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Discussion Starter · #1 ·
Is the 66 temperature sending unit in a special circuit?

Original idiot light sender is dead. searching for a replacement with a new 66 gauge style sender and a new late model Dodge idiot light sender. on either one, when measuring the resistance, nothing happens, you get resistance at a steady state. if I hook up a battery charger to either sending unit with a spare bulb in the circuit, nothing happens for a while (30 seconds), then the bulb will glow. quickly measuring the resistance shows a low value increasing back to the cold state resistance.

so it appears the bench testing is heating the senders.

What are you using for a 66 idiot light sending unit replacement? And what is it I've yet to learn about the sending unit circuit?

Thanks for your help.
 

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Is the 66 temperature sending unit in a special circuit?

Original idiot light sender is dead. searching for a replacement with a new 66 gauge style sender and a new late model Dodge idiot light sender. on either one, when measuring the resistance, nothing happens, you get resistance at a steady state. if I hook up a battery charger to either sending unit with a spare bulb in the circuit, nothing happens for a while (30 seconds), then the bulb will glow. quickly measuring the resistance shows a low value increasing back to the cold state resistance.

so it appears the bench testing is heating the senders.

What are you using for a 66 idiot light sending unit replacement? And what is it I've yet to learn about the sending unit circuit?

Thanks for your help.
First off, do not use a battery charger for a testing power supply. Most battery chargers rely on a load such as the battery being charged as a stabilizer for the voltage output. If you are doing a lot of testing, charge a decent car battery and then disconnect the charger and use the battery itself as a power source (and also fuse things properly so if a short is present, a cheap fuse will blow and not have the wires melting down).

Getting to how a temperature idiot light works, is when the sender itself is below the turn-on value, the sender should show infinite resistance or an open circuit and when the sender is heated to or above it's urn-on point, internally a connection is made (or a short created) between the body of the sender and the wire connection tab on it.

Now to get the temperature idiot light on a car to work, you would have 12V off of the ignition circuit of the car going to a 12V light bulb and then the other terminal on the light bulb would then connect to the sending unit's connection tab. The sender being screwed into the block with no teflon tape or sealant then has the body of the sender going to ground.

Some cars have a check light feature in that the wire between the temperature light and the sender goes to a terminal on the ignition switch that is only grounded when the key is in the crank position. When you turn the key to the run position, the temperature light is not seeing a ground through the sender and the light is off but now with the one wire split going to a ground on the ignition switch, the light should light up when cranking over the motor and once the motor is running the key returns to the run position and the light turns off. This only happens with a cold engine or a sender that is below it's turn-on point. If the motor was running hot but not enough for the temp light to come on and the motor is then shut off and restarted, the light may come on with the key in the run position and crank position and still stay on until the motor is then running and may go out as what is happening was what they call heat soak. When the water in the motor stops flowing after it's at a running temperature, the coolant will rise some and then eventually start dropping.

To keep things straight, an oil idiot light works the opposite way in that the sender in hand will show no resistance (or a dead short) across it's terminal and the threaded body but once oil pressure is put into the sender, this connection opens and the indash idiot light goes off. These senders have a pressure turn-off point depending on what value the switch is rated at.

Also too when using a meter, be careful not to be touching either of the test probes as there is resistance between your one hand and the other. It's doubtful this is affecting any readings of yours but I have no idea what ohm meter you have. Depending on the meter, squeeze with one hand one of the test probes and then squeeze with the other hand, the other probe and you might see a resistance. Now really squeeze tight and the resistance goes down. If the meter has a setting for lower resistances, this may not work but set the meter to a higher resistance and you can see how a loose grip and a tight grip across the leads affects resistance and lowers it with a tighter grip.

Jim
 

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The sender being screwed into the block with no teflon tape or sealant then has the body of the sender going to ground.
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Jim
With all due respect I have to disagree with that.

