Non working speedo

[Chevygold]

On the first drive of my truck I noticed that the speedo doesn't work, one of the jobs I had to do before I could drive the truck was to change out the tranny, original was TH350 replaced with a used TH400 as the original only drove in reverse!
I fitted the speedo cable with the adaptor from the TH350 assuming all would be good, I have no idea whether the speedo worked before the original trans failed, I tried fitting the cable direct to the tranny without the adaptor but still nothing, what I noticed when I had it apart there was a lot of end float on the shaft inside the housing where the adaptor fits, is this normal?
If I spin the cable with my drill which way should I spin it to test the speedo? BTW the adaptor is sound, output turns when I turn the input shaft.
Thanks
Graham

[Chevygold]

Sorry forgot to say '76 C10 Cheyenne Longbed 350 with TH400

[frotosride]

A few things to check, make sure that the drive rod (kinda looks like the internal drive rod of a weed eater) isn't rounded and that it isn't too small for the adapter female socket that drives it (driven gear).
The adapter out of the tranny, when looking directly at it, spins clockwise. So when looking directly at the speedo cable it should spin counter clockwise.
If everything above checks out, and speedo turns then it is probably the drive and/or driven gear for the speedo output in the tranny. The bullet that houses the speedo driven gear ( this is where the speedo cable attaches to) may be worn out and allowing the driven gear to move away from the drive gear and thus no speedo. Performance automatics (PA) actuall makes these bullets out of steel as replacements if you find that to be an issue or just want to stop it from happening later.
The only other option is if the speedo doesn't move. The cable could be broke, disconnected from the speedo, or the spedo itself is bad. You can take the gauge cluster out (not necessary but helpful for first timers) and drive the truck back and forth, watching the speedo cable to see if it's moving.
Last but not least, when you do get the speedo working it will likely be off depending on the final drive ratio (ring and pinion) and tire height of the TH400's original vehicle versus yours.

[Chevygold]

Thanks frotosride,
Thats the exact info I needed, from what you've said I think it could well be the drive gear moving out of mesh but I'll go through the cable etc. hopefully it's not the speedo head as I prefer things I can fix!
Graham

[Chevygold]

OK spun the speedo cable today and the speedo is good so it looks like it's the drive, can I take out the "bullet"without loosing fluid from the tranny?
Graham

[frotosride]

You will loose some trans fluid because the "bullet" adapter is on the bottem of the output shaft. It shouldn't be a whole lot if the trans is at least somewhat level and not overfilled.

[Chevygold]

Thanks I guessed as much, I'll have a clean bucket handy and park it facing down hill on my drive, shouldn't be too much leakage.
Graham

[Chevygold]

Hi again, before I dig deeper into the speedo drive, does anyone have dimensions for the "bullet" so I can see if it's worn or if the plastic gear is worn to cause the end float I mentioned earlier.
Also does anyone have a link to a gear calculator so I can make sure I have the right ratio before I start ordering up parts, I see there are 2 different bullets depending on the tooth count of the driven gear but the drive gear seems to be standard at 15 teeth?
I don't have the SPID so will need to jack it up to get the rear end ratio and I know my tyres are not original size, also my truck already has an adaptor on the cable but I don't know which ratio it is I just swapped it across when we changed the tranny.
Thanks
Graham

[Chevygold]

OK quick update, found the problem, before I pulled the bullet I wanted to measure the end play I had on the gear shaft but when I pulled it came out! the gear shaft was sheared off at the gear.
Was really worried the broken gear might have gone inside but it was sitting inside, managed to fish it out with a blob of sticky grease on my finger!
Other info so far driven gear is 45 tooth, adaptor seems to be 1.125:1 Stewart Warner 666F B6  8974B and tyres are 31 10.50 x 15 I'll post up the final drive ratio once I've worked it out.
Graham

[Chevygold]

Further update, I've managed to repair the broken gear with a piece of M5 stud and Araldite so will put it all back together tomorrow and see where I am with indicated speed against real speed.
Graham

[Chevygold]

When putting it all back together noticed the housing, part No. 1362077, is marked 40,41,42,43, so is the 45 tooth gear too big causing it to break?
I've looked up the part number and it appears to be OK for 40 to 45 tooth gears so perhaps it was careless fitting by the previous user of the tranny which was a used but good no idea what it came off but so far seems to work OK! The bullet slid in easy so will see what happens when I go for a run.
Graham

[Chevygold]

Well it works but my SOTP speedometer thinks it might be reading low, I left my phone in the house so I couldn't check it against my speedo App. I didn't fit the adaptor for the test run but that would have slowed it down even more so I'll leave it off for now.
My final drive ratio came out at roughly 3.5:1 as with one wheel off the ground one turn of the wheel gave 1 3/4 turns of the prop shaft so all I need now is a speedo calculator to work out which gear I should have.
Graham.

[Chevygold]

Found a couple of calculators and the gear I need is coming up 36 teeth which would mean changing the bullet for the 34 to 39 tooth version, can anyone explain the difference between the housings as with shipping etc. it's starting to look expensive :-(
Graham

[frotosride]

Found a couple of calculators and the gear I need is coming up 36 teeth which would mean changing the bullet for the 34 to 39 tooth version, can anyone explain the difference between the housings as with shipping etc. it's starting to look expensive :-(
Graham

I really never thought about the difference between the housings but I know it's clearance for gears being larger. I used your numbers and the TCI calculator and came out with a little different numbers.
https://www.tciauto.com/speedometer-gear-calculator

I also found this housing that will work and it's cheap! But appears OE.
https://www.amazon.com/TH400-NP208-Speedometer-Gear-Housing/dp/B01M2Z4RGF/ref=pd_sbs_263_5/141-4164876-7535605?_encoding=UTF8&pd_rd_i=B01M2Z4RGF&pd_rd_r=8e8baadb-c88b-48ee-ad61-6f11cd02b463&pd_rd_w=7OQ8l&pd_rd_wg=efujM&pf_rd_p=c52600a3-624a-4791-b4c4-3b112e19fbbc&pf_rd_r=NW7EF53KP907GFFP3AW2&psc=1&refRID=NW7EF53KP907GFFP3AW2

Also found a 36 tooth driven gear for $10
https://www.amazon.com/Speedometer-Gear-1359270-Tooth-Driven/dp/B00P8BKMFO/ref=pd_bxgy_2/141-4164876-7535605?_encoding=UTF8&pd_rd_i=B00P8BKMFO&pd_rd_r=9f3908f8-6aec-4a8d-aa33-f362db58e0ac&pd_rd_w=iFQSJ&pd_rd_wg=VGPis&pf_rd_p=f325d01c-4658-4593-be83-3e12ca663f0e&pf_rd_r=RTWBD0E9P2FFNPZR4NWV&psc=1&refRID=RTWBD0E9P2FFNPZR4NWV

[Chevygold]

Hi, I used the TCI calc as well, used 15 for the drive gear teeth, 3.42 for the final drive ratio and 29" for the tyre diameter, I know the 31" tyre should be 31" in diameter but measuring centre to the ground I got 14.5" also the actual diameter of the tyre is a tad under 30", putting those figures in came to just under 36 teeth for the driven gear.
I've got a NOS sleeve for 20 pounds from a UK Corvette specialist! bargain or what!!
I'm going to have a ring round to see if someone has a gear available, I'll have another look at Amazon but when I looked they showed out of stock and no prospect of getting more.
When I get the parts together I'll pull it apart and compare the sleeves to see what is the difference, I think it has to be the shaft axis is in a different position to accomodate the different size gears, could be as the retainer holds the sleeve in a specific position?
Many Thanks
Graham.