oh. right... here's the parking lights, they were on a separate page.



again, this for a 1992 740-940

note the bottom part of the drawing is three versions. the top one is the 744 rear, the middle one is the wagon rear, and the bottom one is the 944 rear

so, the red shows where there is power when the light switch is off and the key is off. when the light switch [3/2] is in the middle position, it connects pins 3 and 7, that brings power to pin 3 on relay [2/37], through fuse 11/21 to the left front parking light and fuse 11/22 to the right front tail light. fuse 11/21 also supplies power on the white wire through connector [B] to both license plate lights, and these two fuses go through the bulb failure module to the rear parking lights on the red (left) and brown-white (right) wires.

when you turn the headlight switch to the 'on' position, in the US version at least, it connects pins 1 and 4 which powers the headlight relay [2/37] if and only if the ignition is turned ON, and then that relay powers BOTH the parking light circuits and the headlight circuits as shown on the other page I pasted earlier.

now, there's yet MORE pages showing the turn signals, and the brake lights. I've not scanned these.

 

OK, I brought the wagon to the garage to look a the o2 sensor, as I now started to have seriously reduced gas mileage. I'm down to about 21mpg from 26 before the 'check engine' light came on.
However, they first checked the brakes and showed me that I had almost nothing left on those. A lot of money later I have several new rotors, pads and calipers and the beast doesn't squeak anymore and breaks nicely. I also got new shocks fitted and it feels much better now. Pierce, I didn't go with the nice shocks you recommended, the garage had some budget versions instead. Still feels much better than before.
BTW, I have no idea still about the light wiring issue and the mechanic told me it would be a pain and costly to figure out. I might have a go at it some other time again when I feel lucky.
I just passed 180K this morning and the wagon is still a pleasure to drive, but I want to get a new o2 sensor. The garage told me it would be cheaper if I got it myself and brought it to them, so I'm scouring the net and found not just one but about 10 different ones! How the hell do I know which one is the right one? Do I have to get the old one out first?
Here's just one store: Volvo 740 Oxygen Sensor at Auto Parts Warehouse
part #s: USOS-4000, USOS-3000, USOS-2000, USOS-1000, ARBV960901, REPV960907, BS15735, W0133-1611960, BS13957, BS15737

Are they all compatible with my 91 wagon???

 

I'd get one of these, Oxygen Sensors

note the cheap 'generic' one requires you splice the original volvo connector onto the new generic one. the bosch one is the exact same part you'd get from Volvo, assuming your car is Bosch LH2.x EFI and not a 16V or Turbo. The Regina part is specific to cars with Regina EFI, which I don't believe was used in 91 US model 740s' (but I've been wrong before).

 

My '86 740 is having a similar headlights problem. When in the parking lights position, the dash lights and the tail lights come on but not the headlights, unless I hold the lever. When in the headlights position, the headlights come on but the dash lights and tail lights don't. Any ideas?

 

That's a funny one, too. It's different from my 740's behavior though. I don't understand what mine does, and yours sounds just as baffling. maybe with Pierce's diagrams you can unpuzzle this?
Pierce, thanks for your links to the sensors. Can they be exchanged 'easily' by myself, or should I ask the garage? And, if I choose a generic one, are there actually significant quality differences? Or is it recommended to get the original Bosch part? I don't have a Regina model, btw.

 

personally, I'd go with the bosch one that already has the right connectors rather than messing around with splicing. if I was money constrained, I'd get the generic bosch 3-wire, its the right sensor, and splice the wires. the additional savings from noname o2 sensors aren't, IMHO, worth the potential hassle of not knowing if its working right when things go sideways.

an old O2 sensor can be a real bitch to get out, and the new one needs to be installed with just a tiny amount of the right sort of anti-seize and you have to be quite careful to NOT get ANY of that anti-sieze on the tip, just on the threads.

 

ps, all those headlight problems sound to me like a loose connection, either on the steering wheel column stalk switches, or the relay panel, or a ground down in and around all that.

 

Thanks Pierce, I've ordered the Bosch part as you recommended from Groton. I'll let the garage put it in, no need to break my head with this. Will report next if that fixes my fuel economy problems.

 

The garage put my new sensor in, finally. I had gotten the original Bosch part, and it appears that it has fixed my 'Check Engine Light' problem. It also appears my idle is smoother than before. Will have to wait a few days before I can say that consumption is back to where it should be.

 

OK, I can now finally report that the new O2 sensor has restored my 740 to its previous great fuel economy. I've gone a mix of highway/city recently of about 270/60 miles, and the mileage came to 25 mpg. On pure highway I expect 26 mpg as before the O2 sensor broke, or even better. Wonderful!
I don't really know how the EPA figures our mileage numbers, the 740 is listed at 20 mpg highway, but I really have much much better economy.