The International Cessna^® 170 Association

Looking for some help with more electrical troubleshooting. I have a standard master switch driven contactor with a pull starter setup. Lately it seems like my battery contactor is a little weak. I replaced my master switch at the last annual because of this issue, but it must not have been the switch. What happens is I turn on the master switch and I see power go to a couple instruments and then when I pull the starter the starter does nothing and the master bus is no longer powered. I have always been able to get it to start by cycling the master switch a time or two. Am I right thinking that it is he contactor that is acting up? Or could it be a weak battery? The battery voltage before I pull the starter is not all 12.4-12.6V.

Any help would be appreciated.

Thanks,
Doug

Doug,

Sounds to me like the contacts in the solenoid/relay are crusty/burnt. If you have the original square plastic covered solenoid, you would be able to remove the cover and file/clean the contacts. Likely you have the metal can style solenoid and for all the best depression era farm tractor mechanic, they are not serviceable.

What I think is happening is the contacts are making contact but only by a few hairs. Enough to run the low amp gauges and lights. But when the starter is engaged the higher amp load burns the fuzz making contact and the electrons stop flowing till the next time you cycle the relay. You could confirm this with a multimeter. With the master on voltage would be flowing to all the low load stuff including the starter motor switch. But pull in that starter switch and the fuzz burns and no more voltage getting through the master relay.

To be certain of your description…. when you say you replaced the “master switch” ….I understand you replaced the switch in the panel that provides a “ground” to your battery master contactor/solenoid. And that had no effect.

You also are saying that energizing your pull-starter seems to remove power from all items… but that “recycling” your cockpit switch sometimes seems to “cure” the problem and the starter will rotate the engine….(which then presumeably starts to run, the battery is re-charged, and everything works properly.)

You are asking if there is a simple method to determine what is causing this problem…and your hypothesis is that the battery master contactor/solenoid is not passing sufficient electricity to the starter unless you re-cycle the cockpit master switch…… the question being “is there a simple test to see if your hypothesis is correct?”

Am I correctly understanding your question?

If I am…. then there is a simple method to by-pass your battery master contactor/solenoid…. and that is (staying CLEAR OF THE PROP….. use ordinary automobile jumper-cables and connect the large terminal on one side of your battery contactor/solenoid to its opposite large terminal. You have now completely by-passed that battery contactor/solenoid …. the main electrical “buss” is active….and pulling your starter should activate the starter.

Replacing that battery master contactor/solenoid is not expensive. Depending upon your comfort-zone…at Spruce you can source the part for as little as $32 for an automotive equivalent … https://www.aircraftspruce.com/catalog/ ... kkey=10293
. or as much as $80 for one that claims to be better: https://www.aircraftspruce.com/catalog/ ... kkey=10293

So, for $32 you can simply and temporarily replace your present contactor/solenoid with one that will prove or disprove your theory (instead of using automotive jumper cables)….and if that works for your test…. you can “forget” to remove the $32 unit and probably never have to deal with it again.

OR…you can buy the $80 unit, install it and achieve the identical happy results.

Why puzzle any further about your questionable contactor/solenoid when a solution is so inexpensive and be done with it?

PS: if you do this yourself…. please DISCONNECT your battery ground-strap while working on the solenoid. Recoonect your battery ground strap after the new solenoid is installed.

Thanks guys, it sounds like you both understood my situation. I will replace the battery solenoid soon. I just didn't want to blindly replace another part like I did the master switch. Sometimes I just need to bounce it off someone else and I don't think my wife is that interested I'll go bug my A&P friend now with a plan of action.

I appreciate the links, George.

Obviously you should FIRST check that the battery-to-solenoid and solenoid-to-pull-starter cables/terminals are “brighty and tighty”. (A poor terminal connection can behave exactly as you have described….the problem connection at-first carrying the small amount of current necessary to “light up” the master buss…but fail under the heavy demands of the starter….. but that heavy demand creating “arcing” which can jump-thru looseness/corrosion sufficiently to work just fine on a subsequent re-activation of the solenoid. This is exactly the same scenario at the terminals to which Bruce was describing as “fuzz” inside the solenoid…… same-o/same-o.) And don’t forget to inspect/brighten/tighten the battery “ground” connection as well as the one that connects the engine-to-airframe.

Keep us appraised of the outcome so we can all learn.