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I spent a lot of time trying to resolve some problems with a 2006 Toyota Aygo.

The most important or significant code it has is U0100: No communication with ECM/PCM A.

Information for 1KR-FE engine, 2WZ-TV (LHD) and 2WZ-TV(RHD) engines and for vehicles with VSC Systems and without VSC systems together is in one diagram making it a bit difficult to follow.

It gets even worse.

Due to changes along the line, in the course of production, the model had quite a long run, from 2005 to 2014, and all the changes were incorporated into the same diagram.

The vehicle I am working on has no VSC.

But using the manufacturers VIN decoder inside the Technical Repair website to narrow down to documents specific to this vehicle fails,it failed to recognize the VIN and show me relevant documentation specific to this vehicle, so I had to pull down everything and determine what's relevant and whats not.

To illustrate my point:

The diagrams in service documents I have are for vehicles with VSC systems

Reference is made to A41 Brake Actuator Assembly, but courtesy of an engine control documentation (please see four color picture below title Engine Control 1KR-FE) actually A21 and not A41 applies to the vehicle I am working on.

A41 Brake Actuator Assembly Diagram in Service Documentation

A21 Brake Actuator Assembly Diagram in Service Documentation While A41 has 46 pins, A21 has 26 pins.

Engine Control 1KR-FE The documentation for resolving this problem tells you to check resistance between the transmission control ECU connector (A28) and the ECM connector (which could be B25 or B26) but it does not tell you which engine or model comes with B25 or B26!

Neither does it say when to follow the diagnostics pointing to ECM B26 instead of ECM B25

When I tried diagnostics based on ECM B26 connector and got no results, I then tried ECM B25, and got results.

in this picture, the ECM Connector is B25

In this picture, the ECM Connector is B26

Depending on which part of the service documentation one consults, the CAN communication lines may run from ECM B26 harness or B25 harness.

High level wiring schematic between the ECM and the Transmission Control ECU and the Skid Control ECU - low resolution

High level wiring schematic between the ECM and the Transmission Control ECU and the Skid Control ECU - high resolution

CAN Communication System Diagnosis - low resolution CAN Communication System Diagnosis - high resolution CAN Communication System Diagnosis - high resolution CAN Communication System Diagnosis - high resolution

CAN Communication System Diagnosis with a B26 ECM Connector

Circuit Diagram from Transmission Control ECU Communication Stop Mode diagnosis Transmission Control ECU Communication Stop Mode Diagnosis Check 1 - Checking if the CAN Branch Wire Connected to the Transmission Control ECU is in order

Transmission Control ECU Communication Stop Mode Diagnosis Check 2 - Checking if the wiring harness and the following connectors are good (IG, GND, PGND)

Now the question or questions?

Is my diagnosis in order?

Initially I established that the two CAN communication lines were open so the was no communication between both the ECM and the Transmission Control ECU.

After fixing that I then fixed communication between the Transmission Control ECU and the CAN junction connector.

But the results for the Transmission control ECU CAN communication lines and body ground is out of specification.

Although the voltage drop reading between the ignition line of the transmission control ECU pins and body ground is within specification.

After fixing the wiring, I got resistance readings, although the readings are a little lesser than what the service documentation says.

Like the expected resistance between the ECM connector and Transmission control ECU connector should be below 1.0 ohms but I get 1.6 ohms.

The major difference is that CAN +/- and body ground should be 10kilohms or higher but I got 50 ohms when I followed the Diagnostics for U0100.

When I followed the diagnostics for CAN Communication System Transmission Control ECU Communication Stop Mode.

I checked the CAN Bus Wire (Transmission Control ECU Branch wire) and while expected resistance should be between 54 and 69, I got 45.2.

When I checked the harness and connector, ignition, ground and pground.

Expected values for Transmission ground and pground are below 1 ohms but I got 1.6 ohms.

Expected values to voltage to transmission control ECU ignition pin is between 11 and 14 volts, I got 12 volts.

Following the U0100 diagnostics guide is inconclusive.

Following the CAN communication System Transmission Control ECU Communication Stop Mode diagnostics tells me that the Transmission Control ECU is bad.

Which do I believe?

My conclusion is that it seems that the transmission control ECU is bad.

Am I correct?

NEW QUESTION:

The ECU connectors are labelled B25 and B26. I was only able to establish communication between B25 and the ECM, was I supposed to be able to establish communication between the B25 ECM connector and the Skid Control ECU and the Transmission Control ECU as well as the B26 ECM connector and the Skid Control ECU and the Transmission Control ECU in the same vehicle? Or does the schematic for B25 and B26 respectively apply to different vehicles?

  • Is there a question here? This just seems like a rant, and BTW most manufacturers do one manual with all engine types / body versions / gearbox versions... – Solar Mike Jul 27 '18 at 9:57
  • As an example, my car has 5 different engines, 3 different gearboxes, 2 body types which means 30 possible variants without starting on trim levels : cloth, leather etc etc And you want one manual per variant .... – Solar Mike Jul 27 '18 at 10:02
  • Will modify to make it a question. The documentation is so convoluted, it is frustrating. – ElectronicsNewbie Jul 27 '18 at 10:48
  • Did you or do you have communication with the tcm using a scantool? The u0100 was in the ecm? I’ll put this out there, module failure on toyota’s are rare. How exactly did you fix this no comm issue? Was the code persistent? Some of this testing seems to be unnecessary. What were the other codes? In my mind, if you have comm with the tcm than a u0100 code would be at the bottom of the list of things to fix. – Ben Jul 27 '18 at 12:25
  • No communication with the TCM. Which was why I did the testing to find out what was wrong...then discovered wiring faults between the Transmission Control ECU and the CAN Junction, which I fixed. Vehicle went in for repairs and I think whatever the other job did to the vehicle resulted in the transmission being messed up or the wiring to and from the transmission. Its a rare type of transmission it's called a multi mode manual transaxle. – ElectronicsNewbie Jul 27 '18 at 13:20

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