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Old 11-18-2017, 01:02 PM   #43
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It actually reads the voltage at the sensor input to the gauge. The gauge should work something like this:


The voltage at the sender input terminal will be higher when the sender is at highest resistance at a full tank.
When the sender goes to lower resistance the voltage goes down also. When it get to a preset value designed into the circuit, it will trigger the dash light.
Theoretically a trim pot could be installed in the new design that would allow a custom level to trigger the light.
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Old 11-18-2017, 01:05 PM   #44
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Any other electronic folks out there who would like to add a circuit function description, I understand the basics, but I am not good at putting it into words. I still need to get some diode types and values and some caps need to be pulled for values. If the components did not die giving them a bath in hot water, I will hook it up and see what frequency the oscillators run at.
Well, analog design isn't my strong suit. Especially in the old days when they liked to pull transistor tricks out of their hats. But here's my take.

Fuel gauge makes sense, simple transistor to pull the ground low on the bulb when the sensor output goes low enough. Why they ran the 100K to the transistor base instead of the V+ I don't quite get,I would have to think about it.

Washer level, are you sure you don't have emitter and collector switched? I haven't sen a transistor wired backwards like that, but it could be a hat trick I don't know.

water in fuel and coolant level I would have to put pencil to paper and think about, are those an AC signal in, I have to assume?


What I would do personally, since analog isn't my strong suit:
Look at the output of the sensors with a DMM and with an oscilloscope, to see what they're doing, and then just make a new circuit using a small microcontroller and some simple code. Whip up a PCB to mount the microcontroller on and add I/O conditioning and off you go. If I had the working sensors in front of me I could probably knock that out before I could finish simulating this analog circuit.

Not necessarily the best or most elegant thing to do, but it's what I would do in my garage, because I'm weak at analog design, and have microcontrollers laying around.

You might consider posting this somewhere like allaboutcircuits.com, you might have someone throw back a detailed explanation off the top of their head.
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Old 11-18-2017, 01:28 PM   #45
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Fuel gauge makes sense, simple transistor to pull the ground low on the bulb when the sensor output goes low enough. Why they ran the 100K to the transistor base instead of the V+ I don't quite get,I would have to think about it.
I am not exactly sure but I think the 100K is to ensure a stable output, when the dash lamp is off (darlington off) the output will have 12 volts through the dash lamp to ensure the input stays high. and just the opposite for when the darlington is on (0 volts to the 100K and input).

Quote:
Washer level, are you sure you don't have emitter and collector switched? I haven't sen a transistor wired backwards like that, but it could be a hat trick I don't know.
Unless the transistor spec sheet is wrong, that is the way it is wired.
I hate PNP transistors, it is like trying to think out writing something on a piece of paper that you are viewing through a mirror.
Again, I am guessing, Since the emitter of the PNP is biased at 12V, any significant resistance on the sensor input will cause the transistor to turn on and trigger the light. Since this is one of the module's failure on my MH, I need to look closer at the design.

Quote:
water in fuel and coolant level I would have to put pencil to paper and think about, are those an AC signal in, I have to assume?
I think these are designed using a AC signal instead of DC, similar to other liquid sensors I saw online, I just cant wrap my head around the circuit either.

Quote:
What I would do personally, since analog isn't my strong suit:
Look at the output of the sensors with a DMM and with an oscilloscope, to see what they're doing, and then just make a new circuit using a small microcontroller and some simple code. Whip up a PCB to mount the microcontroller on and add I/O conditioning and off you go. If I had the working sensors in front of me I could probably knock that out before I could finish simulating this analog circuit.
I aint going there. I will use the KISS method for a cloning
KEEP IT SIMPLE STUPID
Less complexity, less chance for failure
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Old 11-18-2017, 01:43 PM   #46
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Look at you guys!
Impressed!
My head is spinning!
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Old 11-18-2017, 02:08 PM   #47
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I am not exactly sure but I think the 100K is to ensure a stable output, when the dash lamp is off (darlington off) the output will have 12 volts through the dash lamp to ensure the input stays high. and just the opposite for when the darlington is on (0 volts to the 100K and input).

Unless the transistor spec sheet is wrong, that is the way it is wired.
I hate PNP transistors, it is like trying to think out writing something on a piece of paper that you are viewing through a mirror.
Again, I am guessing, Since the emitter of the PNP is biased at 12V, any significant resistance on the sensor input will cause the transistor to turn on and trigger the light. Since this is one of the module's failure on my MH, I need to look closer at the design.

I think these are designed using a AC signal instead of DC, similar to other liquid sensors I saw online, I just cant wrap my head around the circuit either.

