intermittent master warning (?) light

Next time the warning lights and driveability issues appear, immediately go to the nearest Toyota dealer's service dept, leave the car READY, and ask for a tech to bring a diagnostic laptop to the car so that the DTC can be read.

 

There is a latent problem with the accelerator pedal encoder sometimes called the "big hand" syndrome. The accelerator encoder is a pair of dual-potentiometers:

The encoder is that gray unit at the left of the accelerator assembly. Internally it looks like:

Here you see the contacts inside the encoder that contact the gray cover traces:


What happens is the contact springs get a little weak and dust collects that cause the two potentiometers to become 'noisy' and non-linear. I use the jig in the first photo to measure the before and after cleaning effects:


When this happens, the hybrid vehicle control computer sees confusing signals and goes into a 'limp home' mode. It is called the "big hand" syndrome because it feels like a big hand has decided to hold the car back. Some report that stopping the car and rapidly flexing the accelerator can temporarily clear the condition for awhile. But this makes driving the car not so much fun.

Doug Schafer pioneered how to clean the contacts and his instructions are found in the archives of the Yahoo Group "Prius Technical Stuff." However, if you don't feel like cleaning it yourself, you can buy a replacement accelerator assembly from Toyota. The accelerators show up on Ebay from time to time. However, I refurbish them too for $250 with a $100 core charge refunded when the old accelerator is returned.

GOOD LUCK!
Bob Wilson

 

Excellent!

You can find Doug's instructions at:
http://autos.groups.yahoo.com/group/toyota-prius-sat2/files/

This group is one of two 'overflow' areas of "Prius Technical Stuff" that started November 2000. Yahoo groups in the past did not have enough space for all of the images and data files members were submitting. So the "sat1" and "sat2" groups were started to handle the overflow. Doug's procedure is posted in the "sat2" overflow area. Membership in these three groups is via free registration but 'first posts' are moderated to keep down the SPAM.

Doug uses a battery-capacitor-ear phone method to detect the wiper noise. I accomplish a similar goal with the turn-buckle, board and VOM.

Opening the encoder, I use a Dremil tapered grinder to remove the lip around three sides. The flat lip, nearest the connector, is left on:

The remaining lip is to the right and the rest is ground down just barely enough to see bits of the gray plastic. A small flat easily removes the remaining lip holding the gray cover on. At the very center to the larger radius is a small gap that the flat can use to raise the cover.

In all of the cleanings, I've yet to see any appreciable debris although I have seen the impression of the pickup arms and the six pad contacts. Using a wide flat, I gently, re-tension the pickup arms and pad contacts.

Make sure you have a small clamp or large vice grips handy and then mix up the JB Weld. With the cover back in the recess, and the surfaces clean and dry, gently clamp it in place and use a toothpick to apply a solid bead around the edges where the lip was removed. After 8 hours, it will be mechanically strong but I prefer to let it cure for 18-24 hours. Just follow the epoxy instructions and JB Weld has excellent characteristics for this job.

I then test the encoder back in the accelerator assembly using the test jig. If it passes, I put it in my car and run until someone requests a refurbished accelerator.

GOOD LUCK!
Bob Wilson

ps. You are welcome to review my collected works:

http://hiwaay.net/~bzwilson/prius/

To the above URL in your browser, add the following image file names:

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BWILSON:

Seriously. How do you do that?