Master Switch for a 70A Alternator??
- Thread starter KJC
- Start date
Sean007mi5
Registered User
I do not see a link for your PA-18 diagram that you just posted.
Thanks for your help.
Thanks for your help.
Sean007mi5
Registered User
OK I have my LAMAR Master solenoid all installed. No diode is installed and I found the case hot to the touch after say 45 min flight. How hot do these things get?
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How hot do these things get?
quite hot if it is the wrong one. a Master rely should have about 3 -5 ohms minimum across the coil leads. a starter relay will typically be 1 or less. the master needs to be a continuous duty type relay, the start relay is intermittent, so it can be a higher draw, which creates a larger pull in force (to minimize burning the contacts... remember, the master is already closed when the starter engages, so the is no arcing there). if your "master" is so hot that it hurts to touch it, it is the wrong type, and probably wired wrong as well. as for the diode.... there are 2 reasons to install a diode.... spike suppression and reverse polarity protection. neither are required by regulation (on main power, but yes on ground power). so how hot does it get?
quite hot if it is the wrong one. a Master rely should have about 3 -5 ohms minimum across the coil leads. a starter relay will typically be 1 or less. the master needs to be a continuous duty type relay, the start relay is intermittent, so it can be a higher draw, which creates a larger pull in force (to minimize burning the contacts... remember, the master is already closed when the starter engages, so the is no arcing there). if your "master" is so hot that it hurts to touch it, it is the wrong type, and probably wired wrong as well. as for the diode.... there are 2 reasons to install a diode.... spike suppression and reverse polarity protection. neither are required by regulation (on main power, but yes on ground power). so how hot does it get?
one more thing... does it have 2 big poles and one small one, or does it have 2 big and 2 small? if it has 2 small pins, they should read at least 2 ohms between them with no wires connected (to those 2 poles). if it reads lower, it is probably a starter type relay (intermittent duty), if it is 1 small pole and 2 big poles, then one big pole should read continuity to the small pole (about 2 ohms) if it is a master relay. if the small pole reads continuity to the case it is a start relay
Sean007mi5
Registered User
I installed Aircraftspruce.solenoidx610028.php
Part # 07-01875
Its the continious duty with 4 poles. The reason I have no diode installed is that I fried the one I bought from Aircraft spruce part 16050-2
I installed it the wrong way as the ring ends sizes was setup for a starter 3 pole I presume.
Anyway could no diode cause this much heat. How hot? You could not hold your hand on the bottom if the case for say more than 5 secs. Seems way to hot to me. Could that be from induction turning it off? I will do an Ohm check like you suggested.
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Part # 07-01875
Its the continious duty with 4 poles. The reason I have no diode installed is that I fried the one I bought from Aircraft spruce part 16050-2
I installed it the wrong way as the ring ends sizes was setup for a starter 3 pole I presume.
Anyway could no diode cause this much heat. How hot? You could not hold your hand on the bottom if the case for say more than 5 secs. Seems way to hot to me. Could that be from induction turning it off? I will do an Ohm check like you suggested.
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I suspect it is the wrong one... a master should never get that hot. problem is the relays look exactly alike (the 3 pin ones look alike too, but are wired different inside). I looked at the spec for your relay and it is supposed to be 14.5 ohms on the coil & draw .8A (some Cessna relays are as low as 5-6 ohms). anyway, the diode you got is for the 3 pin style relays.... the big ring is the negative side of the diode so it is in reverse bias across the coil (the 3 pin relay shares one end of the coil with the natter terminal, inside.... the small pin is the other end and goes to ground at the master switch). Anyway, it's part of an AD if you have a Bendix mag switch with the starter built into it. not required if you have a push button start or a different brand switch. if you need to install a diode, it goes across the 2 little terminals & in reverse (not conducting)... you can buy one at a local electrics store & crimp on rings (yes, it's legal, FAR 21.9... long story)
Sean007mi5
Registered User
I put a new Diode on with the Cathode on the correct side, just like you described. That works well, thanks. The 'How hot' test revealed that under load with no Alt running, with Avionics on for say 15 Min it heats up to a point that you can keep your fingers on it, but it's uncomfortable. That is the scientific measurement for 'Gets Hot'. I will get a IR temp gauge next time. 
I have installed ACS P/N 07-01875 14.5 Ohm .8 Amp (as you say above) Lamar P/N X61-0028. ACS sells one other, P/N 07-01877 4.2 Ohm 2.9 Amp. Lamar P/N X61-0030.
Do you suggest I use the latter, X61-0030 with lower Ohm which should run cooler? On on second thought maybe not since W = A x V compare < 12 Watts to 34 Watts.
Also do you think I should put a new Starter Solenoid in while I am busy.
I have installed ACS P/N 07-01875 14.5 Ohm .8 Amp (as you say above) Lamar P/N X61-0028. ACS sells one other, P/N 07-01877 4.2 Ohm 2.9 Amp. Lamar P/N X61-0030.
Do you suggest I use the latter, X61-0030 with lower Ohm which should run cooler? On on second thought maybe not since W = A x V compare < 12 Watts to 34 Watts.
Also do you think I should put a new Starter Solenoid in while I am busy.
Part # 11-03162 I expect the one that is in there is from 1960 something. (has no Diode)
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ya, you want the higher ohm one for the master. did you measure the ohms across the 2 small terminals to double check? remember, there is no load when the master closes, so there is no arcing on the big contact points. the start relay on the other hand has to carry 3-400 amps of inrush current as soon as the contacts close... that's why it is low ohm. that pulls a lot of current & gives a strong pull, but makes it intermittent duty because of the heat (it can melt itself if on too long). not a bad idea to replace it too, but you can go on condition. and yep. I had a brain wave of some kind.... the AD I mentioned requires a diode on the start relay, not the master. on the other hand, many master relays have them as spike suppression. they are also used on master relays as protection to prevent reverse polarity (battery installed backwards) and as a feed when to engage the master relay via ground power (when the battery is dead). if you turn off your radios before engaging/disengaging the master then you don't really need one at all. and... modern radios have better self protection these days anyway. if you have a Bendix mag switch with the start function you need to diode the start relay
