wanttaja@ssc-vax.UUCP (Ronald J Wanttaja) (03/31/85)
A while back, I posted a problem to this net, concerning the habit of my '65 C-150 (the last of the straighttails) to turn the power back on after the master was turned off. I got a lot of good suggestions from netters regarding the problem, which turned out to be water getting into the master power solenoid and shorting out the contacts. Now the solenoid has failed completely, and I'm into a bit of a parts problem. I went to a local aircraft parts place to find a replacement, and the guy looked it up in his Cessna parts book, and gave a price of $14.95. As I had paid $100 for one to replace the original last summer, I was a bit shocked. I gave him the parts number off the original one, and he said, "Oh, yeah, the parts book lists a second type of solenoid... it's refered to as 'solenoid w/diode,' and it costs $190!" Gulp. $175 for a diode seems a bit steep. Both solenoids were listed as good numbers for 150Es. Maybe the $190 one is meant for the military version of the 150 (the famed F-150). :-) Anyone have any ideas why that diode might be necessary? From cursory examination of the old diode, it's across the coil, not the main contacts. I've heard of putting a reversed biased diode across the coil to protect IC circuitry (preventing transients into the driving chips caused by the collapsing field when the coil is turned off), but can't see why it would have been necessary in this case- the plane was originally delivered with a Mk IV tube-using "coffeegrinder." The only other possibility would be to protect the coil, somehow. Assuming the replacement solenoid I put on last summer didn't have the diode, that might explain why if failed aft er 30 hours. I have ordered the cheaper solenoid, and, depending how my thought processes go, may install a power diode externally. I've been told I don't need an STC, as the change is not on a flight-critical item, and I have a friendly A&P willing to sign it off. Any thoughts on this option? Send mail only, please, and I'll keep y'all posted... Ron Wanttaja (ssc-vax!wanttaja) After all, who needs an electrical system... I haven't been to a controlled airport for eight years, and I can always teach the wife how to hand-prop ... :-)