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Rising amperage

B

BrianP

Member
Hi guys.

I bought an RV8 that has a Dynon SC700.
When the engine is cold, the amp readout will be reading normally, starting around 13 amps or so, then all of a sudden it will climb quickly to over 50 amps. Sometimes will spontaneously come back to normal, other times I have to cycle the masters to get it back to normal. Voltage never changes. Any ideas where to start looking?
 
BrianP said:
Voltage never changes. Any ideas where to start looking?
Click to expand...

If the voltage truly never changes, not even 0.1 of a volt, then you most likely don't have that large a swing in amperage. If it was real, then something is pulling a LOT of current and your voltage will drop in response, and the alternator will try to pick it up. If the regulator was flaky and telling the alternator to go nuts to make more amps, then the voltage would rise with that much current being produced.

First suspicion falls on the amperage shunt, and the wires leading from it to the panel indicator.

The shunt works by passing the current through a metallic alloy bar with a very slight resistance, creating a very small voltage differential across it (usually 1 millivolt per amp of current) and measuring that difference. It doesn't take much in the way of a bad connection to skew that reading and get something obviously out of wack on the cockpit indicator.
 
Last edited:
BrianP said:
Hi guys.

I bought an RV8 that has a Dynon SC700.
When the engine is cold, the amp readout will be reading normally, starting around 13 amps or so, then all of a sudden it will climb quickly to over 50 amps. Sometimes will spontaneously come back to normal, other times I have to cycle the masters to get it back to normal. Voltage never changes. Any ideas where to start looking?
Click to expand...

The fact that cycling the master makes it go away kind of points away from instrumentation. If the VR is allowing the alt to run away, the volts will spike also. Is that happening?
 
BrianP said:
Hi guys.

I bought an RV8 that has a Dynon SC700.
When the engine is cold, the amp readout will be reading normally, starting around 13 amps or so, then all of a sudden it will climb quickly to over 50 amps. Sometimes will spontaneously come back to normal, other times I have to cycle the masters to get it back to normal. Voltage never changes. Any ideas where to start looking?
Click to expand...

If you have a wiring diagram, identify where the amp shunt is located. It is either in the alternator lead or in the lead to the buss - i.e. all the load demands. In the alternator lead means it reads the buss loads+charging load. And . . . if your alternator stops, you have no reading for demand to manage.

This will help identify the event as an issue or characteristic.
 
I second on Greg suggestion.

A small bad connnection on the shunt may lead to a large change in displayed current - remember that the SC700 is actually measuring the voltage across the shunt terminals. A hot firewall compartment would make metals expand which may lead to a bad connection to go worse. That can make this current indication fluctuate.

I would check all connections from the shunt all the way to the SC700 for proper crimping - specially on the shunt.
 
We don't know what kind of battery is installed in addition to not getting a response on make of alternator.

Further responses may be a waste of time without additional information.
 
koupster said:
We don't know what kind of battery is installed in addition to not getting a response on make of alternator.

Further responses may be a waste of time without additional information.
Click to expand...

There is no battery made, of any chemistry I'm aware of, of a size appropriate to our aircraft, that would NOT register a voltage increase when absorbing an extra 40 amps. The make/model of the battery here is immaterial, the data does not lend itself to a battery problem. If the data changes, then perhaps - but not with the current data.
 
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