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Discussion Starter · #1 ·
Electrical Problem

I need to know what the resistance measure from the center
conductor to the ground on the thermostat for the water temp sender with the line disconnected. I have been having a problem with the temp gauge "pegging" all the way to the right and have narrowed the problem to the sender unit on the thermostat housing. I replaced it with a brand new one but it is still doing the same thing. The only thing we can figure out is that the sender unit is not the proper resistance. I got the new sender from OGTS and it is a 73 GT. Thanks for any help...

Roy
 

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Discussion Starter · #2 ·
More On Electrical Problem

I know that the Temp gauge is OK because we substituted another dash panel with different gauges. It still pegs all the way to the right. There are no other problems with the other gauges, even the clock works!
There is no ground on the wiring because we ran a new set of wires when we tried the other dash panel. I even have a spare wiring harness and ran it through it to the spare dash panel. And it still does the same thing. I can take the wire and place it on the valve cover or the thermostat housing and it pegs all the way. Remove the wire and it goes back to normal position. So that leads us to think that the new sender unit is either bad or not the correct resistance.

Roy
 

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Does your sending sensor "peg" the temp gauge when the engine is cold, off, or otherwise in a "not-running" state? Does the temp spike after the car has been running?

Your thermostat might not be opening up to allow the water to circulate.

You might try taking the temperature of the water in the radiator when the gauge shows spiked.
 

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Discussion Starter · #4 ·
The Temp gauge pegs when the engine is cold or hot. We eliminated the thermostat as a problem because it didn't matter. The only thing we can figure is that the sending unit, at the thermostat housing, is bad or simply the wrong one. That is why we want to check the resistance to see. We actually took the new one out and put heat to the water end of the sender and checked with a meter. You could see the resistance going down as the heat went up. So, the only thing left is the fact that the sending unit's resistance is not where it should be. The old sender did the same thing!
This is the strangest thing I've seen in awhile (Next to my ex's boyfriend)!

Roy
 

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It sounds like the wire to your gauge or sensor is grounding out some where, maybe a short.

I ohmed a brand new sender, still in the wrap, and got .53 I also ohmed the current sender in my GT and the engine was still warm from running about 2 hours ago. It was warm to touch, but not uncomfortable. It ohmed out at .10

Hope that helps you checking it out. You might try removing the sending wire from the gauge in the dash and see if you get different results with the gauge.
 

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Discussion Starter · #6 ·
Thanks for your input. I have an extra dash panel with all the guages and an extra wiring harness for the whole car. I laid all this up on the engine and wired it straight through to check how the guage reacted to another complete set of wires and guage. I wired it straight to the sending unit on the car and it still did the same thing!
But your ohm readings on the sender are much different than the readings I'm getting. I'll check that out and let you know what I find... Hummm, maybe we are zeroing in on this problem... What model GT do you have?

Roy
 

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I've got a 72 GT with a 1.9 bored to 2.0. It has 75 EFI, which does have a different thermostat houseing, but I am using the same gauge that originally came with the car.

I will check the current sender this morning while it's cool.
 

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Group,

Anyone who has bought a recent Wiring Diagram Poster from DESTEC has a small chart in the notes that show Tsender Ohmic values Vs Eng Temp. So if anyone has one handy, maybe you could quote some Temperatures Vs. Resistance reading .


JJ
 

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A chart showing Ohms Vs. Galons is also available on request.
In case you gas gauge goes out, you can see how much gas you have with an Ohm Meter.
You read from the RED jack pin 4 to Ground.

Write for a POSTAL mail copy.

JJ
 

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I just Ohmed my sending unit again and, all I can say is, it was late last night. I'm not sure what my ohm meter was on.

104 in the car in the garage over night at around 68 degrees.
641 for the new sending unit, never installed, in the package. Sure seems way off between the two.

