Ground power plug wiring issue

I installed a Piper type ground power plug on the plane. I followed the wiring sample noted in Marc Ausman's Aircraft Wiring Guide. The ground power example is on page 51 if you have the guide. Here is the problem Im having:

I wired it exactly as in the example. With the battery installed, the ground power contractor is being activated and there is power from the battery to the ground power plug (no ground power applied). I thought maybe the diode was backwards, but when I swap it the other way, the fuse blows. The terminal (for the master switch wire) on the master contactor has power flowing to it. There is a wire that runs from that terminal to a terminal on the ground power contractor. I called the Stein guys and was told the master contactor small terminal will show a current on a tester, but it becomes grounded when the master switch is activated.

They way I belive the syestem in suppose to work is the power from the ground power plug activates the ground power contactor, which allows the current to flow to the battery and starter. When no ground power is applied, no power should be flowing back through the ground power contactor to the plug.

Hope all this make sense. Any ideas what I have wrong. THANK YOU>

I always just wire it directly to the battery, no contactor.

This is how certified planes do it with an extra contactor (aka relay) -



The 12 volts on the exteral power cable switches on the extra contactor.

+ 1 here too.

Jesse said:

I always just wire it directly to the battery, no contactor.

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We put the Cessna type in and ran the ground side to the engine mount in our case and the pos. to the battery side of the master relay. Our battery is on the firewall per Van's plans and parts so it was just two short #2 leads down to the mount and relay. We mounted the plug to the top of the engine mount just behind the oil filler door so that all we have to do is just open the door and plug in. If you place it behind the rear edge of the opening it is not in your way to check oil and do a good preflight. It also does not go through the cowling and you don't have any extra work in taking the cowl off.
Just what we did and works very good for ground power with a 10 Amp charger for those long hangar flying trips and keeping the battery charged if you wish, along with that ramp jump you will need some day.
Yours, R.E.A. III #80888

Looking at the drawing, thanks Gill, here is a question. Does it matter which post the power comes in on the contact. The drawing has the battery on the left post and the external power on the right post. I have it wired opposite also, is the arrow into the bar a fuse or a diode for the contact terminal power.

Jesse, that is definitly the KIS (keep it simple) way. Only concern, dont you have power constant to the external power port?

The diode prevents connecting an external battery with the polarity reversed. Reversed polarity can destroy expensive avionics. It has happened.

The external power relay can only be energized by an external battery (via the diode). The external power relay can NOT be energized by the aircraft battery.

I do not think that it matters if current flows from left to right or from right to left through the contactor.

togaflyer said:

Looking at the drawing, thanks Gill, here is a question. Does it matter which post the power comes in on the contact. The drawing has the battery on the left post and the external power on the right post. I have it wired opposite also, is the arrow into the bar a fuse or a diode for the contact terminal power.

Jesse, that is definitly the KIS (keep it simple) way. Only concern, dont you have power constant to the external power port?

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If you are using the standard aircraft part you should keep the wiring/pinouts standard too.

One (+) pin is larger than the other -



As Joe said, the current can flow either way through the contactor, and since the contactor shown has two isolated coil terminals it will be OK. A three terminal contactor won't work here.

I followed the Bob Nuckols way and it has worked great.

Here is the contactor:


Plug connects to right terminal, Battery contactor to left. I have separate cable from battery contactor to battery. Terminal with no wire in picture runs to switch on panel and even has light to show its connected.



I will look for wiring diagram next time I'm at the hangar.

Scott, do you have a switch to activate the ground power solenoid. The diagram I worked off of shows a power wire coming from the plug. Like the diagram Gil posted. I have a piper style plug, but its wired alike. Also wondering why are your solenoids inverted. Should they be, I did not. Could my problem be the two post solenoid. I bought a generic one. Are they all the same?

togaflyer said:

.... I bought a generic one. Are they all the same?

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Check with an ohm meter.

There should be no continuity from the coil terminals to either of the main power terminals or to the case of the contactor..

Thanks for all the help. Figured the problem out. I have it wired the way a Cessna plug would be, but off a piper style plug. I checked my old Piper manual and realized it is wire differently. Pipers take the ground power directly to the starter solenoid, bypassing the master solenoid. I had both Ground power and the solenoid power coming off the positive end of the plug. So once power was applied to the Ground power solenoid it keeps the circuit closed. I will need to install a switch on the terminal ground wire in order to deactivate the solenoid. Hence why there is the 2nd positive wire on the Cessna ground power plug. Well, just another sad story in the life of an experimental plane builder

Piper External Power

Rich,
My Piper plug (center) is wired to a contactor (master solenoid type) along with a connection to one terminal of the its coil. The other terminal of the coil is wired to a diode(pos) then to the master switch. If the master is on
power is applied to the battery and the reset of the circuit. If the center pin
has a negative voltage on it the circuit will not pull in the external solenoid.
External battery wired backwards, safety feature. The external plug does not go directly to the starter solenoid, it can't, all is wired together from master.

If a happy lineguy comes out with his Tug and he thinks all aircraft are 24Volts then you are in trouble because this circuit will not check for that. Saw the results of this. A blown 12V battery.

What I'm saying is don't just hookup your battery to the piper plug directly and don't have a separate switch. In the rare event 5 years from now you won't remember to activate it.

And, Cessna uses a separate pin for 12V to activate where Piper uses the same power pin with a diode.

my two scents.....John