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Smiths Tach Won't Go Past 1500 RPM 1800 1973

I'm back with a new issue.

I started the engine to test the tachometer. It appears to read accurately at idle and climbs smoothly as I raise the revs. But the needle won't go above 1500 or 1600 RPM, even though I know the engine is revved higher than that. Back off of the throttle and it drops back to 600-700-ish as it should. I'm guessing there's some aged capacitor or transistor causing this issue. Is this something I can fix myself?








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    Smiths Tach Won't Go Past 1500 RPM 1800 1973

    Chris;

    1500-1600 is not very high... but I don't believe this is a symptom of a problem with the electronics (then again, I don't know of all symptoms that could occur)...could it be a mechanical jamming of the Tach Needle?...try tapping on the Tach case and see if that frees it you going up or down...otherwise, remove Tach from Dashboard and turn case quickly to see if inertia of the Needle allows it to swing past that max area...if not, that somewhat confirms a jammed pivot also. Options then are to send it to an instrumentation specialist such as Nisonger or Accutach (no relation).

    See also: http://www.sw-em.com/Smith's%20Tachometer.htm

    Good Hunting








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      Smiths Tach Won't Go Past 1500 RPM 1800 1973

      I tried the "inertia spin" but couldn't determine anything definitive. I opened up the bezel and glass and was able to swing the needle to red line with no mechanical obstruction. So I put it back in the car and now, mysteriously, it will climb with the rev's beyond 1500. I momentarily rev'd it to up to around maybe 4000-ish and the needle followed however it did quirk out jumping up and down in the 3000-to-4000 area. I have two extra tachs and all three of them quirk out in kinda the same way. I'm starting to wonder if the ignition pulses are not clean. I'm still running points but I had to replace the coil - it currently has one of those blue Hecho en Mexico Bosch coils. I think I have another original black coil in the parts heap that I could try, but it's 45 years old. I just thought I could put the timing light on it and see if I can detect any skips in the spark.

      Moving on, i had a situation where the engine temp gauge was pegged out max with a cold engine. The sensor's resistance was only about 1 ohm so I swapped in a different sensor. This one reads about 4500 ohms when cold and now the temp gauge doesn't seem to move off the cold line even after running the engine a few minutes. Does anyone happen to have a temp vs resistance curve or at least a couple data points that I can use to check it?

      And the saga continues....

      Thanks again!








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        Smiths Tach Won't Go Past 1500 RPM 1800 1973

        Chris;

        If simply moving the needle manually past the sticky point and exercising it a few times cleared it, be happy it was a simple fix (how long it will last is another question) ...it still does sound to me like a mechanical issues...it could have been something as simple as a piece of dust or debris had gotten into the pivot enough to bind it up from going past the 1500 mark...it wouldn't take much...were talking about a very fine mechanism (watchmaker stuff!) here...

        I can't help with a sensor, but I recall Temp vs Resistance charts for sensors in the factory green manuals...maybe you could start looking here: http://www.volvoamazonpictures.se/documents/Volvo_Amazon_manuals_en.php

        Good Hunting!








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          Smiths Tach Won't Go Past 1500 RPM 1800 1973

          Check that the wiring connections are clean and secure with good crimps. Both the coil and dizzy ones can get a bit loose and corroded. Clean and reset the points or renew. Hopefully there isn't too much play in the dizzy shaft. The tach is reading the on/off at the points and this action should be precise and accurate. If it starts to jump at higher revs the points may be erratic or floating a bit.








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            Smiths Tach Won't Go Past 1500 RPM 1800 1973

            I have new-ish Bosch points and condenser in it. The car has ~100,000 miles on it so I wouldn't think there would be a whole lot of play in the distro bearings. I hooked up a timing light and ran the engine up and down the RPM range and couldn't detect any skips in the strobe light to correspond with the tach needle jumps. So I'm thinking I'll just run it on the road for a while, whenever that day comes! If the needle jump is being caused by a mechanical glitch then maybe it will smooth out over time.

            Thanks for comments/ideas.
            --
            Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)








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    Smiths Tach Won't Go Past 1500 RPM 1800 1973

    Perhaps the tach thinks that it is in a British car and doesn't want you to blow it up!

    If the revs make sense up to 1500 RPM, I think that your problem is mechanical.








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      Smiths Tach Won't Go Past 1500 RPM 1800 1973

      It might make sense to hook up a portable tachometer to confirm that the indications on the factory tach are Indeed correct up to that 1500-1600rpm.

      I still think there is a problem, but then at least you would have independently tested data.







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