Information about the speedometer system on the late 1980's - early 1990's Camaro, Firebird, Fiero, is often hard to find. When the systems quits working, understanding the components can be helpful.
The system is really very simple. It consist of the following components.
(1) PM Generator (Permanent Magnet)
(2) Buffer Amplifier
(3) Speedometer Circuit Board
The PM Generator is a small AC generator which uses a permanent magnet on a shaft that is driven by a pinion gear in the transmission. Similar to the cable driven for older model year speedometers. The signal is sent to the Buffer Amplifier.
The Buffer Amplifier converts the PM Generator voltage to to a digital voltage for the speedometer and the ECM for engine and transmission control.(The PM Generator provides a voltage of whose frequency is 1.1 cycles per second for every mile per hour driven.) The buffer circuitry is on the speedometer circuit board on some models.
The Speedometer Circuit Board receives the signal from the Buffer Amplifier. It then outputs a wave signal that drives the speedometer aircore motor. The odometer is also driven by the circuit board. The odometer is driven be a special precision DC motor called a stepper motor.
So how can this information be used for trouble shooting and repair?
Here are some quick tips.
(1) If the speedometer is not working, meaning no speed information and no odometer information, and the ECM has not set a "Check Engine" light, then the problem is most likely in the Gauge Cluster, or Speedometer Circuit Board.
(2) If the speedometer is not working, meaning no speed information and no odometer information, and the ECM has set a "Check Engine" light, them the problem is most likely in the PM Generator system (PM Generator, Gear, Wire Harness, Plug).