Let me summarize, to see if I understand the situation correctly. I apologize for the length of this post, but I don't know how to correctly explain the situation briefly.

(1) You have hooked the trailer up to another tow vehicle, and all of the lights on the trailer work properly in that situation.

(2) You have used an ohmmeter to test what you believe is the ground pin on the plug on your tow vehicle, and it has what you believe to be a good ground.

(3) You have turned on the lights on the tow vehicle, and are getting a good 12-volt reading on the pin of the plug on the tow vehicle that supplies power to the trailer's tail lights, license plate light, and side marker lights.

(4) You have not been able to get a reading on the other two pins for left and right turn signal lights, when those lights are turned on in the tow vehicle.

Here is what I suspect is happening:

If you're using a 4-pin connector, there is NO GROUND WIRE hooked up through that connector. The four pins supply the following power:

(a) Power to taillights, license light, and side lights.

(b) Power to left turn signal light.

(c) Power to right turn signal light.

(d) Power to back up (reverse) lights.

Naturally, during emergency flasher operation, power is intermittently applied to both of the turn signal pins simultaneously. Similarly, during braking operation, power is constantly applied to both of the turn signal pins simultaneously as long as the brake is depressed. A 4-pin connector, unlike a 7-pin connector, DOES NOT provide for connection of a ground wire. The system depends on metal-to-metal contact between the two vehicles, usually through the tow bar or an external grounding strap.

A 7-pin connector, in addition to the above 4-wires connected in a 4-pin connector, provides for the following:

(e) Ground connection.

(f) Power to electric trailer brakes.

(g) Power to recharge an emergency break-away battery on the trailer.

So how are you getting a ground reading on one of the pins of your connector? I believe that you are not reading a true ground, but are rather reading a low-resistance path to ground through the light bulbs or other appliances in your tow vehicle.

In order to insure that you are corectly measuring the voltage for the turn signals and the backup lights, make sure that the negative (black) lead from your voltmeter is connected to the FRAME of the tow vehicle. Then connect the positive (red) lead of the voltmeter to each of the pins, in turn, on the towing connector on your tow vehicle.

Here's a tip: If you use an analog voltmeter (with a display needle), it will react fast enough for you to see the fluctuating voltage of turn signals or emergency flashers. However, if you are using as digital voltmeter (with a numerical display), the meter circuitry may not react fast enough for you to get a proper voltage reading. If that is the case, get a friend to step on the brakes while you test the two turn signal pins.