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I asked a question about troubleshooting ECU related short circuits.

No more short circuits in the wiring harness, the only short left appears to be inside the ECU.

This leaves me with a faulty ECU. The diagram below explains how the entire circuit works, what it looks like. The problem appears to be between IMRC Terminal 1 and PCM Pin 42, which is now permanently closed irrespective of engine RPM when it should be in a different state if RPMs are less than 3,300 and in another state if RPMs are higher than 3,300.

Faulty circuit in ECU

Here is the question: ECU Short Circuit Troubleshooting

What is the best way to go about fixing this?

Get a new ECU?

Repair this ECU? Is ECU repair something that can be done by a DIY? or best left to the experts?

Or get a junkyard ECU?

By the way I followed all the manufacturer guidelines for fixing this except one step. I have two error codes P1520 -IMRC Drive Circuit Malfunction and P1512-IMRC Shutter Valve Stuck Closed, and it says if one has followed all the steps and the problem is still there, replace the IMRC Actuator and if that still fails, then the PCM.

Hence this question!

ADDITIONAL DETAIL

Mazda recommended as follows:

Short circuit

If there is continuity, the circuit is short. Repair or replace the harness.

  1. IMRC actuator terminal 2 (harness-side) and body ground
  2. IMRC actuator terminal 1 (harness-side) and body ground
  3. IMRC actuator terminal 1 (harness-side) and power supply
  4. IMRC actuator terminal 5 (harness-side) and body ground
  5. IMRC actuator terminal 5 (harness-side) and power supply
  6. IMRC actuator terminal 6 (harness-side) and power supply

I had four of those shorts involving IMRC actuator terminals 2 5 and 6, and now I have no more short circuit but the IMRC does not work despite eliminating all the short circuits.

The short circuits that were fixed were as a result of chaffed wiring of the left rear oxygen sensor which is part of the performance circuit of the vehicle.

Actually, after going through some of the comments on the earlier referenced question, I decided to carefully inspect my oxygen sensors, one of the them was the cause of all the shorts in the circuit, since I had overlooked them all along.

Had I followed the instruction to simply replace the wiring harness, I would never have thought of the oxygen sensor itself, but just the wiring between the ECU and those devices and would have skipped this insulation damage.

Turns out the o2 sensor is part of the performance circuit of the vehicle but not part of the IMRC circuit and yet it kept giving me error codes in the IMRC circuit, so diagnosis has to be thorough, related circuits should also be considered

Here is the ECU I am talking about: the problem ecu from a Mazda MPV 2000

FURTHER ADDITIONAL DETAIL

Even with the wiring harness fixed, the voltage drop at:

  1. IMRC Terminal 1 which is the IMRC Control Signal from PCM and
  2. IMRC Terminal 5 which is the IMRC Monitor Signal to PCM

does not match manufacturer specifications

The voltage drop readings should be as follows ( manufacturer specifications)

  1. IMRC TERMINAL 1 at idle = B+
  2. IMRC TERMINAL 1 at 3,300RPM = Below 1.0V
  3. IMRC TERMINAL 5 at idle = 5V
  4. IMRC TERMINAL 5 at 3,300RPM = Below 1.0V

But in my vehicle it reads

  1. IMRC TERMINAL 1 at idle = Below 1.0V (about 0.04V to 0.08V)
  2. IMRC TERMINAL 1 at 3,300RPM = Below 1.0V (about 0.04V to 0.08V)
  3. IMRC TERMINAL 5 at idle = 5V
  4. IMRC TERMINAL 5 at 3,300RPM = 5V

So to me the problem appears to be internal to the ECU at Pin 42

HOW THE IMRC WORKS

The IMRCs are programmed to be closed below 3250 RPM and they open above 3250 RPM.

The IMRC Controller has Hot-In-Run power, a ground wire, a control input, an IMRC monitor signal and a signal return wire to provide the reference ground for that monitor signal.

The PCM has an output signal that drives the IMRC controller's control input. A low signal (near ground) causes the IMRC controller to open the valves, and a high level causes the controller to close the IMRC valves.

There is a monitor signal that goes from the IMRC controller to the PCM. The PCM has an internal resistor that tries to pull that input up to 5 volts all the time.

