None of my brake lights are functioning. I did some research online, including this forum, and found that the most common culprit is the bulb failure relay and brake pedal switch. I checked the switch and it is OK and getting continuity. I also replaced the relay and still nothing. What should I check next?

 

Ground(s), short somewhere... Sorry if too general.

 

^^ That is what I was thinking. I just don't know where to start or look.

 

you need the wiring diagram for your year/model car, and a volt meter or test light, and an assistant to step on the brakes for you and work the turn signals on demand, while you probe the wires in back to see if the problem is in the tail light assembly or up stream.

that cuts the problem space in half.

 

Your stumped because you don't have a sound diag. strategy. Your being a parts changer. It's cool to look up common faults, but you have to back it up with testing to prove it before wasting time and money replacing said part.

 

^^ I rarely do it but it is near impossible to check the relay. Have you seen one inside. I took a shot at changing it because almost all of the posts related to the brake lights were resolved by changing the bulb failure relay.

 

I've never heard of a bulb failure sensor causing lights to not work. they do quite frequently cause the bulb failure light to come on when everything is working fine.

 

So did someone step on the brakes while you probed the connector at the tail light assembly. If you have 12V there then check the flex circuit board in the tail light. Start there and see what you get

 

Jag, thanks for the suggestion. I will check that, hopefully soon.

 

Sounds like is time to hard wire your tail lights if you haven't done so already.

 

Did you replace your bulb failure relay with a new or used one? If used, highly probable it also doesn't work. This happened to me recently and I just re-flowed all the solder to get it to work again.

 

Actually that sounds like a good idea. Replacing a part doesn't guarantee it's good.

 

I replaced it with a new one. I was convinced that that was going to solve the problem since most of the people with the same problem have resolved the issue by changing the relay. I have not had time to further diagnose the problem, the next thing I guess would be to check for power at the three back up lights while the brake is applied and also the socket that the relay plugs into. I am hoping that I do not Have to run any new wires. All of the wiring in the car seems to be in exceptional condition, especially for a car that just turned 20 years, but you never know. It could have been rubbing against something over the years or the insulation deteriorated. What I need would be the wiring diagram but how do I get one? Thanks for everyones help so far.

 

is this 1994 940 a wagon or sedan? tail light wiring is different...

 

Does your cruise control work? Its tied directly to the brake light circuit and won't work if there's something wrong. Obviously there's something wrong, but maybe it will help narrow it down.

It may be a shot in the dark, but before you start replacing more parts and rewiring your car, I would try re-soldering your old bulb failure relay. It really is the most common problem with brake lights not working. I also assume the bulb failure warning light didn't come on? With your sudden failure of all the brake lights, it sounds reasonable.

I would like to be able to assume that you new relay works fine, but sometimes you never know. With old cars like ours, even new parts have been sitting on a shelf for a while.

 

lets see. US model 1994 940, the left tail light is yellow-red, to pin 6 on a sedan, pin 1 on a wagon. right tail light is yellow, same pin numbers. ground is black on both sides, pin 5 on a sedan, not labeled on a wagon.

also, a yellow/black wire goes to the heads up brake light.

the full circuit is really simle. power goes from the battery + to the 'positive terminal' (under the driver side dash on my 1992, presumably same on a 1994), and from the positive terminal it goes to the unswitched positive rail in the fuse/relay panel and supplies power to fuse 4 (and others). fuse 4 has a green/yellow wire to connector C7 pin 2 to a green-yellow wire to the brake switch pin 1. so, this brake switch pin 1 should ALWAYS be powered. Connector C7 is a 8 pin connector somewhere under the dashboard.

brake switch pin 2 is a yellow brown wire to connector C7 pin 1 to yellow brown to the bulb-failure-sensor pin 54S/9 (diagram is fuzzy). brake switch pin 2 should be powered when you're stepping on the brake.

The bulb-out-sensor has 3 outputs, 54L/10 is yellow-red to left side, 54R/11 is yellow to right, S/5 is yellow-black to heads-up. all three of these wires go through connector C1, which is a big rectangular bulkhead connector "Instrument Panel Harness - Sill Harness", somewhere near the driver side door hinge (?). The Sill harness travels down the driver side door sill to the trunk area.

 

Thanks for everyones help. The car is a sedan and the cruise control does not work. It stopped working about 3 months ago. I just recently realized that the brake lights don't work. I guess it is very possible that I have not had brake lights for the past 3 months without even knowing it, yikes. I was going to fix the cruise control since I enjoy using it but I never thought that it could be connected to something critical like the brake lights so I have dragged it. I have not had anytime to do anything to the car. Hopefully I will have sometime this weekend. It is good and bad when you have another car available. Good because you can park it and be unaffected, bad because you have no urgency to fix it.

 

cruise control taps into the the brake light circuit via the yellow-brown wire I described above at C7 pin 1. if it was shorted to ground, then it would be blowing fuse 4 when you step on the brakes. if it wasn't shorted to ground, the lights would work.

 

I'd still look into that Bulb Failure Relay as the cause to all this... Make sure the relay is good, may be test it on another car or swap it...

 

It was the bulb failure relay after all. The old one was in fact bad. I had one of the wires that goes to the brake switch loose after I had disconnected it to check for power. Cruise control still does not work so that seems to be a separate issue. After resolving the issue with the brake lights, I took the car out to do some errands and the A/C compressor, which is the original, blew. I now have an even more expensive issue to address. It seems like things do come in threes, first it was the cruise control 3 months ago, then it was the brake lights a couple of weeks ago and now it is the A/C compressor.