So I was a bit bummed that my newly installed interior was in but my speed gauge wasn't working. I've had never driven the car with the speedometer in it usually just going with the speed of traffic.

Over a year ago I remember hooking up the cable to my drill and testing that the speedometer does work. I even replaced the 2 piece cable with a 1 piece to try and eliminate problems. Why replace the cable you ask, well all that time driving without the speedo I did have the cable inside the cabin and could see that it wasn't spinning, even if I pushed the cable as far in as it would go so that it would insert properly into the TH400. So I felt it best to replace the cable.

Then several weeks ago as I was installing my new TH400 gear shift cable, I decided to check the little plastic wheel thing that goes inside the transmittion that would turn the cable. It appears to be in good working order.

So barring any of that, is there a way to rebuild the TH400 so that one could forget to install something that would prevent the speedometer driven gear teeth from meshing with it?

 

Ensure the cable is perfectly plugged in to the rear of the speedo. The drive cable must engage the speedo head. Ditto in the tranny end; drive cable must engage the square hole in the gear.

 

Hey Mike, assuming those two suggestions are not the issue, I see that there's a "drive gear":



and a "driven gear":


I believe mine currently is the 36 teeth. If that's correct that there's a plastic "drive" gear, how would I go about replacing? Does the rear tail shaft have to come off or can you get to it from the pan?

 

Drive gear is inside the tranny. Driven gear is accessible from outside the tranny.

 

Yes....check the speedo driven gear housing. There are 2 that fit into the trans, one for smaller teeth driven gears and one for larger ones (around 40T or so). Both will fit in the trans, but one is shorter than the other. Both will fit large or small teeth gears in them, but if you use the wrong size housing for your gear, it will not mesh properly with the trans gear. The housing has the correct teeth ranges stamped on it. Count your driven gear teeth to make sure you have the right one.

 

Since I'm about to go out and inspect it, let me ask this. Does the rotation of the driven gear sleeve make any difference?

I see there are markings around the rim of the sleeve with different numbers. Is there a marking on the sleeve that's supposed to match up with a marking on the trans housing?

 

Yes, the rotation must be correct, and there are markings on the sleeve to match for correct orientation.

 

Ok, so I knew that my block, heads, and transmission were the only places left for Bubba to hide and hiding he was.

I want things to be as perfect as I can get them on my vette, and not having the speedo work was a real obvious eyesore for me. So with the info from this forum and others, I first pulled the cable off and stuck it to my drill. Put the drill in reverse and wound it up, wooohooo, speedometer moves!

So I know my speedometer works and that the cable is fine. So next I pulled the housing:

The driven gear with 35 teeth that I marked off in black (starting at the top and going counter-clockwise):


The 40-45 teeth housing:


and finally the inside drive gear:


The drive gear looks like it's white which, from what I've found online, means that it has 19 teeth.

Yeah, the housing doesn't match up with the driven gear as far as teeth count. And from what I'm reading, that's going to cause problems because the drive and driven gears will never mesh with that combination.

So the question becomes, do I have the wrong housing or the wrong driven gear?

 

My rear differential is 3.36 with 26" diameter (245/60R15) tires that I measured with a string at 82" in circumference.

I've tried 2 different calculation methods here and here to try and arrive at a somewhat correct setting. But nothing seems right, especially with what *looks* like a 19 tooth gear drive.

I should have noted that my car was orig a 4 spd, so the TH400 came from somewhere else. I think I tried looking once for the #'s and thought it came from a Camaro, but that was many months ago.

I just have to laugh at the bozo (or bozos/bozettes) who worked on my car, almost everything they touched was bad in some way.

 

My markings, are you referring to the numbers 40, 41, 42, and 43 stamped on the housing? And if so, what is my marking reference on the actual transmission housing that these marking must line up with?

 

I don't know how to do the calculations, but I would guess that someone changed the driven gear when they put the TH350 in the vette to try to get the speedometer correct. I'm guessing that you just need to change the housing to get the gears to mesh. I would try calling a transmission shop. They might have a housing laying around and they might be able to tell you if the drive gear and the driven gear you have will work. You might have to find a shop with an old guy still hanging around.

 

So I was a bit concerned that my drive gear was white with 19 teeth, but I wanted to make sure. So I pulled the 6 bolts holding my exhaust on (I was being lazy before) and actually counted it.

This time with the exhaust removed, it looks a lot more gray than white. I took a marker and counted the teeth, it has 15, not the 19 I was thinking.

 

The "markings" are actually the indentations embossed on the sides of the housing where retainer on the trans holds it in place. If the retainer holds the housing flush, pulls against the tongs, and fits in the groove, then you're OK.

 

Most TH400 trannies came with either 15t or 17t drive gears. It appears that your problem is easily solved by getting another housing. (Available at your local GM dealer). Your combo of 3.36 rear, 26" tire, and 15t drive gear takes a 39t driven gear for speedo accuracy.

Here is a link for calculations, scroll down to TH400 speedo parts :http://www.transmissioncenter.net/sp...4AodDjLF9Q#400

Did the math......http://www.bgsoflex.com/cgi-bin/spee...n&givengear=15

Good luck, hope this helps.

