I am working on a 1992 Toyota pickup that has an electric speedometer. It is an analog needle speedometer and is driven by electricity.There is no cable. The transmission has a reed switch that sends a 12 volt pulse to the speedo 4 times per tire rotation.
According to GPS, the speedo is showing speed progressively slower below 50mph and progressively faster above 50mph. This also affects the odometer.
I've included a photo of the findings. The "control" is information I found on the net that correlates expected MPH based on a given hertz.
I removed the speedo and applied 12volt control voltage to the IGN and GND. And connected a function generator (square wave setting) to GND and SEN. This setup reproduced the error exactly as seen in the truck.
I replaced the 10uf and 100uf cap which were both looking pretty rough and cleaned all the circuit board and resoldered a bunch of components.
Also tested all of the resistors and they appeared to be well within specification.
What you see on the board at the top and right side is apparently pretty classic for these speedos. Every one I've seen on the net looks like this with the black discoloration. Must get a bit warm. I ohmed all those paths and found no broken connections and no high resistance.
Upon further testing, I discovered that if I run the control voltage down, the speedometer becomes increasingly accurate. It ceases to function below 9 volts.
I have read in a couple of places that the speedometer uses a stepper, but I'm not sure that mass of copper wire with the metal cup around it is a stepper. I finally saw a writeup somewhere that mentions a "moving coil movement" but gave no explanation.
I am at a bit of a loss and wondering if anyone here has any ideas.
Thanks!
According to GPS, the speedo is showing speed progressively slower below 50mph and progressively faster above 50mph. This also affects the odometer.
I've included a photo of the findings. The "control" is information I found on the net that correlates expected MPH based on a given hertz.
I removed the speedo and applied 12volt control voltage to the IGN and GND. And connected a function generator (square wave setting) to GND and SEN. This setup reproduced the error exactly as seen in the truck.
I replaced the 10uf and 100uf cap which were both looking pretty rough and cleaned all the circuit board and resoldered a bunch of components.
Also tested all of the resistors and they appeared to be well within specification.
What you see on the board at the top and right side is apparently pretty classic for these speedos. Every one I've seen on the net looks like this with the black discoloration. Must get a bit warm. I ohmed all those paths and found no broken connections and no high resistance.
Upon further testing, I discovered that if I run the control voltage down, the speedometer becomes increasingly accurate. It ceases to function below 9 volts.
I have read in a couple of places that the speedometer uses a stepper, but I'm not sure that mass of copper wire with the metal cup around it is a stepper. I finally saw a writeup somewhere that mentions a "moving coil movement" but gave no explanation.
I am at a bit of a loss and wondering if anyone here has any ideas.
Thanks!
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