Member takane2 gives us information on how to make a scan tool for the OBD-II port that emulates the under the hood diagnostic port. This is great news as through the OBD port, with the home made device you can read off your car’s true actual mileage. Warning:This is for 850’s and first year 70 series Volvo’s only – from ’99 the computer changed and potentially, you could do damage to the ECU or other electrical parts in your car if you do this with the wrong car.
“Hello, I found your post very interesting. My 96 850 Volvo’s analogue odometer stopped working last fall so while fixing it I installed a junk yard Volvo 850 Instrument panel into my 850 for temp use. A local Volvo dealer couldn’t read the ECU/Instrument panel ODO mileage from this Junk yard one, but could read digital odometer mileage with their Volvo scan tool when I reinstalled the “original ” Dash, now with a fixed analogue odometer. It sounds from your post below that you were able to read your odometer mileage direct from your under-the-hood ECU pin outs. I’m trying to figure out if my 850’s ECU retains mileage info as well as sending it for storage in my instrument panel. The local Volvo techs all tell me it’s just stored in the Instrument panel. It makes more sense to me that, as with pre-96 models, it is still stored in the ECU even though also in Instrument panel. Would you happen to have tried reading mileage direct from your ECU pins without your Instrument panel connected?
“But I had this crazy idea that since the dash’s port on a obd-i was A7 and I was connected to pin 7 I tried the procedure to get the mileage and it worked! I think, what I got was 154,689 which was higher than what the previous owner or the carfax reports said. My odometer is busted so I can’t tell for sure. Can someone else with odb-ii try it and see if it matches you odometer?”