MD80 drivers:Oil Pressure Light

[md-100]

MD80:
Does anybody really know how the 35/40 warning oil pres light (yellow& red bulbs) on flat panels work when engines are shut down? When do they turn on and when do ONLY the left side turn off???

thanks

[Wodrick]

They are driven from different sources. the 35 light is driven by an oil pressure switch and should be accompanied by L(R) Ol Pressure Low on the EOAP.
The 40 light is driven by a signal derived from an oil pressure transmitter, the computation being carried out in the System Display Panel.
According to my copy of the schematics the left light is the 35 light and I can't see how that can come on without the 40 light except under fault conditions.
Unfortunately It's been a long time since I saw this type so can't help much further.

[md-100]

but when the mad dog is at the gate, no engine, only APU gen, the left 35&40 bulbs are off while the right 35&40 bulbs are on and..... if you turn on the AUX HYD PUMP, then the left 35&40 bulbs turn on.. and after about 5 minutes later the go to sleep again.. WHY???

[F4F]

This "bulb saving feature" was implemented as the bulbs of the SDP are not replaceable
The panel shuts down the power to save on the oil P and oil T warning bulbs when the related side of the hydraulic system pressure drops <200 psi for 5 minutes or more.
Of course the L or R hydraulic system can depressurize before one another. By switching the aux hyd pump to on you increase the hyd P in the R system, thereby activating the oil P and T bulbs. If then you switch the transfer pump to on, the same will happen to the left bulbs.

Hope everything is clear now

[md-100]

thanks, but... only the left side bulbs go to sleep when you turn the right or left hyd sys on, not the related side.. so the right side bulbs are always on.. why??

[F4F]

Probably my explanation was not clear enough, so here I go again

the left side bulbs go to sleep when you turn the right or left hyd sys on, not the related side

As a matter of fact, its the opposite that is happening; the bulbs go off (asleep) 5 minutes after the hyd press in the respective system is <200 PSI. By switching the aux pump, transfer pump or starting an engine you will increase the pressure in the R, or the R and L hyd system, thereby the lights will come on!

so the right side bulbs are always on.. why??

The right side bulbs are not always on. They obey to the same rules and should extinguish after 5 minutes if the hyd pressure is <200 PSI. If they don't extinguish the reasons may be:
A) a fault in the 200 PSI sensor
B) residual pressure in the RH hyd system
C) windmilling RH engine causing residual hyd press
D) other undescribed fault (don't have the time nor the desire to delve into the wiring diagram, plus it would cost a lot of $ )

Have fun!