For the knowledge database I would like to contribute the following.

For a long time I would intermittently get a no start problem, especially if the car had been left parked outside for a while. Over time this intermittent nature became more frequent.

The main relay (aka fuel relay) turned out to be causing it, but probably not the usual way.



Additional info:

In April CLL started a thread which described the same symptoms albeit with a more persistent nature:

"My usually trusty '90 240 wagon is suddenly not starting in the mornings. It cranks fine, has good spark, and initially will fire for half a second but die, and then will just crank after that without trying to fire at all. There are no codes on the self test unit; blinks 1-1-1."

In one of my replies I wrote that I experienced the same symptoms. A workaround I found that worked every time was (partially) unplugging and reseating the ECU plug.

This nuiscance became more and more frequent until one morning I noticed I hadn't heard the usual clicks from the main relay after turning ignition on. Instead of unplugging and reseating the ECU plug, this time I decided to just tap on the relay with my fingers and see what would happen. Sure enough, the relay started to work again and the engine started and ran fine.

A couple of years ago I had replaced the original Volvo relay with an aftermarket one that was in the glovebox when I bought the car. On later inspection with a USB microscope of the Volvo relay I found some of the solder joints had cracked with age.

Cue to the present. I never threw away the Volvo relay knowing that resoldering it would give it another 25 years of service life and so I did. I substituted the aftermarket relay with the repaired original and I am fairly certain that that cured the problem.

It has been over a month now, and the no start situation never returned, not even after having been parked for a few days in damp weather.

Of course I inspected the aftermarket relay by popping off the cover and found it to be nearly identical inside to the original. One thing I noticed was the voltage stamped into the armature saying 9 V instead of the 12 V one would expect.
I'm not at all bothered by that because when it comes to mechanical relays like this, that is a nominal voltage around which there's a wide range in which it will work. In fact datasheets always specify "must operate" and "must release" voltages and those are usually relatively far removed from the nominal voltage.

In this case, the 9 V armature in a 12 V system would ensure proper operation, even if there was some voltage drop. And yet, the lack of clicking during the priming of the pumps that the LH Jetronic 2.4 does at ignition power on leads me to believe that the relay contacts were sticking.
Tapping released them and so did the unseating and reseating of the ECU plug as it would also tug a little on the relay wiring at the same time.
A bad connection could be another cause of a non actuating relay, but I found no evidence of that. What's more, a bad connection can also cut out during driving, and that never happened. Once unstuck, a relay will work fine until it gets stuck in the off-position again during being parked.

I didn't check if the original relay has a voltage stamped into the armature, however, the clicks are less pronounced leading me to believe it might be 12 V nominal.
12 V on a 9 V nominal coil would somewhat "overdrive" the coil making it move faster and with more force and hence click harder, and that doesn't seem to be the case with the Volvo relay.

I'm pretty confident by now that the problem has been cured.
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1992 245 Polar B200F M47