Thorp Air Command - T18.net

I have an idiot light in addition to my oil pressure/temp gauge.

The light has been on indicating no oil pressure for years and it bugs the he-- out of me.

I put a gauge at that port and confirmed same oil pressure as reported by the cockpit unit.

I checked the switch (it must be an normally closed switch which opens with oil pressure and shuts of the idiot light) and there are no markings.

There are three electrical connections.

Anyone able to ID this... please

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

Are there any ID marks stamped on it ? Manuf. model # ,etc. ?

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?

_________________
Scott Emery
EAA Chapter 326
T-18 N18TE

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.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

--- I will remote mount it as soon as the flying Wx is gone and that's too soon (lows are already near 0c ---

Take the bulb out .

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.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Replace good bulb with bad bulb....Have some one do it whilst yer not looking? almost as good as the Idea Rich had ?.....Dan

Rich and Dan.... the light is dim-able with off being the selected at fully dim; that's where its been for years. Unfortunately I want it fixed and working correctly.

John...

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? NO IT IS ON CONTINUALLY (unless dimmed to zero)

How did you Identify this "switch" as being associated with the light? I GUESS I HAVEN'T A CLUE WHAT IT IS OTHER THAN ITS A PRESSURE OPERATED SWITCH AND I HAVE A LIGHT THAT IS ON AND TRIGGERED BY A SWITCH... MAYBE I'M WRONG!

If you disconnect the three wires & run the engine, what happens? I HAVEN'T DONE THAT... WILL DO THAT TODAY.

Is the light on when the master is on but engine off? YES.

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? AT THE RISK OF RIDICULE.... I'M NOT SURE... HERE IS A PICTURE OF THE GAUGE.. THE OFFENDING LIGHT IS ABOVE THE 1 ON THE TRANSPONDER... AND THAT'S 21 INCHES AND 2100 RPM FOR 130 KTS TAS

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. THANKS FOR THE DETECTIVE WORK... I AM STILL LEARNING.

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 !

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!

Humm.... Not working..... Kinda like that stupid light ;\
https://www.dropbox.com/s/ujhvxghrb8uju ... 2.jpg?dl=0

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.).

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

The chronograph is a separate unit... Davtron m800..

Wrong attachment...