Need help with wacky speedometer

[ScottH]

Just got back from picking up a 79 SWB K10. It has 350/350/np203 combo and ran fine on the way home. Problem is when I was doing 65 the speedo said 90! It also goes up and down quite a bit, don't know if it's related but thought I'd mention it.

Any ideas or suggestions on how to figure this one out would be appreciated.

Thanks.

[magnethead]

those are either some big tires or that speedo gear is missing teeth..

my vote is the speedo gear.

[low sierra]

Don't know if this is what is wrong but I had the same problem with 2 speedo's and found that there was transmission fluid coming up the cable getting into the speedo itself. Drove me crazy for several months. Replaced the seal in the speedo drive gear housing and haven't had any more problems. Couldn't even clean the speedo and reinstall, only solution was to replace it. hope this helps

[ScottH]

Thanks guys. Lowseirra, where is the seal, in the trans? Also do you remember where you got a replacement?

[dswancutt]

ScottH;

On a 4x4, the speedometer cable goes into the transfer case. On mine, there is an adpater that makes a right angle, and if I remember, there is no seal. Are you running oversize tires?, that will cause some error.
low sierra, on yours, did your speedo bounce when the tranny fluid went up the cable. My 79 c-10 bounces and growls, but I haven't investigated what the problem is. Most likely my speedometer is going south.

[ScottH]

There is some sort of adapter between the transfer case and the cable, but it is straight. Tires are 31 x 10.50 x 15. The size difference should account for some, but 25 MPH over is excessive, isn't it?

Thanks.

Scott

[low sierra]

Originally Posted by dswancutt

ScottH;

On a 4x4, the speedometer cable goes into the transfer case. On mine, there is an adpater that makes a right angle, and if I remember, there is no seal. Are you running oversize tires?, that will cause some error.
low sierra, on yours, did your speedo bounce when the tranny fluid went up the cable. My 79 c-10 bounces and growls, but I haven't investigated what the problem is. Most likely my speedometer is going south.

on mine which has a turbo 350 it would actually do three things. most of the time it was too fast, sometimes it was slow, and on occasion it would bounce up and down, especially when going less than 25 mph.

on a 85 f150, that I now own, the drive gear in the transmission (granny low) was worn out and it would do the same three things. sorry about the ford reference but the answer for it could be same for a chevy not having fluid up the cable problem.

[ScottH]

Any other ideas or ways to figure this one out?

[Hoods69BadBowTie]

My buddy had the same problem, there was transmission fluid coming up the cable getting into the speedo itself. He also replaced the seal in the speedo drive gear housing and has not had any problems since then. I have no other ideas right now, those small tires should not effect it that much.

[ScottH]

Looks like we have another vote on the fluid leakage being the culprit.

Can you tell me where the seal is? Is the drive gear in the transfer case, or the adapter between the cable and transfer case? Any idea if I can pick this up at a parts store or need to go to GM?

Thanks for the help.

[lurch115]

it could also be a wornout speedometer cable...i've replaced them before when they acted up like that and it usually always cures it.... to change it you have to take the gauges out though...

[lurch115]

it could also be a wornout speedometer cable...i've replaced them before when they acted up like that and it usually always cures it.... to change it you have to take the gauges out though...

another possiblity is like in the case of my truck....my axles have lower gears and when i am doing 60mph the speedometer shows i am doing 85 or 90

[magnethead]

yea, you didnt say what you had for axles. 3.07 to 3.73 or 3.73 to 4.11 or something like that gets messy quick.

[ScottH]

Pretty sure axles are stock. Just not sure what stock is for a 4x4.

Gonna look at it this weekend. Starting to wonder if someone put an adapter on the cable. I came across one on the net to increase the speed to make up for larger tires. There is definitely something between the xfer case and the cable. Will probably take it off and see what happens.

Thanks to all.

BTW could still use some info on the seal referenced above.

[low sierra]

Originally Posted by ScottH

Pretty sure axles are stock. Just not sure what stock is for a 4x4.

Gonna look at it this weekend. Starting to wonder if someone put an adapter on the cable. I came across one on the net to increase the speed to make up for larger tires. There is definitely something between the xfer case and the cable. Will probably take it off and see what happens.

Thanks to all.

BTW could still use some info on the seal referenced above.

the seal is in the housing that holds the speedo driven gear. the speedo cable screws on to this housing. take it out of the transmission and pull the gear. clean it up and you will see where the seal is. it's just a small seal like would be around any shaft.

[ScottH]

Thanks Low.....is this an O ring seal?

[Bill W]

My 78 with 350, 4speed, and 205 does the same thing. Kinda depends on the outside temp. I've replaced the cable with brandnew and the gear teeth in the 205 are fine and I don't think 90 weight gear oil is going up the speedo cable to the dash. In my case the speedo head is worn out and needs a replacement.

Bill

[lilpup69chevy]

maybe someone switched axles? that would also throw the speedo off but i went from 2.73's to 3.73's and i don't think the speedo was quite that dramatic

[low sierra]

Originally Posted by ScottH

Thanks Low.....is this an O ring seal?

it is a small seal like an axel seal with hard outside ring. go to a transmission shop and they will probably have lots of these since they don't usually replace them when you get a rebuild. I went to the shop that did my trans and asked for one. the guy is a friend and I asked him why they didn't put it in when they built mine (seal comes in kit) his reply was they don't think it is necessary because most people don't have a problem. cost me two speedo's.
hope this helps. but if you don't have an auto, you probably have a bad drive gear.