Electric vs Mechanical Sending Unit - JeepForum.com
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post #1 of 18 Old 12-11-2019, 06:31 AM Thread Starter
steve801
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Electric vs Mechanical Sending Unit

My 83 CJ7 inline six 258 positions the water temperature sending unit on the top rear deck of the cylinder head.

I believe the thread for this electrical sending unit is 1/8 NPT (correct?).

Does anyone have a resource for a mechanical water temp sending unit / gauge that would fit this small thread port?

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post #2 of 18 Old 12-11-2019, 03:05 PM
MrPeter63
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Yes, 1/8 NPT. All the Mech temp Gauges i've ever seen require a "Seal Fitting" so the probe has something to bottom out on and hence seal too. I don't think they make a seal fitting in 1/8 NPT. Now you can go to Hardware store and get a 1/8 NPT male to whatever female fitting and then attach the seal fitting, or find a larger coolant port to tap into. My intake has a (i think) 1/2 NPT coolant port on the side forward of the Carb and EGR valve fitting.
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post #3 of 18 Old 12-11-2019, 05:08 PM
John Strenk
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Or get a 4.0 Thermostat housing and put it in the temp opening there.
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post #4 of 18 Old 12-12-2019, 08:23 AM Thread Starter
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i did think of adapting it into that same port but was hoping for something out there that was more of a direct fit...thanks for your input!
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post #5 of 18 Old 12-12-2019, 08:26 AM Thread Starter
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John, I assume the "4.0" you mention is another engine???
I live secluded in my own 258 straight six world...is there a specific year/model vehicle to ask for when I go to the auto parts store?
Thanks
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post #6 of 18 Old 12-12-2019, 11:47 AM
John Strenk
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Originally Posted by steve801 View Post
John, I assume the "4.0" you mention is another engine???
I live secluded in my own 258 straight six world...is there a specific year/model vehicle to ask for when I go to the auto parts store?
Thanks
The 4.0 was the replacement for the 258 4.2
A lot of the parts that fit a 258 can fit a 4.0 I think it started around 1987 to 2006..

Here is the 4.0 thermostat housing. This one is for a 97-06 Jeep Wrangler.



https://www.quadratec.com/products/51209_1008.htm

As you can see, it has a port just perfect for the temperature sensor and it bolts right onto a 258 block.


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post #7 of 18 Old 12-12-2019, 11:52 AM
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Does anyone have a resource for a mechanical water temp sending unit / gauge that would fit this small thread port?
I don't, but a lot of mechanical temperature sensors I've seen are just running a solid piece of copper wire into the cab to conduct the temperature, so I wouldn't expect more accurate results if that is the intention. You are essentially moving the sending unit into the cab.

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post #8 of 18 Old 12-12-2019, 12:09 PM
John Strenk
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I don't, but a lot of mechanical temperature sensors I've seen are just running a solid piece of copper wire into the cab to conduct the temperature, so I wouldn't expect more accurate results if that is the intention. You are essentially moving the sending unit into the cab.
I think that copper wire is actually a copper tube that contains a fluid that pushed the meter movement as it expands.
That is why the sensor end has to be so big and have a big fitting to hold it in place.
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post #9 of 18 Old 12-12-2019, 12:16 PM
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I was so happy when I got rid of my Autometer mechanical water temp and oil pressure gauges that the previous owner installed. The oil pressure gauge used to drip a few drops of oil every once in a while into the cab. I fixed that, but I did not like how the tube had to feed into the cab. For a non race application, it is so much simpler to run a quality aftermarket electric gauge. Modern electric sender are so much better than they were 30 or 40 years ago. If you break a mechanical oil pressure line or the gauge fails, you have a major oil leak in the cab. I really like my new Autometer gauges and they are not that expensive.
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post #10 of 18 Old 12-12-2019, 12:16 PM
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I think that copper wire is actually a copper tube that contains a fluid that pushed the meter movement as it expands.
That is why the sensor end has to be so big and have a big fitting to hold it in place.
Well last one I was messing with was definitely a wire because I cut it to find out. It probably depends on how much you spend on the gauge.

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post #11 of 18 Old 12-12-2019, 12:28 PM
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A capillary tube has a very small passage inside the tube, So if you used a a set of diagonal cutters on it you are basically crimping it and the cut would just look like a thin wire.

You would have to use a cutoff wheel to actually see the passage.


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post #12 of 18 Old 12-12-2019, 12:33 PM
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Those gauges work just like a mechanical air pressure gauge. The oil (or fluid in the case of the temperature gauge) pushes against plunger attached to a spring in the gauge and that causes the needle to move. With mechanical temperature gauges, you cannot generally shorten or replace the tubing. It is a sealed unit and if you cut it , you have to buy a new gauge. The mechanical oil pressure gauge is just a thin tube that hooks into an oil passage in the motor and you can cut it your desired length.
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post #13 of 18 Old 12-12-2019, 01:13 PM
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The 4.0 housing would be your best bet, or like i said, depending on your intake there should be a larger port some where on it. There is also should be a coolant port under your carb on the side towards the back of the intake where the Temp sensor is for the intake heater. If your intake heater is still hooked up you can always T off of it and screw both sensors into each end of the T. They also make Temp sensor adapters for Radiator hoses and heater hoses, but wouldn't be my thing, I'm sure they work fine but kind of look half assly in my opinion....
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post #14 of 18 Old 12-13-2019, 06:32 AM Thread Starter
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Thank you all for your input. My OEM temp gauge has failed so i thought a mechanical replacement would be easier than pulling the cluster (again). So, as I recall from Johns previous posts on the fuel gauge...would a new stand alone electric temp gauge have to be matched (resistance) to the sensor?? I.E. Can i use the stock sensor with an aftermarket electric gauge?
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post #15 of 18 Old 12-13-2019, 12:02 PM
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sometimes...
Check the specifications of the meter. it list the designed resistance range.


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