MGL Avionics User Forum

Finished the retrofit install of the Blaze TP-4 and now have several ground runs and about 2 hours of flight time. There are a couple things I need to tweak and looking for any insight from other users.

Oil Pressure - I'm using the UMA pressure sensor and have selected that sensor in the setup menu. Using a mechanical gauge as reference, the TP-4 matches the mechanical within 1-2 psi from 70 psi up to 100 psi. When I shut down the TP-4 still shows 14 psi. If I want to correct this, I suppose I'll need to go into the custom curve section of the setup and put in the data. Anyone else have this issue?

Oil Temperature - Some day I'll have to use a laser probe to check the accessory case at the probe when the engine is hot to verify the reading. It does read the ambient temp correctly.

Volts - Matches the DVM reading to within 0.1 volts.

Amps - Here is where I'm trying to figure out what to do. I did the cal procedure with the magnetic sensor. With the alternator off and avionics on I see about -5 amps. After an engine start and putting the alternator on line, I see about +10 amps which gradually goes done to close to zero after a couple of minutes. What bothers me is that once everything settles out after about 5 minutes the TP-4 shows a discharge of -1 amp. The magnetic sensor is attached to the battery cable at a position next to the no longer used ammeter shunt. Since the shunt is still there I did a temp connection so that I could do a compare with the old gauge. Where the discharge was about -5 amps, both were very close. Where the charge was +10 down to +5, both gauges were close. But down where the guages were showing +/- 2 amps there was a bias. Once things are steady state, the Westach has its needle on 'zero' while the TP-4 is showing about -1 amp. I don't like to see the ammeter at -1 amp while flying around as I'm running dual LSE ignitions ( I do have a backup buss for one of the ignitions). Any thoughts on this?

I'm doing some cogitating on how the main battery buss was wired up to see if I can come up with a theory. But I built the airplane nearly 35 years ago. After some quick looks under the panel I think I know how the main buss was wired. Batt pos --> main relay --> start relay on hot side of firewall --> back inside cockpit to ammeter shunt (MGL mag current sensor attached to this wire) --> circuit breaker buss. Then we have alternator --> alternator breaker --> circuit breaker buss. I need to crawl in the cockpit upside down so that I look under the panel and confirm how things were connected up at the circuit breaker busses.

Thanks,
Lowell

Are you using an open loop or closed loop current sensor ?
If the open loop be aware that it can react to stray magnetic fields that can easily be emitted by nearby electrical wiring.
Either move the sensor or add windings - the more windings you have the more robust the whole thing gets with respect to external fields.

Also make sure the ground supply of your magentic sensor runs using its own, unshared wire directly to a ground point on the TP-4 - that way it will not be "seeing" additional voltages in series to what the sensor creates - these can easily cause a false reading.

Hi Ranier, thanks for the replies.

As I think you figured out, I'm using the magnetic sensor. I chose that one since I didn't see any documentation as to the ID of the closed loop sensor. Also I didn't want to have to deal with getting up under the panel to disconnect the cable to be able to slide the closed loop sensor over the crimped lug and cable. The cable I'm measuring the current at is a 2awg cable, so can't do multiple windings. About 2" away from the sensor mounting location is an 8 awg alternator output cable. I'm worried that that is causing an issue, but was hoping the inverse square law would be my friend. Rerouting these primary electrical cables is not an easy option. The magnetic sensor ground wire is soldered at a point about 6" away from the RS232 connector on the TP-4 with the ground conductor common to both instrument and sensor then running about 20" back to a ground buss.

High current cable just 5" away ? Hmm - I think that may cause a deviation in your reading as you do not have any windings - that means you are looking at the sensor output with high sensitivity. It's sensitive enough to be affected slightly by the Earths magnetic field.
Of course there are ways to deal with this - one thing you could consider - get a ferrite ring and carefully grind a gap large enough for the sensor and cable to just fit in (dremel works nicely) - the sensor should be located in the gap and the cable in the center of the ferrite. Now you have created a closed loop sensor and it will be much less sensitive to any disturbance. Fix in place with some epoxy.

Ok, I'll look around to see where I can source a ferrite ring of that size. Thanks!

Thousands of options on Amazon, but couldn't find what I wanted.

Ended up installing a pair of clip on suppressors onto the alternator cable. Also got ferrite loops that I cut a gap in so that I could place them behind the battery cable where the sensor was attached to.

Still showing a small discharge on the gauge, 0 to -0.3 amps. Much better than what I was seeing before. Might be as good as I can get. Wish there was a way I could go into the menu and put apply a bias to the reading so that I would see a reading of 0 to +0.3 amps. I have to do some electronics research to see if there is some way to measure actual current at the shunt without having to crawl under the panel to disconnect the main cable.