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Discussion Starter · #1 ·
Well, the wire from the sending unit from end to end has basically no resistance at all, maybe .5 ohm. So, no problem there. I bought a potentiometer and put it in line to a hot ground at the sending unit end. With it set at various ohms levels the gauge matched the expected readings from the chart provided by Dave in my first thread.

Okay students, what have we learned:
-the gauge is okay
-the wiring to the sending unit is okay
-the wiring to the 12v side of the gauge is okay
-the old sending unit and the new sending unit both provide the same ohms levels by temperature

This means that if the new sending unit doesn't somehow make the gauge work properly again the new sending unit is as broken as the old unit!

I'm driving the car to a club meeting tomorrow so we'll see how everything performs in a real world test. Stay tuned!
 

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Well, at least ya picked up the pot like I mentioned a couple of times. Have to admit it's a slick trick if you have the numbers. It's also a little cheaper than ripping out the gage.
When you get back try baking the old sender. A working home oven is usually +/- 5 degrees.
Put the ol' chef's hat on and grap the bar-b-que tongs. Set it in a Pyrex dish and bake for around 20 minutes at various temperatures. Season according to taste.
Take some readings and see how the thing is responding.
Oh, and the same pot is good for fuel gages if it is a multi-turn.
 

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Discussion Starter · #3 ·
AAAAARRRRRGGGGHHHHH!!!

I drove the car today in 95 degree weather. A great real world test for the new sending unit. Before I even got out of my neighborhood it hit it's normal full temp spot (right in the middle of the gauge). I thought it sure warmed up fast but it was hot out. As I got on the highway (about 4 blocks from home) the temp reading on the gauge continued to climb rapidly. In less than a mile is was pegged at full hot. Concerned I may in fact be over heating it I pulled off. Under the hood everything seemed in order, it wasn't puking fluid so it wasn't overheating and the upper radiator hose was hot and pressurized so it would appear the thermostat had openned.

Tentatively I continued on. I really didn't want to overheat the car but everything seemed in order. After about another 3 miles the needle remained pegged at full hot. This continued to make me nervous so I pulled off again. No spewing fluids, nice hot pressurized hoses and it seemed to be running fine. I continued on.

I then turned on the heater full blast. I was met with nice hot air but NOT blistering hot air. I figured it must be running at the right temperature so I continued on to my desitnation. The trip was about 15-20 miles. When I got there I popped the hood again and all was well. I let it sit and it never blew any coolant.

After the meeting I drove through town to a member's house for dinner. It ran great. The temp gauge shot almost immediately to full hot before I even got out of the parking lot. After dinner it was cool enough to check the coolant level. It was completely full. On the way home the gauge again pegged full hot.

When I pulled in at home with the gauge fully pegged, I shut it down and grabbed my multimeter. I checked the sending unit's ohms and it read just a smidge over 100 ohms! According to the chart provided by Dave earlier and confirmed by my own testing of my gauge that should be showing up on the gauge as half-gauge (middle of the gauge).

Okay, all my testing thus far has shown that my gauge is working correctly, is calibrated correctly, the gauge is getting the proper voltage, the sending unit wire is not inducing any additional ohms into the equation, the sending unit is working (perhaps not correctly but both old and new work exactly the same) but the system still is not working properly! This makes no sense!

A club member mentioned that perhaps I was sent the wrong temp sending unit. He said he saw a similar situation happen when a sending unit for a warning light was installed with a gauge. I don't think that could be the situation here since the warning light sending unit is simply an on/off switch that is simply not ON until a certain temperature is attained (right?). Besides, wrong sending unit or not, it is sending 100 omhs down the line, the gauge reads 100 ohms as half-gauge yet I'm getting a full+ gauge reading!

I'm going to re-install the original sending unit that was previously reading too cold to see what results I get with that. I'm so confused!

BTW, I have a 180 degree thermostat and I drilled an air bleed hole in it when I installed it last year. I don't think I have an air bubble trapped in the engine. If that were to happen I wouldn't have been able to get the same amount of coolant back into it when I refilled it anyway.

Any more suggestions?
 

