Bouncing speedometer

[Dutch Rutter]

https://www.tciauto.com/speedometer-gear-calculator

I'd start here and get the right stuff to ditch the adapter box.

I like this plan. With the driven gear attached to the adapter box thing I get some play. I can move the driven gear about 3 teeth either way before the other end of the box thing starts moving. This is with me holding both ends keeping the boxes output stationary.

According to that link I have 15 tooth drive and 40 teeth driven (mainly for me to reference back to) im going to reassemble without the box for the night and so I can drive to work tomorrow. HOPEFULLY the bounce will be gone. I know it probably won't read accurately. But I can hope for no bouncing.

 

[Turbo4whl]

https://www.tciauto.com/speedometer-gear-calculator

I'd start here and get the right stuff to ditch the adapter box.

Something to keep in mind, the transfer case was manufactured by New Process Gear, division of the Chrysler Corp. Not as many internal speedo gear choices like GM's own two wheel automatics.

Most units built by GM came from the factory with a ratio adapter.

@Dutch Rutter your test without the adapter is a good choice. If the speedo smooths out, I would look for a replacement ratio adapter.

 

[SirRobyn0]

@SirRobyn0

Thats a good idea. When I replaced the cable I did the whole thing outter and inner. I felt it was the correct length maybe a 2-4 inches too long.

All though I think you understand I should clarify just in case. The outer and inner can be shorter or longer than original that's fine, where the problem comes in if the inner part is longer than it should be it loads the inner part and makes it work poorly, this can be by as little as 1/8".

 

[Dutch Rutter]

Yep, I'm tracking on the cable length. I think it's good there, I don't notice any binding in the ends. Driving in this morning without the converter box thing I still have a bounce so it's back to drawing board there.. I might try re-routing the cable tonight there are a few gradual turns in it but maybe something there is causing it

As for accuracy, it's reading closer to real speed without the box. If the gauge says 65mph I'm traveling 60mph now.

 

[Bextreme04]

Those gear ratio boxes are the way that 4wd's came from the factory. The 2wd trucks get the different ratio gears, but the 4wd have the speedometer output in the T-case and its a pain to change them out(if you can even find different gears). I still say that any time a speedometer is bouncing, it is due to resistance on the driven side of the system. It has to be either the speedometer or the cable, none of the other drive stuff has any play for it to have a spring action like that.

 

[Dutch Rutter]

Those gear ratio boxes are the way that 4wd's came from the factory. The 2wd trucks get the different ratio gears, but the 4wd have the speedometer output in the T-case and its a pain to change them out(if you can even find different gears). I still say that any time a speedometer is bouncing, it is due to resistance on the driven side of the system. It has to be either the speedometer or the cable, none of the other drive stuff has any play for it to have a spring action like that.

This is my thoughts as well after seeing those gears not chewed up. Since it's acted the same with the 100mph speedo I got used and now this brand new one I'm leaning cable. I made sure to pull the inner completely out and inspect/clean it then lubed it very liberally with graphite, before I installed it. I routed it just like the one that I removed was.

It basically leaves the transfer case on the passenger side, then snakes over the top of the transmission then turns towards the cab, and straight up into the engine compartment where it turns through the firewall.

 

[saltdog]

This happened to me as well. After replacing pretty much everything, I yanked out the gauge cluster. Turned out there was small bit of fuzz up at the speedo head that was interfering with the needle. Blast of air and fuzz gone and works flawlessly.

 

[MikeB]

I've always found that graphite cable lubricant will fix the needle bounce, unless the cable is very old, kinked, or installed using a very tight turn. Even a brand new (especially imported) cable might be dry.

I saw grease and oil suggested in another forum. DO NOT use those.

 

[Standi]

I used that speedo calculator the other day. Worked great. Dead on now. I did replace the drive gear too - easy work after taking off the tail shaft (transfer case). I also lubed the cable as it was making a lot of noise. Lube helps a lot. I still get some bouncing so I’m thinking I have a tight radius I need to widen.

I ordered new drive and driven gears. RTV the housing and should be g2g.
Good luck!

 

[Raider L]

@Dutch Rutter,

I have a box where my speedo cable connects to the transmission also. I've never had mine apart though, and I have a small leak coming from the halves. I think the gasket is bad. The speedo was a electronic speedo when the truck was stock, it was plugged into the printed circuit board like it is from the factory. But when I rebuilt the truck the entire printed circuit board came out and I installed a Auto Meter mechanical speedo and all other gauges are mechanical. I had to go to the dealer where my friend who worked in the parts dept. knew how to look up the length of a mechanical speedo, where they were mechanical. I think my truck was like 38" or something, I'd have to look it up. It connects to the box and goes straight up to the back of the speedo. There is a small bit of play in the length but only because there has to be for when you have to pull the dash out for whatever. It can't be exact to the speedo or you'd never be able to pull the dash out far enough to get hold of the finger nut on the back of the speedo to undo it. You'd have to disconnect the speedo every time to get the instrument cluster out far enough to do work on it, like change a burned out bulb. But it's not so much as to create a big bend in the cable behind the dash.
You could look behind the dash to see how much play in length there actually is where it connects to the back of the speedo.

You must be registered for see images attach

You can see the size of the box on my trans. I suppose there are gears in there, but between the two halves of the box is where mine is leaking. The line down the middle of the box is the left and right halves.

 

[Dutch Rutter]

@Raider L
Thanks for that. Really clean looking setup btw.

I ended up getting this pretty well dialed in. At first I took the cable I had and used a grinder to shorten the part that goes into the back of the speedo like @SirRobyn0 suggested. That helped out a bit but still had a pretty big bounce. I then ran the cable with a drill to and the speedo still showed that same bouncing.

Ended up getting a new cable from napa. The napa cable got super greased up and fit better then the LMC one it replaced. I routed it over the trans instead of the t-case like I has it before. All of that fixed about 99% of the bouncing. I can live with the remaining 1%.

As for the accuracy issue. I found https://transmissioncenter.net/
Which suggested going with a 35 tooth driven gear to go with my 15 drive gear, 32" tall tires (measured not supposed height) and 3:73 gears. I'm installing that today or tomorrow and we'll see how close that gets my accuracy.

With your leak. I'd be getting some gasket material and cut a new gasket to fit that box. It seems finding those is tricky at best.

 

[Dutch Rutter]

I just installed that 35 tooth driven gear. Out of curiosity, does anyone know if I should be able to use the 40 series gear housing with a 30 series driven gear?

That company seemed to think I needed the 30 series housing but it's about 1/8th inch too big for the hole in the t-case. Right now I have the 35 tooth driven gear in my 40 series housing. But I have not yet driven the truck.