 

Some may think why post this, but my reasons for doing so are for the transmission neophytes like myself that don't know much about the innards of a TH400 vs TH350 other than I know I have a TH400 and it makes my car go.

So after getting to a point where I knew what I needed, all that was left to do was get the new housing and driven gear. Thought about the online vendors or Jegs/Summit, but I knew there was a transmission shop in my town where I had bought a used TH400 pan. So I thought if he had that maybe he had some other stuff as well.

So I drove off this morning with what I had in hand and knowledge of what I wanted: a 34-39 housing and a 39T driven gear. I got there and he called someone who said he could have it here this afternoon! Wow, that's awesome. So call comes that the part is in and I make a mad rush to the store. I get there and he hands me this black, plastic thing that is in someways the same as mine but different looking as well. As I said, I don't know these transmissions and was relying on his experience. He said that they switched to plastic in later years to save weight but that the part would work. The driven gear was brown with 39T marked on it so I knew that looked right.

So off I merrily go back home, dreaming of seeing my speedometer working that afternoon.

I get home and jump under the car with my new, used part. Hmm, that's odd, it won't go in the opening. So after a few choice words I get up to inspect the parts. I broke out my calipers to measure the difference in diameter.

At the store I noticed a big difference in alignment, but just attributed that it being for a different "Teeth" setting:



As I said I broke out the calipers.

The old:


The new/used:


Yep, the part is not correct, it's from a TH350. So I run back to the shop and get it straightened out and he should have the correct, aluminum piece in his store tomorrow morning. And yes, of course I'll have with me my caliper and more knowledge of what a correct TH400 34-39 tooth housing should look like.

Should the manager of a transmission shop known it was the incorrect part, sure. But I'll cut him some slack in that it's probably been many a year since he's seen one in his shop.

 

Finally, some success. I now have in my possession the correct sleeve and what I think is my correct driven gear.

Comparing the two: 34-39 and 40-45.







and finally with it in:


So I took it out for a drive and did see the needle move but quickly could tell it was reading low. Got out my GPS/speed app on my phone and on the interstate doing about 60 it read about 52, doing 65 it read about 57/58. So that means I'm between 12 and 15 percent off on my readings, frack. That's off a lot more than I was expecting, maybe my speedometer gauge up front isn't reading what it should as I have no way to test it.

According to everything I read, I need to remove about 5 or 6 teeth to get the reading up. The lowest I can go is 34 so I can only remove 5 teeth and I'll have to be satisfied with the results. Maybe my 3.36 isn't what the rebuilder put in but that's what it says on the receipt by the builder Paul at Northside Corvettes here in the Minneapolis metro.

Anyways, it works, exciting. Just some small tweaking to do.

I appreciate everyone's help.

 

What driven gear did you put in?

At 26.6" diameter, and 15 drive it should by 38
At 26, it should be 39

From what I remember, GM set speed-gearing for 60mph

For what it's worth, (I haven't tried) I was told by the local trans guy that if you put the rear of the car on a set of ramps, you can swap the trans driven gear without loosing virtually any fluid
Mooser

 

One last follow-up. Since my speedometer was low, I needed a lower number of teeth gear. Thinking I may need a 34, I thought about ordering one. And then I remembered the 35T one that came with Bubba's version. Granted the differential gears have changed, but 35 might be close enough to the 34 I calculated I needed, about 3% change with every tooth.

So I took 30 minutes to swap out the 39T driven gear for the 35T driven gear I still had. Took it out on the road and at 60mph on my Android GPS app, my speedometer read 60!!! Now granted, when I get around the 40mph mark, I'm off again by about 4-5 mph, but I can live with that. It feels to me that anything under 40 is off by a bit.

I'm curious if everyone elses speedometer is pretty accurate throught the mph range?

@Mooser, I just jacked up the back end to put in the different gear and no fluid came out. Whereas the other day I had just jacked up the front to change my tie rod setting before opening up my speedo gear sleeve. I really wasn't thinking that day and I had a flood of fluid, almost a quart, come out. I guess I thought that even though I jacked up the front, the fluid would somehow all drain nicely back to the pan. I was wrong.

 

If your getting a good reading at 15 drive and 35 driven your looking at a 3.08 rear gear with 26.6" tires
Mooser

 

There are 2 possible reasons why it's reading what it is. Either I don't have a 3.36 diff like I think I do and it's a 3.08, or the speedo gauge is incorrect.

What I didn't think of doing while I had the 39T driven gear in there was to test mileage. With my 35T in there now, I did a test with my GPS phone. The GPS read I had gone about 6.3 miles while the travel odometer read almost 7.1 (it was between 7.0 and 7.1). If I do my calculations right, that's almost about a 12% different in that my speedo gauge reads higher than it should. No, I never took it out to the highway to measure using the mile markers.

So I've been trying to find a motor of some kind that would give me 1,000 rpms so I could measure if my odometer is working correctly and what the speed is reading at 1,000 rpms.

I know my tach had a slider on the back that I could increase/decrease tension on the spring for the gauge, I assume the speedo has the same?