Alternator conversion causes Master Switch fault ??

PlaneJane1 · March 2019

My 172-A got an alternator this week, but now the old generator light comes on, making the A&P (and PlanePower tech support) think that the Master Switch needs replacement. Master was working fine before the alternator went in. They have replaced some (original ?) wires through the firewall, and now all the amp readings are correct. What should that generator light do now that the alternator is in? I'm told it should just go on in the event of an alternator failure, but it faintly glows right after startup, goes away after increasing RPMs slightly, just like the generator, but quicker.

Cessna doesn't have any new Master Switches, so mechanic thinks he will get a Field Approval to put in a Cessna-supplied toggle switch, used after '65.

Would there be another switch be needed for alternator disconnection, in the event of an alternator failure? Do any older 172s have a 2-position Master? Sort of like the Landing/Taxi pull-and-pull-again switch? What's the rating on the Master, would the light switch handle a Master's load? I can get light switches, if they would work, or work out...??

Scott Sherer · March 2019

Hi Jane,
Would any of these three switches do it for you? Almost every 172 I've ever seen has a split switch; battery master and alternator. https://www.aircraftspruce.com/search/search.php?s=cessna+master+switch&x=0&y=0

PlaneJane1 · March 2019

Thanks for the suggestion, but it's coming into focus now. The Master Switch is a push-pull switch, used prior to 1964-65. It is NOT the same as the Landing/Taxi light switch. I am truly and officially in the realm of Vintage, but it may as well be antique - the company making the switches has been bought/sold a couple of times since then. Cessna is out of them, but last price was somewhere between $2-400!!
The good news: it was also used in older 150 and 170 Cessnas. The company now owning the product, Littelfuse, is still in business, and has a good name in the vintage/antique automotive segment. the International 170 Assn Forum has a very recent discussion about this very topic! It seems they are just getting old, rattling loose on the inside, and making intermittent contact. There are directions for repairing them. While still listed as for sale on the manufacturer's website, I cannot locate any distributors with one available (quick search only).

Now, since I'm not an A&P, my shop, MW Aviation of KLXT, is ready to get a Field Repair from the FAA. But we still need a part. He is searching the obvious: Wentworth, White Industries (local to us), and the big names like Yingling, etc. I am beating the bushes. He seems to want a Certified part to replace this, even with the Field Option available, and even if it's used. He has an alternate plan, for the Field Option, of using a newer Toggle switch, recommended to him by Cessna Parts Dept.
Here's a photo of what I'm looking for. Note the 4 terminals and how they face. DSPT makes it different from the landing light switch, more like a headlight switch on old cars.

Scott Sherer · March 2019

I've looked at 2,000 switches on eBay and I'm taking a break, lol. I'll search more tomorrow. No luck so far.

PlaneJane1 · March 2019

Well, I finally found two switches. One was available from a collector of International tractors, with the same configuration, but a black plastic knob (headlights?). Price was $28 delivered. The other, my mechanic located from Wentworth for $150 + shipping, with a 30-day warranty. The mechanic trumped my hand, he wanted the lower risk of a certified part. FAA wins another round...
The joy of owning a vintage aircraft!

Scott Sherer · March 2019

But the good news is that you found two! Maybe you should buy the $28.00 one and put it in your tool box

Alternator conversion causes Master Switch fault ??

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