73-87 Chevy _ GMC Trucks => Interior & Equipment => Instrumentation => Topic started by: m3talc0re on December 25, 2016, 04:39:55 pm
Title: Speedometer gear?
Post by: m3talc0re on December 25, 2016, 04:39:55 pm
Does anyone know exactly which speedometer gear I need for my 83 gmc Sierra with a TH350 auto trans?
My speedometer doesn't work. Pulling the dash out and spinning the little, um, spinner on the guage seems fine. The little dongle doesn't spin when the truck is in motion. So I replaced the cable. It still doesn't spin. When I told the old one off, the end that went into the transmission seemed stripped. It wasn't square like it should have been. I'm guessing the gear is stripped out. Would the housing cause issues?
Title: Re: Speedometer gear?
Post by: hatzie on December 25, 2016, 07:06:43 pm
I assume 2WD.
Plastic Driven gears for the Chevrolet & GMC TH350/TH375 with 7/8" driven gear speedometer housing hole. There are at least six driven gears that can be identified by color. Be sure to get seals along with the gear.
Which tooth count though? This had me lost looking online
Pull the little bullet and look at the gear. As long as you don't have different axle gears or different size tires get the same color gear. It'll work just fine.
Title: Re: Speedometer gear?
Post by: m3talc0re on December 26, 2016, 12:33:02 pm
Yeah... (http://i.imgur.com/Np3HUnN.jpg)
Title: Re: Speedometer gear?
Post by: VileZambonie on December 26, 2016, 01:01:08 pm
Looks like Hatzie gave you exactly what you needed there. You should get a housing too.
Title: Re: Speedometer gear?
Post by: m3talc0re on December 26, 2016, 01:39:03 pm
I didn't know it just came out that easy or if it had other things going on. I didn't want to pull it out and have all the trans fluid pour out or anything of that sort, lol. Why replace it though, the housing seems fine, just a little dirty on the outside. The teeth on that gear though are completely fubar'd.
Title: Re: Speedometer gear?
Post by: bd on December 26, 2016, 02:22:41 pm
That's an aluminum support bushing. The hole for the gear axle needs to be circular. Wiggle the driven gear from side-to-side. If the gear wiggles further than ~0.006", or if it moves further in one direction, replace the bushing. Next, remove the gear and inspect the axle hole. If the hole isn't perfectly circular, replace the bushing. Otherwise, the speedometer needle will exhibit an annoying bounce.
Title: Re: Speedometer gear?
Post by: hatzie on December 26, 2016, 03:08:03 pm
I didn't know it just came out that easy or if it had other things going on. I didn't want to pull it out and have all the trans fluid pour out or anything of that sort, lol. Why replace it though, the housing seems fine, just a little dirty on the outside. The teeth on that gear though are completely fubar'd.
Dang. That driven gear is chewed right up. The original gear housings were made of pot metal and they tend to wear on the plastic gear shaft. Replacement steel housings are cheap and you can buy the gear with the housing pretty cheap.
With a driven gear that looks like that... what does the plastic drive gear on the end of the mainshaft look like? I'd pull the tail housing off and take a look at the drive gear. Just make sure to clean the crud and dirt off the top of the tail housing to transmission seam before you pull it off. The drive gear isn't much more difficult to replace than the driven gear. It'd be a darn shame to order a driven gear and chew it up instantly with a beat to death drive gear.
GM only lists two drive gears in the catalog for the 1983 2WD TH350. GM 6261782 9 Teeth Green Drive Gear. GM 6261785 10 Teeth Purple Drive Gear. GM 6261781 Drive gear spring clip.
Given the GM 3987921 RED 21 tooth driven gear and assuming it and the transmission are original to the truck... In 1983 the truck could've come stock with P205/75R15 tires, 2.73:1 Rear axle, and the GM 6261785 PURPLE 10 tooth drive gear or P205/75R15 tires, 3.08:1 Rear axle, and the GM 6261782 GREEN 9 tooth drive gear All this info is in the parts books.
Title: Re: Speedometer gear?
Post by: m3talc0re on December 26, 2016, 03:15:38 pm
I'll have to give that a look. I didn't even think about another plastic gear in there.. Why would they use plastics like this? XD I think the stock tire size is P235/75R15 btw, guessing that's a typo?
Title: Re: Speedometer gear?
Post by: hatzie on December 26, 2016, 03:28:07 pm
I'll have to give that a look. I didn't even think about another plastic gear in there.. Why would they use plastics like this? XD I think the stock tire size is P235/75R15 btw, guessing that's a typo?
They're a fuse. Plastics are easy and cheap to manufacture and replace.
Stock base model tires were pretty small. My 1985 came to me with the bone stock 205/75R15 tires. I installed 235/75R15 tires on it and changed the speedo driven gear to match.
If you have 235/75R15 tires I'd change to the BLUE 20 tooth driven gear. Q on the below table...
