Speedometer gear question

[dmw319]

I replaced my TH-350 transmission in my 70 c10 a couple weekends ago and accidentally broke the grey plastic speedometer gear. I bought a new one and matched it up, 22 teeth, same grey color, but now my speedometer reads 10mph too slow. Anyone know why this happened or what I can do to fix it? I guess I could buy a different gear, but it seems strange I would need to do that if I replaced it with the correct one.

thanks in advance!

[hartbraker]

by replaced the TH350 a couple weekends ago, you mean that you got an entirely different transmission from what was original to the truck?

If so, it's likely that, while you may have reinstalled the same gear that you broke... that the gear that came with the "new to you" TH350 was incorrect to begin with. I'd imagine that you need to purchase the gear that is set up for your trucks tire size and differential gearing. Hope that helps.

[68gmsee]

If you have easy or quick access to the different driven gears you can try one with fewer teeth like I did. The fewer the teeth the higher the reading of the speedometer. Try maybe a 21 (red) or 20 (blue) tooth gear.

If you have to order online, then you may be better off doing some calculating to get the exact ones. Here's a site you can look at but there are others online. http://www.tciauto.com/tc/speedomete...alculator/#350

Here's a writeup from an internet site.

TH350C, Powerglide and Muncie 4 Speed: Speedo driven gears have from 17 to 25 teeth. Each tooth added lowers the speedo reading about 5%. Each tooth removed raises the speedo reading about 5%. In our example, we are reading 15.4% high so we would need to add teeth to spin the speedo cable slower. Remember, one tooth = 5% so we need to add about three teeth. (15.4%/5% = 3.1 teeth).

[Tx Firefighter]

Remember, there are two speedometer gears, the drive gear and the driven gear.

The grey one you're talking about it the driven gear. If you speedo is off, it sounds like the drive gear inside the tail housing is different between the two transmissions. They easy to swap by removing the four bolts that hold the tail housing on. If you jack the rear of the truck up, but not the front when you do it, you won't loose any fluid during the process

[dmw319]

So I guess I could either swap the drive gears, or get another drive gear with less teeth. Hmm. Thanks y'all!

[68gmsee]

Originally Posted by dmw319

So I guess I could either swap the drive gears, or get another drive gear with less teeth. Hmm. Thanks y'all!

Note that they are called "drive" for the one inside the transmission tailshaft housing and "driven" for the one going to the speedometer.

[dmw319]

Originally Posted by 68gmsee

Note that they are called "drive" for the one inside the transmission tailshaft housing and "driven" for the one going to the speedometer.

Got it. Thx!

[dmw319]

Another strange issue... my dummy oil pressue light turned on when I installed the new to me transmission. I have an actual gauge and it reads the same way it was before/reading correctly... any idea why this oil pressue light came on?

[68gmsee]

Originally Posted by dmw319

Another strange issue... my dummy oil pressue light turned on when I installed the new to me transmission. I have an actual gauge and it reads the same way it was before/reading correctly... any idea why this oil pressue light came on?

Somewhat confused.. Are you saying that you have an actual gauge that you installed besides the dummy "idiot" light?

If that's the case, you may have a defective oil pressure switch on the engine or a short to ground.

[dmw319]

Originally Posted by 68gmsee

Somewhat confused.. Are you saying that you have an actual gauge that you installed besides the dummy "idiot" light?

If that's the case, you may have a defective oil pressure switch on the engine or a short to ground.

Yes I have an extra actual gauge that I installed.

Those 2 things you mentioned are probably the likely candidates. I guess I just need some time to poke around under there. Just not sure how I would have caused a short in line when I did the trans swap... but I guess I could have nicked something or accidentally disconnected something and didnt reconnect it?

[Hawghauler]

Check the sender's wire. It may have been dislodged/damaged during the trans install.

[dmw319]

Originally Posted by 68gmsee

Note that they are called "drive" for the one inside the transmission tailshaft housing and "driven" for the one going to the speedometer.

thread update! finally got around to purchasing a 20 tooth blue driven gear. Im going to give that one a try here in the next week and see if that fixes it. I don't have access to those gears locally easily, so I'll just do a guess and check!

I still don't know why my oil pressure light is on, doesn't look like any wires are disconnected or damaged. Maybe I'll check again when I do the driven gear.

[dmw319]

uh oh!!! the part is the wrong size (too long). time to order another one! I should have noticed from the picture when I bought it The correct one (grey color) is about 2 and 1/8" long. That's the size I need...

[dmw319]

Well, I installed the (correct length) speedo. The blue one with 20 teeth, and the speedo is STILL off by about 7-8 mph. I guess I need to just to like 18 teeth and see what happens?

