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headlight low/high beam issue (maybe a bad ground)


Mar 14, 2005
Portland OR area
'88 740t sedan, and recently swapped all four sealed beams with the hella H1/H4 setup. When on low beam, both outboard lights work as they should. When I pull the headlight switch to toggle to high beams, the passenger side low beam stays on and the high beam turns on, but the driver's side low beam turns off and the high beam doesn't turn on.

I confirmed all four bulbs are working fine, and swapped them to each side to confirm the issue is only on the driver's side. I checked continuity on all the wires for both lights, and didn't see any loose ground wires or anything out of the ordinary. I'm stumped.
I believe you need to swap the ground and low beam wires in the connector to use H4's.
Very interesting. I'll swap the low beam wires when I get home and see how it goes. I'm curious why the passenger side works fine but the driver side doesn't.

To add, I had converted the low beams using the Hella kit a year ago or so, and had no issues then. Found a good deal on the high beam kit and did that this last weekend. When I noticed the anomaly, I swapped the high beams back to the sealed units and it wasn't working properly. I don't know how long it had been that way either.

I have very rudimentary knowledge of how to use my multimeter, so if swapping the wires doesn't work, I'll probably take it to a local Volvo shop.
In the past the Hella rectangular H4 would be supplied with a small adapter harness to do the wire switching for you. But it's better to just swap the wires in the existing plug because H4 is better lighting.
This is the Portland area, and as much as I have grown to detest it, there is a strong Volvo presence here. I've been going to the same shop for close to 20 years for this and other Volvos. There's also another shop that does a lot of vintage bricks in my work area. Not today though (labor day).
In the past the Hella rectangular H4 would be supplied with a small adapter harness to do the wire switching for you. But it's better to just swap the wires in the existing plug because H4 is better lighting.
There was a conversion harness that came with the low beams. The high beams only have 2 leads, and the driver side doesn't work regardless of how it's connected
For what you need to do there's not a lot of multimeter technical usage.

Voltage testing setting - DC, first highest number above 14V.
Resistance testing settings - start somewhere and then adjust to higher or lower ranges until you get peak resolution.
Same wiring spots for both.

Basically looking along the +12V-----Resistance----|I 0V GND path to see what numbers are. Just confirm that going to the bulbs you've got ~12V where it needs it, and after the bulb it's ~0V. The higher above 0 the worse your ground path is. Otherwise your wiring is probably swapped like hiperfauto posted.
I'm still stumped. I checked the wiring based on the above diagram, and the adapters that came with the H4 conversion are correct. I also verified ground on both sides (via continuity, didn't measure resistence.) I swapped the H4 beams to see if that was the issue, and now neither high beams work when I actuate the high beam stalk. The H1 bulbs are good, but no power is going to them when the high beams are supposed to be on. The blue high beam instrument light does not illuminate either. Googling these symptoms, it could also be a faulty relay. I've had to replace a number of bad relays the past couple years. I'm having someone look at it tomorrow.
Easiest test equipment for you is a test light. It will show you not only if there is power but will also let you test the ground side of the circuit. By using an old school test light you also test the circuits ability to handle the current needed to light up a bulb.

You probably need to get a small file and clean the contacts in the connectors. Use some CRC contact cleaner the one safe for plastic. I think it's called QD cleaner. That and a small file will have clean shiny conducting connections. You can protect the connections with OX gard. it's at most local box stores in the electrical dept. it's used for aluminum wire but also works great to protect all connections.
I also verified ground on both sides (via continuity, didn't measure resistence.)
Continuity is good for determining why things spark at you. Bad for determining if high current stuff is having voltage problems. It's also a minor settings difference on most multimeters. Curiosity kills cats and solves electrical problems.

Even a small resistance can act big when you're dealing with high current but only 12V.

If you have a benchtop power supply, it might be worth separately confirming which side of your lights wants +12 and which wants ground, then verifying they're getting that when plugged in (backprobe the connector if plugged in).
I have to sadly report the culprit.....fuses. I didn't think to look here because the wires to the high beams come from the low beams, and the low beams work. I didn't even know there were specific high beam fuses. I wish it was something more interesting that wouldn't make me look like a complete amateur.

On a side note, it died again and the shop diagnosed the issue stemming from the distributor assembly, so that has to come out and they've had to source parts from afar to rebuild it since many of the parts are no longer available.
Glad you got it! After you have all nice clean new fuses in there. Put some oxgard on the fuses. Just a small amount will help the connection last much longer. Not the best fuse design but it works with a little help.