2005 Prius Woes

Well diagnosed Seilerts. I agree that this is the most likely situation of how the OP’s braking ECU came to be replaced unnecessarily.

Hi Randy72. I’ll just translate into simple terms what Seilerts has figured out. This is what I also believe most likely happened.

• The inverter coolant pump fails causing the inverter/dc-dc converter system to protect itself by limiting the amount of power it will provide. It stops providing any power to 12 volt system (which normally powers all the 12 volt accessories and charges the 12 volt battery). The inverter runs hotter than normal but is not damaged.
• The voltage on the 12V bus, now powered only by the aux battery, gets a bit low which triggers a spurious low accumulator pressure diagnostic code from the brakes. This code is spurious in the sense that once the original problem (overheating inverter) was fixed this code could have just been reset without any ongoing problems.
• Instead of tracing the reported overheating problem (from your description I’m assuming that when you got the warning triangle light that you also got the “thermometer” symbol on the MFD right) the technicians started tracing the spurious brake “problem”.
• Blindly following a troubleshooting guide for the spurious code led them to change the braking ECU.

 

Well said Patrick. Yes this was an unfortunate case but in the end the documentation shows that the ECU was replaced in good faith, even though it almost certainly wasn't the problem.

I agree, but I also think it's asking an awful lot for someone who's probably not very "tech savvy" to go against what the technicians and their computers are telling him.

 

hmm. Should a DTC lead to action even when no symptoms are present to substantiate the DTC ? I'm thinking specifically of this case, where no braking problems are noted.

 

Yes. I'm also wondering if the technicians were made aware of the overheating problem, because it seems strange that they could think that the brake ECU could be behind the Inverter overheating.

Was the information about the over temperature warning passed on to them. Or even if it wasn't, wouldn't that have set a code anyway? Strange.

 

Next question for the OP is did you keep the old skid control ECU? Since this was an out of warranty repair the old part belongs to you. At this point maybe you could swap the part back and prove the old one was good all along and possibly recover some of your money by selling one of the parts. A part with a cost of nearly $2K should have a respectable value in the used market.

 

I understand that mistakes happen, but when you take your car to a dealer, you have an expectation of an expert, factory level repair in exchange for the premium paid for parts and labor. Randy's repair was inconsistent with the his original complaint, that of a triangle code, along with freeze frame data that just doesn't make much sense unless you consider that the 12V battery was dying due to lack of charging from the DC converter due to lack of cooling by the IC pump etc.

We will never know if the tech made an honest mistake, is incompetent, or even greedy. But the dealership has an obligation to recognize and admit that a mistake was made and work something out with Randy.

 

Go right ahead. You may hear complaints from one or both that we are all just armchair wrenchers here, but I think the combination of the freeze frame data, the techs own notes, and the service manual hint are compelling evidence that the ECU was not the problem.

 

Other evidence: The OP did not report any problems with the brake system, and it would be hard to imagine a scenario where a skid control ECU failure would result in an overheating issue.

 

Hi Randy, just one question about something I've been wondering. Did the guys at the dealer know that the original indicated problem was overheating (warning triangle + temperature warning icon). Did they ask you any questions about the cars driveabilty or ask you about braking performance?

As for the report I've just made some quick annotations of where I think their diagnosis was poor in case that helps. See attached jpegs.

 

Direstraits is correct, Did you receive the old ECU? You are entitled to keep any parts they replace on your dime. I'd demand that "bad" ECU back, at which point it could be tested.

I'll be happy to test it for you and sign a statement if it's still good.

 

I totally agree. I think we all know that it a 99+ % chance that the ECU will be in perfect condition.

I'll bet you that if Randy asks for it back that they'll be very reluctant (because they know it will provide proof of their poor diagnostic skills). They'll probably claim to have thrown it out already.

 

Either that or testing will reveal they hooked it up to an arc welder before sending it back. =)

 

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