t50 speedo drive

[timbo]

hey all,

i originally posted this in the regional forums asking for sugestions of good gearbox shops. after much thought im going to pull the box out myself (save $300) and maybe have a crack at finding the problem.

here's the process so far:

-took my car to my mechanic to get the speedo cable fixed
-three days later i'm told its the nylon speedo cable drive gear has worn out and its not possible to get another...
-rung toyota at taringa and order it in (quite a helpful bloke there)
-waited three weeks and fitted it today only to have the gear box chew it straight up after a 100m drive...

so much frustration!

after a look on toyodiy i have come to two conclusions; the metal worm gear (see pic) inside the box is fugged or im using the wrong toothed nylon gear.




now the reason i say this is because the shagged one originally in the box was 19 tooth thus i ordered another 19 tooth but on toyodiy it gives the choice of a 19 or 22 with GT, APX in brackets...



the car is an APEX, could it be that someone has replaced the gear with the wrong one and that's why its chewing them up or is it more likely the work gear inside the gearbox?

any help or thoughts will be much appreciated!

[Javal]

Both the nylon speedo gear and the worm drive gear are different between GT Apex and other models, which is, unfortunately a pain in the butt.

I'd make sure of a few simple things first, like that your speedo drive still has the clip in place that holds the gear and shaft in the speedo drive housing so that it's not launching itself into the gearbox for fun. I've seen it a few times before.

If you really want to spend alot of money, get the box pulled apart, inspected, a new worm drive gear put on and a new speedo nylon gear.

Most sane people wouldn't do that as a diagnosis step though

[timbo]

the speedo drive came with a new rubber seal and a clip and i used both so its definitely not jumping out. i guess the only other choice is to pull it apart... any pointers?

[Skylar]

Pulling the tail housing off the T-50 isn't hard. Getting the shift rod(?) past the selector rod ends(?) is tricky though. "It's all the twist" is how a mate explained it to me.

I don't know the name of the parts so there's a ? after them.

[cbjaust]

19 tooth gear (SPEEDOMETER DRIVEN GEAR) is too small in diameter to properly mesh with the worm drive (TRANSMISSION GEAR). The 19 tooth gear should have the numbers 19 and 5 on it indicating that it matches with a N=5 worm gear which I presume has larger diameter than the "6" size. I have three gears, a 19 5, 20 6 and 22 6. I have two AE86s and the one that goes has the 20 6 speedo drive gear and as far as I can tell it has a 4.556:1 final drive which gives me 83 km/h by the GPS at 3000 RPM and the speedo reads close to 100 km/h. I tried the 19 5 gear in that transmission but that just shaved down some of the teeth on the new gear. My other 86 before it was written off would read way under actual speed - but that was before I had a GPS so I'm not sure the exact number; it was running a 3.909:1 S series diff and the 22 6 gear, so clearly that gear was wrong.

So from all this I concluded that if your speedo is reading high you need to go to a gear that has more teeth due to the fact that the worm drive to gear is a reduction gear and for every rotation of the output shaft that has the worm gear on it the speedo cable will advance by one tooth so the more teeth the gear has the slower it will turn.

Things that I am not sure about are which speedo drive gear sets go with which diff ratios. It seems that the 3.909:1 ADM sprinters have the 20 tooth drives but the listings for the JDM spec cars mention 19 and 22 tooth gears and with 4.1:1 and 4.3:1 ratios being the typical it sort of doesn't make sense if the sequence is 20 6 gear for 3.909:1 diff then say 19 5 for 4.1:1 and then 22 6 for 4.3:1... (Edit 1/71/4: I guess it also depends on the OEM tyre size too)

[timbo]

fark this is going back! i ended up using the 22, however the speedo is out for sure. i guess i have a different final drive ratio than stock... the wonders of imports!

[cbjaust]

Do you have any numbers - Engine RPM, Selected gear, GPS Speed, Speedo Speed? If you have a GPS do 3000 RPM in 5th and see how fast you go vs what the speedo reads. That would be handy. That is what I did to figure out which Final Drive ratio I have (presuming stock T50 gear ratios)

Cheers

[timbo]

ok will do mate, just got the car back today, almost normal...