Thorp Air Command - T18.net :: View topic - What is it and where can I get another one (that works)

Hi Tim,
It definitely looks like a single pole double throw (spdt) switch. One terminal is common, one is normally open (when pressure is below the set point), & the other is closed. They may be marked "c", "no" & "nc", or 1,2 & 3, etc. A switch designed to activate the Hobbs meter will probably have too low of a set-point. If it doesn't have a small external adjustment screw the pressure setting is probably fixed. If it has a screw you might want to try to adjust it. My only question is why you are using three wires.

---The electrical system in the airplane is more complex that I am and mystifying I'm embarrassed to say. There is no contiguity between any of the three poles (master off), nor is there 12V between any of the poles and ground (master on engine off). There is a rudimentary electrical diagram but it provides no guidance for this.---

If the third wire is actually being used & terminates somewhere, you should be able to see what it's doing.

---Tracing wires in this rather shall we say.....rats nest.... is a near impossibility...I have been delaying getting too deep into this since it will ground the airplane for some time---

It may or may not be doing something useful. These type switches are readily available, from automotive suppliers also - check online. Just make sure to get a quality one with a sufficiently high pressure burst rating & long cycle life, designed for an oil pressure sensing application. Also, make sure that the threaded fitting on the end has a very tiny hole. As mentioned, a failure/leak on this major oil galley could be very bad.

Author:	Fly [ Thu Sep 25, 2014 6:06 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
Tim, Looks like a Hobbs pressure switch. Theymake quite a few different models. Take a look at this catalog on page 25 you should be able to find what you are looking for in terms of required pressure to open or close the circuit. Once you find the part number it should be easy to find one for about $20 online. http://www.davidsonsales.com/docs_pdf/S ... atalog.pdf That being said, you may want to consider remotely mounting the transducer. Should that transducer mechanically fail due to vibration you'd have a heck of an oil leak for a minute or two... Marshall

Author:	Rich Brazell [ Mon Sep 29, 2014 11:20 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
Are there any ID marks stamped on it ? Manuf. model # ,etc. ?

Author:	SHIPCHIEF [ Tue Sep 30, 2014 9:32 am ]
Post subject:	Re: What is it and where can I get another one (that works)
That looks like a Catarpillar diesel pressure switch. Here is a link to Skygeek for a Hobbs pressure switch that operates @ 15psi. You can learn about these switches and the different operating pressures they offer. One of the descriptions states they are adjustable as well. http://www.skygeek.com/honeywell-hobbs- ... 5-psi.html The question I have for you: What pressure should turn off/on the light? Lycoming engines run a fairly high pressure while in flight, so under 40 PSI might be a good warning, but it will be 'on' when you reduce RPM on final approach or idling after landing. 15 PSI shouldn't turn on the light at idle, but would be too low for flight? When would a low pressure warning help you the most?

Author:	jrevens [ Tue Sep 30, 2014 2:41 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
Hi Tim, It definitely looks like a single pole double throw (spdt) switch. One terminal is common, one is normally open (when pressure is below the set point), & the other is closed. They may be marked "c", "no" & "nc", or 1,2 & 3, etc. A switch designed to activate the Hobbs meter will probably have too low of a set-point. If it doesn't have a small external adjustment screw the pressure setting is probably fixed. If it has a screw you might want to try to adjust it. My only question is why you are using three wires. If the third wire is actually being used & terminates somewhere, you should be able to see what it's doing. It may or may not be doing something useful. These type switches are readily available, from automotive suppliers also - check online. Just make sure to get a quality one with a sufficiently high pressure burst rating & long cycle life, designed for an oil pressure sensing application. Also, make sure that the threaded fitting on the end has a very tiny hole. As mentioned, a failure/leak on this major oil galley could be very bad.

