Speedo issues... replaced parts

[ukhky17]

Hi,

Looking for some help on my speed calibration project. Bought an 83' and have been getting it up and running. Speedo did not register anything when I bought it and knew the speedo cable was probably shot. Replaced the cable and bullet. Worked OK with limited bouncing to the needle and good pick up. However, the speed was not calibrated correctly.

Here is where it started going south. After ordering a few driven gears with teeth ranging from 18-22 to try and dial it in. I started to get violent bouncing from the needle, extremely inaccurate readings (slower than actual speed) and moments where the speedometer is dormant. Removed cable and put graphite lube throughout and checked cable-speedo with a drill and it seems to transfer revs OK .... Any thoughts on my next move?

 

[Charlie]

 

[1987 GMC Jimmy]

I’m not an authority on the whole gearing thing, which shouldn’t be an issue unless you changed tire size, but it sounds like your speedo head needs to be rebuilt. You can calibrate the speedo by gently pulling the needle off with a fork or gold divot tool and pointing it slightly higher or lower on the gauge depending on which way it needs to go. The one in my car is doing the same damn thing. One day it made a bunch of crunchy, springing noises, and it started working horribly. It bounces like crazy, especially at low speeds, and it goes “click-click-click” whenever the bouncing is more pronounced. It’ll correct to a vibration at higher speed and make a softer ticking sound. I’m 99% sure that it got damaged from not lubing the cable at all or lubing it with literal ****. I replaced the cable, and it helped to keep me going, but I’m getting tired of all the noise and the inaccurate reading at lower speeds. I’m going to have to send a spare one that I have to be rebuilt professionally by Bob’s Speedometer. They’re the ones that seem to do the best work, but that workmanship comes at a price. Maybe it won’t be so bad for you since you only need one thing. My spare is in really bad shape.

 

[GMFORLIFE]

Just use a GPS unit
Makes my life so much simpler.
Gearing and tire changes have no affect.

 

[1987 GMC Jimmy]

I was talking about speedometer gearing not rear end gearing. Should have clarified.

 

[84 Shorty]

What transmission and what color drive gear on input shaft?

 

[ukhky17]

Thank you for the feedback. It has a THM-350 and a blue drive gear. The speedo issue may make sense seeing how there is a LOUD clicking noise when in use.

 

[hatzie]

Noisy speed head with a jumpy needle sounds like the speedometer is not all that it once was.

If the bullet shaped sleeve or the seals in said sleeve are worn the gear will not turn smoothly either. A worn drive gear will cause the same issues as well.

I assume you mean the gear on the transmission end of the speedo cable is blue... that's the "driven gear" using GM terminology. The "drive gear" is the big helical gear on the output shaft of the transmission.

As for which drive and driven gears to use on your rig... look at the speedometer gear charts in the 1979-1984 GM parts books volume 52A at the end of section 4.
You need your tire size and axle ratio. The THM 350 is RPO MV4...
http://gmpartswiki.com/getpage?pageid=59803

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[ukhky17]

Hatzie, thanks for clearing that up and for your input. I have a spare speedo I bought for parts. I will start there and see if that helps. If not I try and tackle the worn drive gear.

 

[Obwonkonobe]

I'd your haven't already, use an online speedo gear calculator. It'll let you rule out incorrect plastic gearing for speedo inaccuracy