I feel like a real idiot now. I was in the middle of replacing my 1990 740 GLE rear wiper motor which wouldn't work after replacing (circuit tested wires) , so I guess there's a short or open circuit somewhere- wiper motor came with tiny relay, so I decided to swap that (position K in relay board), and in my infinite wisdom, pried up the old one with a metal screwdriver, which caused a tiny spark.

So, rear wiper still doesn't work, ok, but, what I then discovered is that the headlights will not shut off when the engine is OFF (had to jump it), they WILL turn off when engine is running, and if I take rear wiper relay out, they don't work at all. Not only that, when engine is running, the headlight switch now works opposite as before: off is now on, on is now off.

Called mechanic I know (not a Volvo specialist) and he guessed it may have fried headlight relay. Ok, so why does everything now work in it's reverse function? Light relay is the big red round one in far left corner of relay board, and, as it appears and says in Haynes manual, not replaceable. Great!

Didn't really need the rear wiper working for all this. Shoulda left alone. Whew.

(Side note: there was a disconnected red wire in tailgate panel, which I reconnected, looks like it is for tail window defrost, don't think it's related, yet thought it worth mentioning).

 

What a day. Ok, put old rear wiper relay back in, situation normal, lights and switch behaving normal.
Thinking that the tiny spark was not the culprit now. Why would a new rear wiper relay cause this in the headlights?

Rear wiper still not working, so I guess I'll look for that fault on a day when my head is clear (mmmhmmm).

Back to the 40 to 50 mph wobble...

 

there's a lot of different part number relays, they are mostly not compatible.

 

so, I'm not concerned about it now since the old one made everything work (sans rear wiper, of course), but, why would putting a new rear wiper relay that came with the new rear wiper motor not be compatible?

 

the big red round thing isn't really a 'relay', its the light failure detector, that lights the warning light if it thinks any bulbs are bad. the low beams, parking lights, turn signals, and brake lights all go 'through' it, and it measures imbalances in the current of the left and right sides, lighting the warning light if there's any such imbalance.

as far as whats going on with your headlights, I dunno, I think I'd want to sit down with a volt meter, and the proper greenbook wiring diagrams, and a big pot of coffee (engineering juice), and sort out whats going on. While I have all the greenbooks for every year 240 and 940, I do NOT have the 740's (except the 91/92 that are the same as the corresponding 940 years). 1990 was a transition year for 740s, where they became less like the older 740 and more like the newer 940s, so there's likely a lot of discrepancies making using the newer year diagrams dangerous.