Hi guys,

my 1991 740 wagon is acting up, and because all those things happened at the same time, I wonder if there's a connection:

a) 4 days ago I got the 'Check Engine light'. I read the codes:
1. 'Battery Voltage to low/high' 1-3-2
2. 'Fuel trim (lambda control) too lean at part load' 2-2-1

b) 2 days ago, suddenly my headlights don't come on - when I turn the switch to medium pos, the 'small lights' come on, and the dash lights, but on 'full', the lights are all off (head, tail, dash).

I measured the battery with everything off - 12.3 volts. Very low - but I never have problems starting the engine. Odd? If the battery was dead, I would expect starting trouble.
I pulled the head light switch - no visible trouble. I had read in another thread that some people had a broken switch. I ohm-metered the switch in the three settings, and found that some contacts open and close, but I don't know which ones should open and close, so not sure. There doesn't appear to be one central fuse for the switch, but separate ones (behind the ashtray). Looked them over, none seems bad.
I looked at the hose assembly at the air flow sensor, trying to locate leaks. Nothing... Fuel economy doesn't seem worse than usual. (25 MPG, mostly highway).
I reset the codes once, drove some 20 miles before the check engine light came on again. No battery warning this time, but again the 2-2-1 code for the lambda control.

I'm confused. Is the battery, the lambda, the light switch trouble all a coincidence?

 

i wonder if you don't have a bad ground or something, that throws everything off, includinig the charging

when the car is running, take a DVM and measure the voltage between the battery - terminal and the engine, also the frame of the car (at a ground screw or whatever). the voltage difference should be really small.

 

O.K, straight off, forget anything being wrong with the o2 sensor. People see the code and "lambda control" and instantly run around changing o2 sensors. All it is reporting is that the o2 sensor detected a too lean/rich condition. Just about ANY fault in the fuel and ignition management will light an o2 code.
I think Pierce is heading in the right direction, I would firstly check all wiring coming from the alternator. I don't think it's a bad ECU earth, it would show up as all sorts of weird stuff. Another possibility is failing alternator brushes, I've seen intermittent voltage spikes on other vehicles when they were wearing low and momentarily losing contact now and then.

 

Thanks Pierce and Typhoon. I haven't had a chance to follow your advice yet, will do so tomorrow. The 'Check Engine' light is often off for the first few miles and lights only afterwards.
As for the main lights not working - is there a possibility that it's the ignition switch? I say this because the switch had been kind of sticky at times, not fully engaging. This once led to me going on the highway for some miles at 55mph but at 4000 rpm as the car wouldn't shift higher. Also the radio didn't work (which wasn't really my biggest problem at the time :-)
Or is there some central relay for the lights that controls multiple bulbs (including the dash)? I got the '1989 Volvo 740 Electrical Wiring Diagram', but can't make much sense of it, I don't even find the headlight switch in there at all...
Thanks again for your help!

 

the volvo wiring diagrams take some getting used to. I'm assuming you have a greenbook?

on my 92, there's a couple relays behind the fuses that control the headlights... here's the schematic page from a 95 940, the headlight switch is 3/2. 3/5 is the lowbeam switch (truth table under it shows its connections). 3/1 is the ignition switch. 2/37 and
2/50 are the headlight relays. 7/75 is the bulb malfunction module.
http://www.hogranch.com/files/volvo/...headlights.pdf

there's an index at the end of the book

 

That's great Pierce. I don't have a 'Green Book', but your comment made me search around a bit and I found this:

Electrical: Circuits, Relays

So my next step will be to look at those relays. Incidentally, I didn't try if switching to high beams makes the lights come on. Apparently, sometimes those work...

 

to get to my relays, I had to put the shifter in 1st (parking brake set), then remove the sh tray to access the fuses, take the cigar lighter out, unsnap the bezel immediately around the cigar lighter, and there's two phillips screws, remove them and the storage tray comes out. NOW I can access my relays...

