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Author Topic: Temperature sensor question  (Read 532 times)
Rufcar
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« on: January 01, 2014, 01:12:54 PM »

I recently finished my console gauges and tested everything and I thought it was all fine until I went for a drive today. The sensor that came with the console kit was too large a dia appox 3/4 dia thread and appox 1-3/4" long overall not including to top screw. The hole in my Dart Head was appox 1/2" or 5/8" dia and appox 1" long so I installed a stock GM 350 sensor. That all said once I drove appox 1mi the sensor went to red hot and I pulled over and the radiator was still cold!
First question is: Is there a difference in these sensors?
2nd If there is ,Can I use a brass adapter from 5/8 to 3/4 to make it work if I do it will be protruding outside the block.
3rd Or can I get the proper sensor with the 5/8" thread and if so where?
Thanks Happy New year to all!  Jim F
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Everett#2390
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« Reply #1 on: January 01, 2014, 07:00:35 PM »

The OE sensor is 1/2 inch NPT (National Pipe Thread}, a tapered thread - tighten tighter to stop leaking.
Sensor is installed without too much pipe sealant as the sensor body needs to be grounded to the head for circuit continuity.
If you use an ohmmeter, measure the resistance from the center terminal to engine ground.
When cold, reads high resistance, above 1000 ohms.
As the sensor heats up, typical resistance value would be about 180-200 ohms for an approximate gauge reading of 180F, the 10 o:clock mark.
Reads like the head is set for a 3/8" NPT.
You might be able to drill out the 3/8 hole for a 1/2 hole, I have never done it, yet. Be careful in tapping, you could make the threaded hole too big.

Yes, there is a difference in the sensors you have.
No, you should not 'adapt' to fit the sensor as the sensor tip needs to be in the coolant flow for a correct reading. If the sensor is out from the coolant flow, the gauge will not sense the coolant temp. It may sense cylinder head material temp, and would be a higher temp indicated.
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L78racer
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« Reply #2 on: January 01, 2014, 09:15:03 PM »

can you put it into the intake manifold near the t-stat?
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Rufcar
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« Reply #3 on: January 01, 2014, 11:12:55 PM »

Everett and L78, the temp end on my sensor is 1/2" and it had the threaded top and its the OE that came in the kit but the thread is too big as you stated. I have checked other forums before I wrote the original post and someone stated that the GM6400759 is the console gauge sensor possibley thats smaller? Tomorrow I will ck the pipe size with my threaders. If I do not have the correct size sensor maybe the the GM part has a a smaller dia? It seems though that my options are remove the head and make the hole bigger because its easier then trying to do that with such close clearance or drill and tap the intake near the thermostate? Best Ideas please?
L78 can you show me a pic of where exactly to tap the intake? my intake is aluminum.
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rich69rs
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« Reply #4 on: January 02, 2014, 08:25:46 AM »

In my mind locating the sensor in the head instead of the intake always made good sense - sensor will always be in the coolant flow; hot or cold engine; normal or low coolant level, etc.  I'd find the correct sensor for the head.
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Richard Thomas
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L78racer
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« Reply #5 on: January 02, 2014, 05:53:23 PM »

L78 can you show me a pic of where exactly to tap the intake? my intake is aluminum.

Here's where Chevrolet put them up until around 1968.

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Rufcar
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« Reply #6 on: January 04, 2014, 01:43:56 PM »

I found a recessed plug in the intake that was covered by a hose. I removed that plug, it was the exact size and I installed the correct sensor. I ran a temporary wire, ran the engine awhile and all seems well. I ran a permanent wire, enclosed same and now all is well! Thanks for all the advise and now off to sort out a rear back up light situation! Jim
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