Electric issue - brake/dashboard interference

[Azazello]

Hello guys, I'm here asking for your help for my green lady freshly on the road.
A couple of days ago a friend during a meeting told me my brake lights were not working at all, and a few hours later the light alarm began beeping every time I pressed the brake pedal (I installed a box that sounds a beep every time I leave the lights on switching off the key, it has two wires linked behind the dashboard to 2 positive contacts, one is the +of the fuel level instruments and the other one I don't remember - I choosed a contacts that turns on switching the key and another that turns on switching on the lights).
Today I made a bit of testing, and turned out that every time the brake switch makes contact (I have the pressure switch on the master cylinder, but even taking off the wires and linking them together works) stop lights are not switching on but the first 2 lights on the dashboard (dynamo and temp/oil) and the fuel stop working. Disconnecting a wire from the cylinder switch makes the dashboard work again as expected, but I need stoplights too...
I will proceed escluding the "beeper" from the dashboard seeing if it is its fault, but in the mealwhile any hint or suggestion is really wellcome. As far as I know brake light wires are not passing through the dashboard...
Electric scheme is 1975 model.

 

[André4Renaults]

This sounds like a defective earth somewhere, allowing electricity to pass through the "lights on" bleeper the wrong way. The most important thing is to get the brake lights working properly, do I would remove the bleeper first. Then check that you have a permanent 12v+ at one of the terminals in the brake master cylinder (it MIGHT only be 12v+ when the ignition is switched on, but an ideal situation is that the terminal is permanently live.) Connect this terminal to one side of the master cylinder switch & check that the other side of the switch becomes live when the brake pedal is depressed. If it is, reconnect the wire that runs to the brake lights and they should light up. If not, check the bulbs and scrape the terminals to make them shiny and all should be well!

The "lights on" warning buzzer should only work when the ignition is switched OFF (obviously, since you don't need to be told the lights are on when you're in the car!) so when I've installed them I fit a 5 pin relay to the ignition circuit, which is energised when the ignition is switched on. 5 pin relays operate as an either/or switch so they can be wired so that when the ignition is switched OFF the power is ON to the "lights on" warning buzzer and vice versa. The buzzer should be connected to the relay and the relay should be connected to the HEADlight (not the sidelight) circuit, ideally at the light switch. The other end of the buzzer should be connected to a good earth, preferably away from the plastic of the dashboard. I can explain more if needed.

Hope this helps!

 

[Azazello]

Yesterday I tested the wires on the brake master cylinder: one has +12v (I tested with ignition switched on), and even disconnecting both wires and linking them together (closing artificially the brake circuit, bypassing the cylinder switch) the problem is here, fuel gauge and partial dashboard current is cut off.
For the buzzer I am not sure I understood what you are trying to explain me, but I used this alarm:


It is told to be wired to two positives: when one of these is missing, the sound happens. So on my other car I linked it on the fuse holder, wired to 2 fuses, one with permanent +12v and one that brings +12v when ignition is switched on. Everything is working fine. On this model I have not this choice of fuses and I choose to link it hidden behind the dashboard...
Later in the evening I will proceed to take the dashboard off (sigh!) and delete the buzzer. I will post test results.

 

[André4Renaults]

I think your brake lights are earthing through the car's tail lights. What I THINK might be happening is when you put your brakes on the electricity is flowing to the brake lights but instead of going to earth it is then running back along the tail light circuit the wrong way and when sounding the buzzer. Try removing the brake light bulbs and seeing if it still happens. Do this before you strip the dashboard & it might save you some hassle!

If the buzzer is the type I think it is the 2 wires should be wired thus: one to a wire that's live when the ignition is on and the other to the HEAD lights (NOT the sidelights) Are you sure you have wired the buzzer to these 2 connections and NOT to the sidelights? They also have to be wired a specific way round, too!

 

[Azazello]

Tiny blue and red wires are from the buzzer.

No way, buzzer definitely not involved...
I forgot to mention that the car worked fine in this configuration intil a couple of days ago, when problems began, so something happened... another problem I had is that I swapped accidentally front sidelight and direction light wires (same bulb, 21w & 5w), but now it is fixed.

Buzzer was wired to the dashboard light (as lights on indication) and (accidentally, what a coincidence) to the wire that brings current to the switch on the brake cylinder (tested with instrument). Now I unplugged the buzzer but no changes, still current flows away from dashboard lights and fuel gauge when I push the brake pedal.

Thanks for your patient investigation...

 

[JonathanT]

Hello Azzazello
Clearly Andrew has good knowledge of such problems. I think the best place to start -forgive me if you've already done this - is to isolate sections of the wiring and test with a meter or bulb to check for continuity and poor earth etc. Disconnect anything confusing the issue. Taking a separate jumper wire direct from the battery to the back of the car you will be able to test the lights independently of the existing loom and therefore isolate where the problem is

 

[Azazello]

Hello Azzazello
Clearly Andrew has good knowledge of such problems. I think the best place to start -forgive me if you've already done this - is to isolate sections of the wiring and test with a meter or bulb to check for continuity and poor earth etc. Disconnect anything confusing the issue. Taking a separate jumper wire direct from the battery to the back of the car you will be able to test the lights independently of the existing loom and therefore isolate where the problem is

I did exactly what you told, and at the end, when nothing helped, turned out to be the wire from fuse box to brake cylinder that was in some way having earth problems. Another wire was passed and the problem seems solved... my worst fear is that if wires are so old and ruined, something similar can happen everywhere in the car in any moment...

Now I noticed grease on the garage floor and it was a driveshaft boot cut... both driveshaft were new, with 1500 kms of life since I put them on the car! I really don't like to have to remove the driveshaft to change the boot, even because earlier shafts are really a pain to refit... will have a try with the easy boot...

 

[JonathanT]

Clean the split in the boot really carefully then superglue it! Will work for a good while