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Discussion Starter #1
Hi,

This is my first post so forgive me if I don't cover all the bases but here goes...

I have a 1967 GTO Convertible. It has a 68 model 400 and a TH400 behind it. I've had issues with the speedometer since I bought the car a few months ago. First, I had to get it dialed in to read the correct speed so I figured out the gears I needed and still couldn't get it close enough. I ended up putting a gear reducer on it and it read perfectly.

It worked for a while and then it just stopped intermittently. It would work fine while the car was cool then after a short drive and things heated up, it would just stop. I'd drive the car a few days later and it would do the same thing. Work for 10 minutes and just stop.

I've replaced the speedometer with a remanufactured unit, and a new cable lubricated properly. The driven gear is new and the gear reduction unit appears to be functioning correctly. The only part I have not replaced or inspected is the actual drive gear. Could this be my problem? Is it possible this has a low spot and functions for a while until it heats up?

Any help is appreciated. I've read many posts in these forums and gained valuable information I just have not come across my specific problem.
 

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Welcome to the forums! :cheers
If it were mine I'd remove the cable from the back of the speedo and route it so you can feel the end when your driving. Then drive around for 15 minutes or so and see if the inner cable is turning. If not and the cable is new I'd be looking at the gears at the tranny. If I remember correctly, the base of the speedo cable mount at the tranny turns to adjust the gear mesh. Possibly it's miss adjusted?
 

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Discussion Starter #3
Thanks for the fast reply. I actually did try checking the cable while driving just like you described and I couldn't manage to duplicate the problem. That's what led me to believe it was the speedomter before I replaced it. I went ahead and completley replaced the gauge cluster with a beautiful new set of Rally gauges that Pete Serio put together for me so I'm betting my problem is not with my new speedometer.

I've tinkered with the connections at the transmission several times but I'm a little confused about what you mean about the gear mesh adjustment. I had to replace the driven gear housing to get a bigger gear in there and, from what I remember, that is just pressed in and sealed with an O ring and is held in place by a Y shaped hold-down bracket. Is there an adjustment somewhere else I'm not aware of?

Thanks again for the help. I've done a lot of digging and I just can't seem to find the answer to this.
 

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Discussion Starter #5
It sounds like my gear may be too far out and a small adjustment might do the trick. I didn't realize that housing was adjustable. Hopefully, my gears are still in good shape if they have been too far apart for a while now.

I'm going to try that adjustment first and see how that works. If it doesn't change anything then I'll remove the gear reduction and take a test drive. I'll let you know how it goes.

Thanks a ton for the fast replies and the new info. I have not read this anywhere else and the thought that the housing was offset never occured to me.
 

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cablegram

Where the cable goes into the plastic square insert at the transmission, does the cable fit tight in the plastic, or is it loose? Sometimes the cable fit is bad, and it winds up spinning around inside the plastic, making the square hole round. This can be remedied, if that's the case. I've seen where the square part of the actual cable is so much smaller than the plastic counterpart (even at the speedometer connection).
 

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Discussion Starter #7
That's another good point. I'm going to check this, along with the possibility of the offset and I've also read it could be that the housing is worn to the point the driven gear could have too much play and cause my issue. Lot's to check out when I get off work today and I'm sure it's one of these issues.

I'll report back soon.
 

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Discussion Starter #8
It looks like it is basically what gjones described. My reduction gear screws right into the gear housing and it has a metal tab that mates with the plastic speedometer gear. The tab would turn freely with little to moderate effort so I'm betting that is my problem. I removed it and cleaned it up then put a little dab of super glue in there to see if that would do the trick. If not, I guess i'll break down and buy a new gear. Cheap but it's a shame these arne't locally available anymore.
 

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Mother McCree's snake oil

What can be done...*cough*... as a temporary fix...*cough*... *ahem*..., well, what I did, was...*ahem*... o.k... O.K... Take you some aluminized foil (the heavy duty stuff) and form it over the end of the cable (or "tab", in your case. But I've never seen that reduction box before, so I'm not sure, what your "tab" looks like), but form it tightly around the cable end (or tab, if you can), just enough to take up the slack in the square hole. (I also did this at the speedometer end, too). Then, assemble. I did this to mine (about 2 years ago), and just looked at it the other day- it looks exactly the way it did when I installed it. Slips on and off the cable end, like a glove! And that old speedometer is as smooth as butter! (Whereas originally, she either jumped all over the place, or didn't work at all!) Good luck to you! Hope that does the trick. :eek:
 

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Discussion Starter #10
It looks like the super glue trick worked for now. I have about 60 miles on it after the fix and no problems yet. Now on to the next broken part...
 
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