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If oil Pressure is allowed to peg for 1 minute on cold Days ... (I've never seen it) Whats the problem with the HYD/OIL light going out prior to 25% N2?

When the oil is cold and thick, (or the jet hasn't flown in a while) isn't the oil pressure going to be higher @ at the lower RPM range as well? and turn out the light?

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It indicates a suspect bad Main Oil Pressure (MOP) transmitter. Below that RPM the engine's oil pump shouldn't be making the required oil pressure.

The oil is only that cold/thick at very low temperatures, and even then shouldn't affect the 25% RPM start rule... I believe the thinking is that even with thick oil the pump wouldn't be able to push enough oil to make the HYD/OIL light go out.

At least there is no provision for it in the technical data...

Your logic is true though for cold days. I've never seen pressure that high either.

Enthusiast Joined: May 09, 2006 Posts: 21 Status: Offline

It actually indicates a bad Low Oil Pressure Switch (little cannon plug that connects on the right hand side). With RPM's under 25% there is enough hydraulic pressure to close the Low Hydraulic Pressure Switches (in the wheel wells), however the oil pressure is not high enough to close the the Low Oil Pressure Switch. So if the light goes out before 25% it more than likely means the switch is bad (I've seen the wiring be a major cause of the problems too)

It is also tied into the two hydraulic low pressure switches on A & B system. They will also cause this problem. More than likely from my experience it's been the cannon plug on the engine low oild pressure switch.

One other thing to check is the 30 second oil pressure timing module.

tbolt2 wrote:

More than likely from my experience it's been the cannon plug on the engine low oild pressure switch.

CCAF

(I've seen the wiring be a major cause of the problems too)

That was the problem, wires on the cannon plug, we changed pressure switch on red ball and light still went out early @ 15% ... I'm assuming with damaged wires and an open on the oil pressure switch cancels that portion of the 3 switch circuit. I know hydraulics will build at a relatively low RPM and cancels the light.

Good catch by the pilots.

CCAF wrote:

It actually indicates a bad Low Oil Pressure Switch (little cannon plug that connects on the right hand side). (I've seen the wiring be a major cause of the problems too)

You're right... Sorry guys... Doh

On the test-cell that oil switch isn't installed, but a temperature probe/direct pressure tap is installed. This is where you compare the MOPT reading to the direct pressure gauge to be sure they jive. (On start if they don't you have a bad MOPT)

The MOPT is what drives the Low/High Pressure fault that will pop a NO-GO BIT through the Engine Diagnostic Unit. It is also what drives the cockpit gauge. (The switch/module/light and MOPT/EDU/gauge are on seperate circuits.)

Sorry for the mix up. Embarassed

I've troubleshot this problem more times than I can count and I've never seen the 30 second timing module anywhere in the tree. I don't think it's involved on start-up.

tbolt2 wrote:

I've troubleshot this problem more times than I can count and I've never seen the 30 second timing module anywhere in the tree. I don't think it's involved on start-up.

More like if you're not getting a light at all.... then it could be the module.
Opposite of the light being on too early.
(I only know of 1 or 2 being changed, mostly during the ops check of the module.)

After the switch detects low oil pressure, the signal is sent to the 30 second timing module. After 30 seconds, the HYD/OIL light and master caution will illuminate.

The PW engines will run 30 seconds at MIL with 0 oil pressure without ANY damage to the engine. (Repeatably!) This was one of the original USAF specifications for the Viper engine during the design stage. This is one of the reasons PW engines are used by the Thunderbirds. They are scheduled to start using Block 52s w/PW-229s in the near future. Twisted Evil

FYI - Watch the Thunderbirds (PW-220 engine) and count how many seconds their aircraft are ever inverted. It's always less than 30 seconds! Cool

Last edited by Mushmouth on Oct 19, 2007 - 08:04 AM; edited 1 time in total

I never liked the fact that the engine switch and the A and B switch tied into the same light. Speaking on GE's but still the same concept when tied to the airframe, you always had to swap the switch (or light assembly depending your effectivity code). After that, move to the wheel wells. The initial wiring on the A and B switches are pretty crappy though. Our electricians were always replacing the wires on that cannon plug with better ones. And if you replace once side, might as well hit up the other side.

Mush

Enthusiast Joined: Aug 24, 2007 Posts: 86 Status: Offline

Once again, brain fart from the alzheimers' setting in... Where was the MOPT, anyway? Could tell ya on a CF-6, but ain't looked in *this* bay fer awhile.

Now, if I could just find my dentures and cane, I can go out and finish this service check...

oldjay wrote:

Where was the MOPT, anyway?

Right-Hand side of the engine, just above and behind the aft-most chip detector.

The MPOT is on a bracket mounted on the RH upper side of the gearbox, (between the oil tank and fuel pump inlet) and the Oil Pressure Switch is mounted in the same general area except more forward on an oil pressure line.

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