Please help me with my brake lights

[SP2]

Hello,
I have an 81 SC. I have done searches and have used previous threads to be somewhat systematic. Both of my brake lights do not work, and the entire left taillight assembly did not work until I did a few things. I will explain what I have done so far.

I switched out the two brake switches on the MC. That did not work. I checked for continuity on each of the 3 prongs on each switch with a multimeter (MM) and there is continuity to two of the prongs with the brake depressed, and infinity with the brake released. When I switch to the third prong, it is reversed. So I believe the switches are working.

I tried jumping the wires to the connection to the brake switch by using a wire with two male ends crimped on. No brake lights for either connection/switch.

I studied the wiring diagrams and it looks like the brake lights share a common ground. So I went to the left rear taillight, the inoperable one, and l think I found the ground wires screwed into the body with a 13mm bolt. It was covered in dirt, so I removed the bolt and cleaned the wire connector. Then I grounded that with a separate wire to the body to be sure I had a good ground. Now the reverse light and turn signal operate (YEAH!, partial victory) but the brake lights still to not work. I tried jumpering the connection back up front at the brake switch, but that still does not work.

But now I notice that my brake pedal is not firm. It is firm for a bit, then sinks to the floor. That is true with the engine on too.

I am very frustrated because I can't even drive the car in to get fixed be a professional. I suppose I could tow the car in, but I have gone this far. I also checked to make sure the brake light bulbs on both sides are new now.

Any advice? Thanks.

[andyt11]

Sounds like you have a few things all together.

If you remove the plug from one of the switches in the master cylinder and stick a wire across the two spades, it should light the brake lights. Note that the ignition needs to be on the first click at least.

If not, I assume you checked the fuse in the fuse panel in the luggage area?

As for the spongey pedal, how long has it been like that?

[SP2]

Thanks Andy,
Yes, I said above that I pulled the plug and jumpered the plug. Should I jumper across the switch? I thought it receives the power from the plug. Yes, the ignition was on.

I forgot to metion that the fuse was the first thing I checked, including the contacts.

The spongy pedal was after I replaced the switches on the MC. I drained out quite a bit of brake fluid and had to top it off with more fluid. Hopefully it is just air in the lines (and not a bad MC).

James

[andyt11]

Yes, you are right jumping the plug should light the lights. I see you did that already now. Ignition on and fuses good only really leaves a ground problem. The bulbs are good right?!

And yes, I would expect to have to bleed the brakes after changing the switches.

I wonder if the contacts inside the light housings are a bit dirty? Maybe jump that plug again and then put your meter to a good ground and also to the contact in the housing. Then you could at least confirm you have 12v there. You could check the other contact is a good ground too.

[kerthunk]

When you replaced the switches, you introduced air into the brake system. You need to bleed your brakes in order to bring the firmness back.

As for the lights not working, you are going to need to dig a little deeper with the multimeter. It is just going to be a matter of tracing the wires to figure out where you lose continuity/voltage.

[SP2]

I did something to the brakes because 1 day after I topped off the fluid, the reservoir is now empty and there is brake fluid in the interior under the pedals. I am failing rapidly!

[kerthunk]

Yikes, that isn't good! Your master cylinder may be leaking from the switches if you didn't set them with the proper torque. I don't remember if they have sealing washers or not. There is nothing around the pedal area that carries brake fluid, but it is pretty typical to get fluid there if your MC is leaking. I would get that all cleaned up before resuming on the hunt of the electrical gremlin.

[andyt11]

I heard the switches have a tapered thread. So don't reef them too hard but you should be able to stop them leaking at least. (I"m assuming that's where the leak is coming from since that's all you changed)

I don't know I'm afraid, I didn't change mine as they don't seem to work properly even when new.

[SP2]

Thanks guys! I have to regroup. Yes, they are tapered ends, like plumbing lines. I tried to open the lid on the left rear tailight assembly and on of the screws is stripped. I think the advice about cleaning up the brake fluid mess is a good one. The weekend is over, so it is back to nighttime mechanic work on the car.

[john70t]

The brake light circuit is just a long chain...starting from the fuse box.

When you break it down to basic components, there should be only a few things to locate and check: fuse box, master cylinder switch, housing to body ground, wires, etc.

Have a friend sit in the car, or use a bar or piece of wood to push the pedal.

Using a DVOM/multimeter with a long piece of wire for chassis/battery ground, first check that the connections of meter itself are good using the battery neg (-).
Then go find each link and subsequently check they are good. You can use alligator clips with a piece of wire to probe difficult connections. Just touching it should be enough to get a 12V reading from the multimeter.

[SP2]

John, this was great advice and I used it tonight. I have the brake lights working now. I think I solved the leaking brake fluid issue as well. I did not torque the brake light switch in tight enough. I refilled the reservoir and hopefully in the morning there will still be brake fluid in there.

I had two problems. The reason my entire left taillight assembly was not working was because of a very dirty ground to the chassis. I used a long wire to connect to a good grounding point and that solved the left taillights (reverse and turn signal.) But the brake lights still did not work.

The second problem were the brake light switches. That's a $10 replacement. I have to push down on the pedal hard to get the brake lights to work, and I think that is because of air in the lines. I will flush them out tomorrow after I pick up some brake fluid.

This was really my first electrical fix on my car. I learned how to use a digital multimeter to check voltage and resistance. And I learned how to read the wiring diagrams in Bentley for my car. Thanks to Gerry for stressing this.

[dshepp806]

Very nice!!!! Just be sure that you never hook that meter up to voltage while she's set to measure "ohms"......some will survive this ,..others, not so..

Good work, BTW!

Doyle

[andyt11]

Glad to read that you sorted it. So the dirty ground was stopping the reverse and tail light working. What was stopping the brake light working? No earth at all?

As for having to press the brake pedal quite hard in order to light the lights, I think its a common problem. Some including myself have installed mechanical switches, some have had success installing new pressure switches and have a good bleed.

[SP2]

Yes Andy, the bad ground I think was why the brake lights didn't work too. I believe the brake lights share a common ground. I hope that the switches require a hard press because of air in the system. I will try to bleed the brakes tomorrow. They are both new switches now.