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Instrument Cluster Dimmer Nerfed by SW update?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by erixsparhawk, Aug 7, 2015.

  1. erixsparhawk

    erixsparhawk New Member

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    Until this year the dimmer wheel on my 2004 Prius would allow me to turn the instrument cluster lights all the way off. Now it only dims a little bit then scrolling to the end of the wheel does not dim it further. What is irritating is that it doesn't go dim enough to eliminate all of the glare cast on the windshield any more. Was there a recent firmware or software update that might have been applied at the dealership during my last oil change?

    I don't know when this started exactly but it must have happened this summer when the days started getting longer and my commute to work was no longer in the dark. I just discovered it this morning on my morning commute now that it is dark again.
     
    #1 erixsparhawk, Aug 7, 2015
  2. bisco

    bisco cookie crumbler

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    welcome! no, the rheostat probably just failed. see how much a replacement is online. if too expensive, try salvage. all the best!(y)
     
    #2 bisco, Aug 7, 2015
  3. erixsparhawk

    erixsparhawk New Member

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    Bisco,
    Update: I've replaced the prius dash dimmer with two different ones from salvage. They both exhibit the same behavior. Do you have any other suggestions?
    Thanks,
    Eric
     
    #3 erixsparhawk, Nov 22, 2015
  4. Patrick Wong

    Patrick Wong DIY Enthusiast

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    I doubt that SW/FW is the issue.

    I don't have access to the electrical wiring diagram right now. If you care about this enough to do some research, get the EWD for your model year from techinfo.toyota.com. You'll probably find that the dimmer variable resistor has one side going to body ground. If so, that connection is probably loose, so that the voltage drop across the resistor is less than it otherwise should be.
     
    #4 Patrick Wong, Nov 22, 2015
  5. goldfinger

    goldfinger Active Member

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    I like turning it all off occasionally. I hope this isn't a software update. The dealer never asks, but just does it.
     
    #5 goldfinger, Nov 22, 2015
  6. erixsparhawk

    erixsparhawk New Member

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    I wasn't able to figure out exactly what was what from the EWD, however I was able to compare the voltages / impedances on a working car of the same make and model to mine. I spliced in a wire to chassis where I thought the ground wire was but that didn't make any difference in observed operation. This is the test results pre splice for the working car:
    NC NC
    NC HI Z
    NC Short / LOW Z
    NC Short / LOW Z
    NC NC
    NC NC
    NC Short / LOW Z
    NC NC
    On the car with the problem the bolded connection read a stead 33 ohms and that is the one I spliced to ground.
    I also ran test with lights on car off, car on lights off, and car on lights on. When the lights were on and the car off the one above the bolded connection read 12V on the working car and 0V on the non working. On the Car on Lights on test that same pin read 14 V on the working and 1.8 on the broken. There were some other .01V differences on different pins and different tests but I believe that these are measurement error.
    Patrick any further thoughts? Am I at the end of fixes that an noob can do? I think that there is something wrong with the hot line bases on the tests above. I haven't retested voltages since splicing in the ground.
    Thanks,
    Eric

    PS. I also tested the working dash dimmer in the broken car and the salvage ones in the working car. The salvage ones work just fine in the working car as well as my original that I thought had broken.
     
    #6 erixsparhawk, Feb 29, 2016
  7. andrewclaus

    andrewclaus Active Member

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    My wiring diagram shows terminal "E" on the rheostat connected to chassis ground via a white wire with a black stripe. Have you checked the resistance of that wire to ground at the rheostat connector?

    Your table isn't clear to me. Does it relate to a pinout of the rheostat? If so, and if you don't have terminal numbers, how about wire colors, as shown on the wiring diagram? What does NC mean?

    Also, is it just the combination meter lights that aren't dimming, or all the other lights on the dash as well?

    I doubt there's a power supply problem, or none of the lamps would work at all.
     
