From: (Denis M Arbeau)
My mechanic recently replaced the landing gear solenoid on another Swift when the owner reported that the gear wouldn't work and troubleshooting confirmed it was a bad solenoid. The owner came up with another (automotive) solenoid and they installed it. Ops check good. Now, however, after some use THIS solenoid has also failed. John took the solenoid apart and said it is rather cheaply made. He was wondering if you have advice (ie. part number and/or application) for a GOOD solenoid that they might buy. Let me know. Thanks! Denis

Aircraft Spruce has p/n 111-226 (continuous) and p/n 22735 (intermittent) relays. Either will work, although the intermittent (starter) relay should be better even if it does have a plastic case. Recently I needed a relay right now so I went to a local auto parts store and bought one, it was about twice the price! Aircraft Spruce gets about $15.00 -- Jim

From: John Tuuri <>
Monty-- I am installing the Merlyn STC for the hydraulic powerpack and can not find my copy of your comments relating to the replacement of the relay that were in one of the GTS Updates recently.. Could you suggest a replacement? Thanks....Jon Tuuri

Get a 111-226 ($16.95) in Aircraft Spruce. Contrary to what I may have said at one time you want a continuous duty solenoid. -- Jim

Subj: Solenoids?
From: Graham Gitlin <>
Monty, I have a quick question for you. I currently have Swift N78230, Charlie's first Swift, which has a Merlyn hydraulic motor. I have used an intermittent duty solenoid for the motor as previously recommended, but now understand that you are recommending a continuous duty solenoid. Could you please explain why the change?, and what are the significant differences between the intermittent duty and continuous duty solenoids? Thanks in advance.

The intermittent duty relay will get hot when engaged for a period of time. The continuous duty relay is wired so there won't be a buildup of heat even when engaged for many hours. (maybe a REAL electric wizard will jump in and explain this better!) The little green light circuit keeps a small load in the gear circuit continuously. I have always used continuous duty relays, but when I got thinking about it, I thought, why not use a starter type relay? I had forgotten about the continuous load. -- Jim

Subj: Landing gear motor fun
From: Ronald Williamson <>
Monty, Here's my analysis of the motor/solenoid mystery...
First the solenoid: The momentary rated solenoid is just fine for the gear motor relay replacement. I changed my original bakelite one when I did the first rebuild of 40K back in 1975 and put in a starter relay. It's still working fine. The difference between a continuous and intermittent is the enthusiasm with which they close. The continuous rated has a higher resistance coil which draws fewer amps when energized. It can thus stay energized and not make too much heat todamage itself. However, it closes and opens with less force because the reduced coil current also means less force to close the contacts. Once the contacts are closed, they can pass the same amount of current as the intermittent type. The intermittent contacts are made to break large starter motor currents and therefore has a lower resistance coil which makes more force to "snap" the main contacts open and closed. However, the higher coil current gets the solenoid hotter and intermittent types can self destruct if left on continuously. The gear motor is a small load to make and break as compared to a starter motor, so either type solenoid will work just fine.

Now for the fun. The Swift landing gear circuitry is detailed in Figure 32 on Page 77 of the old red "Operator's Handbook". The same circuit has been copied into the oft-mentioned manual produced by Mr. Commings. So, assuming Jon's Swift is wired "stock", then it can be used. The green light is NOT part of the solenoid circuit. The green light is only energized when EITHER gear is in the down position and the corresponding micro switch is closed. Thus, the circuit can be split and independent greens for each gear can be wired without affecting the operation of the gear. I think Dick Collins has that circuit detailed out somewhere. More importantly, the red gear light is not a gear UP indicator. It only indicates the gear is NOT DOWN (in 40K the red light is labeled "UNSAFE"). That is, if either down microswitch moves enough to "click" then the red light will be turned on. With either the stock or split green wiring, a red AND a green is a normal indication when the gear is transitioning. Since one gear always goes first, it will turn on the red light while the remaining gear is still green. When both gear move off the bottom microswitch, then only a red indication is seen. When the gear hits the top microswitch, the current to the coil is broken and the hydraulic motor shuts off. Thus, gear UP is only indicated by the yellow hydraulic pump light turning off.

