Master Relay or Solenoid, Battery contactor

How to keep the Cessna 170 flying and airworthy.

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Bruce Fenstermacher
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Master Relay or Solenoid, Battery contactor

Post by Bruce Fenstermacher »

Just wanted to tie some information together and update it.

Regarding the battery contactor better known as the master relay or solenoid. Cessna used all these names depending on what document you’re looking at and we will use all those names interchangeably.

The master solenoid is a continuous duty relay. It should not be confused with the very similar but different starter relay some aircraft have. The master relay is meant for continuous duty, on for long periods of time, and the starter relay is for intermittent duty.

I could not find a part number in the Illustrated Parts Manual (IPC), and believe none is listed, for the '48 Cessna 170. Curious there is no part number listed for the Cessna 140 IPC either. However it would be a good bet it is the same as the 170A and B. The part number for the 170A is 041121-1 and the same in the 170B IPC with a note there to use 0712603-1 when 0411021-1 is exhausted.

In 1965 a Cessna Service Letter (SL), SL 65-89 was released allowing for all Cessna single engine airplanes to use battery contactor S1579-1. It should be noted the SL says all single engine Cessna’s to date but does not specifically list the 170 models but starts the list at the Cessna 150. This is typical.

In 2013 Cessna released Service Letter (SEL), SEL 24-02 to provide parts and instructions for replacement of obsolete 12-volt and 24-volt contactors with Lamar
contactors. In our case this would be Lamar Technology’s and Cessna part P52-0034-1

In summary, the Cessna part numbers for the master relay are, in order of supersession: 0411021-1, 0712603-1, S1579-1, S1579-2, S1579A2, 111-140D and finally P52-0034-1. In addition Sky Tec offers a FAA PMAd replacement part STS-M12 that is a direct replacement for all of the previously listed Cessna part numbers.

Letss talk about other master relays available, which look exactly like the relays Cessna sells. Such parts might come from your Ford or NAPA dealer or you can buy Aircraft Spruce #11-03161, Master Relay 111-226 - Battery Solenoid (12 Volt) and many people have because the price is very attractive. In the case of this Spruce part, at one time, it was advertised as being a part for home built aircraft but used on many certified aircraft. Today this product is advertised as specifically not for use on certified aircraft.

In addition Spruce also currently sells a relay made by Lamar, Spruce Part # 07-01875. Spruce says this is an “Aviation grade solenoid for 12 volt applications in continuous duty, fitting current GA production aircraft as well as legacy aircraft”. However if you dig into the Q&A for the product it says, “the Lamar 12V solenoid (07-01875) is not FAA PMA approved for installation with a log book entry only. It can be installed in a certified aircraft with a 337 field approval.” So this Spruce Lamar solenoid is no different than any other without a Cessna part number or PMA for the same.

The hurdle to overcome with all alternate parts such as those discussed in the last two paragraphs is legality. Many, but not all, will consider these parts legal using AC 23-27 PARTS AND MATERIALS SUBSTITUTION FOR VINTAGE AIRCRAFT as guidance. There may be other FARs used to justify the installation as well. It really depends on whether the glasses you, or more likely your mechanic, are looking through, are opaque or rose colored. The intent of this article is not to argue one way or another in this regard.

Here are the current prices for solenoids mentioned split in two categories:

Legality is gray and must be confirmed by the aircraft owner:

Advanced Auto- Part No. S603, $41
Aircraft Spruce- #11-03161, Master Relay 111-226 $30
Aircraft Spruce- (Lamar) # 07-01875, $66

Clearer legality through superseded part numbers and service letters:

Aircraft Spruce Part # 06-00692 Cessna #S1579A2 (Lamar P52-0034-1 pictured) $150
CessnaParts.com- Cessna # P52-0034-1, $124
Yingling Aviation- Cessna #P52-0034-1, $115
Air Power- Cessna P52-0034-1, $113
Sky Tec- STS-M12, FAA PMAd equivalent P52-0034-1, $79
Aircraft Spruce- Sky Tec STS-M12, Spruce #07-03564, $70

Clearly it pays to talk to your mechanic and shop around.

SL 65-89 Allows all single engine Cessna's to use battery contactor (master relay or solenoid) S1579-1
SEL 24-02 Allows all single engine Cessna's to use Lamar Tech. contactors Cessna part# P52-0034-1

Besides this one here are other threads on the subject.

http://www.cessna170.org/forums/viewtop ... =24&t=6788
http://www.cessna170.org/forums/viewtop ... f=24&t=660
http://www.cessna170.org/forums/viewtop ... =24&t=5550
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n2582d
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Re: Master Relay or Solenoid, Battery contactor

Post by n2582d »

Nice synopsis Bruce. I did find that the C-140 used CR2792-D116A2 master solenoid. Fig. 25-25 in the 120/140 IPC says, "Not available as spare part see R-57" KRN does have one of the CR2792-D116A2 solenoids available. Who knows if it's a match for the 170. It seems that some of the early part numbers in these Cessna manuals were vendor part numbers. Only in the later manuals did Cessna assign their own part numbers.
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Re: Master Relay or Solenoid, Battery contactor

Post by GAHorn »

The Sky Tec document to use in support of their STS-M12 is actually a HARTZELL document:

http://hartzell.aero/pma/sts-m12/
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Re: Master Relay or Solenoid, Battery contactor

Post by Bruce Fenstermacher »

