temperature sending unit
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,347
Likes: 654
From: Tampa, Florida
temperature sending unit
I need to replace the sensor on my '70. Its the one mounted in the block on the left side.
How much coolant is going to come out if I unscrew it without draining the system? What should I put on the threads? Teflon tape?
Thanks,
Jim
How much coolant is going to come out if I unscrew it without draining the system? What should I put on the threads? Teflon tape?
Thanks,
Jim
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
if you use tape or sealant, be sure to check for a good ground connection with an ohm meter before you refill with coolant. you'll lose about a quart or two at most, less if you're quick.
Safety Car
Joined: Apr 2002
Posts: 4,653
Likes: 6
From: North Royalton, OH
St. Jude Donor '06-'07
I don't think you want to use any sealant - the sender gets tighter as you screw it in. I have no leaks in mine after changing it out. Not sure how much fluid you'll lose - jack the car up on the driver's side and you'll lose less...? 
Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
i couldn't get mine to seal without sealant. they are NPT threads which are made in a way that does not completely close the gap between the treads when tightened. i think there is a thread out there similar, can't recall the name, that is supposed to seal, but it isn't the NPT; maybe an overseas spec?
if you use tape or sealer, it gets pushed out of the way for at least several threads so ground isn't a problem. try it out: try dry (maybe you'll get lucky and it'll seal), try tape then remove -see how many threads still are covered with tape; try same with sealant.
i've had more trouble getting a seal, but never getting a ground.
jst one member's experience . . . 0.02
ps drain coolant from the radiator first far enough so that the coolant in the head is below the sensor threads so that you can get the threads in the head clean and dry
if you use tape or sealer, it gets pushed out of the way for at least several threads so ground isn't a problem. try it out: try dry (maybe you'll get lucky and it'll seal), try tape then remove -see how many threads still are covered with tape; try same with sealant.
i've had more trouble getting a seal, but never getting a ground.
jst one member's experience . . . 0.02
ps drain coolant from the radiator first far enough so that the coolant in the head is below the sensor threads so that you can get the threads in the head clean and dry
Last edited by S489; Apr 11, 2007 at 01:08 PM. Reason: ps
Melting Slicks
Joined: Nov 2001
Posts: 2,668
Likes: 43
From: Eastern US XX
You should not use tape on electrical sensors.
NPT is a tapered thread. When first screwed in, it is loose, but as the tapered male goes deeper, the threads become tighter and that's when the sealing starts. Complicating things is the dissimilar metals in this case (brass sensor and CI or Aluminum heads) - which can cause "gauling" and subsequently binding (that's why sometimes the joint "feels" tight but still leaks). You should use a good "pipe joint cement" (aka "pipe dope") on ALL NPT threads - it also acts as a lubricant. One of the best is RectorSeal - I use it on all threads. Available at Home Depot or many hardware stores. Teflon tape is the greatest when there is not an essential conductivity requirement (water pipes, etc.).
Change the sensor when the engine is cool. Have the new sensor "doped up" and ready to screw in. Unscrew the old with one hand and be ready to block the hole with a finger of your other hand. Grab the new sensor and push your finger out of the hold with it so it blocks the hole immediately. Tighten it up and you're done. Shouldn't loose more than a few ounces of coolant.
NPT is a tapered thread. When first screwed in, it is loose, but as the tapered male goes deeper, the threads become tighter and that's when the sealing starts. Complicating things is the dissimilar metals in this case (brass sensor and CI or Aluminum heads) - which can cause "gauling" and subsequently binding (that's why sometimes the joint "feels" tight but still leaks). You should use a good "pipe joint cement" (aka "pipe dope") on ALL NPT threads - it also acts as a lubricant. One of the best is RectorSeal - I use it on all threads. Available at Home Depot or many hardware stores. Teflon tape is the greatest when there is not an essential conductivity requirement (water pipes, etc.).
Change the sensor when the engine is cool. Have the new sensor "doped up" and ready to screw in. Unscrew the old with one hand and be ready to block the hole with a finger of your other hand. Grab the new sensor and push your finger out of the hold with it so it blocks the hole immediately. Tighten it up and you're done. Shouldn't loose more than a few ounces of coolant.
Last edited by pws69; Apr 11, 2007 at 01:24 PM.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
i couldn't get mine to seal without sealant. they are NPT threads which are made in a way that does not completely close the gap between the treads when tightened. i think there is a thread out there similar, can't recall the name, that is supposed to seal, but it isn't the NPT; maybe an overseas spec?
if you use tape or sealer, it gets pushed out of the way for at least several threads so ground isn't a problem. try it out: try dry (maybe you'll get lucky and it'll seal), try tape then remove -see how many threads still are covered with tape; try same with sealant.
i've had more trouble getting a seal, but never getting a ground.
jst one member's experience . . . 0.02
ps drain coolant from the radiator first far enough so that the coolant in the head is below the sensor threads so that you can get the threads in the head clean and dry
if you use tape or sealer, it gets pushed out of the way for at least several threads so ground isn't a problem. try it out: try dry (maybe you'll get lucky and it'll seal), try tape then remove -see how many threads still are covered with tape; try same with sealant.
i've had more trouble getting a seal, but never getting a ground.
jst one member's experience . . . 0.02
ps drain coolant from the radiator first far enough so that the coolant in the head is below the sensor threads so that you can get the threads in the head clean and dry
There should only be 3 wraps of teflon tape used. It will only fill the voids where the leaks would occur and is cut out of the interferance areas so as to not cause a grounding problem.
Some people use so much tape that it could be used as a golf ball.
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Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
Free country - you can certainly be wrong together!
before i waste any more time trying to type a proof beyond the above . . . maybe it would be better if you provided your technical references supporting your position, that we would all love to read, tapered threads never leak when tightened enough. to be fair, no one can count personal experience or anecdotes . . . how fair is that?
Le Mans Master
Joined: Mar 2004
Posts: 7,465
Likes: 1,485
From: Little Rock AR
I believe the factory used pipe thread dope based on the senders I have removed. I would use pipe thread paste if you have it. This might be shown in the AIM but I must confess I did not check. Not worth the time IMHO.
Tape is fine if you do not have any paste. Just confine it to the first half inch of thread or so.
-Mark.
Tape is fine if you do not have any paste. Just confine it to the first half inch of thread or so.
-Mark.
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
Drain your radiator (can be done from the radiator cap with a suction tube) about half way, then loosen the old sending unit and check for drainage. If fluid leaks out, remove more from radiator. I would also use 2 - 3 wraps of teflon tape.
JimT
JimT
Racer
Joined: Jan 2004
Posts: 273
Likes: 0
You may already know this but just in case, the generic replacement which is commonly available is not callibrated to work with your existing stock temperature gauge. It will read hot unless you get the correct one. Lectric limited I know is one supplier of the correct sending unit.
Just don't want for you to have to do it twice
-Andre
Just don't want for you to have to do it twice
-Andre
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,347
Likes: 654
From: Tampa, Florida
You may already know this but just in case, the generic replacement which is commonly available is not callibrated to work with your existing stock temperature gauge. It will read hot unless you get the correct one. Lectric limited I know is one supplier of the correct sending unit.
Just don't want for you to have to do it twice
-Andre
Just don't want for you to have to do it twice
-Andre
Jim
Drifting
Joined: Aug 2005
Posts: 1,430
Likes: 8
From: Hamilton NJ
http://www.ecklers.com/product.asp?p...6&dept_id=1310
For what it's worth, the replacement unit I got for my 79 from Eckler's is pre-wrapped with tape
For what it's worth, the replacement unit I got for my 79 from Eckler's is pre-wrapped with tape