How Your Cluster Works
Many people are curious as to just how the inner workings of the speedometer cluster for the Toronado or Trofeo work. I will explain exactly how these units work.
I'll first start with the analog cluster for the 90/92 model year.
The speedometer cluster for 1990 through 1992 is an analog, digital hybrid. The analog portion is what you see with the needles on a dial. Tach, speed, oil pressure, volts, etc. The digital portion is the green alphanumeric character section that displays information and mileage above the needles. This green display is known as a VFD or vacuum fluorescent display.
The dial portions of the cluster are actually very simple. Each needle on the dial is moved by a low-voltage stepper motor. That needle is attached to the steppers center shaft. That stepper motor consumes approximately 1/2 of one volt in order to swing the needle from its rest position to its max position at the end of the dial. Whether that be 120 mph on the speedometer or your fuel gauge showing your gas tank as full.
The onboard processor in the cluster itself deciphers values being sent from the body computer module or BCM by way of data being shared over circuit 800 known as the dataline.
Those values are changed into very precise voltage measurements on the clusters processor board and sent out to the stepper motors to move the needle in the correct location for you to physically view. For example, if you have a half a tank of gas, the processor sends half the voltage that it takes to push the needle to the max position of the stepper motor, and that is 1/4 of one volt for a 50% reading. GM went with the analog cluster in 90 for cost cutting reasons. It was cheaper to produce.
The backlighting of the analog cluster is just an extended portion of the dash panel lighting circuit.
The only data that goes to the cluster, not on the data line is pulse data for the dial driven tachometer. That is why when these clusters fail, you sometimes see that the tachometer still works, but nothing else does. Because they are on two completely separate data feeds. The data to feed the tachometer on the all-digital cluster comes from the BCM, and not pulse data.
The VFD / vacuum fluorescent display in the analog cluster and everything on the fully digital display (86-92) operate on the exact same principle. Every single pixel or segment in the display has a unique computational address. Think of it like a honeycomb where every single open cell has its own address that corresponds to an individual pixel or segment on the display. It takes a value of ONE to turn that pixel on, and a value of ZERO to turn that pixel off.
That honeycomb of memory cells or addresses are chained together in order; so for example, if the vacuum fluorescent display has 5000 total individual pixels and segments, there will be 5000 individual memory cells with corresponding addresses in a very specific order that can only be a ONE or a ZERO. If that pixel is assigned a ONE it lights up; if it is assigned a ZERO it goes dark.
The processor in the cluster reads the data off of the dataline coming from the BCM and determines what it needs to be displayed to you as the driver in the VFD, the processor then forms a chain of numbers into binary. That binary string of numbers consists of nothing but ones and zeros.
That string of binary numbers is pushed into an array of memory chips behind each VFD that also have specific addresses, and each cell in the array of memory chips has a corresponding honeycomb cell and an assigned segment or pixel.
When the complete string of ones and zeros is formed by the processor, it is shoved into the memory chips. Think of those 5000 characters of ones and zeros as a single file line with a header at the beginning and a footer at the end that tells the memory chips this is where a new packet begins and this is where it ends. Once those memory cell locations on the chips are fully populated the processor sends a command that triggers a latch on the chips to open which then drops out the ones and zeroes to the VFD and that correlates as a pixel being turned either on or off.
Every single time that even just one pixel in that entire display changes, the processor has to reassemble the entire string of binary ones and zeros and send out all 5000 of them to the memory chips. Once the entire 5000 has been populated in the memory chips the latch releases and each corresponding pixel receives either a one or zero that drops out on the display, and the display changes.
For those who have a full digital cluster, While you are driving every single time any number changes in the speedometer, or if you have your bar graph tachometer turned on and the RPM's change. The onboard processor in the cluster is reforming and sending out that string of binary numbers to the display easily tens of thousands of times a minute.
Any time you have a VFD that is dim, missing segments, or is lit when it's not supposed to be; that is caused by either incorrect or corrupted values of that 5000 string that is not leaving or getting to the VFD from the memory chips correctly.
Bonus trivia. All of the clusters used in the Toronado from 86 to 92 model years were made by Yazaki. The VFD elements used in all of them were made by Futaba in Japan.
As Paul Harvey used to say, now you know the rest of the story
