Guest Vetteman61 Posted December 11, 2011 Share Posted December 11, 2011 Hello,I have a 1978 Caprice coupe. For years now once the speedometer hits 45 miles per hour it begins bouncing and does not register correctly. Does anyone have a link or explanation on how I can take this apart and fix the problem?Thanks,Brandon Link to comment Share on other sites More sharing options...
Guest Vetteman61 Posted December 11, 2011 Share Posted December 11, 2011 I meant to add that the speedometer is making noise when bouncing, so according to what I have found so far that means it's not just a cable issue.Brandon Link to comment Share on other sites More sharing options...
hwellens Posted December 11, 2011 Share Posted December 11, 2011 Sounds like the lube on the cable is gone. I think you can take the cable housing off the back of the speedometer and put sum lube (like graphite) down in the housing. Link to comment Share on other sites More sharing options...
Guest fmbjogger Posted December 11, 2011 Share Posted December 11, 2011 Vetteman, I love the picture. The lighting, background building, the feeling of the 70's and of course, the beautiful looking '78 Caprice. Link to comment Share on other sites More sharing options...
Bleach Posted December 11, 2011 Share Posted December 11, 2011 In some cases it can be a build-up of dust, grime or dried out grease on the moving parts inside the speedometer. This is just an example and not your actual speedometer head. Yours will be similar. Link to comment Share on other sites More sharing options...
Richard1 Posted December 11, 2011 Share Posted December 11, 2011 Normally that is the cable (either dry or going through a sharp bend), but you might also want to lube the bearing on the head. It might also not be perfectly seated in the head or slightly rounded and slipping when it gets fast.The cable spins and the faster it spins the more it can fluctuate. The cable spins the gear for the miles and a magnet for the speed. There is a spring on the back (inside) that puts tension on the the spinning magnet to keep the speed steady. But don't just put more tension on it as it will then show a speed too slow. That tension is what calibrates the speedometer. Link to comment Share on other sites More sharing options...
Ben Bruce aka First Born Posted December 12, 2011 Share Posted December 12, 2011 I would lube the cable first. Chances are that will fix it. Ben Link to comment Share on other sites More sharing options...
sfair Posted December 12, 2011 Share Posted December 12, 2011 Also check the cable that goes to the cruise control transducer and the transducer itself for binds/jump. Link to comment Share on other sites More sharing options...
NTX5467 Posted December 12, 2011 Share Posted December 12, 2011 What ALSO might be an issue is internal wear of the cable housing's inner surface. By this time, most cables were of a plastic nature with some metal wire reinforcement.Typically, the bushing in the back of the speedometer head is also plastic. It can deteriorate with time and mileage, too. Lubing it will only add more lube where it normally isn't needed or desired. Replacement bushings (when the head is rebuilt by a speedometer shop) are usually oil-impregnated bronze (similar to door hinge pin bushings), which lasts much longer.As you mention "noise", my suspicion is that the rear plastic bushing has worn, which lets the spinning magnet side (rear) of the speed cup rub the other side (front) of the speed cup (attached to the actual speedometer needle's shaft). This noise should be similar to a scraping and ringing noise when it momentarily happens with each revolution of the speedometer cable. Each "touch" can make the speedometer needle jump.The actual speed calibration is done with "magnetic intensity", meaning a stronger magnet makes the front speed cup want to follow it more closely and weaker magnetic forces do the opposite. At the speedometer shop, to effect such calibrations, it's a cut and try method with a "zap gun" (for those who were around in the earlier days of color television, you might remember the "degause" treatment of the picture tubes . . . the "zap gun" is similar, as it affects the ultimate magnetism of the rear speed cup's magnet.The "clock spring" on the front speed cup ensures that the speed cup's resting position is at the "zero index" point on the speedometer's face plate . . . no more, no less, although it's possible effect upon speed calibration is compensated for by the magnetic force of the rear speed cup's bar magnet, typically.On "certified" speedometers (for law enforcement use vehicles), their calibration is specified to be + or - 1mph over the full speed range, IF the interior temperature is within a normal temperature range. It could well be a special clock spring which makes this close-calibration possible, as well as tighter quality control.In conclusion, I suspect your issue could be a combination of issues. One of possible inner cable housing wear plus normal wear of the speedometer head's rear bushing. IF the car has cruise control, the governor weights in the cruise control "head" might also be an issue, too.Happy Holidays . . . from TEXAS!NTX5467 1 Link to comment Share on other sites More sharing options...
Guest Jim_Edwards Posted December 12, 2011 Share Posted December 12, 2011 (edited) Without knowing how many miles are on the car rather than get into a bunch of speculation I'd simply replace both portions of the cable. They're cheap and don't really take that much time to replace. Worse thing that will happen is the noise doesn't go away. On the other hand, there is probably better than a 50/50 chance replacing the cables will end the problem.I've cars that are over 50 years old and to my knowledge have never had an service done to the speedometer or cables and have yet to suffer what you are experiencing. Edited December 12, 2011 by Jim_Edwards (see edit history) Link to comment Share on other sites More sharing options...
Guest Old48Truck Posted December 12, 2011 Share Posted December 12, 2011 Easy fix: Just keep it under 45... Link to comment Share on other sites More sharing options...
Guest Jim_Edwards Posted December 12, 2011 Share Posted December 12, 2011 Easy fix: Just keep it under 45...Or sell the car and buy one without a bouncing speedometer needle. Link to comment Share on other sites More sharing options...
Guest Vetteman61 Posted December 12, 2011 Share Posted December 12, 2011 Thanks for the responses guys.Haha. I can't sell it, I got it the week after I got my driver's license. That building was my dad's GM dealership. We owned it until GM closed us down at the same time as a LITERAL crooked city council tore the building down and put up a "judicial center." Not a single of one the entire committee was reelected. Our building had been there since the '20s.Anyway, I suppose I'll just try to take the cluster out and using the suggestions here see if I can look at the mechanisms and see what's wrong. Speedometers are still a bit of black magic to me, so I have a fear of breaking them and not being able to put them back together again.Thanks,Brandon Link to comment Share on other sites More sharing options...
sfair Posted December 12, 2011 Share Posted December 12, 2011 As a word of caustion...do not be tempted to clean the number face with anything "wet".The paint might come right off of the face! Link to comment Share on other sites More sharing options...
Coley Posted December 13, 2011 Share Posted December 13, 2011 What needs cleaning and lubing is the small bushing in the end of the main shaft that holds the end if the speed cup.shaft. Remove the speed cup and clean /oil the small bearing with machine oil.The cable isn't the cause as the noise isn't coming from the cable. Jumping can be caused by a dry or kinked cable. Link to comment Share on other sites More sharing options...
Guest Kingoftheroad Posted December 13, 2011 Share Posted December 13, 2011 I would lube the cable first. Chances are that will fix it. BenI agree...Start with the simple & cheap solution. No sense in replacing good parts.. Link to comment Share on other sites More sharing options...
Guest MORRISGAUGE Posted December 13, 2011 Share Posted December 13, 2011 Delving into the speedometer head is likely going to be the fix, but I do not suggest it. Speedometer repair requires specialty equipment and expertise. Begin by cleaning and lubricating the cable, but you probably have a combination of issues. On average, a speedometer rebuild runs $150. If you can purchase a guaranteed speedo for less than that by all means do so. Just keep in mind that the old lubricants, even in an NOS unit, dry up over time. Link to comment Share on other sites More sharing options...
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