Speedo headaches

[Obwonkonobe]

Alright, so I've got a Speedo that's been giving me some weird problems. I rebuilt it from 2 differs,t ones, used the better needle, betrer faceplate, better Speedo guts, etc. The odometer is rotated up just slightly, this may be from where I made a new odometer holder thingy out of a credit card, but im not sure if that would affect the angle the numbers are at.

The other major thing is where the Speedo rests. To rebuild it I had to take off the needle right, well my ******* didn't think about putting it back on. I have it guessed pretty well, right now when I come to a stop the needle will go close to zero, not exact though, and when im driving it its always a little off, and id rather not have to explain that to a cop. Before anyone asks yes i did get new trans gears for my tire size, that's all good.

Last problem is the Speedo just wobbling, always. Even when im going at a constant speed, lets say 45, it'll bounce from 35 to 55 at random. And every now and then itll catch and make a click noise, and then just max out past 85, it the, procesed to take the next moment to travel everywhere at ramdom and then go back to its regular 10mph dance around the actual speed. Ant suggestions appreciated

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[Honky Kong jr]

Go pluck one out of a yard.

 

[chengny]

Last problem is the Speedo just wobbling, always. Even when im going at a constant speed, lets say 45, it'll bounce from 35 to 55 at random. And every now and then itll catch and make a click noise, and then just max out past 85, it the, procesed to take the next moment to travel everywhere at ramdom and then go back to its regular 10mph dance around the actual speed. Ant suggestions appreciated

This last problem might be caused by a binding drive cable.


When you think about it, the main body of the cable is really just a long, thin, stiff spring. Provided the speedometer head is okay, surging of the indicated speed is almost always caused by friction between the actual cable and the inner wall of the sheathing. Whatever the reason this friction exists, it will cause the cable to rotate in fits and starts (usually more pronounced at lower speeds).

The cable is struggling to spin and - if sufficient friction exists - eventually it will jam somewhere in the middle. The end that attaches to the speedometer head slows - or can actually come to a complete stop. When that happens, the indicated speed falls sharply (or goes to zero in extreme cases).

Meanwhile at the other end of the cable, the transmission output gear is still cranking away. So what you have is, a spring being torqued at the driven end while - at some point along it's length - it's rotational speed is being restricted.

This can only go on for so long - even with a spring - at some point, something has to give. The cable builds up enough torque at the restricted point that it is able to beak free. All the built up spring tension is suddenly released and transmitted to the magnet that drives the speed cup (that in turn, drives the speedometer needle). The indicated speed jumps way up past actual speed until all the stored energy is dissipated.

Things return to normal - for a while. But soon the cable gets slowed/stopped again and the cycle repeats.


I have found, that in most cases the restriction to rotation - rather than being caused by lack of lubrication or rust/debris within the sheathing - is due to improper routing of the cable. Often the cable body is pinched, kinked or has been rerouted in a manner that creates overly sharp bends (often to accommodate some kind of drive train modification).

While pumping lubricants or graphite into the cable housing will usually provide a temporary solution to the problem of erratic speed indication, the real fix is to route the cable in a way that is as straight as possible. And maybe more importantly, a way that eliminates any pinch points, kinks or sources of heat.

 

[Obwonkonobe]

This last problem might be caused by a binding drive cable.


When you think about it, the main body of the cable is really just a long, thin, stiff spring. Provided the speedometer head is okay, surging of the indicated speed is almost always caused by friction between the actual cable and the inner wall of the sheathing. Whatever the reason this friction exists, it will cause the cable to rotate in fits and starts (usually more pronounced at lower speeds).

The cable is struggling to spin and - if sufficient friction exists - eventually it will jam somewhere in the middle. The end that attaches to the speedometer head slows - or can actually come to a complete stop. When that happens, the indicated speed falls sharply (or goes to zero in extreme cases).

Meanwhile at the other end of the cable, the transmission output gear is still cranking away. So what you have is, a spring being torqued at the driven end while - at some point along it's length - it's rotational speed is being restricted.

This can only go on for so long - even with a spring - at some point, something has to give. The cable builds up enough torque at the restricted point that it is able to beak free. All the built up spring tension is suddenly released and transmitted to the magnet that drives the speed cup (that in turn, drives the speedometer needle). The indicated speed jumps way up past actual speed until all the stored energy is dissipated.

Things return to normal - for a while. But soon the cable gets slowed/stopped again and the cycle repeats.


I have found, that in most cases the restriction to rotation - rather than being caused by lack of lubrication or rust/debris within the sheathing - is due to improper routing of the cable. Often the cable body is pinched, kinked or has been rerouted in a manner that creates overly sharp bends (often to accommodate some kind of drive train modification).

While pumping lubricants or graphite into the cable housing will usually provide a temporary solution to the problem of erratic speed indication, the real fix is to route the cable in a way that is as straight as possible. And maybe more importantly, a way that eliminates any pinch points, kinks or sources of heat.

That makes tons of sence, I bought a new cable from Amazon but never thought about lubrication. Il check for kinks tomorrow, in the mean time what lubrication should I use? And how would I apply it? I figure it can't hurt to just try both things

 

[Rusty Nail]

Wtf u do all that work on an 85 for?

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[chengny]

That makes tons of sence, I bought a new cable from Amazon but never thought about lubrication. Il check for kinks tomorrow, in the mean time what lubrication should I use? And how would I apply it? I figure it can't hurt to just try both things

They sell lubricant specifically for speedometer cables - I think it's just graphite - at any auto parts store. But if it's a brand new cable, I would be more concerned with kinks or sharp bends in the routing rather than lubrication.

Speedometer cables don't rotate all that fast and changes in speed are easy to detect. If you unclip it from the back of the head - and get the end in an exposed position - you can observe the rotational speed of the inner cable while you drive. Then, if while driving at a steady speed, you notice the square tang slowing down and speeding up (or stopping/starting), you will know that the problem is with the cable and not the speedometer head.

 

[Obwonkonobe]

@Rusty Nail lol its more satisfying when you go past all the numbers.

 

[Obwonkonobe]

@chengny il have to try that out tomorrow, il let you know how it goes

 

[1987 GMC Jimmy]

If rerouting the cable works for you, then great. If you need to lubricate it, I used Dorman 03344 speedo cable lube, which is a good product. When you lubricate it, the truck can't be still or the lube will puddle at the top of the cable. If you have a long driveway or a culdesac, carefully driving it around there and pouring it down would be how I'd do it.