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Discussion Starter #1
Okay, double check me on this one.

First, I disconnected the lead at the sending unit. I got almost no reading at all (super high resistance) between the body of the sending unit and the part of the unit that clips to the wire. I got great continuity between the body of the unit and any other good ground on the car such as the alternator. (So the unit is well grounded.)

I then turned on the key with the wire disconnected from the sending unit and it went to way past cold. I then connected the wire to a good ground and it went way hot. For the time being I assumed the gauge was okay and moved back to the sending unit.

After draining the coolant I removed the sending unit. Making darn sure I got a clean connection I recognized I was getting between 1000 and 2000 ohms with the unit cold. When touching the probes both to the body of the sending unit I got zero ohms. So good so far.

I ran the hottest tap water I could get on the unit and I couldn't get it to go under 500 ohms. I then boiled water and using a cooking thermometer I got the water to just over 200 degrees (boiling). I soaked the sending unit for over a minute. It then read just over 150 ohms on the multitester.

So, I conclude that the sending unit has gone weak and needs to be replaced. Of course I want to convince myself the gauge is not at fault since that would be more difficult and expensive to repair or replace. New sending units are readily available and $12.00 each.

Please confirm for me if I'm right or wrong. According to the charts posted in my other thread this sending unit is way out of whack.

I do have to say that I found that my multitester (an old fashion needle unit) was sticking so I had to tear it down and clean out the meter mechanism. I got it working okay and zeroed it out again. I assume the multitester is working accurately after this cleaning. All I really did was blow through the meter and apparently I blew out some dust that was gumming up the works. Since it can sweep the whole dial from off to zero ohms with the probes touched together I assume it's working fine.
 

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The microwave is a no-no unless you have pita bread for it.
Seriously, if Dave doesn't come back, try emailing him. He has some good numbers.
 

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Rich,

Your gage sounds fine to me but the sender appears to have seen better days. An open (infinite resistance/freezing cold) as when you remove the lead from the sender will try drive your temp needle below its mechanical stop in the cold direction. Conversely, when you short the lead to ground (zero resistance/incredibly hot) the temp gage will go to it's full scale mechanical stop. I'd replace the sender but keep in mind that it may read higher than the original for the reason I listed in your other post. Good luck.
 

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Discussion Starter #5
Follow up: a new sending unit is on it's way. I wish I could find an NOS sending unit. I think the one I had before was a modern one as the temps always read higher than they did before. For quite a while I believed this was due to the engine rebuild causing more heat but now I have reason to believe it was the change in sending units. Last summer I checked the temp with a mechanical gauge with numbers on it so I could get a feel for what temp readings I was actually getting. It never got very hot according to that gauge so at least I now have a way to keep track via the factory gauge since I can reference it against the readings I got on the aftermarket gauge while it was installed. I also plan to test the new sending unit before I install it so I know it's working properly when it goes in.
 
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