No power to passenger taillight

[comanche250]

Hello everyone. This is my first Volvo, I'm used to 80s Toyotas with simple wiring.

This is a 1991 740 wagon. I just bought it a few weeks ago and have been going through the car front to back to fix all the little issues. The bulb failure light has been on since I got it, the passenger brake light is out. Everything else works, tails, signal, reverse, rear fog. I pull the bulb out and it's burnt out, okay, I replace. Still no work.
Very strange. The assembly has been replaced as there is not a spec of corrosion on anything! I probe the yellow wire and there is no power when I hold the brakes.

I'm asking for some advice, I looked at the schematic for the bulb monitoring relay thing and almost threw up. I don't know where to start at testing the unit. Before I spend the big money on replacing it, I would like to test it somehow and that is what I'm asking.

Also yes, all the fuses are good, there is no corrosion on any of the terminals. For now I've just wired a wire going from the driver bulb to the passenger bulb so I don't get pulled over but the brake failure indicator is annoying me.

 

[dl242gt]

Get a test light and download the Volvo wiring diagram if it's on OZVolvo.org. The bulb out relay is easy to check. it's just a lot of wires and seems confusing. But it's a simple signal in and signal out relay. So you connect to the terminal for the right brake light input and see if there is power. If there is then go to the output terminal for the right brake light. No power there with power in would mean a bad relay.

 

[bobxyz]

Try here: https://web.archive.org/web/20160306091053/http://volvowiringdiagrams.com/
The 1987 Volvo greenbook is probably close enough to your 91 for the brake lights and has more details than the non-volvo diagrams.
Post in wanted for a good used 740 bulb failure sensor

 

[MasterBlaster]

The bulb failure light has been on since I got it,

Even when not pressing the brake pedal?
Then there's a problem with some other bulb as well.
Maybe a different wattage rating on one side's bulb.

the passenger brake light is out.

That would trigger it only when pressing the brake.

I probe the yellow wire and there is no power when I hold the brakes.

Possible that the bulb monitor has a bad connection or burned out internal coil.

I looked at the schematic for the bulb monitoring relay thing and almost threw up.

It's actually a simple unit, just a bunch of small circuit pairs.

Brake power comes in on pin 9, then out on 10 to the left bulb, 11 to the right.
The wiggly lines in the diagram are coils of wire inside the housing.

On each "pair", one coil will be coiled to the right, the other coiled to the left.
When powered, each coil will create a magnetic field. When both coils have the same
amount of power flowing though them, the magnetic fields cancel each other out.

When the magnetic fields are out of balance (blown bulb, different wattages, bad connection),
the small difference is enough to make the reed switch in the middle close, and turn on the light.

I don't know where to start at testing the unit. Before I spend the big money on replacing it, I would like to test it somehow and that is what I'm asking.

The sensor should have continuity (maybe with some resistance due to the wire coils) between
the input and output of each "pair" you can see in your diagram (9-10 9-11).

If you have a powered test light (or are careful with a jumper wire), you can apply power to
the socket pins 10 (left) and 11 (right) and see if the proper brake bulb comes on.

but the brake failure indicator is annoying me.

Brake failure or bulb failure? Important difference.

 

[comanche250]

Thanks for the replied. I took the unit apart and yeah, it actually is very simple inside. Upon further inspection the solder pad going to the RR brake light was cracked. I reflowed the joint and everything works now. Very simple, I was expecting ICs and a bunch of random resistors inside but I was pleasantly surprised.