Here are some pictures of the speedo board where you need to make the calibration mod:
This mod will only calibrate the speedo.
The odometer will still be out because this mod only adjusts the bias on the moving coil meter (speedo).
To adjust both the speedo and the odometer, you need to fit a module in the speed sender wire to modify the number of pulses – I’m still workin’ on that.
This photo shows the circuit board and the pins the speedo is connected to.
Carefully prise the speedo away from the circuit board pins evenly.
This shows the board that is mounted on the RHS of the speedo (looking at the face of the speedo).
The IC can be seen on the left.
The stepper motor that drives the odometer is the large round item in about the middle.
R6 is in the bottom left hand corner.
This resistor is used to allow for manufacturing tolerances in the speedo and is used to trim the speedo.
It won’t calibrate the odometer, but it can be used to calibrate the speedo.
R4 is on far the RHS.
If a 100K ohm multi turn pot is fitted here, then the adjustment will calibrate the speedo only because it only adjusts the pulse width, not the frequency.
Measure the value of R4 after removing it and set the pot to this value before fitting it – when you go for a drive ( to the dyno ), at least you’ll be starting with the same error you had before.
This photo shows the pot fitted in place of R4.
It is on flying leads so that it can be run out from the dash to allow for easy calibration.
A fixed resistor of the same value as the pot could be fitted after calibration is complete – it will probably need to be made up of two or more resistors in series and / or parallel to obtain the correct value – check a Jaycar / Dick Smith / RS / Farnell catalouge for available values.
( The photo shows a standard trim pot as I didn’t have a multi turn pot available at the time. The end result is the same. )
This is a 10 turn 100K Ohm pot.
Solder the wires onto the two left hand pins.
Here is the block diagram of the UAF2115 speedo IC.
The resistor that you are changing is the one across pins 8 and 10 of the IC.
The resistor that hangs off pin 4 of the IC is used to allow for manufacturing tolerances in the moving coil meter ( speedo ) – It won’t correct the odometer, but can be used to correct the speedo.
In the factory, the odometer would be calibrated first and then the value of the resistor on pin 4 would be adjusted to trim the speedo.
Here is the calibration pot coming out from under the dash.
It has a couple of layers of heat shrink over it to insulate the connections.
When you have the calibration pot set up, the best way of achieving calibration is to go to a dyno shop and whack the car on their rolling road.
It might cost $20, but it’s much quicker, safer and more accurate than driving along side another car.