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EFIS Voltage before or after engine operation

B

birddog486

Well Known Member
I'm considering changing out my master contactor/relay as it seems the voltage getting to the buss from the battery when the engine is off has slowly declined over time. I'm consistently getting low voltage on the GPS now when the engines off.

I'm thinking the resistance across the contactor is increasing but I'd like to hear from others who have the Odyssey battery and the standard vans contactor.

My setup charges at a consistent 14.4 volts but when the engine is off I'm seeing around 12 volts once stabilized on the EFIS. Testing the battery shows 12.7-12.8 volts and this is now the second battery on this plane in the last 5 years.

Do others see this large of a voltage drop across the contactor? I should add when starting it has no problem swinging the engine over.
 
Why not bypass the solenoid and see what change you get? My money is on an alternator field is drawing the voltage down. pull you alternator breaker and see if the voltage drops.

Check volts on each side of the battery master solenoid, if the voltage is the same on the input and the output you know it's not the solenoid, or simply run a jumper wire around the solenied and see if you get a change
 
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The best way to measure voltage drop is to use the battery positive terminal
as the voltmeter reference point. Connect your voltmeter red lead to the
battery positive terminal, then measure voltage drop at various points using the
black voltmeter lead. The circuit under test should be loaded. If there is no
load, then there won't be any voltage drop.
 
I do gain 0.1 volts when pulling the ALT breaker. I think I'll just put a jumper across the contactor next time the cowls off. That seems to be a fairly quick way to tell if the contactor itself has an issue but, what's the expected voltage drop across a good contactor?
 
Turning off the alternator field is a pretty good test. Mine draws 3.5A. A 0.1V drop implies you have a 0.35 ohm loss somewhere. The measured voltage across my master relay was 0.026V with the alternator field on.
 
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Last month, when I tried to start the engine, thee was a "stutter" in the electronics with a sound like the contator was not making good contact. I thought it was either a low or bad battery, or a bad starter contactor. After a few hours, I again turned the master switch, heard the same thing for a few seconds. Hmmmm, MASTER contactor? Yep, that was the problem, a trip to the auto parts store, (bought typical continous duty contactor for $30.00) and all is well again!

DAR Gary

birddog486 said:
I'm considering changing out my master contactor/relay as it seems the voltage getting to the buss from the battery when the engine is off has slowly declined over time. I'm consistently getting low voltage on the GPS now when the engines off.

I'm thinking the resistance across the contactor is increasing but I'd like to hear from others who have the Odyssey battery and the standard vans contactor.

My setup charges at a consistent 14.4 volts but when the engine is off I'm seeing around 12 volts once stabilized on the EFIS. Testing the battery shows 12.7-12.8 volts and this is now the second battery on this plane in the last 5 years.

Do others see this large of a voltage drop across the contactor? I should add when starting it has no problem swinging the engine over.
Click to expand...
 
I just don't understand why you don't bypass the darn solenoid, just disconnect the battry side and the bus side and temporarly connect them directly, if you have an avonics master you are not going to hurt anything. That will tell you if the solenoid is not making good contact. If you have the same voltage drop then you know it's not the solenoid, my guess is the solenoid IS NOT the problem and you have something taking a load
 
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