The Great Speedo Gear Mystery

[nrt6964]

HELP!
IM TRYING TO GET MY SPEEDOMETER WORKING AND I CANT FIGURE OUT WHAT IS WRONG. THIS IS WHAT I KNOW:
1.THE SPEEDO WORKS, I HOOKED IT UP TO A DRILL AND SAW IT TURN.
2. I BOUGHT A NEW SPEEDO CABLE FOR IT.
3. I HAVE THE CORRECT GREEN PLASTIC SPEEDO GEAR ON THE TRANSMISSION (FOR A 3:70 REAR END)
4. THE STEEL SPEEDO GEAR INSIDE THE TRANNY IS THE CORRECT DIAMETER FOR THE REAR END.
5. THE GREEN PLASTIC GEAR SHOWS WEAR, AND A SOMEWHAT FLAT SPOT ON IT AFTER IT HAS BEEN IN THE TRANNY.
6. THE TRANNY HAS JUST BEEN REBUILT. ITS THE RIGHT MUNCIE M20 FOR MY '64.
7. YOU CAN SEE THE PLASTIC GEAR TURNS WHEN THE YOKE IS TURNED, UNTILL IT GETS TO THE "FLAT SPOT", THEN STOPS TURNING.

ANY SUGGESTIONS?
THANX...

 

[WayneC]

If you're asking where you can get a new green plastic driven gear, try http://www.corvettecentral.com

If you're asking why a new gear develops a flat spot, all I can suggest is that there are two different drive gears that can be used in the transmission, depending on the ratio, and perhaps you have the wrong one, ... or. there is somthing wrong in the speedometer head that causes it to hang up once in a while and the gear stripped rather than the cable breaking,... or something is wrong inside the trans in the area where the gears meet..... if the latter, you could leave the cable unhooked at the speedo to see if the problem recurs with a new gear (to eliminate the speedo itself as the culprit).

 

[nrt6964]

ALL OF THE GEARS ARE RIGHT FOR MY CAR, BUT I THINK I MIGHT TRY UNHOOKING THE CABLE FROM THE SPEEDO TO SEE WHAT HAPPENS.
BUT IF THE SPEEDO TURNS WITH A DRILL (OR EVEN MY FINGERS) HOOKED UP TO IT, THAT SHOULD PRETTY MUCH RULE OUT THE SPEEDO RIGHT THEN AND THERE?

 

[JohnZ]

Make sure you have the correct DRIVE gear on the output shaft - the right one for your (3.70) application has 8 teeth, is 1-3/4" in diameter, and is 17/32" thick.

 

[WayneC]

nrt6964, you kinda avoided giving the most important information... did the green gear become flattened (ie, damaged) on one side AFTER you replaced the cable and the green plastic gear, or are you trying to use the old plastic gear, which likely was flattened previous to installing the new cable? Obviously if the gear is missing one or more of it's teeth it ain't gonna keep turning. That's why I listed several possibilities, since you didn't state the full situation.

Another possibility is that you didn't get the plastic gear properly meshed with the drive gear and the cable end when you assembled the cable at the tranny, thus jamming the gear in the housing and causing it to be damaged by the drive gear.

It's been a long time since I replaced a cable, so I don't remember how delicate you have to be when assembling. I do seem to recall you need to gently push & rotate/wiggle the cable into it's housing at the speedo end to be sure it is engaging the green plastic gear at the tranny end (you can tell it's in place when it's as far in as it will slide and it resists turning with your fingers); then hold it down in position as you slide the other end into the speedometer. I think when the cable is in place, the end of the cable is about even with the end of the housing's screw-on ferrule nut (with the nut slid all the way out).

Leaving the cable loose at the speedo should clue you as to whether the cable is being turned... just place the end of the cable where you can see it while driving, and realize the cable may be able to move upward in the cable housing, disengaging from the green plastic gear, so you may have to apply gentle pressure with a finger on the end that sticks out.

