I have tried several times to get my speedometer functioning. The cable and head are OK as I have driven the speedometer with my drill turning the gear and everything is quiet and smooth. I was wondering if anyone has any experience with the worm gear that drives the speedo cable gear and if it is possible for it to be broken or something else. I am running a '70 302 with a light duty 3 speed manual tranny and the truck is a '66 2x4.

 

How is the gear on the end of the cable that goes into the tranny? These do wear and cause irratic operation. The gears can get concave or dished. Pull the one bolt and pop the cable out. look the gear over. It comes off by pulling the little horseshoe clip off the side. You can shine a light up into the hole to see the drive gear. I'll bet the cable gear is bad.

Barry

 

I'll post a photo but the drive gear wasn't dished too badly. Give me a second to upload it...OK, it's under the Clutch Diagram group.

 

The gear doesn't look too bad, but all it takes is a couple of worn teeth to make it jump. Hook the drill to that end to check the cable. If you can't see the gear up on the output shaft, you may have to drop the driveshaft and pull off the tailshaft housing and check it out. It just pops on or off. I believe there's a ball bearing underneath it.

Barry

 

I agree that the driven gear on your speedo cable looks OK in the pic. Suspect your DRIVE gear in the tailshaft housing is shot. In 65 the whole mouting system changed and the tailshaft housing rests on a crossmember, supporting the tranny and rear end of the engine. At least in a 4 speed. Don't know what the 3 speed looked like. Suspect the same.

PITA to replace the speedo drive gear in a 4 speed T-18 Warner, IMHO. But I did it, and my speedo works.

 

The gear looks a little chewed. What exactly is it or is it not doing?
How many teeth are on the gear?
Can you feel the meashing gear teeth with your finger in the hole?

John

 

Barry - The cable and speedo head are ok. I have driven the entire link from the end of the cable and all is smooth. The stupid hurst shifter mounting bracket nearly makes it impossible to insert and remove the cable and I re-shaped the shift linkage to give the cable a little more freedom.
CDH - Just what should I expect for the tailshaft housing worm gear? Is it nylon also? I can barely see it using a mirror due to the floor shifter bracket but it is in there (that's all I can say for sure because I can't see well enough to get a photo or even a good eyeball on it). The pictures I have (courtesy of jowilker) look like the speedo gear is machined directly into the output shaft but after saying that I haven't looked at the casting to ensure the trans is from 66. I would suspect though that the light duty 3 speed didn't change the output shaft configuration for many years. I
JOWILKER- The gear looks fine with the eye and I was surprised to see how well the digital photo revealed the wear. Says a lot about my eyes! When I first got the truck I had a bouncing and intermittent speedo and an R&R & cleaning of the cable did nothing to make it better. Then it slowly stopped working at all. I wonder what the diameter of the replacement gears should be and if all of the teeth have lost the apex of the tooth. Seems unlikely. This gear is a 17 tooth and the easiest thing would be to replace it and see. Will I still find these at the dealer?

 

If it's a 17 tooth gear, it's part number C3DZ 17271-B. Some if not all are now obsolete, but check with your local dealer, bring the old gear and the part number with you. They may have it on hand. Also I located a few on the locator.

Melvin's Classic Conyers, Ga 1-770-761-6800 qty 2
Wesley Obs. liberty, Ky 1-606-787-5293 qty 1
Dennis Carpenter Charlotte, NC 1-800-476-9653 qty 2
Classique Cars Lakeshore, Ms 1-800-543-8691 qty 4

Try these places if you don;t have sucess locally. I would definitely try one before I opened up the tranny. The early drive gears in the tranny were steel, but yours is probably plastic. In all the years I sold Ford parts, I maybe sold a couple of drive gears.

Barry

 

Your DRIVEN gear (the 17 tooth one) is fine. They are not that hard to find, but you don't need one anyhow. The scenario you describe convinces me that your DRIVE gear, a spiral gear, probably plastic, on the tailshaft, is shot.

You can stick your finger in there and feel it -- I supose someone with lots of experience could tell that way if it was really shot. This is not an easy job to replace, at least in a 4 speed. As I noted above, does your three speed rest on the cross member via the tailhousing? I suspect it does in a 66.

