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Discussion Starter #1
On my first drive after the 60th show i heard a noise under the car, sounding mostly like a playing card flapping against some bicycle spokes. I stopped and looked to see if something was caught rubbing the wheels, but no. As I continued it clicked for another minute or two, then it stopped, as did my speedometer. Am I right to guess that the problem is down at the transmission connection? It never made noise at the gauge cluster, nor did I have shaking or bouncing of the needle.
 

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You are most likely correct. there is a speedometer cable that attaches to the outside of the transmission. It is connected to a gear in the housing of the trans. As the gear spins the cable spins and thus the gauge shows the revolutions. In most cases the cable inside of a plastic sheath fails and the cable needs to be replaced. Another failure could be the gear itself. Either way you can disconnect the cable and determine if it still has integrity. While you are there you can check the speedo gear to see if it still has a full set of teeth.

Both are fairly easy to replace and not an expensive fix.
 

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Discussion Starter #5 (Edited)
Driven gear is broken. Driving gear looks ok. Here are the pics for confirmation. I ran the drill on the cable and that all works fine. Interestingly, the bolt that holds the gear in is exactly 10mm, not 3/8 or 5/16, which suggests to me it’s a newer tranny, right? Did they ever make the 350th with metric fasteners?

Naturally I Managed to push the broken gear down toward the transmission. Can I drive it with that floating around in there until the new gear gets here? I fished around with a coat hanger but no luck.

Also, my speedo was running just a tad high. At 70 it would read 76-77, at 80 it read 90. Should I get a different replacement gear? If so, which one? My tires are 26” diameter and I think the rear gears are 2.56. Looks like that purple drive gear is said to be 10 tooth. The calculator at https://www.tciauto.com/speedometer-gear-calculator#350 gives me 19.83788519173135. 19 teeth is natural color- 20 teeth is blue. So adding a tooth should make it spin a tad slower?
 

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Each tooth represents about 10 MPH so your theory sounds good. AS for driving the truck with broken parts floating around in the trans, I would say don't. they could break up and who knows where they end up. Clean any broken parts out before you go any further.
 

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Discussion Starter #7
I poked around in there again and that gear has disappeared. Not sure where it could have gone from that tail pocket. I need a new method.
 

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Discussion Starter #8
Picked up a harbor freight bore scope, works pretty well, but still couldn’t see the broken gear. It might be time to work on that slow front seal leak.
 

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Each tooth on the driven gear is about 3 mph
 

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Discussion Starter #10 (Edited)
I looked at many pics of th350 tails online and it seems like there is only one place the broken piece could be, which is under the governor gear. If that gear spins counterclockwise, which it appears to, it should be alright. But I'm going to try not to drive it until I can fish it out. The new gears are in the mail from summit (19 and 20 tooth, will try the 20 first). We shall see.
 

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On a turbo 350 there's four bolts that hold the tail cover on....pull driveshaft undo 4 bolts and pull cover.Its pretty much a sealed cavity except for a weep hole for lubrication.Theres a spring clip that holds gear onto output shaft.
 

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Discussion Starter #14
Well, nothing is as easy as it sounds. In addition to the 4 bolts there is the exhaust support beam that has to be removed as well. But, done and done. The new blue gear had broken also, so I tested the cable again. It worked with the reverse drill but I couldn't turn it with my fingers, so I pulled out the whole cable and replace it with the one from the parts car. I took out the inside and greased it with white lithium. Replacing the speedometer back in the housing is a bear--didn't finish before dark. Possibly I had the cable routed incorrectly--does anyone know offhand if it comes through the square hole directly behind it or the round cutout to the right? So that is where I will pick up maybe Tuesday? As a post mortem my son pulled the wire out of the old cable and it was nearly impossible to get out. It wasn't broken or kinked, so I guess it was just frozen somehow.


Oh, also, the broken piece of gear was indeed resting under the governor gear. I couldn't get it out by hook or by crook--several different hooks including the plastic magnet shown above, various coat hangers and wires. Now the tailpiece of the transmission and the exhaust hanger are really clean and nice--should look strange to anyone who might look under there now.
 

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Discussion Starter #15
All wrapped up. Putting the speedo back in the cluster housing was more difficult that it should have been. With the cable coming in through the cutout on the right, the speedo would not sit down straight—the cable wanted to pull it to the left. The pic is from my 75, and the 73 Laguna parts car is similarly routed, and this 77 was also. So I rerouted it through the square center cutout and after several tries got it to sit down straight. Everything works well. In fact, with the newly greased speedo, the needle is so smooth—even my S10 speedo isn’t that smooth. Apparently I’m so used to jerky that it had become natural. Now I will know.
 

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