Any good pipe dope (such as Permatex #2 Form-A-Gasket) used on the sender's pipe threads will be cut through when the sender is tightened into the head or intake.
There is no way it could not have a good ground unless you didn't tighten it.
 

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With all due respect I have to disagree with that.

Any good pipe dope (such as Permatex #2 Form-A-Gasket) used on the sender's pipe threads will be cut through when the sender is tightened into the head or intake.
There is no way it could not have a good ground unless you didn't tighten it.
Oh, I agree that when tight and a seal occurs, the threads off the sender at some points should be in direct contact with the hole in the block, head, or intake but I'm wanting to come across that teflon tape or pipe dope in an electrical sense is not a very good conductor of electricity and if it has to flow just through those materials and there is no direct electrical contact. Once an electrical connection is achieved bypassing or working around the teflon tape or pipe dope, then this is good.
 

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Discussion Starter · #5 ·
Well I got rid of the battery charger. Tested the potential replacement Dodge idiot light with a battery. It did the same thing. You can feel the sender warm. Put it in the car, shows a 7 volt drop across sender, sender warms only a little. Waited for 2 minutes, no bulb lighting when it's in the car circuit. Yes I did test the dash lamp, it's good by grounding the wire going to sender. Thanks for bearing with me, this is getting a little too scientific for me. Dash lamp lights at 20 ohms sender resistance. I acknowledge getting a known acceptable replacement would beat my approach to this point. Thanks for lending me a hand folks
 

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Well I got rid of the battery charger.

Waited for 2 minutes, no bulb lighting when it's in the car circuit.

Yes I did test the dash lamp, it's good by grounding the wire going to sender.

Thanks for bearing with me, this is getting a little too scientific for me. Dash lamp lights at 20 ohms sender resistance. I acknowledge getting a known acceptable replacement would beat my approach to this point. Thanks for lending me a hand folks
When you waited for 2 minutes, and had no bulb lighting when it's in the car circuit, the sender may not be getting heated up enough by the water it is in or monitoring. The sender end that would be in the water jacket, probably needs to see around 200 degree's fahrenheit before the resistance goes down to zero or a very low resistance to allow the light inside the car to light up.

I've seen some people actually rig up the sender as well as an indicator bulb and a power supply, and then immerse the sender into a pot of water on the stove and then with a thermometer, monitor the water temperature when the stove is on heating up the water more and more to the boiling point to then see when the light comes on, and then turn off the burner on the stove and let the water cool and then see when the light goes back off.

Jim
 

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Well I got rid of the battery charger. Tested the potential replacement Dodge idiot light with a battery. It did the same thing. You can feel the sender warm. Put it in the car, shows a 7 volt drop across sender, sender warms only a little. Waited for 2 minutes, no bulb lighting when it's in the car circuit. Yes I did test the dash lamp, it's good by grounding the wire going to sender. Thanks for bearing with me, this is getting a little too scientific for me. Dash lamp lights at 20 ohms sender resistance. I acknowledge getting a known acceptable replacement would beat my approach to this point. Thanks for lending me a hand folks
So you are trying to bench test the sender then?
What is warming up the sender?
Where are you connecting the volt meter leads?
 

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Discussion Starter · #8 ·
For the test I'm doing in the car circuit, the sensor is not installed. The ignition switch is on and I'm just making a connection to a cold engine block.

The sender seems to be warming because it is in the circuit with the voltage applied. I did not expect this, but current it giving a false heat to the sender.

Going back and thinking over the 1st reply, I'm probably diving into a bunch of trivia, because this sender is not an on /off device. It's designated a suitable replacement for an idiot light application, but is behaving like a gauge with changing resistance. I'm going to search for a on/off sender.
 

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Discussion Starter · #9 ·
To clarify, I was resistance testing in boiling water to begin with. Then tested with a bulb and independent voltage source. Then in the car. The false heating will probably make the sender inaccurate low.
 

