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Thread: New owner , coolant temp gauge inop

  1. #1
    Join Date
    May 2011
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    San Jose
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    1985 635CSI

    New owner , coolant temp gauge inop

    I'm the proud new owner of a really nice white 85 635csi I picked up today. I'm reading the forums and linked manuals but maybe someone can answer before I find it. : )

    The coolant temp gauge on the dash is not working and I'm anxious to fix it so I can drive the car with confidence . The top of the thermostat housing has 3 or 4 sensors , one of which I'm assuming is the coolant sensor. Am I on the right track and which one is it ?

    Thanks !

  2. #2
    Join Date
    May 2008
    Location
    Montreal, QC, Canada
    Posts
    2,522
    My Cars
    '85 635 CSi A 5 Sp TCD Stage II
    Have a look at the attached photo. The coolant temp sender that communicates with the on dash coolant temp indicater gauge is the two spade jobbie (unconnected in my photo) on the left.
    The sender is comprised of two components:
    1) a thermistor (the fat spade) wired to the temp gauge whose resistance will vary depending on coolant temp and so provide an analogue signal to the needle.
    2) a high temp switch (the narrow spade) that will close if the temp coolant temperature reaches 243 F (Defcon 1 !! ) and will illuminate the dash idiot light.

    Assuming here that your dash gauge needle is inert and stone dead then you can start to pinpoint the problem very easily. With the key in accessory position pull the connector off the fat spade and ground it to the water manifold. If the gauge needle pegs right then both the gauge and wiring to it are fine and the sender is pooched. If still no needle movement then either the wiring to the gauge is breached or the gauge itself is no good. The same test applies to checking the idiot light .

    It would be best if you did not refer to this sender as a 'sensor' because the Coolant Temp Sensor (aka CTS and the baby blue one on the right in the pic) is quite a different animal and serves the top important function of relaying coolant temp info to the Motronic computer so that fuel can be appropriately metered to the injectors.

    Ignore the cobalt blue sender with the black line running from it on the left of the photo as this is something I added . You will have in its place the stock charcoal cannister purge valve switch with two small vacuum lines runing to/from it.
    The brown plug is the Thermo Time Switch (aka TTS) that controls when to activate the cold start valve. There is another sender on the end of the cylinder head right beside the water manifold. It controls the flux capacitors and should under no circumstances be messed around with.
    Hope this helps and welcome aboard.
    Attached Images Attached Images

  3. #3
    Join Date
    Jan 2005
    Location
    Houston, TX
    Posts
    821
    My Cars
    '85 323i, '88 E24
    if your testing indicates a bad gauge, try cleaning the ground on the gauge (to the circuit board) and putting a slight bend in the threaded rod to make sure it's making good contact. had issues with both my tempand fuel gauges since this type of ground is a terrible design.
    Houston, TX
    1985 323i, 1988 635csi, 1990 Miata, 2003 LW200
    1981 CM200T, 1984 VF700S

  4. #4
    Join Date
    May 2011
    Location
    San Jose
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    68
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    1985 635CSI
    Wow thanks for the help. The small spade turns the idiot light on when grounded, the fatter spade only makes the temp needle twitch . It jumps a little but there is no real movement left to right.

    Does this mean I'm pulling the cluster next? I really don't like driving any car without a functioning temp gauge. How hard is it to get the cluster out ?
    Last edited by tbsbimmer; 05-14-2011 at 02:21 PM. Reason: added question

  5. #5
    Join Date
    Dec 2009
    Location
    Bellingham, WA
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    1,568
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    '00 M5, 79' 633csi, 5.0
    It's not very hard at all. Just a few screws and it pops right out really.

  6. #6
    Join Date
    Feb 2006
    Location
    Columbus. IN (Indy 500 area)
    Posts
    4,693
    My Cars
    '87 635Csi & 3 MG's
    Quote Originally Posted by tbsbimmer View Post
    Wow thanks for the help. The small spade turns the idiot light on when grounded, the fatter spade only makes the temp needle twitch . It jumps a little but there is no real movement left to right.

    Does this mean I'm pulling the cluster next? I really don't like driving any car without a functioning temp gauge. How hard is it to get the cluster out ?
    Here is how to remove the cluster, with pics:
    http://www.z3bimmer.com/OffTopic/E24Bimmer//OdoGear/

    Are the "SI" (Service Indicator) lights working? Any other gauges not working in the cluster? The Temp gauge goes through the SI processor, so if it is bad (Batteries dead or leaked on the circuit board) it may affect the temp gauge.

  7. #7
    Join Date
    May 2011
    Location
    San Jose
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    68
    My Cars
    1985 635CSI
    Thanks for the link, the SI lights are working , at least the yellow and 2 red ones to the right.

  8. #8
    Join Date
    Feb 2006
    Location
    Columbus. IN (Indy 500 area)
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    My Cars
    '87 635Csi & 3 MG's
    Ahh, those need to be reset, and if they don't reset, the batteries may be bad and the SI board could be your problem.

    The green SI lights should come on when the key is first turned, all of them when first reset and then fewer and fewer as the time for service nears. When the cars starts the SI lights should go out.

    When service is required, the yellow light comes on and stays on, along with an "Oil or "Service" light. If the SI lights are not reset, first one red light and then another comes on and stays on.

    Resetting the SI lights:
    http://bigcoupe.com/phpBB2/viewtopic.php?t=1669

    Make sure you do the reset that corresponds with which "Oil or "Service" light is lit up.
    Last edited by CW6er; 05-15-2011 at 01:07 PM.

  9. #9
    Join Date
    May 2011
    Location
    San Jose
    Posts
    68
    My Cars
    1985 635CSI
    I swapped in a used cluster today and the coolant gauge works now : ) as an added bonus so does the odometer

  10. #10
    Join Date
    Jan 2010
    Location
    Ca
    Posts
    537
    My Cars
    81 633CSI
    Quote Originally Posted by tbsbimmer View Post
    I swapped in a used cluster today and the coolant gauge works now : ) as an added bonus so does the odometer
    Current rides:
    1981 633CSI
    1987 M6 U.S.
    1979 635CSI Euro
    1983 633CSI RIP
    1991 735IL RIP
    1986 635CSI parts car RIP
    1995 540i/6 newbie 11/13/11
    Many more to come....................

  11. #11
    Join Date
    Sep 2015
    Location
    asdf
    Posts
    8
    My Cars
    m3
    My coolant temp gauge does not work. When i turn the ignition it moves a little bit. Im after a full restoration everything was taken apart. I wanted to torque the head bolts after the engine warmed up and i saw the temp gauge not moving. The head cover was hot. I grounded the connectors but no luck. Only the idiot light worked. I took the speedo apart but saw no strange things.

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