Hi all,
More of a novelty thing than being crucial to running, but I did find a method of correcting the odometer mileage electronically. This might be useful if your odometer gear has been broken for a while (like mine was for ~2k miles over summer).
I followed the instructions on
Dave's Volvo Page for the odometer gear repair in my 1990 240 GL B200F Auto. I opted to (and recommend to) follow the "procedure with speedo needle in place" section - there is more than enough room to access everything.
Anyways, onto the mileage correction: some people suggest clicking the cogs forward by hand using a small screwdriver or similar instrument; however, I was afraid of snapping/stripping other cog teeth in the process of doing this. Instead, I opted to tap into the speed sensor signals (the one that resides on the rear axle) in order to clock the odometer forward electronically, thus removing any risk of damaging any of the other cogs.
To do this I tapped onto the speedo board in the places shown in the attached photo (I even used the correct wire colours as well - go me 😁). I opted to solder some wires onto the board and desolder them before refitting the speedo unit (less chance of slipping and shorting something) - although one could feasibly use some crocodile clips to achieve the same purpose. I then 'spoofed' the speed sensor signal using a square wave alternating between 0V and 5V. To create the square wave I used an Arduino, although other circuits are available for creating this using a 555 timer, an op-amp relaxation oscillator, or even a two-transistor astable multivibrator circuit. I can post schematics of all the above if people would like.
With regards to the frequency of the square wave, the maths appears to be as follows:
frequency = (K-number * speed(mph)) / 3600
The K-number is the one that's printed in the bottom centre of the speedometer - in my case on a non-ABS 1990 estate/wagon, it's K9800. I believe K10042 is for the saloon/sedan with slightly smaller tyres, and the ABS equivalents had K39200 and K40168 for the estate and saloon respectively).
The 3600 is derived from the number of seconds in one hour.
As a worked example (and with video evidence to prove it, if requested):
100Hz = (9800 * 36.7mph) / 3600s
Thus I'd recommend a frequency of approximately 300Hz - 350Hz to make decent progress clocking up mileage.
I will share the Arduino code in a zip file, along with schematics for all 4 aforementioned approaches in a separate post coming over the next day or two. For now, I will just attach the images showing the speedo connections.
Also note, two pins on one of the speedometer connectors need to be shorted in order for the speedometer + odometer to work at all. In my photo, the inner-most and centre positions need to be shorted.
DO NOT touch the outer-most position - I think this may be to do with cruise control or something and will probably fry the unit if you connect it to anything.
Alternative Link:
https://drive.google.com/file/d/1sNk...ew?usp=sharing