Speedometer/Tachometer Dash Lights Not Working. Which Fuse?

The board looks like it just has standard diodes, I can't read what they are, but I'm sure a 1N4001 would be sufficient. Has to be a few cents on ebay.

Edit: Edit this is assuming they are bad. You can test those in circuit as I had described before, no need to take them out if they are good.

 

Let's go at this from a different angle. Pin 3 connects to the instrument cluster. Pin 5 connects to ground. Going on the assumption that the dimmer shunts the voltage to ground, there must be a short between pin 3 and pin 5. So, check continuity between 3 and 5. Be sure to put your red meter lead on pin 3. If there's continuity, follow the traces and see what's shorted. Look closely at where I have the line for pin 5. It's not the single terminal on top. It's on the bottom row. If there's not continuity, then it must be that when the circuit is energized, pin 3 and 5 are shorted together.

 

Yeah that's not a bad thought, although I think we are determining that it failed open, so testing those pins will result in an open circuit. But in-lieu of desoldering the fets, it would probably be a lot easier just to apply power to the switch, and see if/what voltage comes back out of it. I would suspect 0/open.

 

Since unplugging the dimmer makes the lights work, it has to be a shorted circuit that is shunting the voltage to ground when plugged in and an open circuit, allowing full voltage to the lights when unplugged.

 

0 continuity between 3 and 5 (sorry about the lousy pic. I need 3 hands)

 

OK, it was worth a shot. When you follow the traces from 3 and 5, where do they go?

Post a pic of the other side of the circuit board.

 

That's what I thought/assumed. But I think the switch is acting as a leg of a voltage divider, so even when it is out of the circuit, there is still a full path for the lights, which is why they work with the switch unplugged.

 

Here's something that muddy's the water a bit. I am about 90% sure this is how it works.
There are 3 different things going on here.
1. When you turn the key on, the instrument cluster lights are illuminated and you dim them with the dimmer my shunting voltage to ground.
2. When you turn on the headlights, power applied to the dimmer via the green wire, tells the dimmer circuitry to dim the instrument cluster lights even dimmer.
3. When the headlights are turned on, power is applied to all the switch illumination lights and they are dimmed by providing more resistance in the ground circuit via the dimmer.

So, in summary, cluster lights are dimmed by completing the circuit to ground(shunting voltage) and the switch lights are dimmed by creating more resistance to ground.

Again, I am only about 90% sure this is what's going on.

 

Do they? I don't even recall, I thought they were the same brightness headlights on or off. Hard for me to tell in daylight, I guess I would have to go park in a closed garage.

I think this is what the 2nd fet is for, powering the leds of switches and also the IL signal wire for the head unit. It would make sense because that would be far less load, so you wouldn't need as big of one. Also the advantage of letting the instrument cluster be light all the time, and separating off the switches and head unit.

 

OK, I just checked my truck.
1. Key on, headlights off. Instrument cluster, clock, compass/temp and radio display all lit. Dimmer only dims instrument cluster.
2. Key on, headlights on. Instrument cluster, clock, compass/temp and radio display all dim a little bit dimmer than without the headlights on. Now the dimmer also dims the radio display whereas the dimmer had no effect on the radio display when the headlights were off. The dimmer also dims all the switch lights, radio buttons, and light around the radio knobs. It would appear that voltage or ground applied to the dimmer via improper radio wiring is what fried the dimmer. Now, just need to figure which $1 part is fried.

 

I think my truck might be broken then, or different? I just checked as well. All I can see it do, is it will dim the odo just a touch when the headlights are on, it will dim the radio and rest of the buttons with the wheel and headlights on, but the display is always constant brightness. I just have the base radio if that matters. I just assumed that's the way it was. Which I don't like, I like to dim everything at night so it's just barely visible, but having the radio screen super bight and everything else dim is annoying to look at.

Edit: I also noticed that the radio display is always light for me. I honestly never noticed before I was looking at it without my sunglasses on. My sunglasses are polarized so during the day, when I'm wearing them, and obviously don't have the headlights on, I can't see the display because of the polarization, it's just black looking to me.

 

I noticed that if the dimmer is at full brightness, when you turn the headlights on, the odo display just barely dims and the rest of the gauges don't noticeably dim. However, if you have the dimmer at about 1/2 brightness, when you turn on the headlights, all the gauges dim slightly.

With the headlights on, you can dim all the gauges, switches,radio controls, and radio display down to completely off. The only things that won't dim to off are the clock and compass/temp. Here’s the radio in my ‘06.

 

My 2011 behaves the same way.

@DesertRatliff what's involved in removing that dimmer? Now I'm curious how it works and want to take a closer look at mine.

 

I thought about this too. I think it's pull dead pedal, kick panel, lower dash panel, and finally instrument cluster bezel. Or take a screwdriver and pry out the dimmer and gouge up the bezel.

If someone pulled out a working dimmer and took measurements, we would have a baseline for what normal is.

Whoever figures out the $1 part is going to be a TW God.

 

Yeah that's the same radio I have, except my display brightness will not change. I'm heading out to my truck now, I'll see what happens if I put the dimmer in the middle and see what it does with the lights on then off. Before all I could see was a faint flicker and small dim of the odo and that was is. Same goes for the clock, and I don't have a compass/temp so I can't speak to that.

 

haha. my truck is exactly the same as both yours, my memory is just bad. It's the clock that doesn't dim that's what annoyed me, not the display on the radio. When I was looking earlier I was parked at work and it was too bright to tell anything.

But yeah, if I do half dim and flip the lights on and off, the dimmer change is more noticeable. I wonder if that's what the 2nd fet is for. Although, from a design standpoint, that would seem like an overly complicated way to do that considering how it would just be a static change, a resistor would be way easier you would think. But who knows, the dimmer in general is more complicated than it needs to be to start with.

Also, I think this is the longest conversation I have ever had about a light dimmer... haha.

 

Fixed!

 

Haha. Well I was bored and looking around after I found out the mosfet that's in the switch is kind of a goofy one, it's just old from like 1998. I think a FQP50N06 would work as a replacement, and are far easier to find.

Edit: Probably damn near anything would work. But that one does match decently well, I'm sure it won't be that picky either way though. Pretty much the only difference is the FQP50N06 is about 3x faster, which will make 0 difference in the end.

Edit 2: What's even more crazy is I just looked on ebay for used one. I've counted 3 different switches all with totally different plugs/pins all for gen2s.

 

MORE DIMMER CONVO!!! YAY! Thanks for playing along everyone.

Ok. Cool thing about having two 2nd Gen Tacomas (besides buying Toyota oil filters in discounted bulk) is having a working truck to compare when something is broken, needs contrasting or needs testing. I pulled the dimmer from my 2014 (two thin knives, top and bottom, squeeze, wiggle and pull).

So on the working dimmer, I tested for continuity on pins 3 and 5....nada. Zip. Zero. In fact, 0 continuity between pins on every permutation I could think to try. It was enough to make me run out to her truck and make sure the 2014 dimmer worked on her truck. It works as intended. Hmm... Gonna remove the case and test the fets on the board and see if anything is different. (manta: don't mix up the boards!).

Here's a pic of the back of the board as Muddinfun requested. His truck, year, stereo and even color are the same as my wife's (I think his is an OR, though. Her's is a Sport. And his rear bumper is way cooler )

 

The question I REALLY want to know is: in some engineers infinite wisdom in creating such a complicated, expensive, ingenious little part, why is it not protected with a fusible link? After I started expanding my search to the net, there are plenty of threads out there of amateur stereo installers frying this little hoozle.