940 LED bulbs and Bulb Failure Circuit Question

Hey All,

I tried many searches to answer this question but couldn't find anything definitive. I have a '91 940se wagon. I'd like to replace the brake bulbs with LED bulbs to cut down on heat while driving in the wonderful Seattle traffic. My car has a bulb failure relay. Do I need to buy the CANbus bulbs or non-CANbus.

CANbus link

Non-CANbus

Thanks,
Matt

As far as I am aware it doesn’t matter. You can even swap the third brake light for an led.
However, as there is a small current that will always light the leds, you may want to mod as we found out locally with a ballast linked to the cruise control.

Volvo_Utrecht said:

As far as I am aware it doesn?t matter. You can even swap the third brake light for an led.
However, as there is a small current that will always light the leds, you may want to mod as we found out locally with a ballast linked to the cruise control.

Click to expand...

Thanks for the reply.

I would think that I want the constant current to the socket as the brake lights double as driving lights when the brake isn't depressed but I'm mildly horrible at electronics. I can solder like a champ thought. I'm hoping I won't have to modify but, as always, it may be necessary.

I was planning on doing the third brake light as well. I read on another thread that if you mix bulbs on a specific circuit it can trigger the bulb failure sensor.? Mismatched Ohms or current draw or blah, blah, blah. Figured best practice is to replace all the bulbs on the circuit.

Does the third tail light double as a driving light as well? Hmm. Time for a web search.

Unless anybody has a definitive answer I'll probably just go for the non-CANbus lights. See what happens. It's not a real big deal as $10 a pop for bulbs isn't gonna break the bank but it's always nice not to waste money. I'm just really tired of dealing with melted tail light assemblies.

Third brake light does NOT double as driving light. Should only light up when brakes are applied.

Why don't you convert the rear fogs (the topmost in a wagon) to be your brakelights? That is much more noticeable from behind as they will normally be off (even at night when driving/parking lights are on) and suddenly light up when the brakes are applied. Then the combined parking/brakelight bulbs can be simple parking lamps. If THOSE melt the housing you have other problems.

There was a bulb out relay in the rear of 88 765 that I bypassed after it burned out. The front bulb out relay is still functional. The rear bulb out relay on the 760/940SE/960 wagon is found behind the left rear interior panel near the gas filler.

tintintin said:

Third brake light does NOT double as driving light. Should only light up when brakes are applied.

Why don't you convert the rear fogs (the topmost in a wagon) to be your brakelights? That is much more noticeable from behind as they will normally be off (even at night when driving/parking lights are on) and suddenly light up when the brakes are applied. Then the combined parking/brakelight bulbs can be simple parking lamps. If THOSE melt the housing you have other problems.

Click to expand...

That's something I hadn't thought of. I might be interested in getting them both to come on during braking. I wouldn't want to disconnect the lower ones from their brake function. Maybe something to think about for a spring project.

lummert said:

There was a bulb out relay in the rear of 88 765 that I bypassed after it burned out. The front bulb out relay is still functional. The rear bulb out relay on the 760/940SE/960 wagon is found behind the left rear interior panel near the gas filler.

Click to expand...

In all my reading I have never seen mention of a second relay for the rear bulbs. That actually explains a quark in my cars bulb failure system. The brake light on the rear drivers side isn't working, bad contact, but it hasn't been triggering the bulb failure light. I had no idea about the second relay. Thanks for the info.

I didn't have good luck getting LED brake, side marker or taillight bulbs to play nice with my V70. I think the bulb failure circuit was the issue.

On 740 as long as your tail/stop bulbs are same from left to right you won't have any issues with the bulb failure relay.

Despite what the internet might tell you those bulbs you linked won't be nearly as bright as the stock tail lights. I wouldn't trust any brightness specs they give online. Even the "legit" websites give bogus numbers. I used a light meter to test them on my car. When I ordered something similar I found that the "tail light" function was as bright as normal 5W incandescent tail lights. The "stop" mode though wasn't nearly as bright as a 25W incandescent. Like half as bright when measured behind the car! Not cool. Trust no one.

I ended up using 4 LED bulbs. Two in the normal Tail/Stop socket, these were "dual function" like you linked. Then two additional Brake lights in the fog light spots which were "single function". My testing showed that the marker lights were slightly brighter than the 5W incandescent and the four bulbs all together were about 10-20% brighter than the stock 25W incandescent brake lights.

I also realized that a lot of the wagon tail lights only have the fog lamp socket on the drivers side. I ended up carving a socket hole into the passenger light for the fog bulb.

Trying to remember what I bought. I think they were also from superbrightleds.com but I can't seem to find the order.