I think the chart you were refering to JJ is
90 degrees = 400
110 degrees = 360
175 degrees = 105
192 degrees = 80
REDLINE <= 30

That makes me wonder if my new sender will work. I bought it when I thought my old sender was on the fritz. I don't think my GT is 175 degrees right now, not after sitting in the garage over 12 hours with the temp in the garage at 68.
 

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Discussion Starter · #12 ·
The reading you got this morning is more in line with what mine is reading. You may want to check that new sender unit and see if it pegs your gauge, too. Do you think that OGTS could be selling a sending unit that is rated wrong for the GT? They would not do this if they knew it was the wrong part in the right package. But I would be interested to find out if anyone else has had that problem recently with their sending units.
Also, thanks for the ohms chart. I will use that to check on mine!

Roy
 

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I wouldn't think the new sender I have would peg the gauge to RED because of the ohms it takes. There seems to be plenty of resistance in the sender to keep the needle to the cool side of the gauge.

You might check to see if somehow your gauge is grounding out, such as a wire is worn through.

I just came across a wire that was worn through this weekend. Didn't even know it before I pulled it. My son told me it looked like something had chewed away at it.
 

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teflon tape?

You didn't perchance use teflon tape or RTV on the sender when you installed it did you? If you did, you need to take it back out, scrape the stuff off, clean the threads and reinstall. That unit needs a good electrical connection to work and sealant is not good for electricity.
 

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But if there was teflon tape around his sender, wouldn't it never get above cold level. Meaning that the resistance would never decrease enough to show anything on the temp gauge.

However, if the wire was shorted to ground or there was never any resistance in the circuit, it would peg the gauge as soon as electricity hit it.

JJ, What are your thoughts? You're the electrical guru... You are the one that I saw mapped out the resistance to the temp. What might this be?

Thanks,

Chris
 

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Chris, you are correct. do not use teflon tape of pipe goop on the threads: that is your ground connection.

Though these are not linear devices: generally, the warmer the engine, the less resistance reading you will get.

I got these figures by putting a sender in hot water and monitoring both the temp and resistance.

Then I hooked up a potentiometer to the gauge head to determine where "Redline" was.

JJ
 

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Discussion Starter · #17 ·
Temp Gauge Update

We got the Temp gauge working but it wasn't what we may have thought. By using the information you provided, we were able to eliminate the sending unit as the source of the problem. I had an extra temp gauge and wired it up and it worked fine. The original temp gauge had failed internally. I hate that because it is in pristine condition. It is not faded and looks brand new. The ground wires were all intact on the outside of the gauge that failed. It is a grounding problem for sure but it has to be inside the housing of the gauge.
Do you know of anyone who could repair the temp gauge since it is such great condition on the outside? Or is it possible to simply change the face on the gauges?
Thanks for your help!

Roy
73 GT
 

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Ooooner, on older guages, the mag.coil wire breaks. If you have a 10in. soldering iron, you could fix it. Take your time and examine the case, it should come off fairly easy. If the wire is intact, you have two options: 1). find a instument/motor repair shop (Yell-pages) for a coil re-wire or 2). take the guts out of a new guage (having the same sweeper length) and istall it.
My 2c's
 

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Discussion Starter · #19 ·
Thanks for the input. The gauge I have is from my 1973 model but I will try to look at what you said and give it a try! Thanks! I'll let everyone know what I find out...

Roy
 

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My 73 GT temperature gauge has been acting up.

Within the last couple of weeks, the needle on the temperature
gauge jumps around violently such that you can hear the needle
hit. This occurs at idle no matter the actual temperature. It also
occurs during normal driving. However, when I get up to the
magic 60 mph, the needle stablizes to where it normally sits when
all is well. As soon as I drop the speed down, the needle starts
to jump.

The belt seems to be ok--no different than has been in the past.
If I were to guess, I would guess that it's an electrical issue.

Any thoughts before I go through the hassle of removing the
dash and test the wires and/or sending unit? The only other time
I had a problem with the temperature is when the fan belt broke
about 5 years ago. Luckily I had a spare in back compartment.

thanks

bill
 
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