dimmer switch glitching

While driving today in the rain, headlights on, dimmer adjusted above the "click" (always max dash brightness) I noticed my dashboard lights switching from bright to dim and back a few times. Super annoying. Since they were not going all the way out the dimmer switch was suspect, and sure enough, pushing up on the wheel just a touch was enough to force it from dim to bright. I guess either the contact is dirty/worn or the mechanism that maintains contact above the "click" is loosing its oomph.

Can these switches be opened up and cleaned?

If not, what is the part number?

This is a 2007, in case that matters.

Thanks.

 

$150? The markup on those over the manufacturing costs is what, 30X? (Chunk of plastic, a few wires, thumbwheel rheostat, shouldn't cost more than $5 to make in bulk.) Wonder what the resistance range and current rating is.

Anyway, yeah, looks like a trip to the junkyard once it dries out a bit.

Thanks.

 

In terms of models, 84119-52030 was used in:

Scion XB 07-10
Scion XD 08-10
Yaris 06-14 (only if made in Japan?)
Prius 05-09

The local LKQ has no Yaris and a single Scion XB, but 3 Prius in that age range. So its use in the other cars doesn't help much for availability in this case.


How does this rheostat work?
The description in the "Meters" manual on pages 46 and 47 is confusing. There they describe 3 pins:

3 E (ground)
5 T (Tail)
8 TC (Tail cancel)

I am assuming that "Tail cancel" means "as if the taillights were not on", ie, normal daylight driving illumination.

Pin numbering is left to right, 5 to 1 on top row, 10 to 6 on bottom row, looking into the connector end of the rheostat.

The TC pin is tied to the combination meter, this must be the brightness signal.
The T pin is tied to the fuse for, I believe the taillights, so it is 12V(ish) when the headlights/taillights are on.
The E pin goes to some ground (0V).

Resistance values between T and TC for a working rheostat are said to be:

>10 kOhm "TC position, or fully turned up"
<1 Ohm "not in TC".

All well and good, but it doesn't describe the resistances between E and T and/or E and TC. Hard to see where the variable resistance, presumably between TC and E could do anything to adjust the brightness when driving at night with T and TC shorted together. The connection to E must do something or they wouldn't bother making it.

It looks like "max bright" is when TC is near ground, and "min bright" when it is near 12V. T should be <1V if the taillights are not on, pulling TC near ground regardless of resistor setting, so "max bright" during the day. Naively I would have expected TC to be the wiper position on a three connector variable resistor with T at one end and E at the other. Swipe towards E and the brightness goes up (lower voltages) if the tail lights are on, but for typical variable resistors (somewhere between 1K and 100k Ohms) it doesn't match the "not in TC" resistance value in the manual.

Anybody have one of these rheostats out of the car? What are the resistance values between these three pins for the two ends of the adjustable range and when "clicked" ("TC position")?



Thanks.

 

For a control drove around today during daylight hours on more or less the same path with the headlights on and the dimmer above the click. It didn't dim once on the whole trip. Maybe the switch is cold or humidity sensitive, which it was then but wasn't today?

Our old Honda Civic Hybrid had a similar issue - whenever I drove it on a rainy day the "passenger not belted warning" light would come on, despite that seat being completely empty. It only ever did it on rainy days.

 

Thanks for the explanation.

Well, that explains why the device had so many pins, even though only 3 were described. The "follower" is likely something like a power transistor or FET since it has to control current (making the device as a whole a rheostat). Can't be very much current though, as the device seems to have a case made entirely of plastic, which isn't a great way to dump heat. Since it modulates current, instead of just turning it off or on, it must present a resistance other than zero or infinity, and will generate some heat at the controlling device.

I need to get my hands on one of these from the junkyard and pry it open. The dial part is likely just a typical click on/variable resistor of the sort that were common on small audio devices, like handheld radios, back in the day. Those variable resistors can be had for a couple of bucks plus shipping, and it is the part which seems to be iffy. I suppose it is also possible that there is a cracked solder joint in there, so that pushing up on the dial restored the connection, and there was nothing wrong with the variable resistor itself.

Personally I would have preferred a design where there was a separate button for "max brightness" that didn't require changing the dial away from the previously set "appropriate night brightness" position.

 

One more glitch observed. Yesterday, it had been raining for over 24 hours. The car had been turned off with the headlights on and the dimmer above the click, and they turned off with the car. When the car was turned on the display was dim, even though the dimmer was above the click. Push on the dimmer wheel lightly with a finger and the display became bright. Marginal switch connection somewhere, possibly humidity related. (I don't normally turn the headlights on during the day, but it is required by law when it is raining. Hence "possibly" for the humidity.)