Denney

I have noticed on my 66 BB that the temperature gauge acts alot like a reverse tachometer (it drops with increasing revs and rises with decreasing revs). Very repeatable. Anyone ever experience this? To me it looks like there is some stray capacitance or something. Any chance the Teflon tape on the sending unit is contributing to this? Thanks!
Denney

hot1corvette1

That is a real weird one. You must have some kind of electrical leakage between the two gauges. Hard to believe that the teflon tape is contributing, but maybe.
Try disconnecting the tach and see what happens.
Mark

waynec

Teflon tape on the sending unit??? Seems to me the sending unit body is the electrical ground to the engine block for the sender, so teflon tape could indeed be the problem by insulating the threads, especially if it was put over all the threads... try cleaning the threaded sender hole in the block with a stiff steel brush and then using teflon tape only on the top half of the threads, or none at all (the softer brass body of the sending unit, along with the pipe threads, should seal fine.

ghostrider20

The teflon tape may be the problem, although the threads usually cut through the tape enough to make a good ground. I would check it though. Also you may have a loose lead wire that may be making a good stable contact at low revs and a loose contact at hi revs, or vice-versa. I had a problem were my temp gauge was reading high and would peg at max temp. Had a problem with an old crusty lead wire. Replaced it from the fierwall up to the sending unit. Works fine now.
Mark

Denney

When I put in the new 454 HO I decided to put the Teflon tape on the sending unit (have done this before on other cars to make it easier to get out in the long haul). What can I say? I am always paranoid about coolant leaks (they make such a mess) :U In any event I will remove the tape (or at least a good portion of it this weekend and let you know if it was the cause. As far as the connector/wiring, I put in a brand new Lectric Limited harness in the engine compartment, so I hope :confused: it isn't that.

magicv8

To satisfy yourself that the teflon is not the problem, put a batter jumper cable on the sender base and hook it to the battery ground - then run the engine up and down the scale.

64BB

"temp drops with increasing revs"....
i suppose i'll get flamed for this, but is it not possible that the increasing revs are actually lowering the coolant temperature by incresing airflow through the shroud??? if you can get hold of an infra red temp gun, you can easily check this out. does not really sound electrical as described, unless the temp drop is pretty instantaneous. agree?? is temp drop somewhat gradual??

Denney

Actually, the temperature drop is very much in synch with the engine revs (I would call it pretty close to instantaneous.) It reads pretty high to begin with after it has warmed up (which I attribute to the new replacement sending unit being out of calibration compared to the older units). I would say we are talking swings of 50+ degrees in a matter of the time it takes to rev it up. Up shift and the temp goes back up the 50 or so degrees until it revs back up, and so forth through out the shifts. Realizing I was not very specific in the initial description I can understand how you can interpret it the way you did. Thanks.

wombvette

I would do a check of the alternator voltages and also look at guage cluster grounds. If by chance the alternator voltages are going down with revs. it could explain the problem. I know thats not normal, but I have seen it happen.

oger

On a Corvette if anything electrical goes wrong check the ground. Old statement but very true.

Denney

I had a chance to try Magicv8's advice of hooking up a jumper cable from the sender base to a good ground (the intake manifold) and reved it up after warming it up (there's nothing quite as intoxicating as rapping up a big block in the garage) :cool:. It appears that the temp gauge was relatively steady (it did still move a ~little~ with revs), so it appears that removing the Teflon tape is the next step. Will update once completed with results...

ghostrider20

Might want to check to see if the block is grounded well or grounded at all.
Silly but might be something as simple as that.:smash:
Mark
:smash: