Dim dash lights revisited

Sounds like it's stuck in headlight night mode or something.

Posted via the PriusChat mobile app.

 

My first thought would be that maybe I missed a plug, or didn't completely push a plug all the way in, or stressed some wires when I unplugged them--If it were me, I'd go back into the dash and look at/wiggle all of the wires.
My second thought is: where did the replacement combo meter come from? 04/05 are different than 06-09 units.

 

1. Check for dimmer operation (you have)
2. Headlight switch (the parts of the switch that send signal to combo meter could be bad, yet the headlights still work)
   
3. Check for trouble codes with TechStream (communications between Combo Meter & ECU OK?)
4. Check the following fuses (numbers are from my 2004, yours may be different, *=primary suspects)
   1. Fuse 8* - Gauge (inst panel fusebox)
   2. Fuse 14 - AM1 (inst panel fusebox)
   3. Fuse 20 - ACC (inst panel fusebox)
   4. Fuse 31 - AM2 (engine fusebox)
   5. Fuse 39* - Dome (engine fusebox)
5. *Instrument panel ground points IH & II (Look for loose wires or corrosion)
6. All instrument panel connector (loose or broken/frayed wires)
7. Bad Combo Meter (it controls the dimming) or its connectors
8. Bad Body ECU (works with the Combo Meter for light dimming) or its connectors
9. Bad door switches
10. Ambient light sensor

 

if the dimmer switch works, does that mean it has to be providing full brightness?

 

The dimmer is pass/fail, if it dims and goes to full brightness when you roll the wheel all the way up, then it is not the problem (according to the troubleshooting document attached). The wires between the dimmer and the combo meter need to be checked for high resistance, as do the grounds--but that means taking the dash apart. If the dimmer works and there is no high resistance on any of the wires, then the combo meter needs to be replaced. Since the dimming is actually done by the computer inside the combo meter, pulling the 12v ground cable might reset it. Heck, simply unplugging and then re-plugging (is that a word?) the dimmer and the combo meter might solve the problem, if the connectors are getting a little oxidized (dirty/oxidized connectors = higher resistance).
The OP says:

and that introduces the possibility that the ribbon cables on the combo meter (assuming speedo module = combo meter) are not seated properly.

 

o/p also states; 'it responds properly to the dimmer, it's not locked in the detent at the top'. should it be locked?

 

If it's locked at the top, that should give full brightness at all times. A used dimmer switch is $15 on eBay/Amazon, cheaper (I would think) at a pick-n-pull. I would probably just throw the $15 at it. However, according to the Toyota diagnostic, if the lights respond to the dimmer, then it is not the problem. Step 2 in the troubleshooting, is to unplug the dimmer and the combo meter and check that the wires do not have high resistance and that the ground wire in the connectors are good, if all that checks out, then the cure is to replace the combo meter. The OP said:

either the replacement combo meter is bad, the plugs are not tight, or some wires were disturbed (stretched/broken) during the replacement.
I would also investigate if the replacement combo meter was opened--there are several ribbon cables inside that can be difficult to attach properly.

 

Bisco, if the wheel is locked at the top, but the wires are bad, or have high resistance, then the combo meter can be fooled into thinking it is set to 1/2 (or something less than full on). If the combo meter is not getting a good ground, then the LEDs may not be getting the proper voltage (likely there would be other ill effects of a bad ground, the only complaint seems to be dim "lights"). If the dimmer does not get a good ground (or the correct voltage from the car) due to bad wires, then its signal would be wrong on the dim side.

 

so o/p should try locking it first?

 

The way I read this (maybe I'm wrong), the lights dim when the wheel is rolled and it will go to full brightness, but at full brightness it is very dim and not usable during the day.
So instead of going from 0-100%, it is going something like 0-30%. @dpower is that correct?

 

I’d pop that wheel out and measure the voltage you’re sending to the dash display. In theory all the way up, the voltage is passing through. that will tell you if you are sending to a defective display or sending less than full voltage to a good display.


iPhone ?

 

Hey I just had an idea are you living in one of those states were they always have the lights on during the day? I noticed that my display dims to about half brightness whenever I have the headlights on, I wonder if it’s related?


iPhone ?

 

OK, here is the inside of the dimmer. It appears to be an FET transistor connected to (probably) an A2D (Analog to Digital) chip, connected to a rheostat--your basic home dimmer, just for 12v. Mine (from a 2004) had quite a bit of oxidation on the contacts (in the rheostat and at the connector), but with this circuit, more resistance = brighter light (it's designed that way so if it fails completely, everything goes to full bright.) BUT, that is not the only failure mode that is possible (just the most likely). I would suggest trying a replacement for the dimmer. It is still possible that there is a wiring issue, or that the new combo meter is bad, but if all of the lights are dim, not just the combo meter, then this is likely the culprit. I was able to push the dimmer out of the dash by just removing the air vent and reaching into the dash (the dimmer is visible with the vent trim removed).
The correct part number for 2004 thru early 2006 Prius is 84119-47020 (11/2005 thru all 2009s is 84119-52030 and the visible difference is that the graphic next to the wheel has six lines, not 10 lines). They appear to be ~$140 new, but ~$20 used on eBay.
EDIT (I see you bought it new): Hey, does yours have the six lines or 10 lines? If you bought your Prius used, maybe the PO had theirs go bad and they put the wrong one in???

 

Is there anyone in Austin who could give @dpower a hand with this simple swap out?

 

How can I remove the dimmer switch?

 

See comments here, as long as you are careful not to break the old plastic vent piece, it’s easy. If it breaks you can buy an aftermarket one for not much money
Dash Dimmer Switch Not Working | PriusChat