Cheep fix for blacked out combo (speed, fuel, gear, status) indicators. Putting a diode in place of the dome light fuse drops the voltage (by about .7 volts) to the combination meter. Preventing the buck/boost power supply in the combo-meter from locking up. Presumably locking up due to high ripple (within the buck/boost converters) not being suppressed by worn/weak filter capacitors. Lower voltage to combo/speedo presumably reduces buck/boost ripple and prevents lockup. The test to see if this kluge/tempory/fix will work for you is to drain the 12 volt auxillary battery down to about 12.5 volts or less. Reset the blacked out combo/speedo meter by pulling the dome light fuse for several minutes then replace the dome light fuse then observe the door open indicator within the combo display. Opening and closing the door many times as long as the door open indicator is working your speedometer will also work but don't put the car into ready mode just yet because it will charge the 12 volt battery beyond 12.5 volts and eventually go back into lockup state. While observing battery voltage (with a volt meter) and opening/closing doors apply a trickle charge. While the battery voltage rises continue opening /closing door note at what voltage the door indicator stops working (buck/boost/power supply locks up) . My combo/speedo meter worked perfect for 15 years till I replaced the 12 volt battery that was about 8 years old. The eight year old battery was still working fine with morning voltage at about 12.4 volts. I noticed walmart was selling prius batteries so I bought one. The next day after a single drive and having a new 12volt battery the combo meters didn't work. I would have to reset it every couple weeks (by pulling the dome light fuse) after a few months (and summer temperatures) the combo/meter blacked out every other drive. Remembering when the problem started I thought of reducing the battery voltage with a simple diode in the dome light/combination circuit might be interisting/fix? I had an asortment of diodes I went with a 10 amp diode my largest. I bought an assortment of fuses from autozone and soldered the 10 amp diode on a 7.5 amp fuse at the metering connection points on the top side of the fuse. The cathode side of the diode (banded stripe) attaches in the direction of passenger side of the fuse compartment. Soldering requires the smallest tip and I slightly melted the fuse case. I reformed the fuse case using a common butane cigerate ligher with only 5 seconds of flame and needle nose pliers. The actual 7.5 amp fuse element was still good so I used a reverse voltage/current powersupply to blow open the fuse element. CAUTION THE DOME LIGHT CIRCUIT TECHNICALLY IS NO LONGER FUSE PROTECTED AT THIS POINT. But I believe the 10 amp diode in a 15 amp protected circuit will melt the solder connection if there is a short circuit condition. The combination meter/speedo display works perfectly it's been about 6 weeks all is fine. I was reluctant to post this funky fix on prius chat till I theroized the reduced voltage to other prius systems powered by the dome light fuse (radio, security, keyless entry systems) would probably prolong there capacitors. I measured diode temperatures at 124F radio only. 160F with radio, door open lights, trunk light all on. My dome lights all have leds so I didn't bother turning then on. I operated the door lock/unlock solenoides in rappid sugession and did not notice and sugnificant increase in diode temperature. If the diode ever opens connection the prius can still be operated by inserting key fob into the dashboard slot. Amazeingly the combo cluster will operate without a dome light fuse. Each time the prius is ready-ed without the dome light fuse the fuel gage will recalibrate every time. Maybe for some people the combo meter fix could be done by simply leaving the dome light fuse out of the circuit. But you will have no radio or keyless entry and without the fuse there will be a continous VSC indication (vechicle stablization condition) . Use with caution
Forever, or will it come back when the capacitor in the dash degrades even further? I would suggest putting the diode in series with the fuse. Otherwise you are depending on the diode failing open to protect that circuit from a short. While I agree with ChapmanF that the definitive fix is to replace the problem capacitor, this method does have the advantage that it doesn't require pulling the dash apart or repairing a circuit board.
if this works, it will save a ton of labor (or money) not to mention broken dash vents. I love a good mcguyver
IMHO; I would think that throwing the old battery back into the car and jump starting it (when needed) would be easier and safer than jumping through all those hoops. OR you can just replace that cap... BTW; you'll need to check current flow to make sure the diode faces the correct direction. Incandescent bulbs don't care which direction the current flows.
The dash is easy to remove. After doing it once and following a youtube example. The first time took me an hour. After that, fifteen minutes (because I realized how easy it is so I did other things later.) Now, soldering in that cap is NOT easy - I did it, have some experience with tiny circuit board soldering and found it not easy at all. One person on you tube got another board from a company that fixes those Prius circuits. There are hoops to jump thru to do that but worth it not to put up with that problem and soldering. By the way, the circuits that the dome light's fuse protects are a lot of important systems. I would not by-pass that fuse considering the risk to those items.
I definitely have found that turning the car on in accessory mode first and then putting seatbelt on and then powering car up into ready mode is more likely to get the capacitor to fire, so I can verify your method of messing with the voltage loads on greater vehicle can be helpful, but fixing it for good is more realiable. Problem is everytime I'm determined to tear apart the dash it starts working consistently again... But that game has been going on for several years and I just need to get it fixed, especially because you don't get trip mileage and MPG calculation and a bunch of other features when its not working.
