Speedometer/Tachometer Dash Lights Not Working. Which Fuse?

So, maybe this?

@captaintofuburger

https://www.web-bcs.com/transistor/tmfet/k0/K2935.html

 

I'll have to search pretty hard to find anyone around here who might know how to troubleshoot the finer details of a circuit board, but if you need bong-ripping, ski bum, raft guide bartenders, I know just where to look.

 

Oh cool, sorry for the late response I was out to dinner. So, if you don't see anything visible, check that MOSFET, https://www.utm.edu/staff/leeb/mostest.htm Basically, what would really matter in the test to see if it's failed shorted, so put your multimeter in resistance mode, then make it so it beeps when the two probes touch, then put one probe on the middle pin, and one on the left, wait a bit see if it keeps beeping. Do the same middle to right as well.

 

that's crazy too. Like, I kinda thought that's what was going on, but only based because I thought the tach/speedo wasn't working. Had I not thought that, easily would have just thought bulbs. So, I was kind of correct, jut in the wrong way but also half being right haha.

 

Ugh double post again. Middle and pins on that chip, not on the plug connector, you did say ELI5.

Edit: also, is that a little push button right above the fet? I wonder wtf that's about. Unless I'm just seeing things that are not there.....

Edit 2: Seeing things that are not there lol. That's just the nut holding the fet down. I dumb. Does look like a little spst button though at a first look. Now I clearly see it's just the nut holding the fet in.

 

The dimmer indeed shunts to ground to control illumination.

 

Edited for simplicity...

 

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Very cool! This is super informative. Thanks

 

As per the Mosfet test linked to above, the Mosfet is non-conductive in all states and cannot be charged. So...in Layman's terms, I'm concluding it's fried.

The link mentions that if the Mosfet is damaged in the event, the driver will likely also be damaged. One video I see shows physical signs of damage to the driver (a burn), but the driver on my board does not look burned or physically damaged in any way. I'm having a hard time finding understandable information (at least at my level of understanding) on how to test the functionality of the driver. If anyone has a link to some simple-to-understand info there, please post below.

One step closer and determined to fix this one myself or at least learn a lot in the process! Thanks guys.

 

Does your multimeter have a diode test mode? Checking for a drain to source short is easy enough which is why I said to just ohm it out. Usually, you you can get away without actually using the diode test, but if your meter doesn't have enough voltage, it may not work. Also the leads have to be used correctly when checking that way as well.

Take a look at this, his meter is basically just a multimeter put on the diode test mode

https://www.youtube.com/watch?v=PYxiBAW_zh8

But, I would imagine that if it did fail, it would fail shorted not open. Also, I have tested plenty of fets that are shorted with no outwards physical signs of damage.

 

Also, put your multimeter in ohms mode, and test the 2 outside pins of that tiny transistor that's right next to it. It should not give you a reading.

Edit: And while you're at it. Might as well test those 2 little diodes on the far right. Just stick a probe on each side, and see what the reading is, then flip the probes around, and check again. It should only work 1 way, not both ways.

 

Slowly learning definitions here. Thanks for being patient with me. My

$32 Innova 3320 Auto-Ranging Digital Multimeter, Red & Black

Innova 3320 multi-meter has diode test mode

Resistance measured both ways:

1 has resistance of ~2.57k ohms both ways

2 has resistance of ~3.39k ohms both ways

There's also ~2.99k ohms between the Source and Gate on the Mosfet but when put in to Continuity (beep) mode on the meter, it does not beep between any of the pins.



I have to head to work. Will be messing with this later. Thanks again!

 

Oh that's another fet not a transistor. Same test for that one as the bigger one, just follow where the letters are indicating what legs are which. 2 is just a capacitor, I would doubt that would be suspect in anything unless it was physically exploded, no real way to test those unless you take them off the board.

So, I'm guessing that the switch must be a leg in a voltage divider, I *think* that explains why you would get lights with it taken out of the circuit. If one or both of those fets shorted closed, then you would have lights stuck on and not be able to dim, if one or failed open, then it would be no lights. This seems to make sense to me.
The only thing that seems really odd to me is why it only affects the 2, and not all the lights. I would have expected to see something more absolute, an all or none type of thing. I don't fully understand what is going on there. As I said, I'm really just blind guessing here, and just going off what I know generally in electronics, nothing specific to anything Toyota.

