Hi folks,

One year ago, I had an issue with my odometer which intermittently was stopping.
I resolved that issue since then, which was due to some soldering behind the instrument panel.
During my analysis to identify the above problem, I removed the rear speed sensor to check the wiring and since everything was OK, I simply re-bolted it.

Now, since that moment I do have a 5Km difference from the real speed, essentially GPS shows 95 km/h while the speedometer shows 100 Km/h.

Since the difference started to show after I unbolted that sensor, I am sure that is the reason. Before, it was a perfect match with the GPS.

Today, I found some time to disassemble the sensor, but I cannot figure out how the distance to the gear could have possibly be changed given that there is only one bolt and that's it.

I know the tolerance should be 0.85 +/-0.35mm, but really, how can I adjust it?

Thank you,
Alex

Everything you need to know about speedometer adjustment is here in the 700/900 FAQ in the Electrical: Instruments section and additionally an excellent detailed feature article there on Speedometer Repair.

I’ll add to my previous comment about using a varistor to aid in calibration. The suggestion was to source a varistor of suitable range (as noted in the FAQ) and solder it on fine gauge extension wires so it’s accessible out from or under the dash so it can be adjusted to achieve a correct reading. You would then later replace it with a matching resistor (matched to the resistance of the entire wired extension)
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Just a quick point of interest here. In the UK (and elsewhere?) it is a legal requirement to have speedos which are absolutely accurate or which over-read by a small amount. Speedos which under-read are not legal. Most seem to over-read by circa 5-10 %

Ian F

Just to add to Dave's reply. As long as its within the legal requirements car manufacturers don't accurately adjust the speed reading of speedometers. And I think this tolerance was set up way earlier when cars were still using speedo cables. To think about it, how would you calibrate meters using speedo cables back then? BTW my 1994 940 meter reads 100kmh when its 97kmh by GPS.

Regards,
Amarin.

Adjustment is accomplished with one or two resistors on the back of the speedometer, not at the output shaft speed sensor. When you were doing your re-soldering to fix the odometer you likely inadvertently changed resistance at some connection point or damaged a component like a small cap or diode by overheating it. You can go back and try touching up your soldering, being sure to clean any flux residue. Way back when, there was a posting here on the adjustment specifics from a Vista tech, but I don’t think it got archived. One thought was to use a varistor that could be adjusted by trial and error, then replacing it with a perfectly matching resistor once you had it reading more correctly. If you find something elsewhere, please post the link as I’d be interested.

Depending on tire, speed and inflation, my 940T has always read 5-6 km/hr too high. Volvo wouldn’t adjust it, saying it was within the normal error, so I just learned to live with it. My 940NA is 1-3 too high with the same stock sized wheels, which I expect is very normal as all manufacturers err to the side of caution for safety and liability. I had an early production 740 16-valve that was 10-12 too high when new (seemingly related to the different rear end ratios as a number of the first 16-valves were the same) and had Volvo re-calibrate it under warranty. They had a factory tech do it on his next visit in the region. Took him a while by trial and error to get it perfectly calibrated to actual road speed. I had to sign a waiver.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now