"Backup relay" p/n and operation check?

**flagg** · 30th September 2012, 12:16 PM

Hi all,

I've got no charge out of my alt since redoing my loom in my 86 110. I've found that there is no voltage at any of the 3 wires to the alt - ignition, battery sense or alt light.

Tracing back through the loom, I've ended up at what is described in the wiring diag as a "backup relay", pic below:

This is the only relay I didn't replace as I didn't know a part num or how to check its operation..

There are two brown wires, and a slate wire that are live with the ignition off or on. When I start the engine, the slate on turns off, and the green on is live.

According to the diag I have, slate is the oil pressure, and green is reverse light. Black is ignition feed to the alt - but there is no indication if it is to or from the alt if that makes sense.

**flagg** · 1st October 2012, 07:18 PM

Ended up bypassing it as I needed to remove the regulator from the equation anyway.

Just need to sort the oil pressure warning light now.

**steveG** · 2nd October 2012, 09:47 AM

I ignored the standard oil pressure switch, as from memory its operation is reverse of normal pressure switches (ie its open circuit with no pressure) and needed the relay to work - which I didn't have.
I fitted a generic oil pressure switch to the tapping in the main oil gallery below the oil cooler, and connected it directly to the light. I think it was 1/8NPT thread but its been a while so can't be sure.

Steve

**flagg** · 2nd October 2012, 05:10 PM

Thanks- that's good to know. I was thinking I would fit a gauge and not worry about the light.. that way Ill be able to measure high pressure too in the cold winters we have here.

**steveG** · 2nd October 2012, 09:12 PM

Originally Posted by flagg

Thanks- that's good to know. I was thinking I would fit a gauge and not worry about the light.. that way Ill be able to measure high pressure too in the cold winters we have here.

I'd be more worried about missing the gauge was reading low due to a major oil leak than about the possibility of too high a pressure. Run the correct oil for your conditions and you wont have to worry about pressure being too high.
If you really want to put in a gauge, then fit a tee piece and run a switch for the warning light (or hook it up to some sort of audible alert too).

Steve

**flagg** · 3rd October 2012, 06:15 AM

Good point. (The high pressure thing comes from flying GA.. Lycoming use oil that almost solidifies over night.)

**steveG** · 3rd October 2012, 11:05 AM

Originally Posted by flagg

Good point. (The high pressure thing comes from flying GA.. Lycoming use oil that almost solidifies over night.)

Yes, I know the stuff. Pretty sure its straight 50 grade mineral oil Aeroshell 100 or OM270 etc and goes like treacle when cold.
Not an issue when using the correct multi-grade oil in our engines.

Steve

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