Monday, Jan 20, 2014 at 23:01
This is an earthing problem Martin.
At the risk of boring you with what you may already know, what happens when you have a faulty earth to your lights (let's say at the plug) is that the current path which would normally be through the globe filament to earth now becomes convoluted because that earth is missing, the return takes an extenuated trek through any other globes that are connected to the same earth and this generally includes back through globes in the vehicle to get to a good earth.
An example of the circuit route may be:- 12v to tail light - poor earth so current goes through stop filament - back to car via stop light wire to stop filaments (plural as there is more than one) on car - through filaments to good earth on car.
The filament with the lowest wattage (highest resistance, ie tail light,) will glow brightest, in fact just about the same brightness as if no fault existed.
This is because more voltage, in the above case almost all of the voltage, will be dropped across the tail filament.
Switch a few on together, say indicator and tail and every time the indicator globe gets 12v from the flasher it effectively shuts down the pseudo return path for the tail light (12v+ve on both sides) which will go off. It will normally be the indicators on the car and the tail light on the van that alternate.
When the opposite occurs the tail light will again glow.
This is the effect you are seeing.
Now, the brake magnets have far more wattage than the light globes which means they have less resistance, therefore the globes are dropping all the voltage and the magnets are not energising.
The reason that you were seeing that 11.7v at the brake magnets was because the globes were giving you a return path with a small additional voltage drop but the resistance was too high to allow sufficient current to energise the magnets.
Fix the earth and the problem will go away.
Because of what is happening with the lights we can be reasonably confident that all of the lighting earths and most likely also the brake earth are connected together, so you just have to find where that earth is discontinuous either on the van or the car.
Have you tried that jumper between the 2 chassis since cleaning the earth?
Possibilities are:-
1. The earth in the van plug is not connected to the van chassis.
2. The earth from the lights etc, is not connected to the van chassis.
3. The earth from the lights etc, is not connected to the earth pin in the plug on the van.
4. The two plugs are not connecting properly.
5. Any combination of the above.
You are getting a good earth indication at the car plug, so the problem is most likely in the van.
That's all I can offer at present.
Let us know what you find.
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