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Instrument Panel - What is It? Help! 900 1994

posted by  jd620 on  Wed Sep 19 17:08 CST 2018
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last visit: Fri Oct 17 07:42 CST 2014
[RELATED]

I was repairing the erratic fuel gauge and think I have it fixed but may have damaged something else.

My 4 instrument panel bulbs do not work, the ones controlled by the rheostat. The dimmer rheostat wire is grn/brn and goes to instrument panel C5 which has a foil lead to a little rectangular box with the upper right corner at an angle. The box is shown in the 700/900 faqs under electrical instruments/Fuel Gauge Fluctuates or Fails to Work/ and the writeup is Cluster Cleaning/Disassembly and Connector Wires. There is a picture there and the box i'm referring to is in the the upper right corner. What is it? I think it is something to do with the instrument panel lighting. I wanted to see a foil under this box, took it off and now my 4 instrument panel lights do not work. I did solder the bottom 4 pads of the little box to ensure a better electrical contact (may have damaged it). What is the little box? It looks like there is a little PC board inside.
Another picture of the little box is at the top down a little farther in the faqs by 3. Poor Instrument Panel Contacts. [Response: Tom Irwin]
--

Mine:3-940s running, 1-740, 2-940 parts, dtrs:3-940s running








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    Instrument Panel - What is It? Help! 900 1994

    posted by  artca on  Wed Sep 26 23:24 CST 2018
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    That little box houses the rheostat circuit board. Comprised of two capacitors and some diodes, it's pretty sturdy. You need to make sure it is securely fastened onto the back of the instrument panel when you reassemble it b/c it needs to have a good ground. There should be no reason to take it off- it the bulbs are not working use a electrical multimeter to test continuity on the bulbs- that's the best way to check if they are good-

    also make sure that the contact surfaces are clean on the circuit/conduit sheet. sometimes the bulbs don't make good contact on both legs of the bulbs.








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      Instrument Panel - What is It? Help! 900 1994

      posted by  jd620 on  Thu Sep 27 13:02 CST 2018
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      last visit: Fri Oct 17 07:42 CST 2014
      [RELATED]

      My faulty box had a 14 pin IC, a transistor, 3 diodes, 3 electrolytic capacitors, and a fuse. The fuse is okay so I don't know what is wrong with the box. The 740 box works fine.
      I still can't understand why the actual dash mounted rheostat (or potemtiometer) would't be all that is necessary to control the intensity of the dash lites.
      --
      Mine:3-940s running, 1-740, 2-940 parts, dtrs:4-940s running








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      Instrument Panel - What is It? Help! 900 1994

      posted by  artca on  Wed Sep 26 23:31 CST 2018
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      Also if the "foil " you refer to is the thin copper lining on the circuit sheet?
      if that is torn or ragged you can usually solder thin copper wire to make connections from one terminal to another- has to be done carefully and with not a lot of heat.
      or go to a recycle yard and get a replacement instrument cluster- I have a couple b/c the speedometers go out often ( on 940's esp.)








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    Check for torn foil at board cable connectors 900 1994

    posted by  Amarin on  Thu Sep 20 12:36 CST 2018
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    last visit: Thu Oct 23 22:42 CST 2014
    [RELATED]

    The 4/4 thing in the photo below is the rheostat circuit (which Spook had mentioned) to control the grounding potential for the illumination lamps. More grounding means brighter lamps. The cables for A, B, C and D board connectors must all be connected for the rheostat control (dimmer) to work.

    Sometimes the bend over foil contacts at the board connectors (ABCD) might get torn/damaged during initial disconnection to free the cluster. These made worse during reconnection. A little dielectric grease could preserve the fragile foil contacts.





    Hope this helps,
    Amarin.








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      Check for torn foil at board cable connectors 900 1994

      posted by  jd620 on  Thu Sep 20 17:15 CST 2018
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      last visit: Fri Oct 17 07:42 CST 2014
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      Thanks Amarin for the info on the rheostat. I wonder why such a complex circuit for a simple rheostat circuit (4/4)? All that is wanted is to dim the lights. I replaced the rheostat circuit with one from a junker 740 I have and the dash lights are back on. I may have damaged the rheostat circuit when searching for broken foil.

      I still have one heat sensitive foil connection for the bottom alt., oil pressure, etc. lights with the key in run position but engine not running. That makes 2 corrected and 1 to go.

      Thanks also to Spook.
      --
      Mine:3-940s running, 1-740, 2-940 parts, dtrs:4-940s running








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    Instrument Panel - What is It? Help! 900 1994

    posted by  spook on  Thu Sep 20 08:31 CST 2018
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    last visit: Fri Oct 24 08:06 CST 2014
    [RELATED]

    Dear jd620,

    Hope you're well. Based on your description, Volvo VADIS - a superseded dealer parts/service database - shows the part, to which you refer, as the rheostat (#1398485). This part is no longer available from Volvo.

    However, I'd guess that any rheostat from a 940 cluster (and 960s up to and including 1994) will work properly.

    Hope this helps.

    Yours faithfully,

    Spook








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    Instrument Panel - What is It? Help! 900 1994

    posted by  Blindboy on  Wed Sep 19 19:14 CST 2018
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    last visit: Tue Oct 21 17:56 CST 2014
    [RELATED]

    LOADS OF 940 PARTS,,http://car-part.com/cgi-bin/search.cgi








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      Instrument Panel - What is It? Help! 900 1994

      posted by  jd620 on  Thu Sep 20 06:17 CST 2018
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      last visit: Fri Oct 17 07:42 CST 2014
      [RELATED]

      Blindboy, I have 3 extra instrument panels and just need help with part identification so I know whether I am working on the correct problem. I'd like to keep the instrument panel so the mileage reading is correct.
      --
      Mine:3-940s running, 1-740, 2-940 parts, dtrs:4-940s running







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