Temp Sending Unit

Discussion in 'Troubleshooting & Diagnosis' started by Bob R, Apr 20, 2002.

  1. Bob R

    Bob R R.I.P 11/20/16 Staff Member

    4,219
    1
    Jan 6, 2000
    Flower Mound, Texas
    Does anyone know how to test a Water Temperature Sending Unit? I have the original to my 73 Z and need to see if it is still capable of working correctly. The newer GM Replacement part pegs out my Water Temperature Gauge.

    Thanks in advance,

    Bob R

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    1973 Z28 Original Owner 6 Year Frame Off Restoration
    1971 RS Z28 Frame Off Restoration in Progress
    http://www.geocities.com/bobr73z28
     
  2. FSEVENTY

    FSEVENTY Veteran Member

    1,071
    0
    Jan 31, 2001
    Mesa, AZ USA
    My guess is to put a Ohm meter on it at ambient temperature and note the resistance. Even running it under the hot water from the tab should change the resistance somewhat if it still is functioning. You could also boil some water on the stove or microwave and drop it in for a few minutes, then test the resistance again.

    I remember from looking in the part store catalogs that a chevy water temp sender sould be around 80 Ohms at 100 Deg and 180 Ohms at 220 Deg. My memory has failed me before though, so don't quote me! Just a ballpark range to see if the origional still works.

    I also remember someone suggesting holding the sender under a lighter to warm it up, but
    I'm not sure if that might be too much localized heat. If you want to play it safe, just boil it.

    I've had problems with the parts store senders as well. Someone suggested for me to get a GM replacement directly from the parts dept.

    OOOPS! after reading you post again, I see that you already have the GM part. Sorry.


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    1970 RS
    350, 700R4, 3.73 posi
    2 yr. old loves to cruse with me.
    I ride with <><
    Pic of "Dada Racecar"

    [This message has been edited by FSEVENTY (edited April 20, 2002).]
     
  3. Bob R

    Bob R R.I.P 11/20/16 Staff Member

    4,219
    1
    Jan 6, 2000
    Flower Mound, Texas
    FSEVENTY,

    Thanks for the insight on how to test this thing. I dug out the old Simpson Meter and tested both of them. The original Sending unit appears to be bad. No readings from it Hot or Cold. The newer one gave me some screwy readings. At room temperature the new one shows 52 Ohms, at 210 degrees it shows 8 Ohms. It should go the other way. I wonder if this is a bad part or I have the wrong one of these as well. Have to go to Plan B now.

    Thanks,

    Bob R

    ------------------
    1973 Z28 Original Owner 6 Year Frame Off Restoration
    1971 RS Z28 Frame Off Restoration in Progress
    http://www.geocities.com/bobr73z28
     
  4. FSEVENTY

    FSEVENTY Veteran Member

    1,071
    0
    Jan 31, 2001
    Mesa, AZ USA
    The fact that the replacement unit has readings suggests to me that it does work. I guess I assumed that the reisstance went up with temp, but it looks like it goes down.

    Problem is, you still don't have the correct sender.

    I have posted/read probably 5 or 6 posts regarding the issue of "correct" temp senders and I am getting frustrated. It seems no one can find the correct sender even though the parts houses claim to have the right one. Last time I went looking for one, I shoved my way behind the counter and looked in the catalogs myself. Each sender has an application chart with the corresponding resistance values at two different temps, I believe 100 and 220. If you can find what the stock sender is supposed to be, dig through the cats until you find on that has the same values. I just gave up for awhile as the current sender registers ~ 250 on my gauge, but as long as it does not go much beyond that I know the engine is OK.

    check out this link for more insight. My origional post is no longer available I guess.

    hmbldr and I exchanged e-mails about how to add an inline resistor to get the gauge to read close to correct.

    http://www.nastyz28.com/ubb/Forum9/HTML/001419.html



    ------------------
    1970 RS
    350, 700R4, 3.73 posi
    2 yr. old loves to cruse with me.
    I ride with <><
    Pic of "Dada Racecar"
     
  5. tbill

    tbill Veteran Member

    1,712
    0
    Mar 5, 2001
    durhamville,ny,us
    Bob, i went thru 4 senders on my car untill i got one that registered properly, and that was AFTER my little TCS sender snafu [thanks again for the help on that issue].

