C1 '60 temp reading low

[Mike Tess]

My 1960 temperature gauge reads very cold. I have read the archives regarding this topic and am hoping someone can provide some more info.

As suggested in the archives, I verified the accuracy of the gauge by connecting a resistor (68 and 100 ohms) inline to the temp sender wire, and the gauge read 220 and 180, respectively. This points to a high resistance problem somewhere at the the temp sender: at the wire connection to the temp sender, in the temp sender, or between the sender and the manifold. I have cleaned up the connection at the wire and the temp sender, and installed the temp sender without any sealants. (I have tried two different temp senders, an original and a Lectric Limited calibrated version, and both read cold. Prior to fooling with the temp sender, the gauge read correctly with the original sender. Then I installed a new radiator and as part of that project I removed and reinstalled the original sender with teflon thread sealant. The gauge has ready inaccurately ever since.)

I'm not sure what else could be the problem. Seems I have checked everything as mentioned in previous threads posted in the archives. What have I missed? Is it possible that some sealant residue in the threads of the manifold is causing the high resistance connection between the sender and the engine block? Is it a valid test to check the resistance between the manifold and ground? The engine mount ground straps are in place with good connections. I don't think the original sender is bad, because it worked fine before I messed with it. And the supposedly calibrated sender reads the same way.

Thanks in advance,
Mike Tess

[Roy B.]

Re: C1 '60 temp reading low

Add a temporary ground wire to the temp/sender housing then to the battery ground post and see if it works OK, if so your losing ground some wear on the engine.

[John D.]

Re: C1 '60 temp reading low

Mike, THe following is not really the correct way to test temperature sending units but I do it alot and get away with my crude test most of the time.
This test is done with a very clean temp sender sitting on my workbench at about 72 degrees. I use a good ohm meter and measure the ohms resistance. Grease and dirt change the readings.
Here is an example of local auto store replacement TSU readings. .67 to .69.
This low of a reading won't begin to give you a proper reading.
A NOS one AC one that I tested not long ago that went on a 59 Corvette read .92 and the gauge worked perfect.
Typically I like to see readings in the .80 up to the .90's.
Yes I know you guys have much much better methods but this one has kept me out of hot water so to speak.
The one on my 63 was NOS until a year ago and it reads .933. Gauge works perfect and passed ops. Ops guy said he liked the way my gauge just laid on the 180 degree mark. I believe the judge had blond hair, was tall, and always wears the same shorts. I am working on a 62 FI and the original AC Made in USA TSU reads .921
As you mentioned teflon tape and other sealants on the tsu's really throw it off.
A cheater though is Loctite 545 hydraulic sealant. Just a little bit goes a long way and I have used it successfully on TSU's without it affecting the performance. You can always send Fred Olivia your gauge and temp sender and he will calibrate the two for you. Does a nice job. John

[Jack H.]

Go lose...

the teflon tape sealant you dreamed up for wraping the threaded end of the temp sender! The sender gets its ground reference by native ohmic contact between the brass threads and the intake manifold. If you've managed to increase the contact resistance here, then you've essentially installed an unintentional series resistor that'll raise the overall branch circuit resistance causing the temp gauge to read cold...

Once you've removed the temp sender and cleaned the threads on both sides, reinstall the temp sender and verify nearly zero-ohms resistance between the brass body of the sender and ground at the engine (the base of the coil holder is a good place to get engine ground).

BTW, nice move on substituting a 100 ohm resistor to ground to simulate a 180F gauge reading!!! Many simply take the gauge side of the circuit for granted and concentrate on swapping temp senders to find a 'correct' match. You done GOOD on verifying that aspect of your system first...

Also, bear in mind, it's one thing to 'statically' check the system (engine off, ignition on) and another to dynamically verify the system (engine running with generator actively charging the battery and pushing the system voltage up to 13.5-14 VDC)...

[Mike Tess]

Re: Go lose...

Jack, thanks for the kind words but I can't take credit for the methodology in verifying the accuracy of the gauge...I read your old posts and followed your instructions to the letter, including running the engine while conducting the tests. Your expert knowledge is a great asset to the board!

This morning I checked the resistance between the temp sender and the intake manifold, and got a reading very close to zero. (The last time I attempted to fix this problem, I installed the original temp sender without any thread sealants.) The sender itself, however, has a resistance upwards of 5,000 ohms. Upon removing it and testing it on a work bench, I got the same reading. I shook the temp sender and didn't hear anything rattling around in there, but did notice some water coming out of the bottom of the unit itself, where there is a joint between the threads and the inner part that sticks down into the coolant. I suppose all this means the sending unit is bad? Any way to fix it?

Mike

[Jack H.]

Re: Go lose...

"This morning I checked the resistance between the temp sender and the intake manifold, and got a reading very close to zero. (The last time I attempted to fix this problem, I installed the original temp sender without any thread sealants.) The sender itself, however, has a resistance upwards of 5,000 ohms. Upon removing it and testing it on a work bench, I got the same reading. I shook the temp sender and didn't hear anything rattling around in there, but did notice some water coming out of the bottom of the unit itself, where there is a joint between the threads and the inner part that sticks down into the coolant. I suppose all this means the sending unit is bad? Any way to fix it?"

That's WAY too high of a resistance reading for ambient temperatures! Typically, they'll vary in the 500-800 ohm range. There's early and late versions of the temp sender with the early version having a 'thin' lower brass barrel with clearance between the barrel and the inside ID of the sender's brass upper body.

These units are notorious for aging and leaking at the barrel/body interface. Once coolant enters the lower barrel, all (*&& breaks loose....

The ohmic contact between the conductive spring pressing against the thermistor pellet at the bottom of the barrel corrodes and you build up internal series resistance.

After an extended period of time, coolant in the barrel begins to dissolve the fiche paper insulator that keeps the conductive pellet spring from touching the outer brass wall of the barrel. Once this insulating protection is gone, the spring can touch the barrel wall resulting in the thermistor pellet being 'shorted' to ground.

The first defect mode makes the gauge read 'cold'. The second defect mode makes the gauge read 'hot'. Here's a long shot approach to saving your existing sender (read that improbable)...

Put it in the oven on the 'Keep Warm' setting and let 'er heat soak for the better part of a day. You're trying to 'boil out' any trapped coolant. Remove from the oven and let 'er cool down for several hours.

Now test the sender's resistance and IF you got lucky and it's back in the 500-800 ohm range, cool! Clean the brass surface interface between the barrel and the ID of the brass body. Lay in a bead of epoxy to seal the breach and let the glue set up with the temp sender inverted. You just might have bought yourself more useful life.

If that doesn't work (she still reads high resistance after attempting to 'boil out' trapped moisture), short of sending your existing temp sender to Fred Oliva (fan clutch restoration services) who's devised a means of opening a temp sender and repairing it, I'd say you're pushing on a rope trying to make what you have work...

[Roy B.]

Re: Go lose...

Go find one from a 50's chevy Car Same thing

[John D.]

Re: Go lose...

Roy, Didn't the 50's up to and including 56 had a "stud"-stright pin- sticking up on top of the TSU rather than a nail head? John

[Roy B.]

Re: Go lose...

You may be right I'm getting oldtimers