    #7 andrewclaus, Feb 29, 2016
  8. erixsparhawk

    erixsparhawk New Member

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    Andrew,
    The wire I grounded was white with black stripe. The NC stands for no connection meaning that there isn't a wire there. The 16 connections I listed correspond to the 16 pin connector on the rheostat. I wasn't able to figure out which one was officially pin 1 there were no apparent markings. The wire I grounded appears to be the 4th connector hole down on the right hand side as viewed with the connector unplugged from the back of the rheostat. Fortunately there are only 4 wires in the 16 pin connector. It is all the lights on the dash that are not dimming properly. I say properly because they dim a little then stop and do not go to full black as they used to.
     
    #8 erixsparhawk, Feb 29, 2016
  9. andrewclaus

    andrewclaus Active Member

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    You refer to 1.8V on a contact on the broken car. What's the color of that wire?

    The fact that you measured 33 ohms on the ground wire (presumably measured from connector to ground) shows a grounding problem, which is what Patrick suggested to start with. I think that's still your problem, somehow, somewhere. Be sure that measurement, from pin to chassis, is zero with the ground you added.
     
    #9 andrewclaus, Mar 2, 2016
  10. erixsparhawk

    erixsparhawk New Member

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    I doubled checked the ground I added and it is grounded. The color of the 1.8V is red. The wire above that is blue and the bottom wire below the ground is grey.
     
    #10 erixsparhawk, Mar 8, 2016
  11. andrewclaus

    andrewclaus Active Member

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    If the red wire is 12V on the good car (with the connector unplugged) and 1.8V on the bad car, and you read low resistance from connector to ground on the bad car, that may be your problem. Did you check that resistance on the good car? With car off, it should be high resistance. 12V on a 33 Ohm fault will only draw a fraction of an Amp, and that fuse is 7.5A. One thing I'd try is pulling that "7.5A Panel" fuse and see if the combination meter lights work better, if that isolates a fault.

    Another issue: I just noticed you said previously there were four wires in the connector. My wiring diagram shows five wires. That could be a problem too. Look for a loose one. In addition to the gray, blue, red, and white/black, there should be another blue, either sky blue or dark blue. You said earlier you couldn't figure out the wiring diagram, and it's not clear exactly how the rheostat works, for sure, but at least you could verify the number and color of wires.

    I can see that the gray and blue wires are connected via fuses to relays on the junction block. You can check both of those for 12V when lights are on. It appears that the red and sky blue wires control the illumination levels.
     
    #11 andrewclaus, Mar 8, 2016
  12. erixsparhawk

    erixsparhawk New Member

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    I've only got 1 7.5 amp fuse the other two sockets are empty because no fog lights etc. Replaced the fuse even though it looked good, no change. Re-examined the wiring harness going to the rheostat and can't find any evidence of an additional wire blue or otherwise. My one blue wire does happen to be sky blue. I haven't had a chance to get the volt meter out again to recheck voltages. In the mean time any other suggestions?
     
    #12 erixsparhawk, Mar 9, 2016
  13. andrewclaus

    andrewclaus Active Member

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    In my last post I recommended removing the fuse for a test, not replacing it, to try to isolate a possible fault. The 7.5 PANEL fuse will be in the fuse box under the driver's side dash, not under the hood, and there should be a legend in the box cover, maybe even a puller. It's remotely possible the combination meter lights will dim correctly if you isolate a fault on the red wire, it's an easy thing to try, and that could narrow it down.

    I just noticed you have a 2004 and that does indeed have a four wire system--my apologies for the confusion. Every model after that year has five wires.
     
    #13 andrewclaus, Mar 9, 2016
  14. erixsparhawk

    erixsparhawk New Member

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    Ah, got it. I removed fuse #17 (Panel 7.5A) and the dash lights will no longer dim at all. Thank you for all the help.So this probably means that somewhere on the red line there is some sort of very high resistance short to ground?
     
    #14 erixsparhawk, Mar 10, 2016
  15. andrewclaus

    andrewclaus Active Member

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    Yeah, that's what I would look at next.
     
    #15 andrewclaus, Mar 10, 2016
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