Starter solenoid - main contactor resistance

[Battson]

What resistance would you expect to see through the master solenoid, measured across the main contactor's terminals??

I have measured 4 ohms on mine, and I am thinking that is pretty bad.... However it is over 10 years in service and probably 13 or 14 years since manufacturing.

As some will know, recently I have been chasing a "hard starting" issue in cold weather which has various symptoms, including kickbacks from my electronic ignition system, and the battery getting worn down pretty fast. I was advised by the ignition system manufacturer that it was unlikely to be an issue with their electronic ignition "SIM" mags, and I think they are right. After a little bit of starter motor testing I noticed the master solenoid was running a lot warmer than I expected, so I tested the resistance across it. Sure enough, 4 ohms. I don't have a lot of specialized aircraft electrician type of knowledge, but I have a strong feeling that 4 ohms resistance across my master solenoid main contacts is not good.

Some years ago, the master solenoid had a period where it failed to close correctly a couple of times, I wonder if that has caused damage to the terminals and increased the resistance. Not sure why that would have occurred in the first place, as it appears to be in great condition.

If anyone wants to measure the resistance across their solenoid, I would love to have a few reference points. Just across the main terminals with the solenoid closed (master switch on).

[alaskabearhawk]

I measured mine and it was 0.7

[jaredyates]

You might search the aeroelectric list archives or post there to get more specialized input. Some contactors can be compromised by overtorqueing the terminals, and naturally I'm sure they can wear out. Safe to say you have verified no corrosion on the rings that attach to it?

[Battson]

Yes, it looks really good at the terminals. The body is showing a little light corrosion.

I think if most other people get values like Alaskabearhawk, then I am just going to replace it anyway. Every fraction of an ohm counts when it comes to starting pulse currents in the hundreds of amps.

[kestrel]

"hundreds of amps" times 4 ohms is 4 hundreds of volts dropped across the resistance. You need numbers closer to (or less than) Alaskabearhawk. Simply going with 12 volts/4 ohms is 3 amps max. I suspect that you're not getting as bad as 4 ohms in practice...

[Battson]

I don't think you can apply Ohms law to this situation, otherwise a 12V battery could never run the system.
Starters commonly draw hundreds of amps, motorbikes maybe 150A, cars 150 to 200A, small trucks 300 to 400A, I assume around 250A for my starter.

[Battson]

It's probably worth pointing out that I measured the 4 ohms at a low load, just a handful of amps. I expect the resistance is varying with current in a non-linear way, so Ohms law does not apply.

If people can switch on the master switch and then measure the resistance across the master solenoid, with just a small load applied, that would be really useful for comparison.

[kestrel]

Ohms law definitely applies. Electronics, embedded/control software, solenoids and motors are how I pay the bills. The stuff I work on is normally battery powered.

The resistance may not be consistent. I'm not sure if this is what you meant by "12V could never run the system", but 12V will not turn the engine with the starter through 4 ohms. ...which means that 4 ohms isn't what it was when you attempted a start. Bad contacts may be inconsistent. Potentially, once some current is going through, things warm up and a better contact is made resulting in lower resistance. This is one possible explanation for my suggestion that you aren't getting 4 ohms in practice.

I'm not sure what you mean by "measure the resistance across the master solenoid, with just a small load applied". If you apply power to the circuit, you can't use an ohm meter. If you know the current, you can use a volt meter and ohms law. Maybe that is what you did?

If you can still take measurements in the aircraft, measure the voltage across the solenoid main power contacts when cranking the engine.

If you can't run a load with the starter and engine through the solenoid, find some other load. If you can't get to your assumed starting current, you can use ohms law to calculate resistance and then what the voltage drop would be at full starting current.

You've got me wondering about my own master solenoid now. Mine was built with the starter solenoid drawing power through the master solenoid. When I crank the starter, the Garmin GPS often reboots or just powers off due to the voltage drop. ...but not always. I have an alternate avionics switch that bypasses the solenoids so I can get power to my avionics even if the master switch/solenoid system fails. It is isolated from the normal master solenoid power by a diode. I think the pattern I'm staring to see is that if I also turn the alternate avionics switch on when starting the engine, the GPS doesn't even flicker. I need to look at voltage readings in both conditions (or measure across my master solenoid) and see if it needs some attention.

[kestrel]

Watching the voltage measurements that are effectively on either side of my main master relay, I'm seeing about a 2V drop across that relay while cranking the engine. Getting about 10V on the battery side and 8V on the load side. If I download the data log from the EFIS I will be able to be more precise about those numbers.

[Chewie]

I don't know much about solenoids but are you saying you're measuring across the battery and starter terminals? Why should there be any continuity through there unless you're supplying enough power to close the relay?

[Battson]

Ohms law definitely applies.

Wouldn't we need to know if we have linear resistance or non-linear resistance? Edit to add - if there's a way we can be sure we can use Ohms Law, that would make it a lot simpler to measure.

I think I'll do the starter test, as you suggested.

[kestrel]

Wouldn't we need to know if we have linear resistance or non-linear resistance?

Yes... and no... and disclaimers...

With respect to current or voltage, it will have constant resistance. It will not behave like, for example, a diode. However, its resistance is probably different each cycle or the contact quality may change with temperature, or...? I was trying to get at this when I said that you probably didn't have 4 ohms in practice because it wouldn't allow the motor to turn at all.

We may have been saying about the same thing, except that I wouldn't throw ohms law out the window. Instead, I would try be careful about test setup and interpreting the results.

Measuring voltage drop under the real load will speak directly to how much that is the problem. It will also allow you to see how well the motor cranks under the conditions measured.