I had a similar problem. I replaced the radiator and the temps came down but not much.
At the advice of another vet owner I purchased a mechanical temp gage ($30) and temporarily installed it. It screws into the block where the sending unit goes, no need for the sender with the mechanical gage. I ran the piping/tubing that connects to the gage through a piece of foam pipe insulation and then over the upper fender, through the vent window and wire tied the gage to the passenger side hand grab. I snapped the hood closed on the driver side but not the pass side (it probably would have been easier with an IR gun ). It was a 90 degree day. The mechanical gage showed 180 degress while driving and 195 when idling in my driveway for 20 minutes.
I then went and purchased the Wells TU-5 at Autozone as per John's excellent article...

http://www.dewitts.com/download/cooling.pdf

and my problems were over.

About 6 months before this I bought a repro sender for about 10 times the cost of the Wells TU-5 and that's what I replaced with the Wells unit.

 

These pics tell the story:

 

I don't know what brand of sender I have it was in the car when I bought it and it is accurate. I did notice a "6" (See Below) stamped on it. Does anyone know what brand might have that '6" or do all of them?



George

 

Buns:

Excellent information. Thanks. I always felt that the teflon tape would not significantly affect the readings, but your pictures confirm the story.

Judging by the ohm readings, it must have been downright chilly outside, or that carburetor stud has a lot of corrosion.

Larry

 

Good show,Tim!!!

Dave Z

 

Yes, I should have mentioned, it was about 45 degrees in my shop and the ground was good.

 

Thanksgiving greetings to all our Forum friends! I've been tracking this thread and gleaning lots of great info. Being a data junkie (comes with my job) I have plotted the temp vs. resistance data from the various sources above. I have also compiled info from the various forum posts. I have attached the PDF file of the collective info and hope I have given credit where credit is due. If I missed you, let me know!.

If the thread continues on with more "good stuff", I'll edit the file.

Happy Thanksgiving to all!

Dave Z

 

daayum.

memo to self, never challenge a position Buns has taken, he just might whip up a pretty convincing test to prove his point!

 

To measure the effect of the tape on the resistance, you should have measured from the side of the sending unit. Then the tape would have been the only unknown in your test. You are testing the tape
factor and the resistor in the sending unit. You are assuming that the
resistor never changed during the test. The friction of screwing it
into the intake and the heat of your hands would change the resistor
value.

Ray

 

And that is why I waited for the sender to return to room temperature before I snapped the next pictures.

 

Dave, nice job putting this together.

Thanks.

 

Ummm, Dave, me thinks you may need a new project car to keep you busy, or your house was full of family and you realized your computer was your only den of sanity yesterday.



Nice job as usual.

 

Stratplus,
Thanks.

62Jeff,

It's all Buns' fault!

He and the others dangled that data in front of me and I was like a deer in headlights!

Hope the info gleaned from the "forum folks" makes the rounds.

DZ

 

I've had the same problems with the gauge reading considerably higher than the actual temperature which was checked and re-checked numerous times with an IR gun. After reading this thread, along with several others that have been posted over the years regarding this problem, I proceeded to correct my gauge readings after installing a new Summit aluminum intake manifold and cast iron heads on the 350 replacement engine in my '66.

I began by mapping the resistance readings on the dash gauge using a variable resistor to move the pointer throughout its travel range and found the readings to be very consistent with, and in some cases duplicating those readings previously posted in previous threads for the '66-'67 gauge. I also mapped a spare '66 instrument cluster gauge (actually, the original cluster from this car) and found it also consistent with previously reported readings. In short, about 79 ohms resistance produced a mid-scale reading on the gauge of 210 degrees.

Looking at the various sending units reported (Niehoff, Standard Motor Products, GM AC/Delco replacement) I determined that the GM replacement provided the "closest" value for the gauge at 210 degrees. It was shown to produce 84 ohms resistance at 210 degrees in the tables previously reported.

I installed the GM replacement along with a new GM 195 degree thermostat and found that the reported temperature on the gauge was about 240-250 degrees, obviously in error and confirmed to be so when measured with an IR gun. Temperature at the thermostat housing when the engine reached operating temperature and the thermostat fully opened was measured with the IR gun to be 192-194 degrees. Checking the resistance of the GM temperature sending unit at these conditions showed that an additional 20 or so ohms resistance should produce a gauge reading of about 190-200 degrees.

I had purchased a Wells TU5 from Autozone and mapped the resistance of the unit at various temperatures, which are shown below:

Water Temp Ohms
Degrees Resistance
100 429
110 328
120 283
130 250
160 173
180 135
200 109
211(boiling) 104

I had already planned on returning the Wells unit based upon the resistance readings, since the ohms reading for a 210 degree gauge reading based upon mapping of my two and previously reported gauges indicated that the Wells unit would be incorrect. However, after determining that an additional 20 or so ohms added to the GM replacement sending unit would produce a more nearly accurate reading, I decided instead to install the Wells TU5. So I did.

The Wells TU5 is very accurate at the 100 degree mark on the gauge; however, more importantly it is about spot on at normal operating temperature. After driving a few miles to insure the engine is fully warmed and at normal operating temperature, the temperature at the outflow side of the thermostat housing reads 192-194 degrees and the gauge hand rests comfortably just about the width of the pointer on the left side of the 210 degree mark. I would estimate this location to be about the 190 degree mark.

I have no explanation as to why the GM sender didn't accurately report the temperature, since the original data showed it would produce 84 ohms at 210 degrees. I checked the GM unit I purchased before installation and it also measured almost the same, 85 ohms at 210 degrees.

Long and short, the Wells TU5 works quite well in my application, and after replacing the previous thermostat with the GM 195 degree 'stat, I also find that the "hunting" of the pointer on the temp gauge has been eliminated. It used to overshoot, settle back, overshoot again, and finally dampen out. No more, just one smooth ride to about 190-195 degrees on the gauge, verified correct at the thermostat housing with an IR gun.

I know this is an old thread, but I thought resurrecting it to report my experience might be the most beneficial way to help another forum member.

 

A cheap bubba test would be pitting a 32 Ohm resister in line between temperature sensor and cable to see if your gauge goes back down to a reasonable reading to IR GUN when 240 is displayed. They are cheap and Radio Shack and for 1 Watt version. You will find the gauge reading should go down to something better than seeing 240 vs IR Gun. There are form readings about putting thermo-switch + 2nd resistor in parallel that will help the higher temp readings by reducing the resistor value at higher temps. This would require a lot of creative thinking in putting the extra components in a good area to get accurate temperature readings for the thermo-switch sensor. I did this on my brothers 65 C2 after he had this issue after having the gauge replaced and the readings would go far right when the car is running about 190.

Have you confirm your thermostat is correct? My car ran very hot before (210-220+w/AC) with Harrison aluminum radiator in traffic jambs on hot days. I was going to change my thermostat and drain the coolant /additive/de-ionized) and discovered there was a 210 thermostat installed in my car. Changed to 180 and the car runs lower near 160-180 now w/o flushing the coolant. We haven't gotten our hot 100+ days yet for me to retest.

 

The original thread creator, ddsjoseph, has probably long since corrected his problem, or quit worrying about it as the original post was in 2008. My previous post, #34, was intended only to confirm that the Wells TU5 was a good replacement for someone with a similar problem as ddsjoseph and several other members have had with replacement sending units; that is, dash gauges reading considerably higher than the actual operating temperature of the engine.

Probably should have just started a new post in this regard, and may need to do so yet to alleviate any confusion by reviving this older thread.