I have a 65 coupe, with 327, 350 hp. The temp guage always goes up to max at 240* after 1/2 hour of driving, no matter what the ambient temp. So I changed the radiator to a 3 row high efficiency copper/brass and even added an external cooling fan. Then I changed the temp sending unit 3 times. I finally used an infrared temp gun to check the temp at the sending unit, and it showed 210* instead of 240. I am not sure if it is the temp guage inside the car that is inaccurate or the sending unit. Aslo, if it is the fault of the guage, then would it be better to add a variable temp. sender resitor or just change the temp guage inside.

 

ah, ok... Now I see why you wanted to go to a 6 bladed fan in a previous post... I would've put this all in one post.. But, be that is may, it seems that your sending units suck, which is pretty normal with todays POS repro's.. If I'm correct you should also be checking the temp at the top neck (coming off the engine) and not at the sending unit... You've got other problems it seems.. how is the fan clutch, waterpump, timing etc.. While I's sure someone with more knowledge will jump in and help, you can try and do a seach on "overheating or cooling problems" and pray that the search works...

 

You can check the accuracy of your sending unit by this scale.

220 degrees = 70 ohms
200 degrees = 90 ohms
160 degrees = 140 ohms
120 degrees = 250 ohms
100 degrees = 340 ohms

I think part of your problem may be the radiator; the car should not be running 210 either. I had the same problem with my 62 and switched to aluminum which solved the temp problem. Now it will get up to 200 only when it's 95 outside. There are two repair help items on this on our site, you might take a look at them. http://willcoxcorvette.com/advanced_...words=radiator

Installing a resistor inline can drop the dash gage to where you need it but it will only make the gage accurate at that set point.

Double check your sending unit first, then go looking for the problem.

 

It's almost always the sending unit, as your diagnosis shows; very few replacements are accurate, although you can get lucky occasionally and find one that's somewhere near close.

If you'd get a Dewitt's aluminum reproduction radiator, it'll run at 180*-190* without any extra cooling fans (assuming you have the correct fan, clutch, and shroud). The copper/brass replacements have about 30% less cooling capacity (heat rejection capability) than the Harrison aluminum radiator.

 

Below is what worked for me.


I have a 65 327 / 350hp with new everything and a total of 16 miles after a complete frame off. The first 5 miles (75 deg outside temp) the temp never got past 180, stopped and got a 4 wheel alignment drove it 5 more miles and at the point never really looked at the temp gauge and stopped at a local shop. After a short stop I went to start it and notice the temp gauge reading 240, so I let it set for a couple of hours and the gauge went back down to 150. Drove it 6 miles back to the house (93 deg outside temp) and temp gauge never got over 190.

I pulled it into the garage left it running and started checking temps with my IR gun, radiator inlet was 184, 195 on the gauge and 167 at the radiator outlet. I shut the car off and let it set of a few than turned the key on and the temp gauge was reading 240. At this point with the gauge reading 240, engine not running I put the IR gun back on the engine and never found any part of the cooling system over 175 degrees.

I took out the Limited Lectric sending unit (PN 01513321 / $25) and also picked up a new Wells sending unit (PN TU5 / $6) from Auto Zone and did my own experiment on the stove.

Temp GM Ohm Value LL Ohm Value Wells Ohm Value

100 410 325 375
120 300 266 292
140 187 193 235
160 150 149 176
180 123 112 145
190 112 103 124
200 94 90 116
210 83.5 84 105

The LL and Wells sending units did not really match up every GM value but what I found interesting was the LL sending unit consistently has higher resistance values (or lower depending on how you view it) than the Wells unit.

I installed the new Wells unit and ran the car nosed into the garage (95 degree outside temp) with no fan in front of the car for 45 minuets and the temp gauge was reading just past the 180 mark. I put the IR gun on it and radiator inlet was 183 and outlet was 167. I shut the engine off and after a few turned the key on and temp gauge read 160.

So I guess it was a mismatched sending unit problem. Hope some of this helps others.