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    71 ss 350/4spd 66 chevelle 396/4spd www.americaniron.net
     
  6. 81ZDan

    81ZDan New Member

    8
    0
    Mar 24, 2004
    Utah
    I went through the same thing when mine didn't work. I bought one from autozone and it didn't work right so I went to the dealer. The price was cheaper and it was the right sender. Give the dealers a shot, they know there cars better.
     
  7. danbrennan

    danbrennan Veteran Member Lifetime Gold Member

    2,763
    15
    Mar 13, 1999
    Brighton, MI
    The auto thermistors I've worked with all tended to be NTC(Negative Temperature Coefficient, as opposed to Positive Temperature Coefficient) devices, so if the resistance goes down as the temp goes up, that sounds right(an open in the temp gauge circuit typically shows a cold temperature, not a hot temperature). If the sender sets the guage too high for a given temperature, you could experiment with adding resistors in series with it(Radio Shack). The right value should lower the guage to the correct reading for a given temperature, but there would be no guarantee it would be correct for all the other temperatures.
     
  8. Bob R

    Bob R R.I.P 11/20/16 Staff Member

    4,219
    1
    Jan 6, 2000
    Flower Mound, Texas
    FSEVENTY and everybody else,

    Thanks for all the input on this. Sometimes it is the little things that drive you nuts. My original Sender Unit was working normally when I took the car apart 8 years ago. For some reason now it does not test.

    I put a new one in just because it was probably due for a new one and I had everything apart anyway. The new one is a GM part and if I am correct I purchased it from Year One. However like all Electrical Magic Stuff, it could be defective even though it was new. Wouldn't be the first time either that I have been down that road.

    I will give Year One a call in a bit and order another one (they are cheap) and it shows to be a correct replacement from GM with a part number of 9520N. I have purchased a lot of little stuff from Ground Up Restorations out of NYC and they have one as well that shows to be correct with a part number of GAC-560. So maybe I will go down to the GM Dealership tomorrow and see what they have to say about it. I looked in my parts books from GM and it shows a 546702 to be the correct replacement. One of these things should work out.

    I would be interested to see if anyone has a correctly functioning ORIGINAL Sending Unit and would be able to get a Ohm Reading from it. This data could be left on this site for the next person that encounters this problem.

    tbill, where did you get the correct sending unit from? Do you have a part number for it?

    Thanks to everybody that is trying to help me through this.

    Bob R

    ------------------
    1973 Z28 Original Owner 6 Year Frame Off Restoration
    1971 RS Z28 Frame Off Restoration in Progress
    http://www.geocities.com/bobr73z28
     
  9. gin man

    gin man Veteran Member Gold Member

    2,008
    2
    Feb 24, 2002
    Morris, Il
    The last time I ran my engine on my 70 several years ago, the gauge was working properly. I measured 630 ohms at room temperature at 70 degrees and 120 ohms at close to 212.
     
  10. tbill

    tbill Veteran Member

    1,712
    0
    Mar 5, 2001
    durhamville,ny,us
    Bob, although maybe not correct for an application such as your car [i.e. perfect], i got the temp sender from NAPA, and i also got the tcs sender from NAPA [i think they were disc. from GM], funny part was, all the other tcs parts were still available from GM [this was last year]. if you want/need the tcs sender part # from NAPA, i'll look thru my receipts to find it.

    i think i still have the original temp sender that came out of my 71, only reason i changed it was for 'cosmetics', and i still have the original tcs sender [ per your advice, it's in my tool box at work]

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    71 ss 350/4spd 66 chevelle 396/4spd www.americaniron.net

    [This message has been edited by tbill (edited April 21, 2002).]
     

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