A different kind of hot start issue - help !

What you really don't want is for the battery to somehow get disconnected from the starter with the avionics still connected to the starter. Any stuttering from the master relay with the starter engaged sets up this situation if you somehow have avionics turned on. The collapsing field from the starter generates a positive voltage spike that can wipe out avionics, and no mere diode can handle the "electric water hammer" of a couple hundred starter amps.

Cars don't have this problem because they have no master contactor.
 
nrpetersen said:
What you really don't want is for the battery to somehow get disconnected from the starter with the avionics still connected to the starter. Any stuttering from the master relay with the starter engaged sets up this situation if you somehow have avionics turned on. The collapsing field from the starter generates a positive voltage spike that can wipe out avionics, and no mere diode can handle the "electric water hammer" of a couple hundred starter amps.

Cars don't have this problem because they have no master contactor.
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Exactly. And cars have an ignition switch function that cuts off the accessories during cranking to prevent the starter's solenoid spike from zapping the radio when the key is released. Today's car computers will be using an MOV (metal-oxide varistor) or similar to shunt the spike around them.

I once measured, using an oscilloscope, the spike off the master contactor. 600 volts.
 
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In 8000 hours I have never damaged my electronics turning the master on and off but I do have the diode. Diodes are VERY cheap from an electronics part store.
 
Piper_Six said:
Many thanks y'all, I have a set of Master Relay and Starter Relay ordered, plus a new Concord battery - expected to arrive this week, will replace them all and report the results. Thank you all for some very enlightening suggestions.
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How's the progress?

tenor.gif
 
Here is the progress, got a new battery installed and the relays have arrived. Haven't had time to swing by the hangar and test yet, will get to that this weekend. Despite the lockdown, this working from remote has kept us plenty busy ! But yes, I should have an update later this coming weekend.
 
Phew ! Finally .... replaced battery, replaced both the master solenoid and the starter solenoid... and all is well. It starts now without making that whining sound and struggling too much, even when hot. Thank you all for a great discussion and many great suggestions. I sure learned a bunch, and continue to learn more. Hmm, now I have two old, tired relays, plan to open it to see what's the view inside... my gut feel says, the master is fine but the starter solenoid is probably pitted badly.
 
Here is a pic of the new solenoids installed...
 

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Piper_Six said:
Here is a pic of the new solenoids installed...
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The brackets they attach to hopefully made for a simpler/faster installation. I think mine are attached to the FW, but would love to replace the old solenoids, esp since I have all new Bogerts in there. Would welcome pics of the insides on the olds ones, esp. the starter if you find anything interesting.
 
Whoa ! I tried to open the old starter relay.. can't seem to figure out how to ? It's not too obvious... Its also pretty rusty... might have to take it to the A&P and see what he can do...
 

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Whoa ! I tried to open the old starter relay.. can't seem to figure out how to ? It's not too obvious... Its also pretty rusty... might have to take it to the A&P and see what he can do...
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Hacksaw....
 
Piper_Six said:
Whoa ! I tried to open the old starter relay.. can't seem to figure out how to ? It's not too obvious... Its also pretty rusty... might have to take it to the A&P and see what he can do...
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Looks like the can's top edge is rolled over the edge of the cap. Grind through that rolled-over section and the cap will fall off.
 
Finally.... 5 minutes with a dremel tool, and 4 cutting wheels later.....managed to cut open the old starter relay, and sure enough, the contacts were pitted badly, and the round copper disc that is supposed to make contact seems to appear badly corroded, this could have been the issue all along.cutting open.jpg parts.jpg pitted.jpg
 
That tiny contact area on the one side is likely the biggest contributor to the resistance problem. A small contact area heats up real quick and increases the resistance and can start arcing. Might have been the source of the buzzing you heard.

The contacts appear to be curved on top. Haven't seen that before. Mostly they're machined flat to get lots of contact area with the disc. The drawback with a flat surface is that the mechanic might inadvertently rotate the bolt a bit as he tightens the cable nut, putting a corner in contact with the disc, which makes a really small contact area.
 
Very likely, I was surprised as to the difference in contact area from one side to the other. The disc corrosion was also a surprise, considering the unit is well sealed. This has been an interesting learning experience.
 
Piper_Six said:
The disc corrosion was also a surprise, considering the unit is well sealed.
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Hard to be sure without seeing it in person, but my first impression (and second after taking a closer look) is it's remnants of arcing built over top of pitting. Every time the contacts close and open on start up and shut down respectively of course, there will be an arc. The bigger the loan on start-up or shut-down, the bigger the arc. I see it on air conditioning contractors all the time. Big loads are switched on and off every time the thermostat calls for cooling and again when its satisfied.
It's surprising to see so much arcing, since most check lists have us shutting off almost all the load (except for the beacon) prior to killing the master.
 
Nathan Miller said:
It's surprising to see so much arcing, since most check lists have us shutting off almost all the load (except for the beacon) prior to killing the master.
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There are several types of loads. Resistive and inductive are the common ones. It's the inductive loads that make the big voltage spikes and therefore arcing at contacts when the contactor opens. Inductive loads include motors (like starters) and the contactor coil itself. Radios and lights are mostly resistive loads that don't generate voltage spikes, but we turn the radios off before shutting off the master so that the master contactor's coil spike can't hurt them. Today's radios should have internal protection, and some airplanes have a shunt diode on the contactor coil to kill that spike, but it's still a good habit to shut the avionics off first anyway. I have found contactor diodes broken or blown out. The ACS ignition switch AD demands a diode across the starter contactor coil to suppress the spike so that the switch contacts don't get burned so soon. Those contacts are very thin little plates that burn out easily.

DC systems are much harder on contactors and switches. AC systems are easier, since there are so many zero-flow points in a second (120 in 60 Hz systems) that will stop any arcing almost as soon as it begins. DC's constant flow will make an arc that continues until the contacts are far enough apart to extinguish it. Look at any toggle switch and see that its DC current rating is considerably lower than its AC rating.
 
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