mll1446 Posted June 13, 2020 Share Posted June 13, 2020 Today was driving car down highway cruising along good speed and when I got near home noticed speedometer wasn’t working. Any idea what to check or look for to see what issue is. Thanks in advance Link to comment Share on other sites More sharing options...
JohnD1956 Posted June 13, 2020 Share Posted June 13, 2020 Speedometer cable is the first thought. A stripped speedometer drive gear at the trans is a second thought. Does the car have cruise control? Outside chance that the servo needs attention. Link to comment Share on other sites More sharing options...
Larry Schramm Posted June 15, 2020 Share Posted June 15, 2020 Quick check would be to remove the speedometer driven gear at the transmission where the speedometer goes in to the transmission. Check to see if the driven plastic gear has "apple cored" If not, try to spin the cable. Good idea if the car has cruise control. Was the speedometer needle jumping prior to not working? If so and it has cruise, the transducer could be bad. Link to comment Share on other sites More sharing options...
mll1446 Posted June 15, 2020 Author Share Posted June 15, 2020 I got under car and found this cable is still good. But this doesn’t loook right. No cruise control on car Link to comment Share on other sites More sharing options...
Larry Schramm Posted June 15, 2020 Share Posted June 15, 2020 9 minutes ago, mll1446 said: I got under car and found this cable is still good. But this doesn’t loook right. No cruise control on car Did the speedometer read correct speed on the dash? The part you have there looks very like a speed reducer that would cause the speedometer to read 1/2 the actual and mileage. Did you take anything out of the 90degree adapter or was that as you found it? Link to comment Share on other sites More sharing options...
mll1446 Posted June 16, 2020 Author Share Posted June 16, 2020 Y ty bay was how I found it Link to comment Share on other sites More sharing options...
NTX5467 Posted June 16, 2020 Share Posted June 16, 2020 That "open" mechanism is termed a "ratio adapter". Given the number of teeth on the drive (on the transmission output shaft, held in by a flat-metal clip and indexing tang) and the driven gear (what the speed cable would attach to the housing which holds the driven gear) combination need a bit of "adapting" for the correct revs/mile of the speedometer cable, then the "ratio adapter" is added between the speedometer cable and the "adapter plug" (which holds the driven gear in place). Usually not big ratio differences, but enough to keep the speed/distance measuring pretty correct. The ratio adapters come in lots of variations and output drive angles (as yours is a right-angle version). IF there is not a flat pad on the main body with a ratio stamped into it, as "1.05" or similar, then you'll need to get into a vintage GM/Buick PAPER parts book which was printed before the 1975 model year. There should be a "Speedometer Gear" Chart in it, somewhere. Divided into rear axle ratios and tire sizes. As I recall, GMs came with a G60-15 tire from the factory? Probably a 3.42 rear axle ratio, too. On the rear axle's passenger side tube, facing forward, about 1/2 way between the center and the brake drum, there should be a string of letters and numbers. This is the "birth certificate" for the axle assy. When built, which plant, AND the last two or three letters would be the code for the axle ration and if PosiTraction was factory-installed. You'll need that parts book to decode those things, which should have several pages devoted to such in the front of the parts book. The ratio adapters were not Buick-specific, but were "just used as needed" by any maker. IF you can find a speedometer shop, they might have sources for them. Possibly the speedometer cable "guru" up in Michigan? BUT you'll need to have that ratio number stamped into the side of the main casting to get a new ratio adapter to match what you had. Just some thoughts, NTX5467 2 Link to comment Share on other sites More sharing options...
mll1446 Posted June 16, 2020 Author Share Posted June 16, 2020 Any idea what I’m missing from mine that Speedo stopped working Link to comment Share on other sites More sharing options...
JohnD1956 Posted June 16, 2020 Share Posted June 16, 2020 There is a gear that goes in that empty spot. It fell out of yours. You may be able to find it if you can remember where you first noticed the speedometer stopped working, and can retrace your route from there back to the beginning of your run. Link to comment Share on other sites More sharing options...
NTX5467 Posted June 19, 2020 Share Posted June 19, 2020 No "user-serviceable parts" in that assy. Look for that ratio stamped into the case, IF it was stamped into the case and not the welch plug that covered the parts which are now missing. If you have a factory tach, might be able to compute mph/1000rpm in high gear. NTX5467 Link to comment Share on other sites More sharing options...
EmTee Posted June 19, 2020 Share Posted June 19, 2020 Remove the reduction gearcase and attach the speedometer cable directly to the transmission output. Road test to determine (1) whether the speedometer works and (2) what (if any) error there is. Establish the answer to (2) by driving a measured distance and comparing to the change in odometer reading. An easy way to do this is hop on a nearby interstate and note odometer reading while passing a roadside mile marker. You can do this for one or more miles; longer distance will improve the accuracy of the measurement. Also, if you hold an indicated 60 mph, you should cover one mile per minute. Once you have the error identified, you can determine whether it may be something simple, like the wrong size tires, or maybe the trans or rear end was changed at some point previously. In any case, if the error is not too large you may be able to simply replace the nylon gear in the transmission's speedometer drive with an appropriate step up or down. The different gears are identified by color. https://www.tciauto.com/transmission-accessories/speedometer-gears/general-motors/driven-gears.html 2 1 Link to comment Share on other sites More sharing options...
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