Inside of the IMRC controller controller are two parallel resistors that pull that signal's voltage down to below 1.6 volts when two resistors attached to that signal are connected to ground by two switches close when each bank of IMRCs actually open.

If the IMRC monitor voltage is above 1.6 Volts when the PCM has commanded the IMRCs to open, the PCM assumes one or both of the IMRCs has stuck closed.

If the IMRC monitor voltage is below 3 volts when the PCM has commanded the IMRCs to close, the PCM assumes that one or both of the IMRCs has stuck open.

If the PCM assumes the IMRCs are stuck either way, it sets the appropriate DTC and sets a CEL.

The purpose of the closed IMRC valve is to increase the air charge velocity at low RPMs which fills the cylinders more, giving you more low end torque. They open at higher RPMs in order to allow full air flow through both ports.

Read more at: http://www.modularfords.com/threads/138793-IMRC-Deletes

IMRC Terminals and their functions

RESPONSE TO ANSWER BY @VINI_I

ISSUE 1:

You said: The voltage on the line between pin 1 and pin 42 is provided by the IMRC.

You also said: The sensor on the other hand, inside the IMRC there are no connections to power. The sensor is just a dumb switch. The power connection is inside the PCM and the IMRC pulls the signal low.

Does that not sound contradictory? Can the voltage be provided by the IMRC if the power to the IMRC comes from the PCM?

What is the purpose of IMRC Terminal 2's connection to Main Relay Terminal D?

ISSUE 2:

Next pull pin 42 out of the PCM. Run the engine and check what the voltage is on the pulled out pin. It should be B+.

This was further clarified as follows: When i said pull the pin out i meant physically remove the pin from inside the PCM connector. This will check all the wiring from the PCM to the IMRC.

Should it be pulled out as shown in the picture? ECU with one pin pulled out

ISSUE 3:

Another test you could run. Disconnect the IMRC (Actuator). Attach a 10kohm 1/2W resistor from B+ to pin 1 of the IMRC (Actuator or Harness connector?). Measure at the connection between the resistor and IMRC (Harness ) connector with a meter. Run the engine at the particular speeds to engage the IMRC. If you see the voltage drop below 1v as described by the diagnostic procedure then the PCM is fine.

Does it matter where I take the B+ from? Can I take it from IMRC Terminal 2 of the IMRC harness? or from the +ve battery terminal? Is the resistor connected to the harness connector and not the actuator?

ISSUE 4:

Based on your answer below, the IMRC is dumb, it is the PCM that supplies it power and ground. If then I have a problem with PCM Pin 42, it then means that even if the PCM is perfect, the PCM terminals may be bad - that is damaged, pulled-out or corroded! Which brings me to the realization that while I have done all the other tests, I did not attempt to inspect the PCM Connector as advised in step 7 in the attached diagram.

Mazda MPV 2000 P1520 DTC Troubleshooting Workflow part 1

Mazda MPV 2000 P1520 DTC Troubleshooting Workflow part 2

Any special precautions for repairing PCM Connector pins?

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    What were the sort circuits fixed? In what wires? Were the shorts related to the two circuits in question? – vini_i Dec 27 '16 at 13:03
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    Is that your ECU or a photo of a similar one? It's marked as remanufactured (repaired) and if that is your's it could be indicative of an on-going problem. – dlu Dec 27 '16 at 18:09
  • @dlu it is a picture of a similar one, not my ECU! – ElectronicsNewbie Dec 27 '16 at 19:26
  • @vini_i I have added additional details including a circuit diagram, shorts were fixed in the performance circuit (left rear oxygen sensor), all four wires had broken down insulation at the tip of the oxygen sensor and were touching. Once isolated the shorts disappeared – ElectronicsNewbie Dec 27 '16 at 19:57
  • @vini_i question updated, please kindly update your response, thanks – ElectronicsNewbie Dec 29 '16 at 12:48
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It is much more likely that the IMRC is damaged.

The voltage on the line between pin 1 and pin 42 is provided by the IMRC. The PCM then pulls that voltage to ground to turn the IMRC on. When the IMRC is on the voltage on pin 5 drops to below 1v.

In your case the the voltage between pin 1 and pin 42 is always below 1v. This could mean one of two things.

