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Cracking a Toyota's code

This Prius brake-by-wire system may be new technology, but old-fashioned troubleshooting led to the fix.
Wednesday, June 27, 2012 - 07:29
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With the inverter cover safely removed for access, I load tested the brake-by-wire circuit and discovered it had nearly 1.0 volt of voltage drop. This might be the problem. Wouldn’t that be nice? Find the bad connector somewhere and avoid replacing that $1,300 Skid Control Module? Wouldn’t that be sweet? After following the schematic, I saw where the circuit feeding that brake actuator module traveled from an under hood relay then passed through the bulkhead. Next, the circuit went through a fuse called out in the manual as “F16,” then back through the bulkhead and to the under hood brake actuator assembly.

Now how much sense did this make? Run a wire from a fuse/relay center that’s under the hood through the bulkhead to a fuse and then run it back through the bulkhead to the device needing the power under the hood. That just didn’t make sense to me, but who am I to argue with a service manual that so far has been 100 percent accurate?

Now comes the hard part: finding fuse F16. I checked the under dash fuse panel, removed all sorts of lower dash pads and trim pieces, and finally even removed the entire IP dash pad. There was no sign of a fuse by that name (or continuity in this circuit) in any fuse panel, in-line fuse holder or even the sign of a fusible link or similar protection device anywhere I could find. But I still had a 1 volt drop on that circuit (when powered up with a substitute load) so where to now?

The Human Side of Diagnostics
The driver of the little Prius (as you recall, my wife Teresa) is getting impatient at my embarrassingly long diagnostic process. As an aftermarket trainer, my time at home is sometimes weekends only and on top of that, I’m just plain slow! After a few “discussions” about why she’s not back in her car after several weeks she blurts out, “Honey, maybe the dealer knows what they are doing a little more than you. Couldn’t you just tow it over there?”

My pride hit the floor and then I got mad. Next it was, “Honey, can I do anything to help you with the Prius? I can hold a flashlight or hand you tools — anything.”

“Finally, she understands,” I thought to myself. But now I feel bad. She’s right. I’m getting whipped by this thing. It’s time to just try something, but $1,300 is a lot of money to just try something. So I hand her the Skid Control Module and tell her to find me one of these parts used and cheap.

Thirty minutes later she tells me she found one on an auto recycler’s eBay store for $75! For that money, I’ll try it. “You’re a genius honey!” I screamed to her, forgetting the low blow comment she made about towing it to the dealer.

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