My 82 Z-28 speedo just spins like a top(pin missing to stop at 85)I bought another gauge cluster from an 85 Z on Ebay and the speedo and tach both read well over what the speed or rpm are . My question is if the post that stated pre 89's are solely mechanical is correct. The reason I ask is ,I was wondering what the electrical plug screwed to the back of the speedo is for. It looks like it may create a magnetic force for some purpose. I have no computer functions as this is a bare stripped down street rod. thanks

 

Yes.

The yellow box on the rear of the cluster is the vehicle speed sensor (VSS).

JamesC

 

Do you know what the function of the "ribbon wiring harness" that goes to the back of the speedo is for?

 

The black piece near the speedo cable connection is the 'optic head.' The yellow box is the 'buffer' (the whole business is generally simply called the VSS). Here's the way it works from my 85 GM Shop Manual:

"This is the device that will supply the vehicle speed input to the controller. The optic head portion of the VSS is located in the speedometer frame. A reflective blade is attached to the speedometer cable/head assembly. The reflective blade spins like a propeller, with its blades passing through a light beam from a L.E.D. in the optic head. As each bade enters the L.E.D. light beam, light is reflected back to a photocell in the optic head causing a low power speed signal to be sent to a buffer for amplification and signal conditioning. This amplified signal is then sent to the controller."

In short, the VSS sends a signal to the ECM and to the cruise control module on your car (since your speedo is cable driven). It does not affect how the speedo or trip odometer reads.

BTW, I have no ECM (and all works as designed) and if you ground the cruise, which is originally grounded through the ECM, it will function. More about that should you be interested:

https://www.thirdgen.org/forums/engi...-function.html

More info on why mechanical speedos fail:

https://www.thirdgen.org/forums/inte...eedo-woes.html

There are electrical issues with retro fitting a later model cluster into an earlier model (I usually suggest simply swapping the speedo head to avoid issues):

https://www.thirdgen.org/forums/inte...atability.html

JamesC

 

You are just Too cool. Thanks for the explanation.

 

So is there anything I can do to fix the tach or speedo so it will read correctly? I know that changing the gears on the ends of the cable is only a small adjustment whereas my issues are off alot (speedo and tach climbs regardless of actual speed).

 

I'm unsure what gear ratio your car originally had, but if you're running 4.10's and have never changed the drive and driven gears in the trans, the speedo reading would be considerably off, more so if you're running non-OE size tires. The tach, if the new cluster was from a V6, would also be off.

JamesC

 

I replaced what were the original 3:23 gears and the tires are OE .It was a 305 carb car originally with a th200. The mechanics instructor here said the gears on the cable ends would not effect the readings more than 10%. The cluster I just bought was said to be from an 85 Z (not sure if they ever put a 6 cyl in a Z-28)

 

I am familiar with the plastic gear that goes in the transmission,is that the only gear? If not ,then that is my problem as the is nothing on the end that goes into the back of the speedo.

 

A TH350 with 26-inch tires and 4.10 gears requires a 7-tooth drive gear and 22-tooth driven gear. 3.23's would have a 7-tooth drive gear and an 18-tooth driven gear. If you require parts, check out the following site:

http://www.transmissioncenter.net/sp...____va.htm#350

JamesC

 

Students are here,thanks again

 

Since both 3.23's and 4.10's require the same 7-tooth drive gear, located on the input shaft, you'll only need to replace the driven gear, which is in a housing that the cable itself attaches to.

JamesC