18 replies to this topic

[I_broke_it_again]

My daughters car is now starting to mess with my head! The battery has been tested by the garage and has been given the thumbs up, so I installed a relay 7 days ago and the battery has been fine. After not being used for 3 days it started fine and the voltage has been above 12.4 all the time.

So the interim fix is working and she has to put up with the clock being wrong until the drier weather and can spend more time at night on it, or so I thought.

Here is the head scratcher and I am defeated.

She came home tonight and fuse 3 - 4 driven bits are not working. No horn, headlight flash, brake test switch, radio or clock. So remove the relay and put back to normal and nothing still works. After a lot of testing I discover that if I take the fuse out and replace it after about 10 secs then I get 12v inside the car. Try to Use any item and the voltage drops to 3.8v and nothing works until I take the fuse out and put it back and back to 12v. Introduce a separate bulb from one of the purple power lines to test the circuit and instant drop to 3.8v and it stays there until I take the fuse out. So it can not be a fault in the horn, lights or radio? Then I discover if I wait a couple of minutes, the voltage goes back to 12v just as if i took the fuse out. Scratch head and then try and test the lamp from the fuse box to see if it is the power to the fuse box or something after that is causing the problem. The bulb works and It stays at 12v. I then try the horn etc and everything now works!!!!!

Basically I have not changed anything and it now works??

The 140ma drain is still there, so I have put the relay back in and everything now works.

Is the alarm on this circuit and could it be this faulty?

It is as if something was detecting a load on the circuit or a slight Voltage drop and went into fault mode dropping the voltage on the line.

Any thoughts would be greatly appreciated as I am now out of ideas

Cheers

[KernowCooper]

When you have a circuit that drops voltage drastically and then slowly recovers to battery voltage it points towards a connection with high resistance. This could be corrosion on the rear of the fuse box terminals or the main feed on the starter connection to the fuse box, or a bad earth on the engine or main earths, but because its only on one fuse circuit thats where I'd be checking. Have you had the fuse box off and checked the rear and the rivets where the terminals are mounted, do some volt drop tests on rivet to terminal while trying to move it.

Edited by KernowCooper, 24 February 2014 - 10:36 PM.

[I_broke_it_again]

The voltage drop I measured is instant as soon as I used anything on that circuit. I wait and keep measuring and it is if a switch is suddenly turned on and it jumps back to 12v battery voltage.

I have measured the resistance and I get the following values:-

Engine head bolt to Joint between front grill and wing on bodywork 0.2ohms
Fuse box mount to Joint between front grill and wing on bodywork 0.3ohms
Battery neg terminal to boot lid strap screw into bodywork 0.2ohms

I'll have to wait until the weekend to take the fuse box off, but any thoughts on if it site alarm with a fault?

[KernowCooper]

Do a volt drop test ohms is no good here is a guide if you need it http://www.theminifo...ring-volt-drop/