is the problem, the 543 shouldn't set. The HO2S test will fail but the Fuel Pressure relay will pass.

I've thought that you may be able to disconnect the HO2S and measure the resistance from the HO2S sense lines to the heater, but I don't know what effect the ZrO2 would have on a high impedance DMM. I just threw one away on Saturday so I can't experiment. I would think it should read open with negligible voltage generated at room temp.

Let us know how it works out.

 

[Mic_]

Well.. Here's the pin point test I have for the circuit J94,J95,J96 and H24 for the fuel circuit.
My question is still this circuit #359 GY/R wire is called "sensor signal return" going back to pcm at pin #46 is it a grounding reference for the pcm (compares to O2 sensors signals at pin 44 &45) or does is supply the voltage?

 

[Mic_]

Here's the O2 pin point tests..

I wish I was better at including them in my post then I would have to upload them as attachments.
I don;t even know how to 'host them' somewhere else :thumbdwn:

 

[Mic_]

Here's the O2 pin point tests..

I wish I was better at including them in my post then I would have to upload them as attachments.
I don;t even know how to 'host them' somewhere else :thumbdwn:

added attachements

 

[dogfriend]

Well.. Here's the pin point test I have for the circuit J94,J95,J96 and H24 for the fuel circuit.
My question is still this circuit #359 GY/R wire is called "sensor signal return" going back to pcm at pin #46 is it a grounding reference for the pcm (compares to O2 sensors signals at pin 44 &45) or does is supply the voltage?

Remember that the O2 sensor generates its own voltage depending upon the amount of oxygen in the exhaust stream. So the "sensor signal return" is the reference ground so the PCM can determine the voltage created by the sensor.

 

[shamaal]

Well.. Here's the pin point test I have for the circuit J94,J95,J96 and H24 for the fuel circuit.
My question is still this circuit #359 GY/R wire is called "sensor signal return" going back to pcm at pin #46 is it a grounding reference for the pcm (compares to O2 sensors signals at pin 44 &45) or does is supply the voltage?

Yes, #359 is the "grounding" reference for a wide variety of sensors including the HO2S. I don't see any connection to the battery return so it appears to be what is called a floating ground. As Dogfriend says the HO2S generates voltage in the presence of oxygen (>600 degF) which is what the PCM reads.

 

[shamaal]

If this isn't helping just ignore me.

Looking at J96, another thought occurs to me. I've been assuming that the relay energizes, the PCM measures the voltage coming fromthe relay and all is right with the world. As we know the relay de-energizes if the car doesn't start. It makes sense for the PCM to sense the line again and if there is still voltage, set the error.
To check, Key on, wait for relay to de-energize, pull the relay and check the pin that goes to the computer. It should read, depending upon meter impedance, approx < 1volt. If more voltage is present pull fuse 18 and see if the voltage drops.

 

[Mic_]

A 94 X I use to have did this. It ran down the road fine but idled really bad. It was the throttle position sensor. 20 bucks. It didn't turn my engine light on when it went bad. That's why it took me a while to find the problem.

Thanks for the idea.

I’ll have to look into checking/testing the TPS out…if I remember correctly, mine went out during last winter but threw a code that time( so it was an easy fix).
But who knows, maybe the new one went bad.

 

[MrShorty]

The test you posted as H24 is the same as step 26 in my manual - testing the wiring harness between the PCM and the O2 sensor for a short to power to the O2 sensor circuit. Step 25 is to test the corresponding wires in the O2 sensor for a short. Basically put an ohmmeter across them and, if there is continuity (the manual specifies <10K ohms) between signal and power, then the fault is in the O2 sensor and should be replaced.

Out of curiosity, which manual are you getting those from?

 

[Mic_]

The test you posted as H24 is the same as step 26 in my manual - testing the wiring harness between the PCM and the O2 sensor for a short to power to the O2 sensor circuit. Step 25 is to test the corresponding wires in the O2 sensor for a short. Basically put an ohmmeter across them and, if there is continuity (the manual specifies <10K ohms) between signal and power, then the fault is in the O2 sensor and should be replaced.

Out of curiosity, which manual are you getting those from?