[frankslagoon]

So did you put a whole new tranny in and if you have the old one maybe put the drive gear and driven off the old tranny in. sounds like your oil sending unit is grounded someplace. pretty easy to pinch between the motor and tranny when putting a tranny in?

[dmw319]

Originally Posted by frankslagoon

So did you put a whole new tranny in and if you have the old one maybe put the drive gear and driven off the old tranny in. sounds like your oil sending unit is grounded someplace. pretty easy to pinch between the motor and tranny when putting a tranny in?

yeah, whole new trans. I might have to swap the drive gears, was just trying to do it the simpler way (driven is easier in my opinion).

and I still haven't figured out my grounding issue!

[dmw319]

well, update to an old thread.

I have tried every single lower driven gear they offer (18, 19, and 20) and none of them have corrected my speedo issue. When I drive 38 mph, my gauge shows 30 mph.

[68gmsee]

Originally Posted by dmw319

well, update to an old thread.

I have tried every single lower driven gear they offer (18, 19, and 20) and none of them have corrected my speedo issue. When I drive 38 mph, my gauge shows 30 mph.

Very strange for sure...

If the speedometer reading only changed after the new transmission, there is definitely something strange going on. I did the gear change on mine when my truck was about 10 mph off after I changed tranny years ago and it brought it up to exactly what it should be. I used my GPS to double check.

It is possible that something is binding like the cable itself or maybe the speedometer gauge has gone bad. It's basically a rotating magnet than creates a current that moves the needle. Simple but it may be causing your problem.

How about the idiot light coming on problem. Did you resolve that one?

[dmw319]

Originally Posted by 68gmsee

Very strange for sure...

If the speedometer reading only changed after the new transmission, there is definitely something strange going on. I did the gear change on mine when my truck was about 10 mph off after I changed tranny years ago and it brought it up to exactly what it should be. I used my GPS to double check.

It is possible that something is binding like the cable itself or maybe the speedometer gauge has gone bad. It's basically a rotating magnet than creates a current that moves the needle. Simple but it may be causing your problem.

How about the idiot light coming on problem. Did you resolve that one?

resolved? no.

removed the bulb? yes.

[68gmsee]

Originally Posted by dmw319

resolved? no.

removed the bulb? yes.

Smart move. I would have done the same.

[dmw319]

another update: trans got rebuilt this last weekend and they replaced both the drive and driven gear to match, so my speedo reads correctly now!

[YoungPup1977]

so the fix is, you were replacing the trans with a used one to save money but the speedometer did not work, so you had the used trans rebuilt and now the speedometer works....

so did you provide the old transmission for parts as well and they took the "matched' set of speedo gears from your old trans.

so did you take the tail housing off either trans to swap out the drive gear on the output shaft....its held on with a clip that most folks break because they do not know how they come off....

Did they fix the oil pressure issue too, was it a pinched wire at the bell housing.

Just wanting a clear understanding of this issue so people can gain from your experience....thats what this forum is used for so folks can learn from others...Thanks.

[YoungPup1977]

Dang...this is an old post...LOL!!!!! took a long time to rebuild the speedometers gears.... Transtar on West Street in raleigh......

[68gmsee]

Originally Posted by YoungPup1977

Dang...this is an old post...LOL!!!!! took a long time to rebuild the speedometers gears.....

dmw319 is kinda like me. Don't get into a rush with these old trucks. Been working on my 69 for about 8 years..

[dmw319]

Originally Posted by YoungPup1977

so the fix is, you were replacing the trans with a used one to save money but the speedometer did not work, so you had the used trans rebuilt and now the speedometer works....

so did you provide the old transmission for parts as well and they took the "matched' set of speedo gears from your old trans.

so did you take the tail housing off either trans to swap out the drive gear on the output shaft....its held on with a clip that most folks break because they do not know how they come off....

Did they fix the oil pressure issue too, was it a pinched wire at the bell housing.

Just wanting a clear understanding of this issue so people can gain from your experience....thats what this forum is used for so folks can learn from others...Thanks.

Nope, I just let the shop take care of all of it. I still have the other transmission sitting in my shed. Maybe I ought to sell that thing...

And nope, the oil pressure light was still on so I just yanked the bulb out. Problem solved.

Originally Posted by YoungPup1977

Dang...this is an old post...LOL!!!!! took a long time to rebuild the speedometers gears.... Transtar on West Street in raleigh......

Yeah, I put it off for a while cause I honestly don't ever look at the speedometer. The shop that did the rebuild was Cottman's. They did a great job!

Originally Posted by 68gmsee

dmw319 is kinda like me. Don't get into a rush with these old trucks. Been working on my 69 for about 8 years..

Haha yep. Slow and steady!