Hi all,

I'm about to throw in the towel on this, it's making me crazy, and causing my Volvo to be a hazard on the road...

This '91 740 (non-turbo, no cruise control) has had a long history of brake light issues, and I'm including it all like a hypochondriac visiting the doctor:

-I had the infamous "bulb failure relay" failure back in '02 shortly after buying the car. This was a simple fix - buy a new one. I kept the old one, resoldered everything on it, and keep it in the glovebox.

-I had a very difficult to track down random short somewhere that kept blowing the #4 fuse. This was happening on and off for at least a year. It would never do it for any mechanic I brought it to, and they all traced everything out and pronounced the harness and connectors as "perfect". As I had a spool of wire in my hand and was contemplating running my own new lines from the trunk up to the front, I discovered the problem. I took out the rear lamp assemblies and was looking at the "traces" very carefully. There were a few dark spots that looked like burn marks. Then I looked at the lens assembly still in the car. There were metal dividers between each lens. They protruded enough to sometimes touch a ground trace and a trace for the brake light. I covered all these metal dividers with electrical tape and haven't blown a fuse since. To this day I don't know if I hit on a serious design flaw or if there is supposed to be a plastic sheet or something dividing these parts and it's just missing on both lens assemblies...

So back to now. Twice in the past month I've had the brake lights and the shift interlock fail. The first time I instinctively went for the #4 fuse. It looked ok, but I popped in another anyhow. Still no lights, no interlock. I then (wrongly) suspected the bulb failure relay. I popped it out, inspected it, put it back in. Lights and interlock worked again. I (wrongly) suspected that perhaps there was a bad connection and removing it and putting it back in cleaned it up enough to work. Today, I drove into NYC and left the car at a lot. As I'm walking away, the attendant yells "how do I get this out of park?!". Grrrrr... I look at the fuse, yank the relay, put in the replacement and still no go. I pull the manual release for the shifter, put it in neutral, yank the parking brake and tell him to do the same if they want to be able to move the car. Apologizing profusely for what I'm quickly starting to feel is a cursed junker, I go to work.

In the office, I pull up the wiring diagram on Alldata. I realize that I'd wrongly diagnosed things both times. The power from the #4 fuse goes to the brake switch on the pedal, then splits there to two locations - one line goes back to the fusebox and the bulb failure relay and then travels on back to the lights. The other line goes to a relay that activates the shift interlock solenoid. So if the shift interlock is not kicking in, I've got either a freakish dual failure where the bulb failure relay, and either the shift interlock relay or the solenoid both failed at the same time, or I've got a bad brake switch.

Prepared for the worst, I came back with plans to run a wire from the 3rd brake light and one from the cigar lighter. When I say "brake", my co-pilot is to touch those, making the 3rd brake light and left brake light come on (2 out of 3 being better than none in manhattan traffic).

Instead, I get in the car, pull the fuse to look at it one last time, put it in and tap the brake pedal. The brake lights come on, the solenoid clicks. I drive home and the lights work the entire way home. The only thing of note is that this was a warm day and by nighttime the temp had dropped about 15 degrees.

I can replace the brake switch, but I would still be pretty damn paranoid. In the back of my mind, I'm imagining something is fishy inside the fusebox/relay assembly. Something intermittent and something that will never present itself within a mile of my mechanic.

Is there anything I'm missing? I'm going to print this message out and bring it to my mechanic tomorrow, but he'll be the third or fourth to be presented with mysterious brake light issues. I've seen switches fail before and it seems like it's more of an all-or-nothing affair than this. I would also imagine if the switch were marginal that when I sit there and hit the pedal over and over at least ONCE the lights would come on, and that sometimes during normal driving the lights would occasionally fail to light. And I'd notice, because in my brake light paranoia, I have installed a small light on the 3rd brake light that I can see in the rearview mirror. :)

How can I fix this in a way that I can be 100% confident that when I'm stopping in bumper-to-bumper traffic on the interstate at 75MPH that my brake lights will inform tailgaters that I'm hitting the brakes?

Thanks for the 20 minutes I took from you to read this...

Charles