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Troubleshooting voltage issue on Bus

J

jeffsvan

Well Known Member
Members,

This is a bit off topic. I am trying to help my hangar neighbor with his aircraft. He is having an odd voltage issue. The plane is a 1974 Cessna 172 Skyhawk. YES this is a certified aircraft. Please keep in mind we are not repairing, modifying or adjusting anything. We are simply trying to diagnose using a hand held voltmeter.

He has a normal bus, an avionics bus, and a Zeftronics regulator.

PROBLEM: This aircraft has three instruments in the panel that can each display bus voltage. Master on/ engine off -they are displaying 11.3, 11.8 and 12.3 volts. The avionics bus reads the lowest.
We checked the sources with a fluke voltmeter and these readings are correct. This does not seem normal to either of us.

With the engine running they display 12.3, 13.8 and 14.1 volts

QUESTION: what could be causing the difference?
I am thinking this electronic (Zeftronics) regulator, or perhaps some bad grounds?
 
The problem: Shouldn't the voltage be reasonably the same throughout the aircraft like my Vans? 12 volts engine off, 14 engine on ?

The instrument in his aircraft which displays 11.3 volts flashes a low voltage warning. We are trying to find something before he calls in his A&P.
 
Last edited:
jeffsvan said:
Members,

With the engine running they display 12.3, 13.8 and 14.1 volts

QUESTION: what could be causing the difference?
I am thinking this electronic (Zeftronics) regulator, or perhaps some bad grounds?
Click to expand...

What you are seeing is the result of a voltage drop as the power is distributed from some main tie point. Each bus is providing current (amps) to equipment. The wiring/bussing or other devices (switches, relays breakers etc.) have resistance, which equals a voltage drop and the voltage drop increases as current increases. If the voltage drop is large enough you might see is as differing voltages when measured at different points. Probably the case here. If you are seeing 14.1V anywhere then the VR is fine. The 12.3 would be a flag for me that there is to much of a drop in the power being supplied to that point. 13.8V is fine. The standard is that there shouldn't be more than a .5V drop from the power source (alternator or battery) to the equipment.
 
Depending on where the black lead of your voltmeter is placed... you could also have voltage drops in the ground circuits.

Measure everything using the battery negative terminal as a reference.
 
What are the instruments? Are any of their power inputs diode-isolated, either internally or externally?
 
It flashes low voltage because 11.3 is low voltage. If I understand you correctly, this scenario is with the engine off and when the engine is started, the low voltage warning disappears?

You are diagnosing an old battery. Put in a new one and the voltage will come up and the warning will disappear.

How old is the battery?

:cool: CJ
 
You are seeing different voltages at different places due to unwanted resistance in the wires and/or connections or switches. e.g., measure the voltage on either side of the avionics master (switch on). If you see 0.1 volt difference that's acceptable; 0.5 volt drop is not. If some buss bars are interconnected by wires, and you see a significant drop there, try loosening the connections, spray contact cleaner to remove oxidation, re-tighten. If there's a lot of oxidation you may need to completely remove the wire and clean the contact surfaces with emery cloth.
 
The vast majority of electrical problems are caused by bad connections. The best way to find a bad connection in the positive side of the electrical system is by measuring the voltage drop using the positive post of the battery as the reference point. In other words, connect the red voltmeter lead to the positive battery post and use the black voltmeter lead to test various terminals and connections until a voltage drop is found. In older aircraft, the connections may have loosened over time. Tightening screws as little as 1/8 turn can make big improvements. Some Cessnas have an avionics contactor that could have some corrosion on its contacts.
 
Like Bob mentioned, we use .1 drop under load per connection, wire to terminal, terminal to lug ect, but .5 max in one circuit. on the battery cables we use .2 and .7 max, things like this can take hours to find when you go through each load path. Start at the battery and test every + lead and then the - side of the house. just remember a VDT is under load.
 
We found this nice EAA article on their website concerning "grounds":

http://www.eaa.org/en/eaa/aviation-communities-and-interests/homebuilt-aircraft-and-homebuilt-aircraft-kits/resources-for-while-youre-building/building-articles/electrical/electrical-grounding

After some examination we believe he has some poor grounds.
We just carefully traced wiring up under the panel and there is a point where 5 ground lugs are tied together by a bolt and nut, then a single wire of the same gage going to the airframe. Another where grounds are chained in series. I suppose this is what accumulated over 42 years by many different people having their hands up under there. Owner said the radio stack has been updated twice and a few E.I. instruments added during the 20 years he has owned the plane.

The grounding issue has been added to his squawk list for annual and the A&P will correct. Glad we figured it out.
 
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