Gary, KRN is calling that a regulator. Wonder if they know what they have?
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Re: Master Relay or Solenoid, Battery contactor

Post by Bruce Fenstermacher »

gahorn wrote:The Sky Tec document to use in support of their STS-M12 is actually a HARTZELL document:

http://hartzell.aero/pma/sts-m12/
Sky Tec is owned by Hartzell Technologies
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Re: Master Relay or Solenoid, Battery contactor

Post by n2582d »

Bruce Fenstermacher wrote:Gary, KRN is calling that a regulator. Wonder if they know what they have?
Probably not. I see that it is listed as "As Removed" so it's not NOS. Here's a picture of what was on my battery box -- no p/n to be found anywhere.
IMG_0319.JPG
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Re: Master Relay or Solenoid, Battery contactor

Post by nippaero »

The master solenoid is a continuous duty relay. It should not be confused with the very similar but different starter relay some aircraft have. The master relay is meant for continuous duty, on for long periods of time, and the starter relay is for intermittent duty.
Isn't one grounded to the S terminal to activate and the other uses positive + to the S terminal to activate? Or vice versa, I can't recall.


I just bought a SkyTech Solenoid for my 170. Is looks IDENTICAL to the $23 "experimental" one I used on my RV7.
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Re: Master Relay or Solenoid, Battery contactor

Post by GAHorn »

It probably IS identical. The addt'l cost for approval paperwork makes up the difference.
Yes, Harzell owns SkyTec...and that is why you must go to Hartzell to find the approval basis was my only point.

I got an email recently about this very subject, in which my reading of the email led me to believe the non-member was looking for a cheap alternative to the very expensive solution some IA was requiring of him, so I suggested he consider AC23-27 and an automotive $23 unit. After several back-and-forths he finally clarified that he was searching for the PMA'd unit. (He actually used the phrase "more correct" or "most correct"). Searching for that PMA documentation is what led me to Hartzell's website as it did not exist at SkyTec.
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Re: Master Relay or Solenoid, Battery contactor

Post by 170C »

Hartzell now owns Sky-Tec (starters) and Air Power (alternators & voltage regulators) and I believe those two entities are now based in Alabama.
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Re: Master Relay or Solenoid, Battery contactor

Post by MoonlightVFR »

My master relay has worked perfectly for decades.

Have always been puzzled though

Appears to be a transparent cover protecting the contact assembly. The puzzle is fact that it is safety wired together.
attaches next to battery case.

Did Cessna ever have a clear cover on master relay?

Unorthodox closure just tells me that some AP was inside the box many decades ago.

It isn't broke.
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Re: Master Relay or Solenoid, Battery contactor

Post by cessna170bdriver »

My master contactor had a clear(ish) cover on it when I first got the airplane. Sometime between 1982 and 2010 it got misplaced...
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Re: Master Relay or Solenoid, Battery contactor

Post by gfeher »

The cover of the master contactor/relay (original - box shaped) of my B model is also clear-ish, and removable.

When i bought my plane a couple of years ago, there was no indication in the logs that the master contactor was ever replaced and it appeared that it, along with the battery and starter cables and ground strap were original and needed to be replaced (along with the fact that there was no diode in the system). Also, the master contactor was pretty grimy. So I figured I would replace it when I replaced the cables and installed the diode. Well, when I removed the master contactor and cleaned the outside, I saw that the formerly black cover was transparent and removeable. When I removed the cover, the insides were clean as a whistle, no corrosion, and the contacts looked like they were new. Everyone in the shop,was amazed at the condition of it. It went right back on my plane, where it is now. I have a lot of respect for that little unit.
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Re: Master Relay or Solenoid, Battery contactor

Post by DaveF »

Same here, Gene! My master contactor is the original with the translucent cover. It had a slight problem a few years ago and I found that it was clean and nice inside. Even the contact points were in perfect condition. It's easily disassembled, repaired, and reassembled, so that's what I did. The $100 replacement I had overnighted is still sitting unused on the shelf.
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Re: Master Relay or Solenoid, Battery contactor

Post by gfeher »

I put the SkyTec replacement I had bought to good use. About a month after cleaning up and reInstalling my original master contactor as I described above, a guy at the airport who owns a C-150 was about to leave for his private checkride in the morning. If he didn't have enough stress at the time, he went to start his plane and no electrical power. My IA friend quickly determined that his master relay was shot. Panic of course. On a hunch, I checked the eligibility list of the SkyTec replacement I had bought a month earlier and lo and behold his C-150 was on it. We had his plane ready to go in less than a half hour. I'm pleased to say that he made it to his check ride and passed it.
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'52 170B N2315D s/n 20467 C-145-2
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Re: Master Relay or Solenoid, Battery contactor

Post by bagarre »

DaveF wrote:Same here, Gene! My master contactor is the original with the translucent cover. It had a slight problem a few years ago and I found that it was clean and nice inside. Even the contact points were in perfect condition. It's easily disassembled, repaired, and reassembled, so that's what I did. The $100 replacement I had overnighted is still sitting unused on the shelf.
Does anyone know of a source for parts? Mine is in great shape but one of the threaded main posts is half stripped.
FWIW the cover on mine is clear plastic with a sprung wire clip to hold it on.


Also, do we know what these relays are rated for? The new motor has a 65 amp alternator. Everything turned on wont draw half of that but I'd rather not risk over loading the old thing.
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