bouncing speedometer

[leander77]

Speedometer on my '77 reads fine up to around 40, then bounces wildly at higher speeds. Is the problem in the cable or the instrument cluster? Info on the fix would be helpful.

 

[iwlbcnu]

Usually the cable, pull it out and lube it good.

 

[leander77]

I'll give it a try. Thanks

 

[broncosbybart]

Usually the cable, pull it out and lube it good.

ditto on what he said. i usually replace them with new.

 

[MarsChariot]

I just replaced the cable on the '76 for the same reason. Did it on the '71 a couple of decades ago. Usually the cable sheathing just gets tires, kinked, and rubbed the wrong way after all the years and lube will only be a temporary fix. Best to just replace it. It is only 20 bucks or so and replacement is not the nightmare it is on some vehicles.

 

[Mao]

Mine does that too, but it's just from the flat spots in the tires. ;D

 

[leander77]

Yup, cable replace is probably in order. I need to change the speedo out anyway. I've gone from P235 75 15's to 33 10.50's. Where can I find the chart to know what speedo to order?

 

[mo-bronco]

Don't mean to hi-jack, but I have the same problem. Can the cable be gotten from a local place or one of the vendors?

 

[leander77]

auto zone shows one on their website for around $10 for the '77

 

[oleguy74]

Yup, cable replace is probably in order. I need to change the speedo out anyway. I've gone from P235 75 15's to 33 10.50's. Where can I find the chart to know what speedo to order?

there is a chart not sure where it is.some have posted it on here but for the most part it can be hard to find one exact but you can get close.it is a gear change on t-case end of the cable.

 

[MarsChariot]

Tom's Bronco will sell you the cable and your choice of gear as a package.
Here is the chart from the DC catalog, reproduced from Bronco Driver (Chris Bradley, author).

How Fast Am I Really Going?
If you ever change the gears or changed tire size
on your Bronco, you can calculate your speed by
the following formula:
Old Tire x Old Gear / New Tire = New Gear
OT x OG / NT = NG
Example:
Old Tire - 29”
Old Gear - 19 tooth
New Tire - 33”
New Gear - ?
235/75-15 = 28.9”
OT OG NT NG
28.9 x 19 = 549.1 / 33 = 16.639 (17 tooth gear)
29 x 19 = 551 / 33 = 16.6969 (17 tooth gear)
31 x 19 = 589 / 33 = 17.848 (18 tooth gear)
If you change tire size and gears (or just gears), you
have to go one step further...
New Gear / Old Ratio x New Ratio = New Gear
NG / OR x NR = NG
17 / 3.55 x 4.56 = 21.8
21 / 33.5 x 28.8 = 18.1
18 / 3.55 x 4.56 = 23.1
Each tooth on the speedometer gear adjusts your
speedometer reading by approximately 5%. When
you add a tooth, you reduce the speedometer read-
ing by 5%.
Information by Chris Bradley, as published in Bronco Driver
Magazine.

 

[Green76]

Bouncing

Speedometer on my '77 reads fine up to around 40, then bounces wildly at higher speeds. Is the problem in the cable or the instrument cluster? Info on the fix would be helpful.

Mine was bouncing a new cable fixed it...;D

 

[sriha]

I let mine bounce, I don't drive on the road that much and there are plenty of cars to judge speed on ;D There are probably other reasons a cop would pull me over for anyways. My problem is with higher speeds but on back roads only I don't get that fast.

About the calculating thing, how do you find the right gear when you don't know what you started with? A few years ago I melted/stripped my spedo gear and went to the Ford dealer for a new one. They didn't know the size and couldn't match from what was left of the old one so I just picked a number and went with it.

 

[fallingdown]

As other have already stated a new cable is definitely in order after 30+ years. In time the factory grease dries out and dirt infiltrates every nook and cranny of your vehicle including your gauge cluster. When I refurbish a head unit I generally let it soak in a parts bath for 24 - 48 hours and then lube everything with a good synthetic grease. There are a few things that can cause your head unit to bind. The most common things are the speedo cable and a lack of grease. Other things include the magnet needing to be recharged or the gears binding up. Its also very common for the head unit to loosen up from people over tightening the speedo cable. If you over tighten your speedo cable you will sheer off the three rivets that hold your head unit in place and then you will have a bouncing speedometer needle.

If a new cable doesn't fix your problem I would be happy to take a look at it for you.


classychassisrestoration.com

 

[leander77]

Thanks for all the info.
Got the cable from the Zone today, and of coarse it didn't fit. Wrong fitting on the underside. I'll need to order the new cable and speedo. Old speedo was 18 tooth, so I'll have to do the math. According to a guy at the tranny shop, one tooth = around 3mph, so you could drive it with the old one and compare to a gps speed or something.
Fallingdown,
I'll see if this fixes it first, but may still send the unit to you for refurb. The work looks nice. Everything else seems to work except the temp reading. For now I've installed a separate gage, but would like to get it working again in the console unit.

 

[MarsChariot]

About the calculating thing, how do you find the right gear when you don't know what you started with? A few years ago I melted/stripped my spedo gear and went to the Ford dealer for a new one. They didn't know the size and couldn't match from what was left of the old one so I just picked a number and went with it.

The number of teeth is labeled on the collar of the pointed end, "17T", "19T", and so on. Also they are color coded.

 

[sriha]

The number of teeth is labeled on the collar of the pointed end, "17T", "19T", and so on. Also they are color coded.

I think that part was melted on mine. I think the one I had was dark red.

 

[HoldMyPocket]

pull it out and lube it good.

Thats what she said.

 

[MarsChariot]

I think that part was melted on mine. I think the one I had was dark red.

Dark Red = 16T, at least in my old parts bin

 

[GusI]

Pulling up an old thread. It appears I need to clean and lube my speedo cable. I have Silicone Lube Spray and Silicone Grease - which should I use?