I flew yesterday and today and during the last two flights had two instances of loss of all electrical power on the ground. Yesterday was after doing the electrics walk around. Today when cranking over.
A few cycles of the “Master” switches the coil in the Main Contactor/ Relay. After a couple of “clunks” this provides a temporary fix. It is definitely “clunking” each time so I assumed the main contacts are starting to burn out. I went flying with the backup portable radio sitting on the front seat just in case it happened in the air.
This evening I flew a few circuits and landed after 45 minutes. I checked the connections on the contactor were all tight and was a bit shocked that the coil was surprisingly hot (almost too hot to touch comfortably). I know how to fault find – One test is to put a voltmeter across the “switching contacts” and when cranking there should be zero volts – if a voltage is present the contacts are going high resistance…………that’s if I can replicate the failure.
Does anybody know how hot the coil should run? Most relays don’t run hot so if they are supposed to run cool (or just warm) I won’t even waste my time investigating and just order a new contactor.
I just looked at the Maintenance Manual and it says the Master Relay should draw 0.5 Amp. That means about 6 watts of power to dissipate. It felt like more that 6 watts to me so I presume I was feeling the heat from slowly burning contacts!
Also there doesn’t seem to be a diode across the coil to soak up back EMF. They have one on the 28 volt Archer 3 but unless I’ve missed it, not on my 12 volt Archer 2.
Bloody awful design – if the regulations were sensible this is exactly the sort of thing a competent person should be able to install without doing minor mods etc. Having said that we’d all have solid state relays by now!
I have also just seen the price is US$ 390 for this relay. That is more than my first car! – I had assumed it was a £ 30 auto part which would cost £ 100 when adjusted for aircraft pricing………….I think I’ll do some proper fault finding after all!
Most US-made GA planes use relays from Stancor. This might be of interest, and you will find some data sheets there.
6W will heat up a component of that size very nicely. The thermal resistance to ambient of the relay package is of the order of 10degC per watt, so you are looking at it getting too hot to touch. Mounting lugs will carry some away but still… For comparison, the thermal resistance of a 30cm x 30cm piece of 1.6mm aluminium, natural finish, vertically held in free air, and with the “device” mounted centrally, is 1 degC per watt.
A diode is definitely desirable but not something most designers in GA know about 
Not having the diode will burn out anything switching the coil a lot faster. It is also why the red autopilot disconnect button on the KFC225 autopilot burns out pretty fast – it is switching the clutch solenoids of all three servos!
Socata use more obscure relays (being French, usually from Valeo which is a French company making car parts) but all can be replaced with Stancor or other makes (e.g. the Ametek one shown).
I found a Lamar relay on LAS Aero but unfortunately it’s not the Piper part. The Lamar website did lead to this interesting video.
To be honest, if a diode is good enough for an Archer 3, I think I’ll just install one, especially after watching the video!
The first thing I would do is check all the battery earth ( grounding ) points, contacters are normally very reliable and in 35 years in the business I have yet to see one fail ( not yet anyhow ! ) but I have seen a number of contacter problems solved by sorting out earthing problems.
Thanks A_and_C that is really useful input. I’ll haul out my ancient Avo 8 & do some proper fault finding tomorrow. As the Aircraft is 39 years old it seems fitting to use equally old test equipment!
I don’t think Lamar actually make anything like relays. Those look like rebadged Stancor relays.
Lamar are just about capable of throwing assembling some components onto a PCB, soldering some wires to it, and throwing the whole thing into a box. They have managed to keep doing this for decades because they got in on the ground floor; they got certified and once you get a part certified, every mug will specify it for ever because it is easy.
Brilliant video – everybody should watch it!
Make sure the diode is the right way round 
If you can reproduce the failure, finding the offending part will be trivial. As you say, there should be no voltage drop across closed relay contacts. Well, their resistance should be a few milliohms. And as A&C says, check grounds, wires broken inside the cable insulation, etc.
If you do put a diode in, make sure you spec a appropriately rated high quality component. If it fails to short it will get really hot really quick!
Unrelated to aviation, I am looking at a resistor and capacitor across a relay (a snubber, which effectively does the same thing). The capacitor appears to be failing as a short circuit and then the resistor catches fire. It seems that the spec on the cap might not have been adequate… oops.
I have been trying to replicate the total loss of power fault but as with all intermittent faults it won’t go wrong when you want it to!
When cranking over I measured the voltage across the contacts of the main relay. I am assuming about 180 Amps to crank the starter. There was consistently about 0.2 volts across the contacts when cranking and this drop was quite consistent. If there was an issue with burnt contacts, I would expect a lot more variation.
So V divided by I = R – so 0.2 volts/ 180 Amps = 0.0011 Ohms – that seems quite good to me! If I have made a mistake with my Ohms Law or the starting current assumption please jump in and comment.
Finally I opened up the base of the relay and you can partially see the contacts. They looked nice and clean and there did not appear to be any pitting or blackness so I now have to assume the issue is a loose connection/ the master switch has become slightly intermittent/ an earthing problem.
I am not sure if one of the wires was a bit loose on the coil or if it came loose as the battery box was removed. I’ll keep looking tomorrow to properly check this out. Thanks to @ A_and_C for making me look at the relay more carefully. I want to examine every part of the circuit to be sure so the next step it to check the resistance of the master switch and earthing everywhere.
0.2V drop is excellent, if the starter current really flows through those contacts.
Definitely worth undoing and cleaning any screw terminals. It could be a dirty battery terminal. Or even a bad connection inside the battery… I am told this has been known to happen with some battery types which had lead terminals (more normally on aircraft batts they would be brass). But then I would hope the starting current ought to expose that.
I may be mis-understanding the problem, but the master relay is not the same as the starter relay, plus it doesn’t follow whether the starter current flows through the master relay. It is aircraft dependent.