Author:	Tim Brown [ Wed Oct 01, 2014 7:17 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
jrevens wrote: Hi Tim, It definitely looks like a single pole double throw (spdt) switch. One terminal is common, one is normally open (when pressure is below the set point), & the other is closed. They may be marked "c", "no" & "nc", or 1,2 & 3, etc. A switch designed to activate the Hobbs meter will probably have too low of a set-point. If it doesn't have a small external adjustment screw the pressure setting is probably fixed. If it has a screw you might want to try to adjust it. My only question is why you are using three wires. ---The electrical system in the airplane is more complex that I am and mystifying I'm embarrassed to say. There is no contiguity between any of the three poles (master off), nor is there 12V between any of the poles and ground (master on engine off). There is a rudimentary electrical diagram but it provides no guidance for this.--- If the third wire is actually being used & terminates somewhere, you should be able to see what it's doing. ---Tracing wires in this rather shall we say.....rats nest.... is a near impossibility...I have been delaying getting too deep into this since it will ground the airplane for some time--- It may or may not be doing something useful. These type switches are readily available, from automotive suppliers also - check online. Just make sure to get a quality one with a sufficiently high pressure burst rating & long cycle life, designed for an oil pressure sensing application. Also, make sure that the threaded fitting on the end has a very tiny hole. As mentioned, a failure/leak on this major oil galley could be very bad. --- I will remote mount it as soon as the flying Wx is gone and that's too soon (lows are already near 0c ---

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What is it and where can I get another one (that works) http://t18.net/thorpforum/viewtopic.php?f=35&t=7664	Page 1 of 2

Author:	Rich Brazell [ Wed Oct 01, 2014 9:58 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
Take the bulb out .

Author:	jrevens [ Thu Oct 02, 2014 12:09 am ]
Post subject:	Re: What is it and where can I get another one (that works)
Rich's idea doesn't sound too bad, but I would want to know what's going on. I'm curious - has the light ever gone off? How did you Identify this "switch" as being associated with the light? If you disconnect the three wires & run the engine, what happens? Is the light on when the master is on but engine off? Do you have an oil pressure gauge and does it have a sensor somewhere else (port on accessory housing, back of engine), or is it a mechanical pressure gauge with a pressure line direct to the gauge? If there is no other sensor or line, there is a possibility that this is a pressure transducer for an oil pressure gauge, which might explain the lack of measured continuity. If this possible transducer isn't showing any voltage input it may be because it is not powered until the engine is actually running, perhaps by another pressure switch or? It may have nothing to do with the light directly. I love a good mystery... wish I was close by to take a look at it with you.

Author:	dan [ Thu Oct 02, 2014 7:04 am ]
Post subject:	Re: What is it and where can I get another one (that works)
Replace good bulb with bad bulb....Have some one do it whilst yer not looking? almost as good as the Idea Rich had ?.....Dan

Author:	Rich Brazell [ Thu Oct 02, 2014 11:22 am ]
Post subject:	Re: What is it and where can I get another one (that works)
Van's sells an oil pressure switch that runs the HOBBS meter AND a 15 psi idiot lite ? Oil pressure below 15 psi and the lite comes on . A little too low for me . Do we know if the switch is good ? BAD SWITCH , BAD SWITCH !

Author:	Tim Brown [ Thu Oct 02, 2014 8:19 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
Hi Rich... I pulled out the electrical diagram andi think it might be related to my DOA chronograph... The chronograph has been dead since day one... Coincidentally as long as the idiot light has been on. I'm beginning to thing the oil pressure switch is supposed to start the timer at the same time it turns off the idiot light. I'll try and post a wiring diagram!

Author:	Tim Brown [ Thu Oct 02, 2014 8:33 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
Humm.... Not working..... Kinda like that stupid light ;\ https://www.dropbox.com/s/ujhvxghrb8uju ... 2.jpg?dl=0

Author:	jrevens [ Thu Oct 02, 2014 8:39 pm ]
Post subject:	Re: What is it and where can I get another one (that works)
OK, I think we may be getting somewhere. You have a very common mechanical oil pressure/temperature gauge (used on a lot of older aircraft). No electrical sensors for that gauge... just a temperature sensing bulb that goes into your filter screen housing (normally) and a small diameter cap tube with a flare nut on the end for pressure. No connection with the switch or electrical system. Do you by any chance mean the small digital hour meter (Hobbs meter) when you talked about the chronograph? If so I think Rich hit it on the head, & it's probably a SPDT switch like we talked about in the beginning. When the pressure comes up it powers the Hobbs & shuts off the light. BOOM! Rich, you did it again! If the light goes out when you disconnect the wires, the switch from Vans is probably what you need. Be sure to mark the wires in relation to your switch so that you can reinstall them on a new switch easily. The "common" wire will be +12volts & the other 2 go to the light (N.C.) & to the Hobbs (N.O.).

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