GP7 Electrical


Notes on electrical system:

Like many USAT diesels that have been in production a long time, there are 2 versions of the electrics. It's usually easy to tell early from late, the early versions have flat square smoke units, a heater only, no fan. Later units have a fan-driven smoke unit, you can see the fan motor, and in addition, there are boards the smoke units plug into, that supply the regulated voltage for the smoke units. In the fan driven ones, when there are 2 smoke units, often one "power supply" is on the main board or the chassis, and the second "power supply" board is attached to the roof the the body.

The unit in this article is a later type, and thus has more LEDs, where in the early units they were all incandescent bulbs EXCEPT the "classification" lights.

The GP7, like virtually all USAT diesels, has bicolor leds for "classification" lights that are red in reverse, and green in forwards. They use a 300 ohm resistor, so it appears that they can draw 60 ma. I need to verify this. The wiring is similar to the F7, so check that section for more details on lighting. Normally they run from track power.

The front and rear boards in the nose have the typical USAT weirdness:

1. the headlights appear to be track voltage, up to 18v or so, fine for DCC conversion

2. the screw in bulbs on the circuit board are 12 volts, both in parallel, judging by my testing and the 300 ohm resistor and appear to be in series with the LEDs below,

3. the leds for the classification lighs are 3 pin bicolor leds, with a common cathode.

note, when working on the board, it is wise to remove the screw in bulbs, polish up the end contact (rub on your jeans until shiny) and put some diaelectric grease in the socket... if these unscrew or vibrate loose, the classification lights are affected. They are hard to remove, try using a paper towel with a spritz of brake cleaner, so you have good friction twisting the bulb. Don't over tighten either, I apply power to see when the light goes on when inserting.

Some modifications:

I have a friend that needed to have his loco changed from battery to track powered DCC. It already had a Phoenix 2K2, so I used an NCE D408. While doing this, I wanted to change something that has always bothered me, the goofy red/green classification lights. Red was often used on the rear of a loco running with no train, but normally there was a red light in one of the headlight housings. So, not so bad, wrong location, but prototypical color. But GREEN in the forward direction for a classification light? That means that not only is the train an extra, but there's at least one more section behind this train. Well, that was not uncommon with steam trains, since they would split a long train into two or more sections because one loco could not handle it, but for diesel? All you do is add more motive power.

Another reason to replace the stock LEDs is that they are 3 terminal common cathod (negative) LEDs, which makes it practically impossible to drive them from DCC, which uses positive as common.

I found some small surface mount LEDs that are red/white. This are also bicolor, in that there are only 2 terminals. I have detailed the connections and setup in the NCE page:

The picture below shows the front 2 classification lights held in place with a bit of black silicon sealant. The lights are wired with fine gauge magnet wire.

Weather Underground PWS KCACARLS78