I just installed a third brake light on my 83 245 Turbo. The bulb out warning light now comes on every time I hit the brakes. I realize that if the bulbs are different it will cause this to happen, so I made sure that both bulbs are the same. I am wondering if it is because of the way I hooked it up. I simply spliced into one of the brake light wires. Now I am thinking it is happening because it is pulling more from one side than the other. Is this right? If so, could I run a wire from the other side to the third light also? Is it ok to pull from both regular brake lights to the third brake light? Any advice is appreciated! Thanks!

The instructions from Volvo for adding a Third brake light to the early cars is to wire it directly to the brake pedal switch.
Nelson

You can wire the high-mount-stop-lamp to the brake switch as Cody suggests, or try my method which was to use a little relay in the trunk to feed the HMSL. I used a high-current, printed circuit relay with a 400-ohm coil resistance (Rayex LEG-12F). This doesn't pass enough coil current to trigger the BFS relay. The relay coil is triggered from the brake light wire (either side), and the relay's power feed comes from the green wire that supplies the trunk light (and power antenna).

I tried this first with a Bosch-type relay but its coil drew enough juice (150ma) to set off the BFS.

The method is a little fussy but avoids having to pull carpet or door sills and run a lot of wire...everything happens right in the trunk.
--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Now I am thinking it is happening because it is pulling more from one side than the other. Is this right?
Yes.

If so, could I run a wire from the other side to the third light also?
Sorry, no.

The Bulb sensor uses opposing wire coils to check for a current balance between the Left bulb (e.g., a 10 turn coil) and the other 2 bulbs (e.g., two 5 turn coils.).

These coils are wound in opposition (10 vs 5+5) around a reed relay (switch) in the sensor. As long as all 3 bulbs are drawing current, the 10 vs 5+5 balance out and the relay doesn't switch. But if any bulb fails, the current imbalance switches the relay, which lights the warning lamp.

--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Thanks Bruce! That makes sense. So, I think what you are saying is that I need to balance the bulbs out. The two bulbs on the one side must equal the other one on the other side. Ok, does it matter that the third brake light is one of those little two-pronged bulbs that you simply push into place (not sure what they are called)? I am trying to understand this electrical stuff! LOL Thanks for your help!

I'm sorry if I gave you (false) hope. I didn't mean to suggest that you could balance the 2 sides yourself. I only meant to show how the system works.

First of all you'd need a Bulb Failure Sensor from an '86 (maybe '85?) or later, when the 3rd light started, in order to have the 3-coil setup. (Your '83 BFS has just 2 coils, one for each bulb.) Then you'd need to run a wire from the High bulb to the BFS, basically recreating the '86-on system.

As to High bulb differences, my only experience was in replacing the '86 Halogen bulb (only used in '86) with the later incandescent type. This change didn't bother the BFS at all, much to my surprise.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

I think the solution would be to wire the light to the brake light switch under the dash. That's how you get past the bulb sensor when you wire taillights for a trailer.
--
1991 245, 61k miles, looking for a 5 speed 92-93 245 cheap.

GREAT IDEA!!! Thanks so much!!

Excellent idea! I'm giving myself 3 fast dope-slaps for not thinking of it myself.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.