For the Carrizo Gorge Railway Inc. Am try to convert (2) EMD F7A original equipment wire to Dynamic Brake via Field Loop (old) on a second set between F7A. To do is you put potential (new) on 24T of 27 MU cable.
This require a LRC Relay and redoing the Brake regulate system.
I was to follow the GP9 but I need the print in detail.
I have the GP9 print but I don't see how it work.
This will let the F7A work F7A,GP9,ect.
50 year Field Loop by EMD is a long time.

Sorry, I'm not an electrician and cannot answer your question, but I have one also. When the dynamic brakes were still in use on the GP9s we had (before I worked there), they had an extra heavy cable between the units for the dynamic brake. Did F units equipped with dynamics have these also?

From about 1965 all Loco had just 1 (27 MU) not 2. Even the GP9 aft shop had (1) not (2).
The problem with Field Loop is you could not go beyond (4) Loco by EMD
With Potential Control that GE mfr for ALCO via 24T could go 9-10 Loco
Look in <www.railpictures.com> for Tehachapi,CA for BNSF to San Bernardino,CA.
It don't make a different what type of loco follow it the lead as long the HP on the Grid on the roof don't get to hot.
The two system are not compared and will not work together.

Use <www.railpictures.net> for BNSF San Bernardino.

Huh ?

Yes, the additional 2 wire jumper on earlier EMD production is the field loop jumper. The function of the jumper is to connect the motor fields
of the locomotives in series. By varying the current passing through
the 'loop' (which was done via the DB rheostat in the control stand), the amount of braking effort could be controlled.

FL control involved the manipulation of a setup switch....

However, all of this begs the question-why would you want to convert a locomotive back to an inferior and outdated control system ?

I want to remove the second set of MU with F.L. and replace with Potential Control (LRC) (new) a 24T of 27 MU cable.

While I'm not strictly an 'EMD man', I suspect that there is more to adding potential control dynamic braking than just adding a relay. I'm sure that one would need some type of transductor to sense braking current , as well as a closed loop control system AND a new braking rheostat in the control stand. If the unit in question has the drum type master controller, you will be out of luck on upgrades.

To the best of my knowledge, the way this type of upgrade is accomplished out in the 'world' is to use EMD Dash-2 components.

So, I would suggest that you contact either of the following: Elcon in Minooka, Il or RSI in Louisiania. Elcon is the OEM for many control components as used by EMD and RSI is the shortline distributor of EMD parts. They may be able to set you up with a kit to do as you wish to do.

OLG