No Grounds For Impeachment

John Diagnostician Redlands, California Posted   Latest   Edited  
Case Study
Driveability
Electrical
2002 Toyota Avalon XL 3.0L (1MZ-FE) 4-spd (A541E)
P1135
P1155
Air Fuel Sensor Hearer Circuits

The vehicle was brought in with codes P1135 and P1155 (Air/Fuel Sensor Heater Malfunction Bank 1 and 2). The codes would sometimes reset soon after clearing, and would sometimes not reset until after several restarts with long periods of idle in between. Another shop had attempted to repair the problem. The vehicle had two new Air/Fuel sensors and a new PCM. It is always a good idea to see what the code set criteria are: diag​.​net/file/f2t07czzz… Here is the wiring diagram for the Air/Fuel (A/F) sensors: diag​.​net/file/flb4hta4l… This particular Toyota doesn't have very helpful A/F sensor heater PIDs. There are no heater command or heater current PIDs, so we decided to connect the lab scope to take a look at the circuit activity when the problem occurred. A current clamp was connected around the heater B+ supply wire at the bank 2 A/F sensor (channel 1). Channel 2 is connected to the same wire in order to view the heater supply voltage. Channel 3 is connected to the PCM controlled heater ground circuit. diag​.​net/file/f57wdig7c… It is clear that the peak amperage is too high at almost 15 amps. The code will set if the amperage exceeds 8 amps. Now we know in which way the failure is causing the codes to set.

After seeing the scope pattern, we decided to access the PCM for further testing. We checked the circuits and A/F sensors heater elements by testing resistance between A/F relay cavity 3 in the fuse box, and PCM connector terminals 3 and 4 in connector E5. The normal heater resistance for the A/F heaters is 0.8 to 1.4 ohms Each sensor read 1.3 ohms. At this point, we know that the circuits appear to be good and the PCM appears to be capable of controlling the transistors to ground the A/F heaters. The heater amperage at times is too high. At this point, we decided to do some closer checking of the grounds. We used the cursors in the measure and deep record function of the ATS ESCOPE to check the ground voltage drop on the waveform. It was approximately 0.8v This is close to what we have seen for typical voltage drops across transistor driver circuits. Checking the wiring diagram shows that the emmiter pins of the A/F heater transistors are grounded through the PCM using grounds E04 and E05. Here is a ground distribution diagram. diag​.​net/file/f4d7ode8s… diag​.​net/file/f5vxisxpj… According to the diagram; grounds E04 and E05 are connected to ground point EB which is on the right hand side of the intake manifold. A visual inspection of the grounds at this point showed that they were properly connected and tight. Voltage drop was 30mv (0.030v). The grounds at point EC were a different story. These grounds are not easily visible (without some help from an inspection mirror). They are supposed to connect directly to the intake through a threaded bolt hole next to the intake support bracket. On this vehicle, someone had connected them to a clamp for the power steering hose, which is connected to the intake support bracket. To make matters worse, the bolt securing the support bracket was loose. The voltage drop was 300mv (0.300v) at Key On Engine Off and rose to over 1v during cranking. We relocated and connected the ground to the correct location. The voltage drop on this ground was now 30mv. After clearing the codes, we connected the lab scope to the A/F heater circuits at the PCM. The waveform shows the current of each heater and the PCM controlled ground for each heater. diag​.​net/file/f4muyylku… … The cursors were again used to measure the ground voltage drop in the waveform. Interestingly, it showed about the same 0.8v. Also of interest is that according to the wring diagram,; the ground that we repaired is not the ground used by the PCM for the heater circuits. It is clear that the heater current is now within specifications. None of the other components which used this ground exhibited any problems. This case study makes a good case for checking oower and ground circuits early in the diagnostic process. They should be checked both visually and using voltage drop testing (especially while cranking the vehicle so circuits are stressed). This is especially true if you have a module controlled output that is not working properly, or sensors that are not providing accurate information. You may not know which power or ground circuits that the module is using for a specific actuator(s) or sensor(s). If anyone needs further instruction or review on voltage drop testing, there several good on line seminars available that cover the subject. Check out the many excellent presentations by TST and Motorage magazine. We hope that you found this case study both interesting and helpful. Thanks for reading.

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Interesting
Randy Diagnostician
Glendora, California
Randy Default
 

Yes I have run in to this same problem before . Heater codes always check ground at back of intake

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Stuart Mobile Technician
Blue Island, Illinois
Stuart Default
 

Yes it was interesting and helpful​.​Thanks for sharing

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James Owner/Technician
Pike, Pennsylvania
James Default
 

Great write up John. Thank you Do you recommend any place in particular for training on the ATS ESCOPE ELITE. I've had mine for over 1 1/2 years now and find it too easy to grab one of my other scopes because of time restrictions.