I aint going there. I will use the KISS method for a cloning
KEEP IT SIMPLE STUPID
Less complexity, less chance for failure
haha, KISS is my go to method, hence the microcontroller for me! It's a nice little black box, I say "when this input gets to this level, turn the light on" and boom, it does it. Plus changes and tweaks are a lot faster turnaround.

I also hate PNP. These things would all be a lot easier IMO with a little comparator and some modern switching MOSFETS.

What does the AC signal look like, does it vary with frequency, or amplitude as the sensor changes? For frequency a counter chip should be easy, for amplitude an integrator.

One trimpot per sensor to tune the level and wrap the whole thing up in a nice 3d printed case.

What are the stock indicators for these lights, LED's? I thought I saw a neon somewhere? LED is easy, neon is prettier

Still puzzled by the NPN with the emitter on the positive rail.
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Old 11-18-2017, 02:27 PM   #48
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Here is something I CAN input on....
Stock warning lamps are incandescent... prob 3 or 5w.

LED all the way.
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Old 11-18-2017, 02:30 PM   #49
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What are the stock indicators for these lights, LED's? I thought I saw a neon somewhere? LED is easy, neon is prettier

Still puzzled by the NPN with the emitter on the positive rail.
AH HA, I got one of the transistors wrong in the Washer Fluid circuit.
The first NPN Darlington MPSA13 should also be a 2N4403 PNP
I will redo the circuit images tomorrow.
As for the dash lights, they are standard filament bulbs in a plastic housing. I replaced all mine with LED's and a resistor. That may cause me problems with the back biasing on the low fuel circuit if the current is too low.
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Old 11-18-2017, 03:11 PM   #50
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I got the update done, it is on the website and here:
Still hate PNP transistors
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Old 11-18-2017, 03:31 PM   #51
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Originally Posted by Keyair View Post
Here is something I CAN input on....
Stock warning lamps are incandescent... prob 3 or 5w.

LED all the way.
Yep, LED all the way then, so much easier!!

Quote:
Originally Posted by WayneG View Post
AH HA, I got one of the transistors wrong in the Washer Fluid circuit.
The first NPN Darlington MPSA13 should also be a 2N4403 PNP
I will redo the circuit images tomorrow.
As for the dash lights, they are standard filament bulbs in a plastic housing. I replaced all mine with LED's and a resistor. That may cause me problems with the back biasing on the low fuel circuit if the current is too low.
Quote:
Originally Posted by WayneG View Post
I got the update done, it is on the website and here:
Still hate PNP transistors
Yep, that's the transistor I was talking about!! Makes much more sense now.
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Old 11-20-2017, 01:04 PM   #52
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A bit more work today checking components and doing some testing on the board.
I did get some circuits to show signs of life others, nothin!
I took a close look at the board and found a bunch of bad solder joints.
Some may be from all my epoxy removal work, but others see to be bad workmanship!


I need to do more testing, After resoldering bad connections, I did get more action out of the board. The oscillator is about 2KHZ and when grounding an input the output slowly flips state
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Old 11-20-2017, 02:17 PM   #53
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Ha!
That would do it!
For reference, Ive been doing something similar this weekend!
My step daughters 2001 Jeep Cherokee windows and locks have been getting erratic.... a search on the internet told me a Common problem is the drivers side window module goes bad on the child lockout button. I pulled it apart and sure enough, look at the solder joints!
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Old 11-21-2017, 06:22 AM   #54
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Yep, that looks like a problem!
Douse the joints with a flux pen and apply some fresh solder to all of them, see if it comes back to life!
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Old 11-21-2017, 09:19 PM   #55
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Dash sensor modules schematic

You can probably thank the lead-free solder rules. Fix it with real lead/tin 63/37 RMA flux solder instead, to keep it fixed...wash the flux off with proper solvents and you should be in better shape.

The lead free stuff tends to be brittle and also grow tin whiskers rapidly in the presence of electricity and moisture.

A planned obsolescence side effect when Im feeling real cynical.
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Old 11-22-2017, 03:35 AM   #56
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You can probably thank the lead-free solder rules. Fix it with real lead/tin 63/37 RMA flux solder instead, to keep it fixed...wash the flux off with proper solvents and you should be in better shape.

The lead free stuff tends to be brittle and also grow tin whiskers rapidly in the presence of electricity and moisture.

A planned obsolescence side effect when Im feeling real cynical.
I dont think they did the lead free stuff in the 80's, maybe for plumbing, but doubtfully for electronics.
In either case I try to use soft solder with silver in it. It has better flow and adhesion.

No new photos today, I think I have all the componet values and semiconductor part numbers now. While home for the holidaZe I am reworking the circuit board pattern to use a edge connector for easier swap out. I will also start tinkering with trim pots so the trip range can be customized.
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