47-55.1 Cable Speedo sticking

[nvrdone]

ok, kevin - tell us what the problem and we can help.

[Kevin Mac]

Not sure why my question didn't post. Anyway my cable driven speedo sticks around 35. The odometer keeps working. I have rerouted the cable and it has more bends, but it was kinda catchy before then. Sometimes it will stick at 35 when you come to a stop and then all at once fall to zero. I believe they are stock rebuild because I have a receipt for them from PO. Not sure how a speedo works, must have some kind of clutch mechanism for the needle to stick but odometer still racks miles.

[dsraven]

sounds like a speedo head problem. if the needle isn't visibly sticking on the face, like on a high spot on the face for some reason, then it has to be in the head. when you stop so does the cable so the speedo is sticking internally. one thing to check would be to ensure the speedo cable isn't in at a weird angle or routed at a weird angle to the speedo as this may cause the cable connection end of the actual speedo to be forced at an angle to the mechanism so it could be causing the issue, if that is the case.

[nvrdone]

if i remember correctly, in oem speedos there is a magnet in the head that is driven by the cable. there is also a spring that stabilizes the needle. Id pull the speedo out & see if there is some way to lube the internals.

[dsraven]

If you take it apart you could check for dirty parts or a dead bug etc but it is an instrument so after that just take it to an instrument/speedo repair shop. If lubing be careful what you use because a "wet" product will attract dust etc.

[mr48chev]

If the odometer works right it isn't the cable but in the head as Dsraven said, I'd go along with the rust, dirt debris concept.

I've had a few odometers out to turn them back to 0 or in one case set one at the original vehicle mileage at the request of the owner when I swapped speedometers but haven't had much to do with the actual speedometer.

[Kevin Mac]

Thanks, I will play with the cable input angle first, it seams to make a difference. If that doesn't work then I will take it apart. The thing looks brand new and wasn't hooked up with zero miles when I bought it 4 years ago. I have had it hooked up for several years now and at one time worked ok.

[Kevin Mac]

Thanks, I will play with the cable input angle first, it seams to make a difference. If that doesn't work then I will take it apart. The thing looks brand new and wasn't hooked up with zero miles when I bought it 4 years ago. I have had it hooked up for several years now and at one time worked ok.

[nvrdone]

another thought - disconnect the cable at the trans & hook up your portable drill. run the drill in reverse to see if the speedo works correctly.

[mr48chev]

From experience if the cable is dry or binds up a tad the speedometer needle bounces and doesn't get stuck in the same spot. If you go one mile at 60 and the odometer reads one mile but the speedometer only went to 30 you do not have a problem with the cable.

If you have a smart phone there are dozens of gps speedometer apps available that are spot on speed and distance wise. I actually use my phone for the speedometer in my 77 because something is screwed up and it breaks the short section of cable down at the speedometer.

[dsraven]

like mr48 says, run it a mile (by your phone gps) and if the odometer says a mile then the cable is likely not the problem. it's likely the mechanism of the speedo head at fault or, like mentioned earlier, the cable is jamming the speedo head parts because it isn't connected inline with the speedo.