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For the test I'm doing in the car circuit, the sensor is not installed. The ignition switch is on and I'm just making a connection to a cold engine block.

The sender seems to be warming because it is in the circuit with the voltage applied. I did not expect this, but current it giving a false heat to the sender.

Going back and thinking over the 1st reply, I'm probably diving into a bunch of trivia, because this sender is not an on /off device. It's designated a suitable replacement for an idiot light application, but is behaving like a gauge with changing resistance. I'm going to search for a on/off sender.
Any time there is current flow through a resistance, there will be heat generated. The higher the resistance, the higher the heat.

I'm wondering, on your sender, let's say it has a cold resistance of 20 ohms. With an idiot light connected to this, there will be current flow but the light may be very very dim to where it looks off in daylight. Now the sensor when warmed up has it's resistance lowering and the bulb now becomes brighter. With more heat applied to the sensor the resistance continues to fall with the bulb becoming brighter and brighter the higher the heat is on the sensor/sender.

Now I have to think that most of the time you would want the temperature bulb to pop right on when the sensor is at or above it's activation temperature and then pop off when the temperature lowers below the senders activation point.

I have to think the idiot light sender is on more of a higher or quicker resistance curve in that at the time the sender is coming close to it's activation point, the resistance drops like a rock while a sender for a gauge has more of a gentler or flatter resistance curve.

Jim
 

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Original idiot light sender is dead. searching for a replacement with a new 66 gauge style sender and a new late model Dodge idiot light sender...

What are you using for a 66 idiot light sending unit replacement?

WHY are you trying to use a "gauge" sender in place of the "idiot light" sender?

First, the idiot light sender will act more as an "on-off" switch. illuminating the light when the set temperature has been reached.

The "gauge" sender will have variable resistance, so that the gauge can read from cold to hot, varying with the temp. Using this sender with the idiot light, will cause the bulb to illuminate way to early, before the coolant is even up to operating temperature.


The "late model Dodge" sender is going to act as a variable resistance, much like the "66 gauge" sender. This late model sender does not directly controol the bulb, but rather it tells the computer what temp the coolant is at at any given time... The computer will determine if the temp is too hot, and illuminate tyhe bulb accordingly...


Why not use a standard replacement sender? These are typically under $10, and the 6-cylinder uses the same sender as the V8s, fo the '1960's Chevelles... You should only need the single blade style sender that is specified for "light" (and NOT "gauge")...


I did read you other post regarding the turn-on temp listed, but I think you are misinterpretting the numbers... While they do seem high, you do not want the light to illuminate at 180 or 190, as those aren't really indicative of a problem...

Also remember that while water boils at 212 degrees when open to the atmosphere, when under pressure in a closed cooling system, the boiling point is much higher... IIRC, you add 3 degrees for each pound of pressure. Therefore, with a 15 psi radiator cap, pure "water" in the cooling system may reach up to 257 degrees before is boils and/or pressure is released... I don't recall, but I think "coolant" has a higher boiling point that pure water...
 

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Any time there is current flow through a resistance, there will be heat generated. The higher the resistance, the higher the heat.


Jim
Not trying to be picky here, but the above is true only if an equal amount of current is flowing through both the low and high resistance values. A higher voltage would be required to push the same amount of current through the higher resistance and cause increased heat. Ohm was a funny guy, I don't know who passed his law.:D
 

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Discussion Starter · #13 ·
Hey Tony, after a while into this, I also ask myself WHY I was attempting to use a resistance sender as well. Here is the answer: back in the day when the six would run hotter because of a low capacity radiator the "temp" light would glow dim, and brighten as the situation got worse. This car went thru a decade of neglect as a "someday car" So it all about distant remembering how it used to work rather than reasoning with free license. You could talk me into a max 250 degree warning, that is why I come here and ask other folks, but dead wrong as I may be, I'm not ready to accept a 275 degree warning. The engine now has an on/off sender.

Thanks for the help.
 
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