The International Cessna^® 170 Association

Here’s a brain teaser for the electricity nerds who enjoy dissecting anomaly’s:
This scenario seems to be a ‘one-off’, never seen it before, hasn’t happened again.

Airplane is a ‘55, hangared, mostly stock with original wiring, rebuilt 35 amp generator, everything in good condition. The Delco voltage regulator has been replaced by a Zeftronics solid state unit. Master solenoid is clean with no corroded posts. Master switch always solidly clicks into place, both on and off (never had a reliability problem with any of the push-pull switches). Starter looks in good shape, probably replaced/overhauled with the engine a couple hundred hours ago. Nothing recorded in the logs. Have never felt the need to inspect the brushes or commutator
(my bad....). Only mod to the starter is the installation of a Canadian Aero Products 5-roller clutch. Everything is adjusted correctly - I’m extremely anal about stuff like that.

THE EVENT: First flight of the day, all loaded up and ready to go last weekend, fully charged battery (I checked it later, just to be sure), master on, gyro spinning, no other electrical load (I leave the “Grimes Grinder” off during start).
With skill and dexterity that a surgeon would envy, I smoothly pulled on the starter cable, feeling the extra resistance caused by the actuating arm having to push a little harder against the spring inside the pinion shaft (in a perfect world, my gear teeth would always mesh cleanly on the first attempt, but such is not my luck). Pulling the cable further, as it neared the end of it’s travel, I fully expected a normal start as soon as the actuator lug pressed down on the contacter button and the starter commenced turning. Then......nothing! With the cable held all the way out, the prop didn’t budge and mysteriously, the T&B gyro began winding down as if all electrical power to the battery bus had been interrupted. Tried again, same results. On the third attempt, after cycling the master switch, all electrical to the cockpit appeared completely cut off and the engine still wouldn’t turn over.
Not knowing the root cause and after a little fiddling around, some head scratching and some rudimentary trouble shooting, nobody could figure it out. Then, after a bit more brainstorming which of course included jiggling the prop (you ALWAYS jiggle the prop when something’s wrong, don’t you?) and some last minute guesswork, I gave it another shot and - voila! - everything back to normal. Weird.
So, after some more serious head scratching, I concluded it just didn’t make sense that, unless possessed by evil spirits, an old airplane with the most basic gadgetry could somehow randomly turn itself off and back on again seemingly absent any obvious malfunction. Having previously flown the Airbus A320 - which I consider an electronic nightmare (no offense to any Airbus guys out there) - for my former employer, I couldn’t get past the notion based on past experience that I had an ELECTRONIC problem vs. a simple electrical issue. OK, the voltage regulator, but why? So I contacted Zeftronics tech support and asked if there was a possibility that hidden somewhere inside the regulator there might be some sort of secret circuit breaker and he said “yes there in fact is - it’s to protect against over-voltage - but we don’t tell people it’s there.” He also told me that the master needs to be cycled to cause it to reset. Ah ha!

All this now makes perfect sense, except I still can’t quite figure out why it happened in the first place. All I did was try to start the engine, same way I always do.
Any guesses?

I don’t think the built in Over Voltage Protect could interrupt the starting circuit. I had a flaky OVP in my comanche. When it would drop out it disconnected the alternator from the circuit, not the battery. That would be a bad thing for an OVP to do in flight. The OVP is there to protect the avionics from overvoltage. It’s unlikely that a battery will supply more than 13 volts without the charging system engaged. It is true that the common way to reset an OVP is to cycle the master if you don’t have a field switch.

So, I think your master relay is the issue. Slightly high resistance on the contact that fails when the high current draw of the starter appears. Flipping the master switch off and on perhaps allows the contacts to find a happy place. Ohms law E=IxR. Voltage drop is the product of current times resistance. Think about your car when the battery terminals get corrosion. Turn the key and a clunk and then all electric is dead. Until you clean the terminals. Meanwhile if you didn’t try to start the car but just turned the radio on and open the door, it would have worked fine. Low current, low voltage drop. High current (Starter) the drop can approach 12 volts. Watts is the product of volts and amps. Now you’ve turned your battery connection into a heater. All that energy that Delco wanted to use to turn the starter is used up heating the connection at the battery.

This is a very simple system. I would clean the battery terminals and suspect the master relay.

Just an educated guess. When you find the cause, I’ll be interested to know.

Rich

I would agree with the high resistance somewhere. Cable to battery, switch on starter? First places I would look. Finally, is your battery losing it? Sometime internal resistance increases, which would be cause for the gyro to slow down. Just a few things to check out.

