speedo help

78CaminoBen · Posted: Mon Mar 20, 2006 11:14 pm Post subject: speedo help

I have a 78 Camino with a 350ci and a 350tt both from a 71 Camaro. I bought it with the speedo working and about a week later i was cruisin and it just droped to 0 and stayed there. I bought a new cable, replaced it, and it wouldnt spin. At this point i took it to a local auto shop, they said "no prob". They got it to spin about $100 worth of parts a labor later, but i told myself it was cheaper than a bad speeding ticket.

So with this spinning cable, i re-atatch the speedo and SNAP! there went the needle! I bought another one off of ebay ('81) thinking as long as i didnt go newer than '83 (the dash cluster change) then i was alright. So i carefully put this one on and guess what? Its no where near acurate.
In the mean time that auto shop that did the work decided to go out of buisness and leave me SOL on information.

Ive narrowed my problem down to 2 causes:
1) the shop just used random parts they had laying around cause they to too lazy to order it and that desroyed my speedo reading
2) the speedo i have ('81) is either no good, or just isnt what i need

but before i just go buyin parts, i would like to know your input on what is the more logical cause. THANKS A MILLION

ps: sorry for such a long post...

Kerno · Posted: Tue Mar 21, 2006 12:35 am Post subject:

OK, I'm going to dig around in my memory for this one. On GM mechanical speedometers, the cable needed to rotate at a given speed to indicate 60 MPH. By making all the speedos work the same, you changed the speedo gears in the tranny to make up for different rear end ratios and tire sizes. So, you new speedo and your old speedo should read the same. The speedo works by spinning a magnet around a hub that the needle is attached to. The hub is restrained by a clock spring, but as the cable speed increases, the magnetic "drag" on the hub causes it to overcome the resistance of the spring. If the cable is well lubed and runs smoothly, the needle is steady. If the cable is dry or kinked, it jumps and the needle jumps around with it. If you remove the cable from the trans, you can use a drill motor to turn the cable a and the speedo should move accordingly. If it does, the problem may be in the gears in the trans. They come in many different sizes and need to be matched in given ratio pairs. You can change the driven gears easily by removing the gear out of the side of the trans. But the drive gear is pressed on the output shaft of the trans and is tough to change. You can also diconnect the cable from the speedo and bring it out to where you can see it and drive the car to be sure it is rotating.

jayachtee · Posted: Tue Mar 21, 2006 6:52 am Post subject:

Cmino · Posted: Tue Mar 21, 2006 7:38 pm Post subject:

I believe jayachtee is on target. After I had my '87 200r4 rebuilt my speedo was off about 5mph too high at 30mph and off 10mph at 45mph. I forgot to tell them I changed the rear gears. They then put a different Driven gear in to compensate but put the wrong one in - and theybuild race trannies. So I'm buying the next size Driven gear and installing it myself. It's a 10 minute job. Each tooth of the Driven gear is worth about 3mph at 60mph so in my case of reading too high, if I buy a Driven gear with 3 extra teeth, it should reduce the speedo reading by 9mph at 60. Without changing the Drive gear also, I believe you'll get close within 1-3 mph but not dead on.

This is info from research but I haven't tried the next Driven gear yet.

Something else to keep in mind if you have the same type of cruise control system like my'83. The speedo is a 2 cable system. 1 cable from the trans to the cruise control gear box on the left fender and 1 cable from the cruise gear box to the dash. My speedo needle dropped dead because the end of the trans cable in the gear box stripped. It's normaly square but got a bit rounded. There is no real resistance here so it could only of happened with a loose connection allowing the square end to slide out of it's slot in the gear box.

Kerno · Posted: Wed Mar 22, 2006 12:10 am Post subject:

Here's a decent resource on calculating what speedo gears you need:
http://www.novaresource.org/speedo.htm

TRIK-56 · Posted: Wed Mar 22, 2006 6:10 am Post subject:

Just a side note to add to Kerno's post about using a drill to turn the speedo cable.
If you do this you will need a reversible drill because the drill will have to run in reverse to move the speedometer needle.
_________________
Bobby
56 Chevy 2-door sedan
82 El Camino Conquista

Cmino · Posted: Wed Mar 22, 2006 5:25 pm Post subject:

Here's another related link.

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

Kerno · Posted: Wed Mar 22, 2006 8:56 pm Post subject:

TRIK-56: You're absolutely right and you beat me to it! I tried it today.

Another note on the speedo gear combinations. I have a Turbo 400 with a 16 tooth drive gear and a 43 tooth driven gear. That makes the gear ratio 2.68 to 1. Here's the interesting part: My 235/60-16's turn 774 revolutions per mile. Since I've got 3.42's in the rear end, it takes 3.42 x 774 or 2647 engine rev's per mile. That also means I'm taching 2647 at 60 mph. I remembered that a GM speedo turns 1000 rpm to indicate 60 mph. Divide 2647 by 1000 and you get 2.647 which is darn close to my 2.68 gearing. So if you really know what you tach at 60, divide by 1000 and you have your desired gear ratio

78CaminoBen · Posted: Sat Apr 15, 2006 7:32 pm Post subject: rear axel ratio

Does anyone no where i can find the rear axel ratio? Id rather look it up then have to take it apart and count them...

I googled but came up with nothing. Does anyone know a site that has this information? or do you know off hand(if u have a 7 Cool

I can find out all the other stuff that i need to (tire diameter etc.) I am just short the rear axel ratio.

THANKS A MILLION IN ADVANCE!!!