  1. The PCM is always grounding the pin. In this case the voltage will always be below 1v. But this also means that the IMRC will be on all the time and the voltage on pin 5 will also always be below 1v. This is not the case.
  2. The IMRC is no longer supplying power and regardless of whether the PCM commands the IMRC or not the voltage will always be below 1v. In this case the IMRC will always be off. This is signified by that the voltage on pin 5 is always high.

First make sure that the voltage on pin 2 of the IMRC is B+. If it's not then diagnose this first. Next pull pin 42 out of the PCM. Run the engine and check what the voltage is on the pulled out pin. It should be B+. If it's not but the voltage on pin 2 from the previous test is OK the the IMRC is damaged.

Issue 1. The IMRC has two circuits inside. A high power circuit and a low power circuit. There is no electrical link between the two circuits, only a mechanical link. The high power circuit contains the control transistor and motor. The power comes through pin 2 to the control transistor. Then goes to the motor and the gets grounded through pin 3. To turn on the motor pin 1 gets grounded. The PCM can not provide high power. All the PCM provides is a low power control signal which in this case is a ground. The PCM ground pin 1 turning on the IMRC control transistor and therefore the motor. In this case the current flows from the base of the IMRC control transistor to the PCM. The low power circuit is the sensor. The sensor is just a dumb switch. The PCM contains a pull up resistor to 5v. When the switch is open the PCM sees 5v. When the switch closes current flows from the PCM to the IMRC and the PCM sees less than 1v.

Issue 2. Yes pull the pin out as pictured.

Issue 3. The B+ could be used from anywhere. Getting it from pin 2 of the IMRC harness would probably be the most convenient. Yes the resistor is connected to the harness and not the IMRC.

Issue 4. Yes the IMRC is dumb but the PCM only provides a control signal. See issue 1 answer. What the diagnostic procedure i gave you provide is a way to figure out whether the IMRC control transistor is bad or the PCM control transistor is bad. It could be one, the other or both. It could also be the wiring between the PCM and IMRC. Check the continuity of the wires while the PCM is disconnected. To repair these the best bet is to go to a junk yard and cut the connector out of another MPV. Then pull the pins out as pictured and replace them in your connector.

  • The voltage on IMRC Terminal 2 is always B+, I guess you mean that I should back-probe PCM pin 42, I did and noticed that it is B+ but somehow at IMRC Terminal 1, it is not B+ but below 1v. If indeed the IMRC is damaged, then I need other units to swap to confirm, especially now that I finally fixed all the shorts – ElectronicsNewbie Dec 28 '16 at 1:11
  • I had the impression before now that when it says: IMRC Terminal 1 which is the IMRC Control Signal from PCM (that it means the voltage is provided by the PCM) and when it says IMRC Terminal 5 which is the IMRC Monitor Signal to PCM (that it means the voltage is provided by the IMRC) – ElectronicsNewbie Dec 28 '16 at 1:14
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    @ElectronicsNewbie In reality it works backwards. The IMRC has a PNP transistor per diagram. To turn on a PNP transistor you ground the base. The PCM does this with an NPN transistor. Note that the transistor in the PCM is only attached to ground. The sensor on the other hand, inside the IMRC there are no connections to power. The sensor is just a dumb switch. The power connection is inside the PCM and the IMRC pulls the signal low. – vini_i Dec 28 '16 at 10:38
  • @ElectronicsNewbie could you provide a more detailed description of your first comment? I'm confused on what your seeing and where. – vini_i Dec 28 '16 at 10:39
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    @ElectronicsNewbie Yes it is clearer. When i said pull the pin out i meant physically remove the pin from inside the PCM connector. This will check all the wiring from the PCM to the IMRC. Another test you could run. Disconnect the IMRC. Attach a 10kohm 1/2W resistor from B+ to pin 1 of the IMRC. Measure at the connection between the resistor and IMRC connector with a meter. Run the engine at the particular speeds to engage the IMRC. If you see the voltage drop below 1v as described by the diagnostic procedure then the PCM is fine. – vini_i Dec 28 '16 at 11:40
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I'd advise against repairing ECU's, and splurging on a brand-spanking new one is, in my opinion, excessive. Get one from the junkyard, make sure the vehicle you are pulling it from has the exact same options, exact same engine, exact same transmission.

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