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Old 03-16-2019, 09:33 PM   #1
Nick_R_23
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Join Date: May 2016
Location: Willow, Alaska
Posts: 836
Temp sensors and LS swaps

Boy there is sure a lot of confusion in this area! Been searching for two days and still can't find a solid answer. My question is really in two parts here.

I have a 1978 GMC 2500 that is a very base model truck other than it had gauges instead of idiot lights. I'm planning on a carb'd LS swap soon with this truck, so I really don't need much as far as electronics go, other than I'd like the factory gauges to function (not willing to swap to aftermarket PERIOD, I want to retain the complete stock look of the dash).

The oil pressure gauge isn't a huge concern, as there are adapters to put a '78 oil sensor into the stock LS position at the top rear of the block. Easy install there. However, there seems to be little in the way of information on the temp sensor. Obviously there is a massive difference in the thread size of the '78 and the LS sensors, and no adapters are available for this.

I've done this nearly identical swap before on an '86 C10, and my solution then was to take the temp sensor out of the original engine, turn it down to match the LS thread size, and install it like a normal LS temp sensor. The difference was that was a previously running vehicle, and I knew that the sensor functioned correctly. This '78 however, was overheated to the point of completely cooking the old 350, so I really don't trust the original sensor to work anymore.

This leads me to basically two options:

1. Take my chances on a parts store temp sensor and hope that the Ohm values are within the same solar system as the original sensors spec. I have not heard of too many of these sensors reading correctly, which makes the gauge basically worthless. I would have to turn this down on the lathe as the previous sensor which is not a big deal, but would obviously make it nonreturnable. I've also noticed that the parts stores show both 1/2-14NPT and 1/2-18NPT for the thread size on the OEM sensor. I cannot find any information that determines which is accurate or why/when a change would have occurred.

2. Source a GM 3 wire temp sensor (I've been told 98 Camaro among others uses this) and only use the 'Green' wire, which is used to feed the gauge, just like older vehicles. This would be a thread on deal, but the Ohm scale wouldn't match the '78 gauge. HOWEVER, according to some internet information I found (http://forum.73-87chevytrucks.com/sm...?topic=25339.0), in 1979 GM went to the standardized 1365-55 Ohm scale on the temp gauges, which is apparently what is still used today, so the 3 wire sensor should be exactly compatible in theory. So, I would be able to use a bolt in sensor with a change to the '79-80 gauge to match the older design (or '81+ if you need to match the newer design). I've seen a little bit of talk on the subject of using the 3 wire sensor, but nobody I've found has reported that's what they ended up using.

I know there are plenty of people on here that have gone through a similar dilemma, so I would appreciate some input and opinions on what route you'd take.
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