bouncy speedometer

[Jarhead79]

My speedo bounces quite a bit before u get to around 20/30 mph, curious if this is a simple fix, or just normal for these trucks, I seem to remember this about my 80 I had when I was 16, actually brings back memories of my youth and my truck but now being older it annoys the hell out of me! Lmao

 

[Georgeb]

First thing I would do is lube the cable and make sure it is properly connected to the back of the speedo. Also that the cable isn't being pulled on by somthing causing missalignment at the speedo connection or it's damaged in any way.

 

[Jarhead79]

I'll give it a try

 

[HotRodPC]

Your drive and/or driven gears in the transmission can also be damaged.

 

[Jarhead79]

I thought of that actually, I've got a small leak there in the side of the trans where the cable goes in.

 

[quaddriver]

if you are talking about your 79 square in your 'data' my first idea wont work, but if you have a bad gear in the drive - yea that can cause bounce, BUT, it tends to eat itself on a permanent basis - quickly. Easy to check tho. PM if you want steps. If you had a 'head driven VSS' you can check pulses so see if the signal is interrupted, in fact, the ECM on those primitive systems will gripe about it. but a 79 wont have this.

Aside from that, non electric speedos work on a close clearance between a rotating drum and the inner 'cicrle', I have seen viscous fluid, magnets, static charge etc. I wont profess to know for sure what is inside yours but to tell you, a fluctuation when the driven pulse is constant is nearly ALWAYS (on the non-fluid ones) due to corrosion. rust or crud on one of the surfaces and to be honest, its not repairable. With a 79 square, you don't give a squat about your indicated mileage right? if it aint super low and you wanna prove it for a concours truck, just boneyard one and replace it. besides, dicking with the speedo and contained mileage indicator is illegal as all get out, but replacement is not.

 

[Jarhead79]

Yeah I'm talking about the 79, it's been sort of pieced together and basically everything works but the speedo bounces, the oil gauge doesn't work but o believe that's because it doesn't have a sending unit on the back of the motor, someone wired a small gauge pod but it's been fritzy, and needs replacing. Will probably be a nightmare but I'm gonna give it a whirl

 

[Georgeb]

What was the outcome of this?

 

[Jarhead79]

Have been too busy lately to get to it, been working on the interior mostly. Lol

 

[Jarhead79]

Messed with the gauges today, still bouncy speedo, going to try and check the gear in the trans tomorrow.

 

[smoothandlow84]

Replace the speedo cable. A bouncy speedo usually indicates a cable ready to snap. The cable could be frayed somewhere causing it to bind up at low speeds. Since you're in the rust belt.....its probably. ...well....rust.

 

[chengny]

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 (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 actually comes to a complete stop - 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 little DC generator that drives the speedometer needle. The indicated sped jumps up past actual speed until all the stored energy is dissipated.

Things return to normal - for a while. But soon the cable gets 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 (usually 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.

 

[Jarhead79]

Yeah I figured it's the cable, but ultimately it's an old truck, going to have problems, only things that really keep me happy is that it's finally running better, will actually spin the rear wheels when it wouldn't before, shifts into passing hear and trans shift good but could be better with a shift kit, rear end feels strong, and most of the hard body work has been done, just need some money thrown at her, and some tlc. I'm putting exhaust on her next week, duals with an xpipe and jones performance mufflers, should finally sound as good as she runs, it's so quiet I can't here her even idle in the cab, couple times it stalled and I wasn't even aware of it, and I like driving by the feel of it, lol