Speedo and Odometer not working


Asked by Mar 07, 2014 at 06:30 AM about the 2004 Subaru Forester X

Question type: Maintenance & Repair

Hi Im looking at buying an 2004 Forester and apparently the speedo and the odometer are not working. I realise that this is not a particularly difficult fix in other cars but my main question is if these are not working will the digital mileage clock tick over regardless? They aparently have not worked in atleast 4 years?

1 Answer


Many vehicles use a standardized communication protocol, called SAE J1850, to enable all of the different electronics modules to communicate with each other. The engine control unit counts all of the pulses and keeps track of the overall distance traveled by the car. This means that if someone tries to "roll back" the odometer, the value stored in the ECU will disagree. This value can be read using a diagnostic computer, which all car- dealership service departments have. Several times per second, the ECU sends out a packet of information consisting of a header and the data. The header is just a number that identifies the packet as a distance reading, and the data is a number corresponding to the distance traveled. The instrument panel contains another computer that knows to look for this particular packet, and whenever it sees one it updates the odometer with the new value. In cars with digital odometers, the dashboard simply displays the new value. Cars with analog odometers have a small stepper motor that turns the dials on the odometer.

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