But, just because the cable turns and drives the speedo mechanism, there's no guarantee that the needle moves, could be a bad speedo head, unless you were getting a healthy needle reading on the speedo while using the drill.

 

[nrt6964]

I THINK I REPLACED THE CABLE AND GEAR AT THE SAME TIME.
BOTH PLASTIC GEARS WERE SLIGHTLY WORN AFTER I TOOK THEM BACK OUT TO LOOK AT THEM.

IM NOT SURE IF I UNDERSTAND YOUR 3RD PARAGRAPH TO YOUR LAST REPLY "you need to gently push & rotate/wiggle the cable into it's housing at the speedo end to be sure it is engaging the green plastic gear at the tranny end - then hold it down in position as you slide the other end into the speedometer."

IM GONNA TRY LOOKING AT THE CABLE WHEN I FIX MY OTHER PROBLEM (CLUTCH FORK SPRING IS GETTING STUCK ON THE THROW OUT BEARING WHEN I PUSH THE CLUTCH IN AND OUT PREVENTING ME FROM GETTING IT IN AND OUT OF GEARS)

THE SPEEDO WORKED FINE WHEN I HOOKED IT UP TO A DRILL
AND THE STEEL DRIVEN GEAR INSIDE THE TRANNY IS THE RIGHT SIZE...

 

[WayneC]

**** IM NOT SURE IF I UNDERSTAND YOUR 3RD PARAGRAPH TO YOUR LAST REPLY "you need to gently push & rotate/wiggle the cable into it's housing at the speedo end to be sure it is engaging the green plastic gear at the tranny end - then hold it down in position as you slide the other end into the speedometer." ****

OK, this is going to be wordy, and my memory may very well be faulty...

As I recall, the (round) cable ends are "squared" to fit into a square hole in the plastic gear at one end and the speedo at the other.It seems to me that the cable is "loose" in it's housing, ie, while it's housing is still assembled at the tranny the cable can be pulled all the way out of the housing from the speedo end. It can be slid back into the housing again and, by wriggling, turning and pushing the cable at the speedo end, it can be coaxed to slide right back into the plastic gear at the other end. When you do pull it up a bit, it (of course) disengages from the green plastic gear. If the cable ends are not properly slid into the square receptacles (speedo head and gear) then the cable is cramped, or stuffed into the cable housing and binds so it turns hard, if at all, causing the plastic gear to be damaged (it is made of plastic on purpose, so that it will be the weakest link).

If the speedo cable end is left loose while driving the car, the rotary action of the cable in it's housing can cause the cable to back out of the plastic gear and it will appear that something is wrong because the cable is not rotating. Thus you may have to "hold" the speedo end of the cable in with the tip of your finger to keep it from backing out while test driving, in order to be sure the cable is being turned by the tranny.

Likewise you may need to hold the cable in with the tip of a finger whilst assembing the cable housing "nut" to the speedo (ensuring that the cable stays engaged with the plastic gear and that the flats on the cable end slide smoothly into the speedo receptacle). If a cable end slips out of it's receptacle at either end while you are assembling the cable at the speedo, then it may be crammed against the end of the square hole it is supposed to slide into (rather than sliding in) or "cocked" partway into the hole, and you get a binding cable.

This may not be your problem at all, I am just trying to point out that you need to have deft fingers while assembling the cable to the speedo, orienting the rotating shaft on the speedo head to ensure that the cable slides smoothly into it as it is supposed to, and not allowing the cable to slide out at the tranny end while doing this speedo assembly. Tough to do, because it's a very cramped space for your hands and you are hard pressed to see it while you're assembling it. If it feels like you're forcing the nut onto the speedo, or it doen't seem to want to go on as far as it should, the cable is probably binding at one of it's ends (ie, it's square ends are not smoothly seated all the way into the square holes).

Good luck

 

[nrt6964]

GOT IT. THANX!