1983 is the second or so year or the rotting insulation of the wire harness.

Also, use a multimeter to verify wire harness connection between the ECUs and the connected sensors. You can also check the sensor, such as the engine control (not temp gauge) coolant sensor (ECT) and other connected things.

The Bentley 240 manual and the errate thread here on the brickboard helps you.

The FAQ also offers proper value ranges for most 240 years. You have the Chrysler MPG ignition hanging on the washer fluid basket frame. 700/900 used Bosch ignition of your same year.

The OZ Volvo archive may have some factory green manuals for your years.

https://ozvolvo.org/archive/

You also have wiring diagrams from the Mitchell Service also. Yet for your it is a copy and paste of some terrible Volvo diagrams.

http://www.v8volvo.se/mekartips/volvo/index.html

You may want a multimeter that uses a 9 volt battery for testing.

Yes, please, as Phil provides guidance here for you.

Yet there is more than rotor and cap replacement on time. So check engine control through the wire harness.

If corrosion, usually invisible or a white milky nasty stuff on connector pins, you pal is DeoxIT-D spray or dilution oc conetrate in alchol applied with lint free something.

And check ground wire collections under screw and bolt heads.

Found cotton swabs to be linty a bit.
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Beh.