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Rich...I had a similar problem with an electric gauge. The sending unit had to be installed into a bushing. The bushing was then installed into the intake. When the sender was installed in the bushing, the bulb was not in the flow of the water. It was reading the temp of the intake. I took an 1/8 x 1/2 galvanized bushing, cut off the hex, tapped the threads all the way through, and screwed in the sending unit. This placed the sender down into the intake another 1/2 inch, into the flow. Gauge works fine now. Before I figured this out, I bought a mechanical gauge, installed it in the some port and drove till it was up to temp. Never showed more than 190. With the sender in the bushing, it showed 220.....Tony
 

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Are you using a Teflon type tape when you install the sender? That could be causing issues by the sender not getting a good ground to the block? Just fishing, as it sounds like you've already got that covered. GOod luck!
 

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Discussion Starter · #7 ·
I've used no sealer of any type. It is a brass piece going into aluminum. The threads are tapered so if you snug it up tight it will never leak. Originally I had used some teflon tape but when I started this whole mess I checked and I had great continuity between the body of the sending unit and any other engine or body ground. The real kickers are:

-it worked perfectly just a few weeks ago
-the new sending unit at full temp was sending 100 ohms and I KNOW the gauge reads 100 ohms as half gauge yet instead I'm getting full gauge pluss. It doesn't add up.

This morning I reinstalled the old sending unit. It's supposed to hit over 100 today and tomorrow so I'll probably wait a few days to drive it and test it again (100 outside is just too hot to be playing with a hot engine). Hopefully it will all check out.

The original problem was I was getting too low of a reading from the old sending unit. Perhaps I just had a bad connection at the unit or the gauge which was adding resistance to the system thus making it read low. I've cleaned all the contacts to eliminate that possibility so perhaps it will work properly with the old but cleaned up sending unit.

Stay tuned.....
 

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Discussion Starter · #9 ·
John, I've boiled the thing two times and took measurements at various temperatures. The new unit provided ohms readings in line with the chart Dave provided in one of my (many) earlier posts. That is what has me so confused: the sending unit is sending out a given ohm level, that gauge provideds a given reading at that ohm level yet in a real world setting the reading at the gauges does not match!

As the car cooled down the other night I set the old sending unit on the engine right next to the other one. After 10 minutes or so it gathered the heat of the manifold and both sending units were roughly the same temperature. I then measured the ohms of each unit. I now notice a roughly 25-30% difference in the output provided by each with the new unit reading lower ohms (which would account for a higher gauge reading). Perhaps these units work differently in a real engine environment than in a boiled water on the stove environment, or the units were not even close in temperature since one was IN the engine and the other was ON the engine, or my previous testing was not very accurate.

I do know that my testing of the gauge was accurate. I checked the ohms of the in line resistor before and after each reading. But then I also know the new sending unit was putting out 100 ohms when I got home which my testing showed equates to a half gauge reading but yet the gauge was showing a full reading. Perhaps I need to do my gauge test again. Maybe my test aparatus added some resistance to the system the skewed the results.

Hopefully the old unit having been reinstalled will solve the problem and I won't have to worry about it again (until the sending unit wears out some day).
 

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Have you considered trying something like this.
The next time your engine is hot note the reading on the gage.
Disconnect the sender wire from the sender.
Measure the sender value across the sender itself.
Dial in that value on the pot.
Hook up the sender wire to the pot and ground the other leg of the pot to the sender base.
The reading on the gage should be the same.
Next ground the pot to a test wire coming from battery (-).
The reading on the gage should be the same.

Purpose is to check how everything is working under real life conditions.
 

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Discussion Starter · #11 ·
It's worth a shot. I hope to drive it a bit tomorrow and see how the old unit works now. I'm hopeful I just had a bad connection at the sending unit/sending unit wire connection that was adding ohms to the picture thus explaining the low readings. I touched the top of the sending unit connector with a wire brush to clean off any and all corrosion to insure a clean strong connection at the wire clip. I cleaned the wire clip too. If it works fine now I've been wasting my time looking for a problem I should have solved easily before doing further testing and buying parts I don't need!
 
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