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  #31 (permalink)  
Old 08-11-2006, 06:54 PM
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Quote:
Originally Posted by oldguy829
Well Doc. With your kind and generous assistance, I would like to continue my education. I am making progress with the temp gauge, here is where I am.

1. On the bench, I have used a variety of Ohms resistors to plot the curve of my temp gauge.


Quote:
2. Using a pot of water that I can both control and record the temp of the water, I have tracked the ohms resistance present in 3 different sending units through a variety of temperatures from 100 to 212.

(all 3 had the approximate high and low values of my gauge, but varied in the linear curve)
Linearity errors are to be expected..the best you can do, is work the resistance Around the "Center" or "Working" part of the gauge, and know that the ends have some + / - errors...(or replace it for a better linearity gauge.)

Quote:
3. (This is where I previously hit the wall ) When I hook up a sender to the gauge and the heat the water, the gauge does not respond properly.

For example, at 50 ohms resistance the gauge reads about 130 degrees. But when the water reaches about 145 degrees (where the sending unit has a reading of about 50 ohms)

The gauge starts to climb until it reads about 190. With the water temp above 145 degrees, it just pegs the gauge.
You DO have a good ground to the sender unit body , CORRECT? (required..)
Quote:

Last night (out of frustration mostly) I added ohms resistors to the sending unit wire. It worked! With a resistor, any resistor, in the line - the gauge would give me a stable reading that was in line with the bench test guidelines.
This was where I was headed if you could not match units..you just went on a step farther and solved it.

Quote:
I finally settled on 7 Ohms as the necessary resistor for one of the sending units. So my question is... What gives??
First, the 7 ohms you read, may not be 7 ohms Et and Al..you must add the resistance of the sender AND the resistance of the gauge circuit to get a total reading..which may just "Happen to be in the proper range you require for close to accurate readings..

The old sender over time may have changed value from mechanical wear, and not you have added that what was missing...
Quote:

Why does the resistor in the line stabilize the feedback from the sender?
Did the original 41 sending unit have something that is missing from modern units to accomplish this?
Simply because you are changing the value of the circuit around the sender/gauge, by the addition of resistance..The Same would apply , say, with a light bulb, by adding resistance to the filament , you can come up with different combinations of bright and dim..

Quote:
Now for step 4. To permanently install a 7 ohm resistor in the sender line.
This leads to my next question. I didn't have a 7 ohm resistor, but in experimenting came upon another mystery.

When I wire 2 - 10 ohm resistors end to end, it creates 20 ohms of resistance. When I wire 2 of them together side by side, it creates 5 ohms of resistance?

When I wired a 10 and a 22 side by side, I got 7 ohms of resistance?

Can you explain this?
Yes, I can..But I'd have to do a chapter on series, parallel, and series/Parallel resistances here, And this post is a major read as it is.. , best not to confuse more than one thing at a time ..

It would be better for you to do an online search for basic electronic theory, under Resistance , and it's properties..OR: get a basic electrical / Electronic book and read that chapter..

In Short, in certain configurations, resistances (series, parallel, series/parallel) become Additive, Divisible, or remain the same and wattage (usable current ) Doubles..or any three like resistances to ground will 1/2 the voltage..and so on..You find all your answers in that chapter..A good read should you be interested..

Quote:
I will be looking for a thermal shielded 7 ohm resistor today, but if I can't find one a 10 and a 20 wired side by side may be close enough. any problem with that scenario?

Thanks again Doc.
OK, since this post you have added more..so I'll add what I was going to tell you in the first postings a while ago..

If you can't Find the exact value Resistance you need to perform within the range you desire..figure the "RANGE" of resistance it operates within..For Instance, 0 to 100 ohms..and you need a 17 ohm resistor..you can try, 15 or 20, but if not close enough..go this way:

Find a "Linear Taper" Variable potentiometer , (A Volume control pot..) , within the range you require..In this case probably 0 to 500 ohms is as close as you'll find at the local rat shack..

Using Half the pot..Center wiper to the gauge, and either end tap to the sender..(this will effectively 1/2 the value of the pot..if it's 500 ohms, one end to center will be approx 250 ohms .. and so on..) Install on the sender line..power up the gauge and "Adjust" the "Volume Control" for the most accurate reading..Once satisfied..Glue the shaft at that point..and get a large shrink tubing to fit over the pot, shrink it down and tie wrap it out of the way..

The only thing you need to be aware of is the wattage of the pot as opposed to the resistor..If you are running a 1 watt resistor , you must use a 1 watt pot or higher..else you will let out the factory installed smoke ..and it won't function.

If all else fails for you, and I doubt it will, your on the right track..let me know, I can design a circuit for you that uses an op amp to control the gauge..but it will require some circuit card building and the Chip and some added electronic components to fabricate it..

Doc

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  #32 (permalink)  
Old 08-21-2006, 09:33 AM
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6v 12 v help

Sorry Doc, just can't check this off the list. I'm missing something basic I think. On the bench all is fine. Installed in the car - way off. I had the entire bezel on the bench, hooked up to a car battery, sender in water, etc. When I installed it I even used the same wire from the unit to the sender (with the extra resistors soldered in line). When I start the car, cold, it reads 180 degrees and pegs in a couple of minutes. Why would it change in the car?
My first thought was poor ground, but I've beat that idea to death.
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  #33 (permalink)  
Old 08-21-2006, 06:02 PM
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Quote:
Originally Posted by oldguy829
Sorry Doc, just can't check this off the list. I'm missing something basic I think. On the bench all is fine. Installed in the car - way off. I had the entire bezel on the bench, hooked up to a car battery, sender in water, etc. When I installed it I even used the same wire from the unit to the sender (with the extra resistors soldered in line). When I start the car, cold, it reads 180 degrees and pegs in a couple of minutes. Why would it change in the car?
My first thought was poor ground, but I've beat that idea to death.
Doc here,

I would tend to concur on the bad ground..

Try (if you haven't already) running a ground wire from the tank to the gauge to the sender body and see if the situation doesn't go back to the way it tested on the bench..

Doc
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