I am using both an electronic version of 'Ford Technical Service Publications' and an electronic version of 'G - TESTS W/CODES - EEC-IV (4.0L)'.

Is there something I'm missing?

 

[MrShorty]

No, I just liked the layout of the pages you posted, so I was wondering where they were coming from. (I might even be a little jealous)

If you have a copy of the "tests with codes" article, then I would start by finding the test associated with a KOEO 543 and follow the steps outlined. That's where I've been looking (exept I don't have the article for a '94).

 

[Mic_]

Well this week end was busy.

Got the RA bushings doen and decide to chase done this electrical situation I have.

Still getting the KOEO code of 543.

This is what I checked: (using the pinpoint tests)
voltage and resistance at each (2) HO2 sensor coneector
voltage and resistance at each (2) HO2 sensor harness
ran with Ho2 disconnected.
Throttle position sensor (varies .9 to 4.5v) manual says .5 to 4.5 is what to look for (not sure is I should b e concerned or not)
contenuity (or lack of) from pcm to fuel pump relay contacts.

The only thing I can try is to either measure the voltage on the pcm input while runnig. or cut that wire (create open circuit) going to the pcm in order to verify that I still get the 543 code.


Any ideas?

 

[dogfriend]

When you checked resistance at the O2 sensor, did you check from the heater power wire to the other terminals in the connector? You should be looking for a short between the power wire and the signal wire.

I would not cut any wires; if you want to try without the wire connected to the PCM, you may be able to remove the terminal from the PCM connector without cutting it.

 

[Mic_]

When you checked resistance at the O2 sensor, did you check from the heater power wire to the other terminals in the connector? You should be looking for a short between the power wire and the signal wire.

I would not cut any wires; if you want to try without the wire connected to the PCM, you may be able to remove the terminal from the PCM connector without cutting it.

I would have to verify my notes for what terminals I checked, but I followed the pin point tests used to check for short of power to return.

I was unsure if I could pull that terminal out of the connecter with out damage, or just cut it and reconnect. This would definitely show if it (the power short) is internal to the PCM or not.

 

[dogfriend]

On most Ford connector shells, if you take the connection apart and look at the terminal side of the connector, you will see a plastic retaining piece, usually a different color than the shell. Remove the retainer by pulling it out. Then you should be able to release the terminal that you want to remove by pushing back the locking piece in the shell.

I'm not sure if this applies to the PCM connector on your truck, but most of the larger Ford connectors work this way.

I recommend not cutting any wires if possible because of potential problems later (corrosion, increased resistance thru the repair point, etc.)

 

[Mic_]

On most Ford connector shells, if you take the connection apart and look at the terminal side of the connector, you will see a plastic retaining piece, usually a different color than the shell. Remove the retainer by pulling it out. Then you should be able to release the terminal that you want to remove by pushing back the locking piece in the shell.

Your were right on the money!

Was able to pull them at my will

OK so I pull wire#8, plug in PCM and KOEO code of 543 :thumbdwn:

But this tells me the pcm does not 'see a real world' input shorted to power.
It therefore must be in the way the pcm is interpreting another?

SO I got to thinking....I pulled both wire #44 & #43 (the HO2 signal returns to pcm).
Plug pcm back in again, get these codes.
KOEO:543 (no surprise at first)
Then I thought I do not have a short, but that leaves the pcm at fault?

Am I thinking this out right?

Oh. and a code..
CM:512 (that must come with all the plugging and unplugging)

 

[Mic_]

Regarding the 543: according to the Haynes schematic on the 4.0 SOHC, there is a wire that comes off of the Fuel Pump Relay (DG/Y) that goes to the PCM. The PCM senses the wire and determines the fuel pump relay closed.
The fact that the fuel pump's running (relay's good) would point to a faulty wire/connection on the sense line leaving the relay socket or at the PCM, a bad sense input on the PCM; or an old stored code that's no longer valid.

I pulled wire#8 from the harness, plugged the PCM back in and got a KOEO code of 543 again.

But this tells me the pcm does not 'see' that input shorted to power.
It therefore must be in the way the pcm is interpreting another?