 

Have you tried to jiggle the ignition key after you start the car. You may have a bad contact in the electrical part of the switch. If the headlights are out after the car is running just move the key a little and see if they come on. Our switch went after 300,000 miles. Start the car OK and had no windows or heater fans or lights or radio.

 

I jiggled the key but no dice.
I now hold that big fat red round 'lamp failure relay', I read that this sometimes goes bad and that this would cause exactly what I see - low beams off, but while I pull the high beam lever, high beams come on for as long as I hold the lever. I'll have to figure out how to pull the cover off the thing and check the solder joints.

http://cgi.ebay.com/ebaymotors/ws/eB...m=260780502789

 

the lockup function of the highbeams isn't in the big red round lamp failure thing, its more likely the headlight relay, which is, I believe, "K", the 2nd relay from the driver side on the lower row of the relay panel behind the ashtray on a 740/940.... the red can is strictly the bulb failure sensor, its not in the path of the lamp power, rather its on its own set of wires that run from the lamps back to the failure module and are used to sense current flow. you could, I believe, REMOVE the failure module entirely, and everything would work without it.

 

BTW, guys, I don't mean to annoy you by my narrative, please accept my apologies. You might want to wait until I figure it out to see the conclusion...

I disassembled the red lamp failure relay, and found nothing clearly broken. No bad solder joints as far as one can tell.
I pulled the 'K' and 'J' relays, and looked them over, nothing appears wrong with those either.

And now it gets weird:

I put back all the relays, and switched the light switch to the middle position (I assume it's 'parking lights or whatever that setting is called). Before I did my fiddling around, this would light the small lights on the sides of the big headlights. (Please pardon my lack of proper terminology). Now, the head lights come on!
And, if I pull the high beam lever, high beam comes on and stays on!

Now I switch the light to the 'full' position, and, as before, all lights turn off, including the dash. When I pull the high beam lever, it activates headlights just for as long as I hold the lever.

What gives??

I'm happy to seem to have my lights back, but have absolutely no clue why they suddenly work in the old 'parking light' setting or whatever the middle position is called. I can't find my owners manual

 

is this a US, Candian, or UK market car, or some other? There's variations on some years at least as to how the different versions work.

i'm too lazy to go back and re-read this whole thread, whats the history here, did this car work fine before, then got squirrely, or did this happen after some level of under-dash repair work, or was this car this way when you got it, so you have no history if someone was under there messing around?

 

Pierce, I hear you and you make perfect sense. I only got to wondering about the lamp failure relay because of the thread on the other forum:

start quote:

Headlamp Failure:

[Inquiry] Driving headlights only work when high beams are selected. Please help. [Response: John Sargent] Are your low beam fuses okay? If they are -(and they probably are) you either have a bad high/low switch (unlikely), or a bad Lamp Failure Relay. The Lamp Failure Relay supervises the Low -- Beams, the Tail Lights, and the Brake Lights, but not the High Beams. Lamp Failure Relays do go bad. Remove the Lamp Failure Relay from it's socket, and test for voltage at the pin connection for terminal 56b, which is one pin clockwise from the pin socket labeled S in the relay base of the Central Electrical Unit. Terminal 56b is the output for low beams from the high/low switch. If you have voltage there, the high/low switch is good on low beams. Re-install the Lamp Failure Relay and test for voltage at the input side the the low beam fuses. If you don't have voltage there, the Lamp Failure Relay is bad. You can pry the cover off and attempt to re-solder it or get a used one. The headlight relay is energized by the light switch and supplies current to the high/low switch. If it was bad you would only have high beams when pulling the high/low switch towards yourself. In this momentary position, the current for the headlights comes from fuse 2.

end quote

It appears illogical to lose your lights because the failure relay is bad, and I assume above info is wrong. In any case I didn't try the lights without the failure relay.

I actually tested both 'J' and 'K' relays by applying 12 V on the coil and verified (you can hear them click rather loudly anyways) that contact is made with an ohm-meter. Maybe I 'unfroze' the relays with the 12V jolt, which is maybe explaining why *something* works again. Nevertheless, I never noticed the main lights being on in the parking (middle) position, but maybe I'm losing my mind...

