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Old 06-15-2007, 12:34 PM   #1 (permalink)
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My temp gauge is always reading real high on my 69 when the engine has warmed up. I will be measuring the actual temp this weekend, but I would like to know what I should expect the resistance to be from the temp sending unit.

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Old 06-15-2007, 01:32 PM   #2 (permalink)
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About 17 ohms at full hot and about 75 ohms fully cold.
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Old 06-15-2007, 01:36 PM   #3 (permalink)
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thanks
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Old 06-15-2007, 01:49 PM   #4 (permalink)
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Quote:
Originally Posted by Midlife
About 17 ohms at full hot and about 75 ohms fully cold.
Any idea what temps those correspond to, and is it a linear relationship? (I'm trying to guesstimate my running temp, obviously).
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Old 06-15-2007, 06:20 PM   #5 (permalink)
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The readings are not linear. 17 ohms would peg the gauge, and 75 ohms is a null reading.
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Old 06-18-2007, 08:25 AM   #6 (permalink)
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Guess you were wrong or you gave me the resistance to one for a dummy light. Tested an original sending unit. 24 ohms in boiling water. 375 ohms under tap water (approx 60 degrees).



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Old 06-18-2007, 01:16 PM   #7 (permalink)
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Quote:
Originally Posted by manyc
Guess you were wrong or you gave me the resistance to one for a dummy light. Tested an original sending unit. 24 ohms in boiling water. 375 ohms under tap water (approx 60 degrees).


Wow! That's really strange. I thought all Ford gauges (exception of the ammeter) were roughly 23 - 78 ohms. The higher ohm reading corresponds to low temperatures, which would make no difference to the gauge until the TSU began to read about 75 ohms, and then the gauge would creep up above its lower peg level.
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Old 06-18-2007, 01:55 PM   #8 (permalink)
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sounds like we may be refering to two different gauge systems. 69/70 is more like an ammeter system. the gauge is connected in series between a voltage regulator and a grounded temp sensor. as temp goes up, resistance of the sensor goes down , which yields higher current, which inturn increase the deflection of the needle on the gauge. yada yada yada. either way its important for people to know that the vendors aren't selling the correct parts.
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Old 06-18-2007, 02:56 PM   #9 (permalink)
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Wow. This is an interesting issue. You guys have something here.
I just called O'Rielly's and they told me one P/N fits all in their books.
Now when I look at the FMPC, well...

65/66 6 Cyl before 9/1/65 C3AZ-10884-A (SW-344)
65/66 6 cyl after 9/1/65 C6DZ-10884-B (SW-552)

65/66 8 Cyl before 9/1/65 C5DZ-10884-A (SW-427)
65/68 8 Cyl after 9/1/65 C6DZ-10884-B (SW-552)

69 8 Cyl exc. 302,351 C6DZ-10884-B (SW-552)
69 8 Cyl 302,351 w/ gauges C9WY-10884-A (SW-888)

70 302 exc Boss D0ZZ-10884-A (SW-925)
70/71 428,429 D0ZZ-10884-A (SW-925)
70 6 Cyl D0ZZ-10884-A (SW-925)

70 351 with gauges D0WY-10884-A (SW-924)
70/71 302 Boss with gauges D0WY-10884-A (SW-924)

I have to wonder now what might be the case with oil gauges.
Time to do some research.


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Old 06-18-2007, 04:02 PM   #10 (permalink)
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Quote:
Originally Posted by manyc
sounds like we may be refering to two different gauge systems. 69/70 is more like an ammeter system. the gauge is connected in series between a voltage regulator and a grounded temp sensor. as temp goes up, resistance of the sensor goes down , which yields higher current, which inturn increase the deflection of the needle on the gauge. yada yada yada. either way its important for people to know that the vendors aren't selling the correct parts.
I'm sorry if I confused you. The Ford gauges respond to sending units of the 20-85 ohm range, and indeed are in-line with the voltage source and the ground (sending unit). The only difference, it appears, is the range of resistance of the sending unit itself. I know (been there, done that) that most non-OEM sending units do not have the right resistance range, and you're often better off trying to get the Ford replacement unit.
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Old 06-18-2007, 04:51 PM   #11 (permalink)
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I started with a sending unit from Advanced auto parts. Gauge read almost to H at 200 degree engine temp. Took the sending unit back and exchanged for another one. Same results. So I went to the local Ford dealer. Got an Motorcraft factory replacement. Results are the same.

I have read that installing a NOS sending unit made 1974 or earlier will produce the correct reading. Not sure why or if it is true.

I need to get a meter on the sensor and take some measurements at various temps and then compare to needle position.

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Old 06-19-2007, 08:53 AM   #12 (permalink)
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after reading Lance's reply I am even more convinced that the dealer gave you the wrong part. compare your Motorcraft number to his chart.
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Old 06-19-2007, 05:58 PM   #13 (permalink)
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Guys, I just checked with my Ford parts connection. The following P/N's are still good:
C6DZ-B
D0ZZ-A
D0WY-A but has changed to F1SZ-A

I have two each coming of the D0ZZ-A and F1SZ-A. Now to make sure I'm clear in checking these resistance values, cold out of the box it's just from threaded stud to threaded housing. Correct? I am interested in seeing what the values are compared to one from O'Reilly's.

Thanks.

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Old 06-19-2007, 08:08 PM   #14 (permalink)
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I just checked the part that I got from the Ford dealer. The number is a F1SZ-10884-A. When I picked it up I had the guy look it up again to make sure it was correct for the application since it was my third attempt at getting a correct temp gauge.

So tonight after reading this post I decided to take my handy Fluke 179 TrueFMS Multimeter out to the car and take some readings. This is what I determined based on a mechanical temp gauge I installed this past weekend and the ohm readings.

Cold engine key off 273 ohms
Cold engine key on 130 ohms

Engine temp.............Ohms
130.................................18
150..................................8
170................................-19
180................................-23
190................................-28
200................................-30

The reading I took was from the sensor connecting tab to neg side of the battery. Is that correct? I am confused why the reading jumped when turning the key on. Is that normal. Also, why the negative numbers. At 200 degrees, ohms at -30 the factory gauge reads a little past the M in TEMP (Close to the H line) All confused now.

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Old 06-19-2007, 09:08 PM   #15 (permalink)
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Wrong way to measure resistance. Measure it from the sending wire post to the body of the sending unit where it screws into the block (ground). You need to measure without the sending wire attached, as the Constant Voltage Regulator will probably give you faulty resistance readings. Reading shouldn't change with key on/off if the wire isn't attached!

BTW, one cannot have a negative resistance! Shame on Fluke for providing that number.
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