This is the online version of the PDF parts book page 1043 that Ian Harding setup from my scans. You can download offline PDF from the link in my signature if you want. http://gmpartswiki.com/getpage?pageid=59803 (http://gmpartswiki.com/getbigpage?pageid=59803)
Title: Re: Speedometer gear?
Post by: m3talc0re on December 26, 2016, 06:38:46 pm
When I pulled that gear housing out, should fluid have come out?
Title: Re: Speedometer gear?
Post by: hatzie on December 27, 2016, 12:35:06 am
I don't think any came out.. There was some on the gear and housing, but none really dripped or poured out.
I wouldn't worry about that. You can check the ATF level and top off after you button everything up.
Title: Re: Speedometer gear?
Post by: m3talc0re on December 27, 2016, 05:12:12 am
From what I can tell, changing the driving gear is a pita.. I haven't had the chance to look at the gear yet, though. I'm hoping it's not eat up like that driven gear XD
Title: Re: Speedometer gear?
Post by: hatzie on December 27, 2016, 11:00:20 am
From what I can tell, changing the driving gear is a pita.. I haven't had the chance to look at the gear yet, though. I'm hoping it's not eat up like that driven gear XD
You have to pull the driveshaft and then pull off the extension housing. Really not that bad. Hopefully it's not chewed to pieces but it wouldn't be shocking.
Title: Re: Speedometer gear?
Post by: Omegadog on December 28, 2016, 05:01:05 pm
I'm having a similar issue with my truck. It's a '84 305 with a 700R tranny. I have and will continue to use 235 75R15's. My speedo reads 45 when doing 55MPH. I spent some time trying to get the right color coded gear but can't seem to figure it out. I will look for the gear ratio next time I can get under the truck. Also, I think that my tranny is a bit slow to upshift so does that mean I need to replace that gear as well? Thanks for any input and Happy New Year!
Title: Re: Speedometer gear?
Post by: hatzie on December 28, 2016, 10:45:47 pm
The 1984 700R4 has FIVE pages with tables of possible Drive gear-Driven Gear-Ratio adapter combinations for axle ratio and tire size combos in the parts manual. You'll need the axle ratio to go with your tire size. Once you have that it's not rocket science to read the tables. As long as nobody has changed the axle ratio in the last 33 years... The axle ratio RPO may be on the SPID label in the glovebox. Another option is to feed your VIN to http://www.compnine.com (http://www.compnine.com) to get the original build equipment list.
The TH700R4 uses the 2" diameter driven gear adapter that has to match the tooth count of the driven gear. The BOP TH350, TH400, & NP208/NP241 use this same setup.
This is the Chevy TH350 with the 7/8" bullet driven gear adapter.
Title: Re: Speedometer gear?
Post by: Omegadog on December 29, 2016, 05:28:44 am
Thanks hatzie. I'll follow through and report back. Very helpful videos. I need to replace my rear seal on the housing anyways. I know that the 305 is not original to the engine but I'm not sure about the tranny. The SPID is long gone and there's a new glovebox in her. I'll look up the VIN to see about that and if the gear ratio is the same, I suppose it may be the original tranny. You folks are very helpful and knowledgeable here. I think that I'll be hanging here a while.
Title: Re: Speedometer gear?
Post by: ehjorten on December 29, 2016, 08:29:07 am
This is super-easy to do! Pull the driveshaft and then pull the tailshaft extension housing. While you are in there, get a new o-ring to reseal the extension housing and you will want to replace the tailshaft seal and possibly the slip-yoke bushing too while you are in there.
Title: Re: Speedometer gear?
Post by: 82silverado on December 30, 2016, 07:18:40 am
http://www.laspeedometergear.com/
I went through all this with my 82 with a 700R. The link above is where I got my parts.
This link is a page to calculate what you might need for a GM application.
Title: Re: Speedometer gear?
Post by: m3talc0re on December 30, 2016, 12:48:27 pm
I didn't pull the housing off, I just used a mirror and light to look into the hole. The drive gear is green and seems okay. I could barely get my finger in there to feel it, but it didn't look or feel chewed up.
Title: Re: Speedometer gear?
Post by: m3talc0re on February 14, 2017, 12:42:27 am
Well, I got my stuff in and... it's not the right combo XD I got the red gear. Speedo is off. It says I'm doing 50 and actually doing 60. 58 for 70. It's also a little bouncy. There was minimal play in the housing, but I guess it's enough. I didn't change the driven gear, it seems okay.
hatzie, I forgot all about your blue gear suggestion :/ Guess I'll need to order it, lol.
Title: Re: Speedometer gear?
Post by: m3talc0re on June 03, 2017, 02:44:57 pm
Well, I got the gear replaced. The drive gear was fine, changed it anyway. I put the blue driven gear on there, but the speedometer reads 10mph slow (going 60 says 50).