I've been working on a 1991 240 wagon project car I got out of an estate for the last few months... I have no idea of any prior history from the deceased PO. I'm pretty much done reassembling all of the trim / wiring and doing preventive maintenance.

Question:
I installed LED bulbs in the brake light housing. Each time I step on the brake the bulb failure light comes on in the dash. Is the bulb failure sensor looking for a voltage higher than what the LEDs consume and thus showing a failure even though the LED bulbs light? The lights work great, but the warning light is a nuisance.

Should I just ignore the bulb warning / remove the light bulb in the cluster?

Please advise. Thanks!

Pete

 

Its more of an electro mechanical thing. It has 2 opposing coils for each light (one left and one right) the coils produce a magnetic field and as long as the pull across the coil is the same they cancel each other out. If your lights are pulling higher voltage, it'll pull the switch too far and close the circuit. Also, the switches default position is closed, so if its pulling too little wattage, then it will trigger the circuit as well.

I have found that pulling the indicator lights from the back side of the cluster can cause some charging issues, it seems that the accumulative resistance is needed to properly energize the alternator (although technically only the battery light is needed) if 2 or more bulbs go out, your alternator may not energize.

I bypassed my bulb integrity sensor for this reason.

volvo 240 Mods and Fixes

 

actually, its current, not voltage... the voltage is determined by the battery+alternator and is fixed at around 14 V when the engine is running, 12.x V otherwise.

but the rest of whatcha all said, yeah, carry on. oh, did you also replace the 3rd brake light with an LED ? thats a 3-way balanced circuit for the brake light failure. most of the other monitored bulb circuits are 2-way.

 

The "3rd" (liftgate) stop light is still incandescent.

So are you saying that if I remove the bulb failure cluster light, the car charging system will not work?

What do you mean by "bypassing the bulb integrity sensor"?

 

actually, the bulb failure light probably won't impact the charging circuit.

re: bypassing the bulb failure module, see the link Nichols gave, volvo 240 Mods and Fixes

I'd suggest scoring a spare sensor, then cutting it open, gutting it, and wiring in the correct set of jumpers per the diagrams given on that link.

 

Right, Pierce is on point.

Rather than modifying the bulb sensor, I made a series of wires to jump between the terminals. I used a 14 or 12 gauge wire and some bullet connectors, I wanted to make the load as light as I could across the plug because mine was melted slightly. I then went and cleaned all grounds and electrical connections I could find.

Believe it or not, something as a slight difference in natural corrosion between the left and right sides can cause the bulb sensor to trigger.