Help Diagnosing Unintended/Uncommanded Acceleration Issue

FYI, this sounds a lot different than the Audi issue. In that case what was concluded was that the brake and accelerator pedals may have been placed too close together causing users to mistake one for the other. There was no unintended acceleration it was user error magnified by questionable pedal placement.

 

Actually, you may have answered the question I had previously; "Would anything other than the accelerator pedal cause an acceleration command?" I would think that the cruise control could fit that possibility....if other failure conditions were to occur as well. Time for more thinking, and understanding cruise control operation.

nylion - You have done the same thing that any pilot does when experiencing an aircraft issue. You reported all the details to the mechanics and maintenance experts....and have answered every question asked....and have not given up. Hopefully someone will figure out this puzzle.

 

When I was thinking about the Cruise Control, I thought it might be worth considering the role that the Combination Meter (the electronics in the dash) plays in relaying the speed to other ECUs. The Cruise Control isn't supposed to work below a certain speed and not from stopped, and I was puzzling over what might need to be intermittently faulty to allow the CC to engage when it shouldn't. It's probably a red-herring...

The following link doesn't seemingly relate to this problem but about half way down it talks about weird symptoms including various ECUs not knowing the speed.

Luscious Garage | Blog | LG TV - Prius Dead Dashboard

 

Sorry to hear about the trouble you've been having with your vehicle and the lack of help coming from Toyota. I understand that you are not in a position to either trade in the car or stop driving it.

If I were in your position, I would replace the hybrid vehicle ECU as that is the computer that interprets the accelerator pedal position, controls the cruise function, and tells the engine ECU how much power the engine should produce.

To give you an idea of pricing, a salvage HV ECU for earlier Prius costs ~$250.
http://motors.shop.ebay.com/__?_from=R40&_trksid=p3859.m39.l1313&_nkw=prius+hv+ecu&_sacat=

You'll need to obtain the latest revision HV ECU (not the traction battery ECU) that will work with your model year, but this is not an exorbitant expenditure. Good luck.

 

If you're really serious about this: the next time it happens, immediately shut off the car, have it towed to a dealer (or perhaps call your insurance adjuster?), and have them query the "black box" which records applications of the brake and gas pedals along with speed. If there was an uncommanded acceleration that will show it. Unambiguous diagnosis without the expense and bother of shotgun-replacement of assemblies.

 

Good call. A good ambiguity group includes everything that touches or may have some effect on the problem.

Bob Wilson

 

The 'test' accelerator assembly arrived ...

I've got to take it home; mount it; clean it up; and start 'blue printing.'

Bob Wilson

 

I doubt that an accelerator pedal failure is the cause of the OP's problem esp with no reported DTC. Try flooring the accelerator pedal when at speed, then press hard on the brake pedal with your left foot. The braking system action will definitely dominate over the drivetrain's efforts to accelerate.

If the OP found that the brake pedal did not do its job of dramatically slowing down the car and that the drivetrain was accelerating the vehicle although the accelerator pedal was not depressed, it seems more likely that the hybrid vehicle ECU (or maybe the skid control ECU) is the problem.

 

FYI:

The upper one is from an NHW20 and the lower from an NHW11.



I'll mount the NHW20 version on a board and rig up a simple circuit so we can test the electronics. Once we know how it works, it looks like a simple process to pry some tabs and take it apart. Then we'll look at what mechanisms might cause a problem.

Experimentation is how we refine a hypothesis. But mostly, I'm curious about what I'll find inside.

Bob Wilson

 

From the photos, the top of the two accelerator pedals looks as if it is entirely made of plastic?

 

Do the dealers have equipment to do this? I'd have to check the manual to be sure, but I thought the section on the black box mentioned that it had to be sent to Toyota Corporate.

Aside from the applicability to this issue, if the dealer can read the box, I'm not sure I like the idea that any mechanic could pull the data (though I'm not sure what, if anything, they could do with it).

 

The NHW20 accelerator rod and housing is made from plastic.

Just 'hand compression,' the spring is pretty strong too. I'll try to measure the resistance force at 'first movement' and 'just before end' as well as the travel distance.

I really want to make sure I understand how the electronics works before opening the case. The earlier schematic will help but I don't remember seeing a value for the external resistance ... any guess on the resistor values in the hybrid vehicle ECU?

Does anyone know the color codes for these signals:

1. VPA - color - swing??
2. EPA - color - ground??
3. VCPA - color - 5 VDC
4. VPA2 - color - swing??
5. EPA2 - color - ground ??
6. VCPA - color - 5 VCD

I'll throw a 5 V linear on the back size of one of my 'wall warts'. But I would prefer to know which signal is 'low' or ground. Right now, I'm guessing EPA and EPA2 are ground but that is just a guess based upon usual schematic practice.

Bob Wilson

 

Hi Bob,

Here are the colors for the six terminals:

1. VPA - L - swing??
2. EPA - B - ground??
3. VCPA - Y - 5 VDC
4. VPA2 - W - swing??
5. EP2 - R - ground ??
6. VCP2 - G - 5 VDC

I don't know whether EPA and EP2 are connected to ground. The repair manual does not show this to be the case.

 

I had a horrifying experience yesterday with my 2008 Prius suddenly accelerating.

I was pulling into a parking space at a store. The parking space was on a slight uphill incline. When I got to where I wanted to be, I thought I was pressing my brake, but the car suddenly roared across the median in front of me. The median contained large rocks, small bushes (maybe creosote bushes?), and a sidewalk. I went flying across the median to the other side, frantically trying to avoid cars parked on the opposite side of the median.

Fortunately there was no one on the sidewalk and there was an empty space between 2 cars sort of across from me that I could steer through. When I was able to finally stop the car, I was very shaken up, needless to say. Incredibly, no one appears to have seen me. I sat in my car for a long time with my head in my hands.

I guess I must have hit the accelerator instead of the brake. At least I’m hoping that’s what it is. As embarrassing as that is, I guess I would rather that be the case than something seriously wrong with the car. I have done extensive research on “unexplained accelerations,†but there seems to be no definitive proof, especially on newer cars.

According to what I read, Priuses don’t have any more “unexplained accelerations†than any other car. I thought about taking the car to the Toyota dealer to at least check to make sure there was no damage done to the underside.

I have read about old people hitting the wrong pedal and having accidents and thought “How stupid can they be?†but now I am humbled. When this acceleration thing happened, I was wearing a pair of shoes that are a little wider than others I usually wear. I swear I will never wear them again when I’m driving!

Afterwards, I realized how incredibly lucky I was that a) I didn’t hit a person on the sidewalk, b) I didn’t hit another car, and c) so far it appears that I did not damage my own car, although I ran over a pretty large rock. I kept looking for an oil spill under my car but so far, so good. I wonder, though, if I could have damaged the batteries or something else.

I felt so lucky that before I went home I bought 5 lottery tickets. Years ago, I used to buy just one maybe once a month (or less often than that), but I thought the least I could do is fatten the coffers of the State of Arizona. (No, I didn't win any money.)
Any feedback is appreciated.