 

Pierce, it's a US car. I had some electrical trouble with the rear lights, those contacts were terrible and kept requiring me to jiggle them left and right. Eventually I got sick of it and soldered the wires directly to the bulb carrier.
That worked for some time, eventually one of the bulbs failed and I got a verification that the failure relay did its job.
I put in a new radio a month ago to replace some old crappy walmart thing that kept skipping. I know that this is kind of a red flag that I did something wrong, but I would have no idea what. I'm an electrical engineer, so it's even more baffling.
I then got the engine check lights about a week ago, which we discussed already. The battery warning code happened just once, and currently the warning is again off. It seems not really connected to the light switch.
I still have to measure the grounding as you suggested, and the diff between battery terminals and chassis. If that is the cause though I'm amazed...

 

k, re: US car, that 1995 schematic page I pasted is apparently for somewhere else, as the relay labeled 2/50 doesn't exist on US cars. the source I got that page from shows a Canadian configuration w/o relay 2/50, but it has always-on headlights (#$@$#@$ DRL's, I hate hate). So as of now, I don't have the right schematic. I've ordered the factory 'green book' schematics for a 92 740/940 (as thats what I have) and expect its *extremely* close to your 91, but I don't know when I'll get them.

when you installed that stereo, did you use adapter plugs that jacked into the factory harness, or did you run your own speaker wiring and power and stuff?

I wonder if you didn't knock the wiring a bit askew behind the headlight switch, or to the highbeam switch on the steering column (said highbeam switch being 3/5 on that schematic)?

 

Thanks Pierce, I'll check them over again tomorrow. The headlight switch wiring seems bullet proof and also keyed to prevent wrong connection. I didn't get near the highbeam switch cable, don't even know where to access that.
Thanks for all your help, more news tomorrow!

 

here's the *right* diagram for the 92 740/940 US model headlights. first time I've seen this diagram, been looking for awhile

 

Many thanks, Pierce! I'll check it out tonight. Still baffled by the lights working in the old 'park light' position.

I had my oil changed meanwhile and got a veritable list of things the garage doesn't like.

1. rear struts 'both are leaking' - I know, it's pretty mushy, have to replace the shocks soon
2. fuel filter 'rusty' - as long as it doesn't rust through or inside, I don't mind
3. air filter dirty - my motto is that as long as fuel economy doesn't suffer, the filter is likely letting enough through :-) but seriously I might put a new one in soon. Pamper the wagon!

Thanks guys, and you Pierce in particular!

 

rear *shocks* not struts. nearly trivial to change, one bolt each top and bottom on each side. jack the car up, put it on stands, put the jack under the rear axle so the shocks aren't holding it up, and unbolt the bottom, then the top, and bolt up the new one, torque down, and you're done. I'd suggest Bilstein TC (touring) if you want a stock equivalent ride for a reasonable price.

air filters are cheap and easy to install, be silly not to replace it if its dirty.

 

Yeah, I also wondered how anything else could leak. But they are the experts, so who am I to correct them.
Thanks so much for those diagrams, even though they continue to baffle me. They unfortunately do not show how the light switch is connecting things when in 'parking' position:
My hypothesis is that in parking, pin 3 is connected to 7. This seems to be what I measured when I pulled the switch, I have to repeat this to verify. Now, following the paths, I see that the main light relay 2/37 remains open, as its coil is not energized. Now, I'm going out on a limb, since I remember the relay having just one set of contacts, while the diagram tries to fool me into thinking there are two parallel ones. I believe 'W' coming from the switch 3/2 pin 7 must be connected to 'Y', i.e. 87 is connected to 87 b. I'd have to look at the relays again to make sure. But otherwise '7' from the light switch goes nowhere!
OK, following Y, we get through the high/low switch 3/5, through the 'Y-W' cable to the 7/75 bulb failure sensor. And now I don't really know what's next.
Which bulbs are providing the parking lights? Do you have more diagrams explaining the flow in parking?
Never mind if you don't, I'll pull the switch again to see what's connected to what...
Good night!