69 Straight Pin Temp. Sender

[Chuck G.]

Thought I was home free, as I found a NOS straight pin (not thumbtack) temp. sender at Bloomington last year. Well, I installed it last week, with a rigged up straight connector. The temp gauge absolutely will not budge...it will not even reach 100 degrees on the mark. Reinstalled my thumb tack model and I'm running 180-200 degrees. I've never seen anything like this. This is a NOS AC sender. There are no other markings on it...no numbers, etc. I installed it without teflon tape. Could this sender have been designed for use with an idiot light? I realize that mismatched senders can cause temp. errors of 20 or so degrees, but I've never seen one so mismatched that the gauge would not move at all. Any thoughts? Chuck

[Loren L.]

Re: 69 Straight Pin Temp. Sender

Chuck, does the correct one say "AC-MADE IN U.S.A.-12 VOLT- X"? "X" equals a symbol that looks like a capital P facing backward that shares a "spine" with a capital B.

[Chuck G.]

Re: 69 Straight Pin Temp. Sender

Hi Loren. This one says nothing except AC 12V. It has no other markings at all on it, and to be honest, I don't know what, if any correct markings are on an original. I look for straight pin senders...this is the only one I've seen at flea markets in about 2 years of looking. I'm going junk yarding next month when it warms up. I'm going to check under the hood of any/all GM cars with a gauge. Is this "X" --- reversePB marking customary? Chuck

[Jack H.]

Why guess?

Give away may well be the nomemclature on the body, but why guess? Inside 'guts' of temp sender consist of 'pellet' that's a therompile. Resistance drops with temperature increase according to GM P/N specs (SEVERAL engineering changes to the T/R curve over the years, hence the disclaimer of +/- 30F accuracy by many of the catalog houses stocking any kind of AC temp sensor). Pellet is compressed against the end of the brass barrel that extends into the coolant flow by a low resistance spring which is, in-turn, electrically connected to the connector terminal (don't matter whether it's pin or button style).

Putting an ohm meter between the brass case and the terminal centered in the Bakelite insulator tip, should render a resistance reading that varies with temperature. Range is 1200 ohms or so COLD room temp to a lower threshold of 40 ohms at off-scale deflection of the gauge (+230F). Nominal reading for normal crusing temp (around 'pop' point of thermostat, 160-190F range) is roughly 85 ohms.

If you've found an 'idiot' light switch, you'll see infinite resistance until the internal switch contacts 'make' (overheat range of +230F) and then darn near zero ohms. So, no need to guess about what you've got, amigo, test and verify, as President Regan once told the ole Ruskies....

[Chuck G.]

Re Thank you, Jack

Thank you Jack. That's the very info I was looking for. How do you know all this scientific info! I'll get out the ohm meter, the thermometer, and a pot of water and check it out. Chuck

[Fred Oliva]

Re: 69 Straight Pin Temp. Sender

Chuck,

With regards to the markings on the sender, the sender should nominally have the markings '12V AC Delco Made in USA' around the body. In addition, there is sometimes an additional mark such as the P/B marking mentioned, a star, a dot, double dot, or (I think) a triple dot. My theory is that these markings indicate which specific vendor or AC plant made the sender.

Jack's discussion of the theory of operation of the sender & the resistance you should expect to see is accurate, however, be aware that a cold resistance of 1200 ohms, while possible statistically, is a bit high nominally. As he states, as long as the resistance varys with temperature, the sender should make the gauge move.

Hope this helps.

Fred O

[Gary F. LeDuc]

Re: 69 Straight Pin Temp. Sender

As a side note to your problem - There was an article:Corvette Fever,1995, March p32,Temp Guage Troubleshooting/Getting a grip on guage ... '68-'72 Corvettes Molatch, Henery He provides what maybe a good replacement AC G1872. The sensor has threads instead of a strait bullet. Simply file down a little of the threads and the bullet connector slips on.

[Jack H.]

BREAKTHROUGH!

Calling around today, I stumbled across what may be a source for 'correct' '68-71 pin-head temp sender connectors. Ordered a half dozen to see how they compare to factory originals. Should have 'em next week and can report results of 'da hunt'....