However, I'm going to hedge my bet, I *think* one or both of those fets are to blame, but I'm not 100% confident in that. I'm also not certain if running the test to charge it up is reliable while it is in the circuit as well. I don't want to give out bad or otherwise incorrect advice. If it were me I would just swap those both out, and see what happens. But that's equivalent to just hanging parts, and not doing a proper diagnosis, and isn't a good thing to recommend to anyone for anything. It doesn't help I'm really dyslexic, which causes me to think about things backward and do generally dumb stuff as a result haha. I'm just a systems administrator, jack of all trades, master of none.

Ill shoot jsi a message, see what he says. I'm not familiar at all with the electrical system in these trucks, I'm sure he knows a lot more on that, and also a load more about electronics than I ever will. Maybe he can chime in if I'm just being really dumb here or if all of the above makes sense.

 

It looks like ya'll are doing pretty fine job without me. (not that I'd be that much help ) One of the problems with trying to fix things like this is even if you find the bad part where do you get a replacement for a reasonable price? Electronics is a hobby of mine so I'd probably give it a go just for the opportunity to learn.

Here are my tips:

• If at all possible isolate the part electrically from the circuit. You can cut leg/s and re-solder it/them or remove it entirely.
• voltage control circuits often use a special diode called a zener diode. Zeners are typically made out of glass and are kind of redish/orange. Testing them requires it to be removed from the circuit and a power supply. It's hard to say from the pictures if that's what you've got.
• I see an IC chip hiding under the heat sink. All kinds of devilry can be hidden there and for most people testing them is out of the question.
• The control is messed up already, so there isn't much to lose by trying.
  
• Don't feel bad if you can't figure it out. Component level repair is something that almost nobody does. Just trying puts you ahead of everyone I know.

cheers

 

Awesome guys. Thank you for taking the time to educate me. Armed with the motivation to fix this, it's been a great way for me to learn about something I take for granted everyday.

-I'll remove the mosfets from the board and bench test them using the methods I've learned here. Even if I can't determine whether or not they are operating correctly, both together are less than $10. I'll chalk that up as cheap education.

-the two diodes up front don't appear red or orange to me (like other Zener diodes I searched images of). They are black with silver markings/stripes, similar to the Zener diode pictured below.


-As specific of a function as the IC chip likely carries out, if it's damaged, I see now that my level of understanding dictates that I just replace it. It has a big 041 R marking and a smaller BP5N6. However, typing the markings in to a search yielded nothing. Any idea where I can source a replacement given the markings listed?

Even if I can't fix this dimmer, the learning done here has been fun. Without really having done anything yet, it's been way more satisfying than just throwing the part in the trash and replacing it with a new one. Thanks again for the help.

 

This right here. If the IC is fried, it's 99.9% sure the whole thing is junk. Manufacturers often source custom proprietary ICs that can not be sourced by common man.

If I were to lay odds, I would bet that 1 or both of those commonly used transistors are cooked.

 

I'll take the challenge.

 

Excellent. I saw that too, but I would assume the diodes would be protection for the IC from current flowing back from the fets. Not to say it's impossible to still fry the IC, but if the diodes test out good, I would have more faith that the IC is intact.

 

@Muddinfun

The most likely replacement source is inside a new controller. The IC is could be an op amp or a 555 timer, but Like Muddinfun said it's also probably proprietary to Toyota. I googled around and didn't get so much as one hit. YouTube guys like Big Clive and Dave Jones will sometimes reverse engineer simple circuits like these. That's a great way to to trouble shoot and really understand what's going on in the circuit.

This right here.

 

It's just going to be reading the encoding wheel and driving the fets, even if it couldn't be sourced, it would be simple enough to make a replacement. An ATTiny or a load of other things would work

https://www.ebay.com/itm/5-PCS-ATTI...SHIPPED-FROM-USA/282315166074?epid=1474099925

Code would have to be written, but that's why I said I would take the challenge haha.