You can try some of the places Barry suggests. I can tell you that Dennis Carpenter does NOT have any driven gears.

I got mine eventually from Inland Truck, based out of KC. They are a wholesaler for truck tranny parts. I will also try to post a pic of the spiral DRIVE gear from a 4 speed. I think they are similar and will give you a reference for what you are dealing with.

 

I uploaded pics to my gallery, titled Speedo DRIVE gear. I think they will help you understand what I am saying.

 

LIst the particulars of your truck, such as model, engine, trans code off door or tag on tranny itself and tire size. I have a sppedo gear book and the parts books to look up the proper drive and driven gear.

Barry

 

CDH - The pictures are great and give me a good idea of what the replaceable drive gear would look like. I have pulled many trans but never had to open one. Just a lot of swapped out clutches and gearboxes. This one is a three speed with a slip yoke and the tailshaft rests on the '66 crossmember.
Barry - Because I can't say for certain the year of the tranny I will post the number of it, differential and tire specs, and go from there. As a general rule of thumb, what difference does a tooth make in displayed mph? Also, how many digits for the tranny code?

 

One tooth averages about 10 miles per hour. It's not precise, but an average as tires, gearing all come into play. You can use any amount of teeth, but the speedo will read incorrectly. They also come in 3 different lengths. Your trans if it came with the 302, may have a tag with "RAN-*" on the tranny itself. On your door tag it should say "Body" then "Trans" and then "Axle". It should be a single letter code. If you post the trans code, axle code( so I get the ratio) tire size, I can nail it down. Did the speedo work at one time then fail? Did you do the engine swap? I'm wondering if that driven gear matches the drive gear in the tranny. It probably does as the gear pic is not really destroyed, just worn. The outer edge of the teeth should almost come to a point, if they're flat at all, they're worn. There are several types of drive and driven gears, mainly which direction the teeth lean on both.

Barry

 

RaleighDad --- there is a VERY precise method of working through speedo and gearing etc. that I have basically figured out.

Principles:

1) Speedometers/Odometers like to have the speedo cable at 1000 rotations per mile. At 60 mph, of course, the speedo cable is rotating at 1000 rpm or "rotations per minute" as well. This is the way that speedos have been for quite a while, only now changing to electronic technologies.

2) DRIVE gears, the ones on the tail of the tranny, have spiral gears. Spiral gears work like this in the calculation: 6 "tooth" spiral = 6000 "teeth" go by per 1000 rpm of tailshaft. 7=7000 etc. The larger the tooth count on a spiral geat, the FASTER the speedo runs (FYI -- the spiral gear in my pic is a 7 toother, and teeth are counted by looking at the end of the gear and NOT by the number that are present on the "side" -- Its like counting petals on a flower)

3) DRIVEN gears (at the end of the speedo cable) actually vary the speedo by 1/15 to 1/18 in a logical fashion per single tooth change. That amounts to more like a 5-7% change in speed shown, per tooth change. 15 teeth are the fastest gears (show fast speed), 19 are the slowest. You would use a 19 with a rear end with very high gear ratios, or very small tires.

4) There are good tables that give the effective circumference of most tires. Using that data, in inches, you can calculate the RPM that your driveshaft will be turning to get you 60 mph.

5) Once you have that #, you can then determine the DRIVE and DRIVEN gear sizes that will give you close to a perfect 1000rpm in the speedo cable.


So, for instance: I have 205/75R15 tires which have a rolling circumference of 85.16" 1 mile = 63,360 inches So my tires rotate 744 times per mile.

Since I have a 3.25 rear ratio, the driveshaft is rotating 3.25 times as fast as the tires, or 2418 rpm.

Since I have a 7 tooth DRIVE gear, there are 16,926 "teeth" engaging per mile (2418 x 7).

To get to the desired 1000 rotations per mile of the odometer, I can use a 17 tooth DRIVEN gear. (16,926 divided by 17 equals 996 which is pretty close to correct)



Here's a link to a decent "rolling circumference table" for future reference if anyone needs it.

http://www.gnttype.org/techarea/susp...irechart2.html

 

Dang CD, That made my head hurt just trying to read that.

John