Detecting constant very low residual voltage in my 78 series brake circuit
Submitted: Monday, Dec 19, 2011 at 13:30
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John
have recently fitted a Tekonsha P3 brake controller into my 2005 78 series turbo diesel troopie. I have wired the sense wire into the brake stoplight circuit. the readout shows normmal battery voltage when the brake pedal depressed but also shows 0.04 volts in the brake light circuit when the break pedal not depressed , with or without the engine running. Does any one know where this voltage may be leaking from?. also at what level will the tekonsha detect voltage in the brake circuit and apply braking, i think this is below the very low reading i have as trailer brakes only applied once normal battery voltage in the circuit detected. other worrry is that this voltage leakage will flatten my battery. greatly appreciate any wise heads advise.
Reply By: TerraFirma - Monday, Dec 19, 2011 at 13:45
Monday, Dec 19, 2011 at 13:45
John, Doesn't the Tekonsha unit itself use a small amount of power when not in use..? Recently had one installed and thought I read this in the instructions? I could be wrong, just thought I read this somwhere.?
AnswerID:
472899
Reply By: Member - Phil G (SA) - Monday, Dec 19, 2011 at 21:56
Monday, Dec 19, 2011 at 21:56
It will work fine. 0.04V is just a few stray electrons.
My P3 is wired into the same wire, and works fine.
From memory the P3 uses about 110mA when its turned on and not doing much, but reduces to very little once it turns itself off. If it worries you then unplug it. I pull
mine out of its cradle and store it in the shed when not towing.
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472937
Reply By: Ross M - Tuesday, Dec 20, 2011 at 00:40
Tuesday, Dec 20, 2011 at 00:40
I have a Prodigy and it uses about 8ma at idle in sleeo mode and slightly more after a brake press and alive and ready. The circuit to the trailer will likely have a very slight voltage in it, as
mine can tell if there is trailer or no trailer. NC is shown with no trailer. It must do this by measuring a small drain via the brake circuit to the magnets at
wheels.
Therefore the small voltage you measured would be normal I suspect. It is possible a voltage level and not a current flow which is being sensed.
all good
Ross M
AnswerID:
472949
Follow Up By: John - Tuesday, Dec 20, 2011 at 11:28
Tuesday, Dec 20, 2011 at 11:28
Many thanks for all the replies, i understand the the P3 detects the trailer with as
Ross says a small voltage in the brake (blue) wire that flows through the magnets then to ground. i am showing a constant small voltage (0.04v) in the red sense wire connected to the brage stoplight circuit. i would not think this would have any voltage unless the break switch had activated the stop lights. this voltage measured even with motor off so poss maybe something to do with the controller but am worried i may have an earth leak somewhere.
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Follow Up By: TerraFirma - Tuesday, Dec 20, 2011 at 14:55
Tuesday, Dec 20, 2011 at 14:55
Hi John, Test it both ways, with and without the controller plugged in. Use the tekonsha removal plug and then test it again if you like. Other than that take it to an Autoelec who can test it while you wait.
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Reply By: Ross M - Tuesday, Dec 20, 2011 at 20:21
Tuesday, Dec 20, 2011 at 20:21
Hello John.
What you are asking about is a voltage of only 4/100ths of 1 volt. Not very much at all but the critical thing if you are concerned is to check the current in the red line as this is what will flatten a battery.
Remove the red wire to the brake switch and insert in the line, a small 12v dash light globe or similar small globe. Even a torch globe will do.
Sometimes I use an LED in the line to see if it lights but you must try it both directions to make sure you are detecting the current flow that may be there.
If you have a multimeter set it on 200 milliamp setting and put that in circuit in place of the globe. This will measure the current flow. If you get a reading of just a millamp or two you have no problem.
The thing to remember is this wire is effectively connected to the stop light globe filaments. If they are not lighting or glowing with the stop light switch not activated and your battery is not going flat then there is no problem.
Regards
Ross M
AnswerID:
473011
Follow Up By: John - Wednesday, Dec 21, 2011 at 11:11
Wednesday, Dec 21, 2011 at 11:11
Hi
Ross, many thanks for your help, tried with the mutimeter in series with the red line and neglible reading, now much more relaxed, again many thanks, john
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