Speedometer Connection?

[Longboarder57]

I just had a 1987 350TPI motor put in my 1986 Trans Am.

It was upgraded from 305TPI, which was locked up.

It runs great, but the speedometer does not indicate MPH.

I am just looking for suggestions on where to look first for proper hookups.

My car has a digital dash installed, which had some other issues but always showed the speed correctly.

Does anyone know of a company that still repairs the digital instrument panels?

The motor I used came from a 1987 Trans Am that had been totaled.

Luckily, it did not damage the motor.

The motor has been modified, but I do not know what was altered besides putting a Holly intake on it.

I believe the heads were changed because the mounting bolts for valve covers were in the center of the valve cover, not on the sides.

Is there any way to tell which heads may have been used?

 

[86 Silverbird]

This information is from the 1988 service manual.

Is this still your original transmission?

If your new motor also came with a different transmission, and the donor car did not have a digital dash, the speedometer in your car won't work.

If it's still your original transmission, check to make sure the cable for the speed sensor is hooked up and not damaged.

Speedometer: The Digital Speedometer is operated by an electronic circuit. The Vehicle Speed Sensor, located in the transmission, generates an AC voltage whose frequency is proportional to the vehicle's speed. This goes to the Vehicle Speed Sensor Buffer and then to the Speedometer circuit board in the Instrument Cluster. The solid-state circuit processes this signal into a control signal for the Speedometer display.

Digital Speedometer: Digital clusters utilize two types of displays: LCD (Liquid Crystal Display) and VTF (Vacuum Tube Fluorescent). They are used in digital speedometers, bar-graph tachometers, fuel gages, etc. Speed information entering the cluster from the buffer amplifier is interpreted by a microcomputer, which controls the speed indication, the tachometer display, and the odometer reading.

The odometers associated with these instruments utilize either numbered wheels driven by a small motor or electronic displays. With an electronic display, the mileage reading is stored in a computer chip (called a non-volatile RAM chip; NVRAM) that does not become 'erased' when the vehicle is turned off, as the display does not retain the information.

Vehicle Speed Sensor: The vehicle speed sensor (VSS), part of the digital cluster, sends a pulsing voltage signal to the ECM, which the ECM converts to miles per hour. This sensor mainly controls the operation of the TCC system. See Code 24 or Section "C8" for more information.

Your friendly GM dealer might be able to help.

 

[Longboarder57]

Thanks for the information.

I will have to check with Hawks Motorsport, where I bought the engine and trans, to see if it had a digital dash.

I should have kept my original trans.

My 86 trans am had been specially ordered by the local dealer's son then.

About 50 extra options were coming out the following next year for the GTA's.