Gear Warning System question

[CL605]

I'm restoring a 1977 M20J that I have never flown. While conducting a gear retraction, I expected to have to depress the bypass switch to get the gear to come up. I also expected to hear the warning horn until the switch was depressed. Instead it came up immediately and without sounding the horn. I'm guessing that the airspeed safety switch is a normally open circuit, as well as the bypass switch? If that is the case, then if either switch is failed closed, will that allow the gear to operate without the horn sounding?

Thanks in advance for any help!

[KSMooniac]

You should have a squat switch on the left MLG, not the airspeed switch, unless it was added later. My 77 retracts just as you describe, but if the throttle is mostly closed then I'll get an aural warning due to the microswitch on the throttle. If the throttle is more than ~ 1/4 open then no beep.

Sent from my motorola edge plus 2023 using Tapatalk

[CL605]

It has the airspeed safety switch. I assume that replaces the squat switch? And even with the throttle at idle, no warning horn.

[EricJ]

If you have the bypass switch (red button) you should have the airspeed switch, and the gear should not come up unless the bypass switch is pressed.   The airspeed switch replaced the squat switch.  It suggests that the airspeed switch is faulty.    That's worth fixing, since it's the main thing preventing inadvertent retraction of the gear on the ground.

 

[CL605]

Thanks, that is helpful. Am I correct in thinking that it is a normally open switch and it closes with airspeed, completing a circuit to operate the gear? 

[EricJ]

19 hours ago, CL605 said:

Thanks, that is helpful. Am I correct in thinking that it is a normally open switch and it closes with airspeed, completing a circuit to operate the gear? 

I think they're actually dual-pole, but wired to the normally-open side for retraction.   The airspeed switch is wired in parallel with the override switch so that if either is closed the gear will come up.

Volume 2 of the M20J SMM has the wiring diagrams, and the second diagram for serial nos 24-0238 through 24-0377 shows the circuit as it should be wired with the airspeed switch and the override button.    SBM20-196a covers the conversion from the squat switch to the airspeed switch but the circuit diagrams aren't very helpful for understanding the functionality.

[CL605]

Thanks for the info, I'll post when I get a chance to do a bit more diagnostics..