inside the speedometer is a circular/disc shaped magnet driven by the speedo cable. the round disc magnet sits inside a metal drum shaped part, with close tolerances between the 2 parts, but it doesn't actually touch the metallic drum. there is an air gap. turning the magnet doesn't mean the drum has to turn. the drum has a needle attached to it that becomes the part you read the speedo by. as the speedo cable turns the attached magnet wants to turn the drum due to magnetism and eddy currents induced inside the drum. the eddy currents have nowhere to go but they do induce movement of the drum part through some scientific laws of electromagnetism. the drum and needle are balanced/calibrated against movement with a coiled spring that resists the turning of the metal drum, otherwise the magnet would simply wind the drum and the needle around in circles as it spins inside the drum. as the magnet spins faster, when the vehicle moves faster, it induces more electromagnetic currents inside the drum to make it want to turn more with the magnet. sort of like a torque converter does in an automatic transmission. as this happens the force on the drum overcomes the balance spring's resistance slightly and the drum and needle turn a little more showing a higher speed on the face of the speedometer. the magnet and balance spring are calibrated to provide the correct resistance for the setup it is designed to monitor. this whole setup also includes a gear drive set connected to the cable input that is used to work the odometer. the odometer moves if the cable turns because it is directly connected to the cable input. the needle portion can be broken but the odometer can still work because it is directly geared to the cable input.
it makes sense then, that if the odometer is reading correctly but the needle is jumpy, that it could be some interference between the spinning magnet and the drum. something like a rust spot on the drum or maybe some metallic dust stuck to the magnet. something that allows the magnet to drag periodically on the drum. sometimes a dead bug (or part thereof) or other dirt gets in there. if the truck, or speedometer from the truck, was left in an area where there was metal work being done and the speedo was not sealed against ambient dust (like there were no bulbs in the back or something like that) then some metallic dust from the air in the shop may be stuck to the magnet. the whole mechanism is pretty small and light duty so if the speedo were not handled well, if it was dropped or the dash were hammered or welded on with the speedo still intact or if the speedo head were forcibly connected to the speedo cable at an angle then the drum or shafting could also be slightly bent or out of round. sort of like a brake drum that pulsates or makes noise as you drive if it is not quite true or has some dirt inside it jamming the parts up slightly. that is why I suggested to check the alignment of the cable with the speedo head and also to check for dirt or debris inside the mechanism. the same could happen if the cable were to be routed in such a way that interference of the cable with moving parts could cause the cable to be stretched or pulled on with engine or driveline torque (when the engine moves against it's mounts under torque) . a cable that is tied up too tightly or is too long or too short can also cause undue tension against the speedo head. this is sometimes the cause of a jumpy needle because the extra stress inside the cable, caused by a quick bend or tight inner cable to sheath tolerances, causes the cable to "wind up and release" which in turn causes the speedo input to become non constant. the odometer reads correctly because when the unit finally stops turning the end result is that the number of turns the cable was made to go still ends up happening, just not in a nice consistently even rotational movement. an old cable can be damaged or worn inside which can cause the inner cable to bind. I have seen plastic coated liners inside the sheath that had worn grooves in the bends from the inner cable rubbing and this has caused the inner cable to "wind up and release" which makes a jumpy speedo needle. driveline torque on a mechanically tight or a tightly retained cable can causing it to pull on the speedo head. tight bends in the cable, causing the cable to be hard to turn, can also cause premature wear or take the teeth off the plastic gear drives inside the trans or inside the speed correction gear boxes attached to the trans. this usually means the odometer reading would be off but, really, with the number of turns it takes to make a mile, the cable could realistically skip a few times and you wouldn't really tell. I have also seen the squared off ends of the inner cable become rounded off in a certain spot because the area they contact inside the speedo or trans adapter isn't very big/long so the stress over time and/or the cable "wind up" in some circumstances has caused that small contact spot to become rounded off. a "tight to turn" cable usually accelerates that rounded off cable or worn out square hole in the adapter/speedo head connection.
sometimes the cable will move laterally inside the sheath and the squared off sections will contact properly again causing the speedo to get a more constant rotational input. this happens more with cheap cables that don't have a stopper on the inner cable part to keep it from moving up and down inside the outer sheath. its a good idea to check both ends of the cable plus the insides of the trans adapter and speedo connection for wear plus check how much the inner cable can move in and out of the outer cable and whether or not there is enough cable sticking out from both ends of the outer sheath to make proper connections at both ends. sometimes this lateral movement of the inner part allows the inner cable to become disconnected from one end or the other for a few turns and can cause a jumpy needle.
here is a link to a possibly better explanation of how the analog speedometer works.

https://www.howacarworks.com/accesso...a-speedo-works

or if you want actual pics and a more in depth look at real parts and longer more precise explanation, try this link. I think it is a motorcycle speedo or a "stand alone" encased speedo that could be used in an individual panel gauge scenario instead of a cluster with more than a single gauge inside the unit.

https://www.engineersgarage.com/insi...dometer-works/

hope that helped somebody, fee-oof. my brain hurts, haha.

[Kevin Mac]

Thanks dsraven and others for the input. Calling for snow today so will be a little bit for a road test but will look into it. The back is plastic and I assume the original was metal.
Mac

[Second Series]

I had a similar issue recently with a 1948 speedometer. The needle would not go higher than 20mph. I discovered the stop on the speedcup was contacting the stop on the housing. I tried to reposition the needle and discovered the spindle was no longer attached to the speedcup. This is a good time to replace the speedometer, but I decided to glue the needle back to the speedcup. In the end it works, but it was a lot of trouble. I found that this was a common problem with the earlier speedometers with a steel spindle soldered to an aluminum speedcup. The design was changed for later years. You didn’t say what year your speedometer is?

[Kevin Mac]

Originally Posted by Second Series

I had a similar issue recently with a 1948 speedometer. The needle would not go higher than 20mph. I discovered the stop on the speedcup was contacting the stop on the housing. I tried to reposition the needle and discovered the spindle was no longer attached to the speedcup. This is a good time to replace the speedometer, but I decided to glue the needle back to the speedcup. In the end it works, but it was a lot of trouble. I found that this was a common problem with the earlier speedometers with a steel spindle soldered to an aluminum speedcup. The design was changed for later years. You didn’t say what year your speedometer is?

It's a 48

[flyingdet444]

The cable might be dry, oil the inside of the cable housing.