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Testing speedo removed from instrument cluster 900

Hi all. I’ve just removed the speedometer from my ’92 940T, using the excellent description from the FAQs, to fix the infamous capacitor problem. The FAQs described how the speedo could be tested in the car by disconnecting the connectors in the trunk and that rapidly shorting the two wires together that go to the speedo from the speed sensor would simulate the pulses it generates and cause the speedo to register speed. Now that I have both the cluster and the speedo out, it seems that it would be really easy to test the instrument by doing the same test on my desk. The 4 pin connector on the cluster is marked as “+” “-“ “S-“ and “S+”. The four main connections to the unit, through the four screws, are marked “IGN”, “48P”, “SRL” and “GND”. The fifth connection is marked “E - PULSE” and connects directly to “S+” (probably the output to the ODO circuit). There is also a two pin spade connector marked “12P” that has a separate plug going to it. One of the wires in this plug is red which, along with the “12P” would suggest that 12 volts should go to it, but I don’t want to test this theory unless I know that for sure. Anybody know what this plug is for? The “IGN” connection on the speedo would suggest that 12 volts should be sent to it as well to simulate a turned-on ignition switch. I don’t know what the “48P” and “SRL” would mean. Has anybody ever tested the speedo on their bench by connecting 12 volts to the “+” and “-“ connections and rapidly connecting the “S-“ and “S+” together on the speedo, possibly connecting 12 volts to the “IGN” and “GND” at the screws as well? Thanks.








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    Testing speedo removed from instrument cluster 900

    Never tried it, but when you get this figured out kindly write it up for the FAQ. And if you want the latest file on speedos, send me your email:

    Stephenringlee (at) hotmail (dot) com

    BTW, a wave generator with adjustable frequency output is probably just what you need.
    --
    See the 700/900 "FAQ" at the menu bar top screen left side.








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      Testing speedo removed from instrument cluster 900

      Will do Steve. A signal generator woud be perfect all right, but I don’t know when I’d ever get to use it again. Maybe I can borrow or rent one. I’ve just ordered the capacitors from Mouser - they seem to be a really good place - $1.40 for 5 capacitors - most places won’t even deal with small orders like that. I couldn’t find the big orange one on their site (although I’m sure they would have it), but I think I can get one locally. I never would have attempted repairing the speedo myself if it wasn’t for the great description and pictures on the FAQ site. I just lifted the pointer over the 0 stop - I don’t think I would have tried that on my own either - you have to lift it pretty high to clear the peg and it’s pretty scary. No problem though it went over the peg beautifully - a lot easier than unsoldering the board. I’ll keep the forum informed of my progress. It will probably be a few weeks before I get the caps.
      Rick








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        Testing speedo removed from instrument cluster 900

        Have taken speedo out of instrument cluster and taken circuit board off the back of the back using the procedure of lifting the pointer needle over the 0 stop, letting it come to it’s natural rest spot, marking it on masking tape and pulling the needle off. I unsoldered the two leads to the interval microswitch and removed the board. Everything worked out just as it was described in the FAQs. I’ve confirmed that the capacitors are almost certainly the problem as I can see some corrosion around the IC pins and the solder joints. Generally the board and speedo is in very good condition. The ODO gears all look like they’re OK - no teeth stripped or binding so I’m not going to do anything with the ODO. The speedo is very delicate and the slightest slip could destroy it. The capacitors have been shipped from Mouser so all I have to do now is wait until they arrive. I put the instrument cluster back in the car without the speedo. All the exterior lights work so I can still drive it, but nothing on the dash does, no gas gauge, clock, temp gauge or instrument lights. I guess things are wired in series with the speedo connectors, which makes it seem unlikely that I’ll be able to test the speedo gauge by itself outside of the cluster.







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