No, that wasn't a mouse in the heater

It sounded pretty convincingly like a mouse scrabbling around in the heater, but it was just the air mix damper mechanism jittering back and forth because (like so many other things in the Gen 1 that use a potentiometer to give position feedback), the contacts had become noisy and it was trying to "hold" position while the position signal jumped around.

PriusChat won't let me upload servo.webm as a video, but if you want to see/hear the action you can download servo.webm.zip and unzip it and view it.

The manual says to remove the instrument panel (!!) to reach this guy, which is ludicrous. Duck under the steering column and you're looking right at it; three Phillips screws out and it's in your hand. It's trivial to take apart, no Dremeling or gluing. A cotton swab with a contact cleaner/lube like GC Jiffy Bath and it's all steady and quiet again.



-Chap

 

I guess something that hasn't been clear to me is ... does every instance of potentiometer noisiness turn out to be an instance of tin whisker growth? The way I thought I understood it, tin whiskers have been an increasing nuisance since the elimination of lead from solders and tinning alloys, but even old vintage stereos and such that go way back into the tin-lead days still get "noisy" pots and switches, which the old-timey way to fix was to squirt in contact cleaner and work it around, which does still seem to work. (In fact, I need to do that on my old vintage stereo again when I get a chance.)

Does the 'contacts get dirty/oxidized' hypothesis still account for anything these days, or do tin whiskers always turn out to be the real culprits?

-Chap

p.s. I guess when my steering gear was going, I did drive around for a while with a chart recorder and catch that the glitches really did look like spikes toward ground, as if perhaps being shorted by a whisker. I didn't do any similar data gathering to see just what the signal looked like from this servo ... it's too easy to just take it out, clean it up, and say 'next project'.

 

I listened to yours when you first posted it, and I found the audio so faint I couldn't decide anything from it ... that's why I linked from your thread to here.

It's odd that you're not getting audio from my webm here. I just downloaded it straight from PriusChat myself and played it to see if something might have happened to it on PC's servers, but it plays just fine for me, after unzipping, either in Firefox or with mpv. The audio is rather loud, as I boosted the level before uploading it.

Maybe try another browser or media player for playing it?

-Chap

 

All the same, pots do just get dirty/noisy with time, and have done since long before ROHS. I have an elderly vintage stereo preamp whose volume and balance knobs will inevitably get noisy and need the occasional teardown and spray with Jiffy Bath, and I had a turntable whose speed regulation would get all jumpy every few years until I sprayed down the 33/45 button contacts and the fine-adjust pot. Ask me if that was annoying! (And not just because it sounded awful, but the disassembly to reach those parts was really tedious.)

My method for cleaning up the heater servo was just to open it up and swab down the pot contacts and paths with Jiffy Bath, same as my lifelong procedure for cleaning any similar thing, and it worked and the thing held rock-solid position after. Now, Bob's speculations about tin whiskers are attractive because they suggest a possible fix with a nine-volt battery and no disassembly needed, and I am not saying for sure that approach wouldn't work; I never tried, so I don't know. All I'm saying is I approached the problem as a classic dirty pot scenario, and that approach worked out well for me.

-Chap

 

A satisfied customer.

For future reference, the Gen 3 Prius replaced these actuators having potentiometer-style feedback with a new design using Hall-effect sensors for the feedback. So this should be a very uncommon issue in Gen 3 or later. Gen 1s and 2s are likely to experience it as they age, and as you can see, it's very easy to fix.

The one exception might be if the air-inlet damper is the one that gets noisy. The other two servos (air mix for temperature, and outlet select) are on the easy-to-reach side of the heater case. The air inlet one is much harder to reach on the other side of the case, sandwiched between it and the blower. The dash might really have to come off to reach that one. I never had to do it (had my Gen 1 up to 230,something thousand), but it is built the same as the other two, so somebody keeping a Gen 1 or 2 super long might eventually have to deal with it.

-Chap

 

This is the Gen 1 forum (2001–2003), and the access is very easy for those.

Your 2007 Gen 2 is very similar, except, as you've seen, there's an extra box in the way. You'll want to see the way yitznewton handled it.

 

That's what this thread is about. Each of those servos has a position sensor attached, sending its position back to the control unit.

When the control unit wants that door to be at position X, it looks at the present position signal. If that's above X, the control unit powers the motor to move the door downward. If the current position is below X, the control unit powers the motor to move the door upward. Normally, the control unit stops moving the door once the reported position is X.

If the position sensor has become electrically dirty, so it is sending back a position signal that keeps jumping around above and below the actual position, then the control unit will be looking at that position signal, and keep thinking the door position has shifted away from X, and keep sending power to the motor one way or the other to drive the door back toward the wanted position.

That's why cleaning the electrically dirty position sensor solves the problem.

 

I believe the one you did first is for temperature blend, the one you are looking at doing is for outlet select, and the one on the far side of the unit is for inlet select (fresh vs. recirculate).