 

John,

That is the same thing I thought. The link I posted was to your paper on our site "Corvette Radiator Cooling System, How it Works 1963- 1982 By John Hinckley" and the bone headed move I made years ago with the brass replacement rad in my own car.

It won't fix his sending unit problem, but it may help him understand why he is running at those temps.

Willcox

 

mine does.........

 

Hay Guys, Will the TU-5 fit small and big blocks both intake and head applications?
ddsjoseph, Let me apologize if this is a Hi-Jack, but I thought it might help others as well as myself.

Earl

 

Take the teflon tape off of the sending unit....if its on there

 

Thank you everyone. I will order a new radiator from Dewitt and hope that fixes my overheating problem. I also found that the engine runs much smoother before it really heats up. It would be nice to enjoy driving the car, with confidence. I will post another thread once the aluminum radioator is in.

 

This is about $29 from CC, worked great, you will need a IR to check actual temp and adj,

 

Remember that installing a variable resistor lets you tune your gauge to be accurate for a given temp, at the expense of accuracy in the other ranges.

 

Jeff,

You are 100 percent correct! Adding resistance only makes the instrument correct at one setting! Resistors are available from Radio Shack for a few bucks if you want to go this route. Or once you know the resistance variable, you can purchase the needed resistor from the same place for about 1.00. This will dial you in for one temperature but the gauge is not linier and other readings will suffer.

Willcox.
18243

 

Do you really have an overheating problem, or just a gauge that is reading high? I would first try a different sending unit. Look at the chart below. The "typical" sender values are supposedly from the original G.M. specs. Look at the specs of the AC DELCO #G1852. My test shows they match at 200 degrees.

I don't know why Wells chose 220 degrees as a referance temperature. Once water in an open container starts boiling at 212 degrees, it won't get any hotter.





You also need to look at your temperature gauge. There is a 90 ohm resistor on the back of it. Make sure it isn't burnt out. Also make sure that the silver terminal on the gauge has a good ground to the cluster.











In this picture you can see that the Standard Brands, Niehoff and G.M. sending units look identical. I would surmise they were all built by the same manufacturer. You can also see that two of them already have teflon tape on them. Lots of people will tell you not to use it but I disagree.

 

why do you disagree on not using the teflon tape? There really isn't an argument here to disagree about.
The temp sender unit gets it's ground from the threads when it's screwed into the intake manifold. By adding teflon sealing tape you are increasing the resistance of the ground (by somewhat isolating the metal contact between the threads of the sender unit and the intake). By doing this you affect the temp reading from sender. In effect it's about the same as if you just wired in a resistor in line with the sender unit.

BTW, the AC Delco sender unit pictured above may have been sold under the Delco name, but it's obviously a newer style sender unit and as you mentioned, probably identical to the others and most likely off the same production line as the others shown. It is NOT the same as a real original Delco sending unit. You can see it doesn't have the correct markings on it.

Here is a pic showing three real, original GM Delco temp sender units. The one on the left was the one that went bad out of my '65, the other two are used units I located that are correct original units also from the 60's, not aftermarket replacements or new style Delco units. Finding used or NOS original units was the only was I finally found to get accurate temp readings when the first unit in my car went bad. I refused to go with the wired in resistor method for the simple reason as already mentioned by Jeff - it's only accurate at one temp.

 

The chances of teflon tape affecting the gauge reading are remote - the pipe threads are very sharp, and cut through the tape with little effort. Even if the tape did affect the grounding of the sender to the manifold, it would make the gauge read LOWER, not higher (higher resistance = lower gauge reading).

 

The original GM #1513321 senders (that still work) are hard to find; I have a "Standard" ST-6 in my '67, and it's right on the money - just lucky, I think.

 

 

Nice post Buns!!! Got it in my collection!

DZ

 

Buns,

That is quite a good post!

Teflon tape is one of the most over used tapes in the world IMHO. There is nothing wrong with using Teflon tape on your sender it’s how you use it that counts!

Teflon tape us supposed to be used on the first two or three threads and is not used as a sealing tape! It is used as a lubricant to allow the pipe threads to seat further in the mating surface.

Willcox Inc