What it the button on side of shifter for 1990 Volvo wagon?

The button on the side of the shifter in a 1990 Volvo wagon is used to engage the overdrive function of the transmission. When the overdrive is engaged, the transmission will shift into a higher gear at a lower speed, which can help to improve fuel economy. To engage the overdrive, simply press the button on the side of the shifter. When the overdrive is engaged, the indicator light on the dashboard will illuminate. To disengage the overdrive, simply press the button again.