Like Bruce said, you might also want to check the switch in the side of the starter; it’s very simple to remove and inspect. It could even be a loose or corroded terminal in one of the heavy gauge wires in the circuit. Also check for intact ground straps from the engine to the mount and from the mount to the firewall. If/when it happens again check all the connections from the battery to the starter and back including the master contractor, starter switch, and grounds; something somewhere along the line will likely be warmer that it should be.

Thanks for the thoughtful suggestions! I failed to mention the battery is a new Concorde and it’s actually the first thing I suspected. I do keep all my connections clean and tight but everything’s worth double checking. Interestingly, Miles suggested checking the starter contacter button and that’s the one thing I hadn’t considered. Might just start with a look there first....so far, the problem hasn’t repeated itself. I suspect the best way to cause it to act up again would be to land at some remote dirt strip in the middle of nowhere, miles from any civilization.

I don’t believe the starter-contactor could cause the complete shut-down of the electrical system.... it would only prevent the starter from rotating.

I agree with Bruce’s theory... adding only that your cockpit master switch internal contacts could be dirty or worn. That cockpit master switch has two circuits inside it... one grounds the battery-relay on the battery box (which is the main suspect along with battery cabling/terminals)... and the other circuit controls the generator field via the regulator.

There is no way the OVP can cause a complete electrical shutdown....all it does is shut down the generator if an overvoltage should occur due to a grounded field.

The increased load imposed by the starter could overwhelm a poor connection at the starter, at the starter-to-batt-relay terminal, inside the batt-relay, at the battery-to=relay cable terminals or at the battery terminals, or at the battery-cable-to-ground terminal.

The prime suspect is the battery relay. How old is it? $32 at Spruce if you aren’t determined to spend more https://www.aircraftspruce.com/catalog/ ... kkey=35605

Othewise identical in form/function as the $70 Lamar unit: https://www.aircraftspruce.com/catalog/ ... kkey=35605

What Rich said about voltage drop across the master relay.

GAHorn wrote:I don’t believe the starter-contactor could cause the complete shut-down of the electrical system.... it would only prevent the starter from rotating. ...

It would if it shorted to ground internally.

OK, just for clarification voltage drop through/across the master relay is measured at the “cranking” terminal (or whatever it’s called) between the post itself and a good ground whilst the starter is engaged? Will a resistance check tell you anything?

Using a VOM measure the volts with the master switch on, negative lead to ground and the positive lead to the solenoid terminal were the positive cable from the battery comes it and the terminal where the positive cable leaves the solenoid on the other side. The difference is the voltage drop caused by resistance in the contacts inside the solenoid. So your measuring the voltage going in and comparing it to the voltage going out.

Bruce Fenstermacher wrote:Using a VOM measure the volts with the master switch on, negative lead to ground and the positive lead to the solenoid terminal were the positive cable from the battery comes it and the terminal where the positive cable leaves the solenoid on the other side. The difference is the voltage drop caused by resistance in the contacts inside the solenoid. So your measuring the voltage going in and comparing it to the voltage going out.

And a more definitive answer would be provided with a load applied such as with land/taxi lights activated (or if you can stay clear of the prop ...with the starter activated.). Jumper leads are a good idea if this test is planned.

When you perform voltage drop tests, you need to load the circuit. What I do is hook my DVOM across the suspected circuit - in this case the Master Switch. Positive lead on one side and negative lead on the other side. Engage the starter as this is when you experienced the issue. The DVOM will show the Voltage Difference between both sides. I would expect to see .5 volts or less. I would also be checking the ground side. Hook one lead to the negative side of the batter and the other on the starter case. Again, I would expect .5 volts or less. Just my 2 cents worth.

Sounds like the solenoid has poor contacts to me. The high current draw from the starter caused it to break circuit. But it could be any of your connections from battery to solenoid also.

If you lost all power to bus it is not Zeftronics OVP.

Check all connections. Do you have a good earth from Battery to motor? The little aluminum jumpers around the engine mounts can fail. I added a ground from negative to motor case. (Though this was probably not this problem)

The master switch on my 53B has two sets of terminal posts to provide the ground to the Contactor. During install of a new contactor, I discovered the terminal posts wired had a fair amount of resistance (when on) and the unused posts had no resistance when on. I moved the wires to the other terminals. Based on my experience with a switch, I would test your master switch. Good luck

Todd,
I'm not understanding "the unused posts". Is your generator/alternator not wired through the master switch?