Temp. Sending for a 64, 327/300hp

[James B. West]

Hello,

I need know who sells a correctly calibrated temperature sending unit for a 327/300hp engine that I have in my 64.

Thanks,

James West

[Jack H.]

Re: Temp. Sending for a 64, 327/300hp

The service replacement temp senders are a bit 'sloppy' and catalog folks like Paragon/Corvette Central place a disclaimer that these are AC units that can be off by +/- 30F. NCRS member Fred Oliva just started a temp sensor rebuild (original temp sender bodies, opened, re-worked, and precision calibrated) program that got some visibility at the NCRS Regional in Orlando.

I don't think it's appropriate to 'plug' a given business on the NDB, but Fred's company Vintage Automotive Research does advertise in Driveline...

[Jack H.]

Re: Temp. Sending for a 64, 327/300hp

Oops! Forgot to tell you, temp sender is just one one level in the feeding chain. You can see wrong readings (using an IR themometer to determine EXACTLY what the coolant temp is vs. gage reading is a very handy place to start -- science/not guess work) for a number of common reasons that include:

(1) Wrong fan (2) Worn fan clutch (3) Inefficient cooling system (both rad & block/heads) (4) Poor wiring integrity (5) Wrong gage

Item five is less well understood. In the '63-64 era temp gauges were 'trimmed' at the factory by hand picking shunt resistors to attach to the meter's contact leads. If yours is 'bye bye' or some historical owner before you has incorrectly done maintenance on the dash and/or swapped temp gages, you may be chasing windmills by replacing temp senders. Best approach is science -- what temp is it vs. what does the gage say....

[Gary Schisler]

Re: Temp. Sending for a 64, 327/300hp

In discussing this area with others before, I am pretty certain that Fred Oliva's rebuild service is cosmetic only! This information is several months old, however. If there is new information out there on this rebuild service that includes an internal rebuild, then we need to hear of it.

It seems incredibly difficult to open up a brass housing and "rebuild" the internals, calibrate it, and reseal it so that it appears original. Is this so?

[Fred Oliva]

Re: Temp. Sending for a 64, 327/300hp

Temp senders CAN be internally rebuilt & recalibrated. Saying anything more about this is inappropriate here, however, I'd like to share some info about how they work & how I believe the factory attempted to calibrate the sender & gauge system.

Temperature senders function thru use of an electrical device known as a thermistor. A thermistor changes it's electrical resistance with changes in temperature. As the temperature becomes higher, the resistance goes lower. The temperature gauge is a simple ammeter connected in series with the sender and the hot side of the electircal system. The gauge reads current flow thru the sender. As the temp changes, the resistance changes, which changes the current being read by the gauge, which is calibrated in degrees F.

One quick way to determine if your temp sender is calibrated correctly is to place it in a pot of boiling water with one lead of a digital ohmmeter connected to the top terminal & the other connected to the body. When the water boils, the ohmmeter should read anywhere between 75 - 85 ohms. Make sure to give the sender enough time to 'heat soak' once the water begins to boil.

During the years 1963-64, the factory calibrated the gauges to the sender by physically positioning the gauge needle in the 'correct' place. From 1965 to 1967, the factory used a 'shunt resistor', which basically bled off some of the current that would normally go thru the gauge, thus shifting the calibration curve of the gauge up or down (depending on the shunt value).

All these gyrations with the gauges were, I suspect, because the senders could not consistently be manufactured to GM's specs. I've made measurements on about 20 senders (including 2 NOS units) & none were within GM's original specs (which I have). The NOS units measured 90 ohms at 212 deg F.

Sorry for the long post. Hope this info helps.

Fred Oliva

[Alex Gilmour]

Re: Temp. Sending for a 64, 327/300hp

I've followed this thread with interest as my temp gauge is currently inoperative and is one of the things that I am going to replace as I pull the dash out. Now, though, Fred's post has me worried. I have a replacement sender that didn't cure the problem. If I end up replacing the gauge, is this going to give me a totally inaccurate temperature reading? How can I check the replacement gauge to be sure it is accurate before I replace it?

Thanks, Alex

'66 A/C Coupe