How does the solenoid get energized? The center post of the gear switch on the backside of the operating handle is connected to the solenoid coil. The top post of the gear switch is connected to the red gear lamp through the resistor which limits the brightness to extend the bulb life - and make it less obtrusive during night ops) and to wire #26 on the diagram. The bottom post of the switch is connected to wire #27. The astute student will note that Up and Down labels on the switch in Figure 32 are reversed. That is, closing the top contact causes the gear to go down and vice versa. When the Swift is on the ground and the gear selector is in the down position, the red light circuit is connected to the coil of the gear solenoid. If either down microswitch opens, then the red light comes on which also powers the solenoid coil and runs the pump to force the gear against the down stop which then opens the down microswitch to turn off the solenoid and stop the motor. Conversely, when the gear switch is set to Up (only after suitable altitude and airspeed), then the solenoid is energized because that side of the circuit is energized when either of the Up microswitches is closed. When BOTH gear hit the top switch, then the coil current is interrupted. If either gear falls out of the well during flight due to misadjustment of the emergency cable or who knows what, then the Up microswitch closes, energizing the solenoid and running the motor until the gear is against the Up stop again. 40K used to do that at 20 minute intervals until I got the extension cable adjusted properly.

So, after all the above blather, what's going on with Jon's Swift? The hydraulic pump is clearly running because the flaps are operating. Figure 34 shows how that is possible, since the flap microswitches in the belly are used to switch the hydraulic motor current directly!!! Not a good idea in my opinion, but it's worked for many years. Out of a need for purity, I used a diode across the back of my flap switch to isolate the gear/flap circuits and 40K now uses the solenoid to start the motor for both gear and flaps. But, I digress. What this means is that the gear and flap circuits are essentially independent. If the motor runs, we know that the 30A breaker on the instrument panel is good and that the wiring to the yellow light and to the motor is correct. If the solenoid doesn't close regardless of the gear switch position, then it is possible that the solenoid isn't wired correctly. The solenoid coil has 2 posts. One post is connected to wire #24 which leads back to the center post of the gear selector switch. Continuity can be checked to verify that connection. Open the circuit breakers and turn the master off before checking with an ohmmeter. The other coil post connects to ground to complete the circuit when the gear switch applies the 12V. If the coil isn't grounded, the motor will never run regardless of the gear handle position. As far as the red/green indication, I'd guess a sticky microswitch in the wheel well. Since the lights are powered from the 5A circuit breaker, we know the breaker is working or we'd see no lights.

In summary:

1. Verify continuity between the solenoid coil and the center of the gear handle toggle switch behind the panel.

2. Verify the second coil post on the solenoid is solidly connected to ground. The mounting screw on the firewall is typically used.

3. Jack the Swift (with suitable precautions about sandbags on the horizontal) and verify the microswitches are free and operating.

4. With the gear down and the gear circuit breaker (5A) on, should have only a green light. Check the voltage at the bottom terminal of the gear selector toggle switch and verify it has 12V.

5. If all checks ok, then gear should retract normally.


Subj: Mystery solved
From: Ron Williamson <>
Turns out that Jon had purchased a "Master Switch" solenoid which has only 3 terminals. In that configuration, the coil power is internally connected to the BAT terminal and the small coil post is grounded to activate the solenoid. So, when he connected the small post to the gear switch, it provided +12V back to the selector switch. With the gear in the "Down" position, that illuminates the red light! The green is illuminated correctly because it's powered off the other contact of the down microswitches. Of course the solenoid coil will never energize because the gear switch also provides +12V so the coil sees a net of zero volts. Jon is going to get the isolated coil version (4 terminals), hook up the ground per the original configuration and he'll be good to go. I always enjoy it when there's a sensible explanation for an impossible situation. -- Ron

p.s. Please pass this info along to Jon if you can. This part will work for the pump motor. Grainger stocks this part so it should be easy to get. Around $22 list, but nobody pays list at Grainger.
Grainger Item No: 6C028
Mfg. Model No: 70-111224-5