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Old 12-30-2016, 12:47 AM   #1
dmjlambert
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Temperature switch for 350 SBC with modern heads

Hi
If this has been discussed before, I was unable to find a thread about it, so please bear with me or point me in the right direction if you can.

I have a gauge cluster and we recently discussed in another thread about getting the right sensor going for the stock instrument cluster gauge, and put it in a 3/8 NPT hole in the updated heads of the 350 SBC. Now, let's talk about the passenger side head and its 3/8 NPT hole. I would like to put an overheat temperature switch in that hole and wire it to an idiot light as described in this other thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=490161

So, what is the deal with temperature switches? I know they are sold in a bunch of different temperature ranges, and they come on at a certain high temperature and go off at a slightly lower temperature, and they are used for either overhead warning or to switch an electric fan on/off as needed.

What I can't seem to find anywhere is what is the temperature range of the stock 350 SBC overheat switch, which is a 1/2 NPT size, because I would like to find an appropriate 3/8 NPT switch with the same temperature range.

If I want to run an electric fan, what is the temperature range of its switch? I assume that is a different range since you want it to switch on and off at lower temperatures than a warning light would be switched on.

When I'm all done with this truck I might just have 3 temperature senders, one for the gauge, one for the idiot light, and one for an electric fan. At least a couple of them will be 3/8 NPT size and installed in the heads.
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Old 01-08-2017, 11:09 AM   #2
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Re: Temperature switch for 350 SBC with modern heads

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Old 01-08-2017, 11:55 AM   #3
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Re: Temperature switch for 350 SBC with modern heads

Last time I looked into the thermal switches, you could get them with multiple on/off temperature combinations. With a 180 thermostat, I'd want it on at 190 or a little more so it's not running all the time when the engine is warmed up.

https://www.summitracing.com/search/...switch%203%2F8
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Old 01-08-2017, 12:43 PM   #4
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Re: Temperature switch for 350 SBC with modern heads

OK, great, thanks, that takes care of fan control. Does anybody know about the sender for the temp idiot light in a 3/8 NPT size? The stock one would have been 1/2 NPT and turns on and off at some unknown temperature, and as I understand it those switch senders can't be machined down and re-threaded in the same way done for the gauge senders.
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Old 01-21-2017, 12:16 PM   #5
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Re: Temperature switch for 350 SBC with modern heads

In this old thread where we were talking about gauge senders:
http://67-72chevytrucks.com/vboard/s...d.php?t=714839
davepl shows a factory chart showing characteristics of the sender at various temperatures.

Is there similar documentation about the characteristics of the temperature warning light sensor? I realize it does not vary in resistance, and it just switches on at a specific (mystery?) temperature. I would like to find a sensor that switches on at the same temperature as the factory sensor for a 350 SBC, but with 3/8 NPT instead of 1/2 NPT so it fits in the new head.
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Old 01-21-2017, 06:12 PM   #6
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Re: Temperature switch for 350 SBC with modern heads

Here is one. On at 200* off 185*.
https://www.amazon.com/dp/B01E52LZE8?psc=1
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Old 01-22-2017, 11:58 PM   #7
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Re: Temperature switch for 350 SBC with modern heads

Thanks for your reply. The only problem I see with that is I don't want the temperature warning light coming on every time the fan comes on. I only want the temp warning light to come on when the engine is overheating at the temperature Chevrolet considered too hot.
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Old 01-23-2017, 12:26 AM   #8
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Re: Temperature switch for 350 SBC with modern heads

I've been searching on the Internet for a long time, but can't find any mention of at what temperature the TEMP light should light up on the instrument cluster according to GM. There is a lot of talk about "you should be OK up to [insert various opinions here] temperature."
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Old 01-23-2017, 06:40 AM   #9
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Re: Temperature switch for 350 SBC with modern heads

I have interest in this also as I too have the instrument lights and an engine with late model heads with the small temp switch hole. I presently have the correct temp switch installed in the thermostat housing. The tstat is a 180* and the efan comes on at 190*. Somewhere in there the temp light comes on and stays on.
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Old 01-23-2017, 11:10 AM   #10
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Re: Temperature switch for 350 SBC with modern heads

OK, if you want to take the time you can test a stock sender for the temp light using a meter, a thermometer & a pot of water on a stove. Measure continuity (ohms) when cold (should be open - infinity) then watch meter as sender warms up you will see the switch close, meter should read very little or no resistance (zero ohms) Read the temp on the thermometer. In perfect world, as it cools down it should go open as it cools below this temp, in reality it may not re-set until a cooler temp. You can watch & record this also.
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Old 01-23-2017, 11:24 AM   #11
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Re: Temperature switch for 350 SBC with modern heads

Man, I'd try to find an over-temp switch that closes at 200 or more. I'd say 210 would be more like it. Why? Because it's not uncommon for coolant temp to go up to 200 or more on a hot day in traffic, and even 210 isn't going to hurt your engine or blow coolant out of the radiator. And to me, a red warning light means "danger".

I have no idea at what temp a GM switch closed, but I'd guess it's 200 or higher.
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Old 01-23-2017, 11:29 AM   #12
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Re: Temperature switch for 350 SBC with modern heads

Here's one that's adjustable up to 210 degrees. I'm sure there are many others available.

https://www.summitracing.com/parts/hda-3651
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Old 01-23-2017, 11:53 AM   #13
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Re: Temperature switch for 350 SBC with modern heads

I have an adjustable thermo switch for the fan. Its good. My concern, as is Lamberts, is the temp light in the instrument panel. Id like to make it work properly.
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Old 01-23-2017, 01:31 PM   #14
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Re: Temperature switch for 350 SBC with modern heads

I believe the fan switch for a '88 Firebird with a Tune Port Injection is 3/8 and closes around 235.