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John Diagnostician
Redlands, California
John Default
 

Hi James​.​When I purchased my Escape Pro, a book was included titled "Getting to Know the Automotive Lab Scope". I think it was written by G Truglia of TST. You could contact Automotive Test Solutions … and see if they can get you a copy. The Automotive Training Group (800I-233-3182) has a good book titled "Essential Lab Scope Techniques". It is used in a class where…

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James Owner/Technician
Pike, Pennsylvania
James Default
 

Thanks for the reply John Jeff, from ATS, reached out to me today and sent some info and said he will continue to send me some things to work with. He also mentioned a class coming up in June or July which sounds like the one you mentioned. I actually was going thru my different scopes this past weekend getting everything back together as things had been mixed around from different techs use…

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Grant Technician
Loves Park, Illinois
Grant Default
 

Thank you for sharing. I enjoyed following your process. In your attached file (1/6) ...the DTC chart wiring diagram... the lower half of the ECM is cut off which would show the heater circuit for B1S2. Inside the ECM on that cut off portion I noticed ground point E03 for B1S2 heater. See my attached file. In your attached file (5/6) ...the factory wiring diagram screen shot... I see E03 next to…

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John Diagnostician
Redlands, California
John Default
 

Hi Grant. Thanks for catching my error. I'm not sure how that happened. diag​.​net/file/f768uothp… This is the screen capture that shows the circuits after the repair. I am sorry if this caused any confusion.

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Bill Owner/Technician
Jackson, Michigan
Bill Default
 

Great work and write up John 👍 - thanks for sharing!

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John Diagnostician
Redlands, California
John Default
 

Thank you Bill.

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Eric Owner/Technician
Grand Rapids, Michigan
Eric Default
 

Gotta ask, how did improving the ground path decrease current draw? High resistance of course should result in lower current flow. What am I missing from the case.

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Agree
John Diagnostician
Redlands, California
John Default
 

Hi Eric. I have been struggling with this myself. I don't know that I have a good answer. I will speculate here: Possibly the heater circuits were using a different (unintended) ground that had a different resistive path. That might explain why the current was higher with the wrong ground. This is one reason that I posted the case study. I wanted to show how ground and voltage supply problems…

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

Postulating on your speculation; Your testing proved that the ground for the front heaters was good. That is backed up by the system returning to normal with nothing being done to that ground. Adding a ground to a patent ground can’t increase current flow for that device. The current reading of 15A is not possible with the other factors given. ie- 14v(system)-.8V drop-13.2v across 1.3ohm=10A…

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John Diagnostician
Redlands, California
John Default
 

Hi Stephen. I believe the amperage reading to be accurate. It was obtained using a trusted and properly calibrated amp probe

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

I wonder why a loose ground for the rear sensor was causing high/lo current on the front sensors. The ECM had to be commanding the high/lo current, as no fault was found on that circuit. The code sets for high or low current so, the actual fault that the ECM coded for is unknown. It has to be inferred by testing. I wonder if the loose ground caused to ECM too get a false reading from the front…

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John Diagnostician
Redlands, California
John Default
 

I agree, the math gets tricky and weird, and still probably does not provide a clear cut answer. Lets use the Toyota spec for the heater acceptable resistance. So if we disconnected the A/F sensor and measured across the heater elements with a ohm meter, we should have a reading of 0.8 - 1.4 ohms if the heaters are good. Now lets use the voltage from my first scope capture (channel 2) when the…

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Grant Technician
Loves Park, Illinois
Grant Default
 

I think you are assuming B1S1 heater, B2S1 heater, and B1S2 heater are in a simple 3 branch parallel circuit. But they are not. B1S1 and B2S1 heaters are in parallel with each other and form 1 branch. B1S2 heater forms the other branch. This is a combination circuit that requires you calculate resistance of the AF heaters' branch first. See this video for how to calculate V/I/R in this type of…

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

Hi John, I am trying to figure out how the current went from 14A to 7.5A with the same voltage applied and no changes made to the circuit. The circuit goes from .99 ohms to 1.8ohms. That doesn’t seem like much but it’s almost double the original value. The Toyota spec resistances don’t explain it because the situation at hand is fixed and did not have any changes made to it. So, what explains…

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John Diagnostician
Redlands, California
John Default
 

Hi Stephen. If you read my reply to your earlier post,I think it may answer some of your questions. The wiring diagram that I chose was the best one I could find as it shows some of what is going on inside the PCM. The amp clamp was placed on the heater supply voltage wire at B2S1. Although it would not matter which heater wire I used. As you know, current flow in this circuit is the same…

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JJ Technician
Quincy, Massachusetts
JJ Default
 

I feel like being able to put problems like this one in a pretty package would require disassembly of the PCM and tracing the heater circuits out on the PCB. The diagram gives you a good idea of what's happening inside the magic box, but it doesn't show the whole picture. Without an actual PCB schematic it's tough to prove that something about that EC ground was or wasn't affecting the upstream…

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John Diagnostician
Redlands, California
John Default
 

Thanks JJ. I agree. I have already beat myself pretty good over this one.

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

John, Thanks for your patience and consideration. I was looking for the smoking gun. I’m glad that the patient made a full recovery. Yours, until someone posts a screenshot,

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Grant Technician
Loves Park, Illinois
Grant Default
 

I have an idea that may explain the high amperage in B2S1 heater. The B1S1 was unplugged during the B2S1 amperage test? As far as why EC Vd causing the P1135 and P1155, I have no explanation.

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John Diagnostician
Redlands, California
John Default
 

Hi Grant. Thanks for the reply and the you tube link. As I am sure you know. The heaters are in parallel. We can treat each leg of a parallel circuit as a separate series circuit when calculating total amperage and resistance. That was a good theory about the high amperage on B2S1. However, I never touched that sensor as it is much harder to access. I left it connected the whole time. Thanks…

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Grant Technician
Loves Park, Illinois
Grant Default
 

John, I apologize, I was wrong when I said the circuit is a combination circuit. I hope this did not waste too much of your time. I got confused during by research and thought I found a mistake. The mistake was mine.

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