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Old 01-23-2017, 03:27 PM   #15
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Re: Temperature switch for 350 SBC with modern heads

Here is the sensor for a 1970. NAPA says 220 degrees. https://www.napaonline.com/napa/en/p...9SB_0223304370
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Old 01-23-2017, 04:27 PM   #16
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Re: Temperature switch for 350 SBC with modern heads

Aren't the sending units different between temp lights and temp gauges?
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Old 01-23-2017, 04:46 PM   #17
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Re: Temperature switch for 350 SBC with modern heads

Yes they are. Temp switches are used on warning lights and have a switch inside that operates at a specific temperature. A temp sensor is used on guages and is manufactured to provide resistance that is linear in response to changes in temperature within a specified range. (ie 150 degrees will always equal "X" resistance and 220 degrees will equal "Y" resistance.)
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Old 01-23-2017, 05:36 PM   #18
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Re: Temperature switch for 350 SBC with modern heads

Good info here fellas.
So we can use the fan switch for the 88tpi engine to operate our stock temp lights?
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Old 01-23-2017, 06:58 PM   #19
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Re: Temperature switch for 350 SBC with modern heads

The 1988 sensor would work but it will require the connector from the later model engine.
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Last edited by HO455; 01-23-2017 at 07:04 PM.
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Old 01-23-2017, 11:24 PM   #20
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Re: Temperature switch for 350 SBC with modern heads

Well I said earlier I had the correct temp switch in my truck. After doing some researching I find they sold me the wrong switch. This one is for a temp gauge, not a light. So my temp light isn't working right. Easy fix though. I'm looking at one from Napa for the 69 truck. It closes at 220* too. It's too large to thread into the late model heads but it will fit in the chrome thermostat housing. Thanks guys.
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Old 01-24-2017, 12:03 AM   #21
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Re: Temperature switch for 350 SBC with modern heads

I am still researching and considering the switch from later model trucks around 1987 and 1988. The switch would be a Wells TU64 or Standard TS-66 and those have the 3/8 threads and flat blade connector. I have written to tech support at Standard and at Classic Industries to see what they think and if they can give the temperatures at which it closes and opens. Based on vague info I read in listings for the parts, I think they close at 273 and open at 253.
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Old 01-24-2017, 12:19 AM   #22
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Re: Temperature switch for 350 SBC with modern heads

Here are some out of the Stewart Warner catalog. Jegs carries them too! The 1/4" ones can be bushed to fit 3/8".

http://www.stewartwarner.com/en-US/P...-switches.aspx
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Old 01-24-2017, 12:49 AM   #23
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Re: Temperature switch for 350 SBC with modern heads

Yes, thanks. I am most interested in finding the factory spec temperature for what is considered overheating for the temp warning light according to the GM engineers. Most temperature switches, especially those found on hot rod parts sites, appear to be for switching electric fans. So, I'm looking for known fitment for vehicles and engines for the dash warning light specifically. Once I finally find definitive information, I will then roll it around in my head and consider whether to go with the factory spec, or warn at a cooler temperature using a non-standard or fan switch.
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Old 01-25-2017, 08:07 PM   #24
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Re: Temperature switch for 350 SBC with modern heads

Update:

Using a couple of online parts store application searches, I found out what temperature switch w/light is supposed to fit a 1969 Chevy truck with a 350 engine. For example, on Autozone's website, searching for this gives a TU51 part number. From my bucket loads of online research, I recognize that as a Wells part number. So it is a Wells switch put in a Duralast box for AutoZone.

Now, over to the Wells website to find out the specs. After looking around for a while, I figure out that the Sales Drawings are what gives the specs. It switches at 251 +/- 10 degrees.
http://www.wellsve.com/custom_search...ings/TU51A.pdf

I wrote to Wells for technical support and asked this:

I have a 1969 Chevrolet C10 Pickup with a 350 engine, and it calls for a Wells TU51 temperature switch. I’ve been looking at the specs on your website and that switches my warning light on at 251 degrees. I had my engine rebuilt, and it has new heads installed that work with modern gasoline. The thing is the new head has a 3/8 NPT hole for the temperature switch, where the old head had a 1/2 NPT hole and accepted a TU51. I had a look at fitment for later model trucks and found that the temperature they switch at is 263 degrees in 1970 and above, and in 1981 when they switched from 1/2 NPT to 3/8 NPT they continue to be 263 degrees, so I guess that is 12 degrees higher.

What I’m wondering is do you have a switch for about 251 degrees with 3/8 NPT?


They wrote back:

David, p/n TU78 closes between 230 & 250* and has 3/8-18 thread, this was used on '82 to '89 Chev & GMC trucks. - Glenn

So, I think I'll put a TU78 in the passenger side head and connect to the TEMP light.

http://www.wellsve.com/custom_search...wings/TU78.pdf

Last edited by dmjlambert; 01-25-2017 at 08:19 PM.
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Old 01-25-2017, 08:39 PM   #25
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Re: Temperature switch for 350 SBC with modern heads

Good deal, who would have thought it would take that much work to find a sender for a chevy!
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