When Do I Need To Consider Couplers Causing A Short Between Locos?

[peahrens]

I've been merrily converting some DC locos lately and yesterday noted that some of my Kadee couplers on hand have plastic shanks.  That triggered a memory cell about the possibility of shorts (via all metal couplers) between consisted locos.  It made me realize that I don't know when this possibility occurs, and I need to know when I might need to be sure to install plastic shank couplers, as I usually just add a Kadee #5 or whisker equivalent if that fits ok. 

The only info I could find was a 2014 thread that explained the issue can occur with older Athearn BB locos, as the frame is hot to one rail.  

http://cs.trains.com/mrr/f/744/t/230072.aspx

 

I am not intending to convert to DCC any that old, and most of the locos I have been converting are DCC ready Proto 2000, Kato, Intermountain, etc.  I understand that to mean the motor is isolated from the frame.  But I don't know if the frame might have some polarity (isolated from the motor) that can cause shorting between loco frames via metal couplers.  Or does the frame float by itself in space, isolated from all electronics (rail power, motor leads, lighting common polarity, etc) such that it's a non-issue for the DCC ready locos?  If a possible concern, is there a continuity test I should be doing between the frame and whatever, to tell when I need to use a plastic shank coupler?

Any clarification is much appreciated! 

[hobo9941]

Or the simple solution is to run two locos elephant style, so both frames are shorted to the same rail. But if you run them back to back, then you will get the short.

[RR_Mel]

 

I have a dozen Athearn Blue Box PAs and SD40-2 frames with E7 Shells.  When I couple of pair “A”s back to back I swap the trucks on the trailing “A” front to back.  That keeps the frames the same polarity.  All of my Athearn PAs and PBs are DC only and swapping the trucks reverses the motor so you need to reverse the wires to the motor on a trailing “A”.   I have always isolated the motors in my Athearn Blue Box locomotives and soldered wires direct to the trucks.  I never liked the Athearn spring steel power connection so I removed it before I ever run them on the rails.

 

Most of my Athearns are A&B paired for passenger service, I only have to reverse the motor wires in a trailing “A”, I keep the A&Bs paired with the frames on the same rail.  Because I only uncouple my paired diesels for maintenance I use solid brass bars between locomotives to prevent accidental uncoupling.   

 

 

 

This is an Athearn Blue Box SD40-2

 

 

I brightened up this shot so you could see the black wire connected to the frame.

 

This mod works very good, it stops erratic operation from the steel power pickup.

 

 

Mel

 

 

Modeling the SP in HO scale since 1951

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

[peahrens]

What about non-Atheran BB, DCC ready locos?  I direct wire the trucks to the decoder, the motor to the decoder, and the lights to the decoder.  So does the frame get any charge that might vary between like wired loco frames in consist or is it isolated from all the electrical goings on so no problem will occur. Or are the frames neutral?

[JoeinPA]

I'm currently remotoring an Athearn RTR DCC ready GP 35 and the negative pickup is via the frame through the truck bolster as on the blue box locos.

Joe

[gmpullman]

peahrens

What about non-Atheran BB, DCC ready locos?

I recall some of the early Proto 2000 locomotives, maybe the Alco FAs, perhaps the P-1000 C-Liners and Eriebuilts and some of the geeps had "Hot" frames.

The plastic shank Kadees will do the job of insulating the frames from shorting unless you want to go through the bother of replacing the trucks with ones that have the bronxe axel-tip pickups that are found on the newer Life-Likes and have pretty much become the standard practice for other manufacturers as well.

I never really trusted the bolster contact point for power pickup. Too many places for the current to be interrupted. Doing the installs I recall having to drill and tap a hole to make a binding point to wire the other rail pickup to the decoder.

You can always place the locomotive (s) on a length of track and use your meter to check that the frame is isolated from the rails.

Good Luck, Ed

[peahrens]

gmpullman

You can always place the locomotive (s) on a length of track and use your meter to check that the frame is isolated from the rails.

Ed, this is what I was looking for.  I think I'll check this way when uncertain.

[modelmaker51]

I just put a KD #5 in a KD draft gear box then mount it to the loco. This isolates the coupler and voila no shorts.

[modelmaker51]

In that case I would either grind down the pad or use an overset coupler.

[gmpullman]

Hi, Paul

Any time I am doing a decoder install on one of the older locomotives, weather it is an Athearn Blue Box or an early Life-Like, AHM or even brass I always do some poking around with my ohm meter.

Start with one probe on the left rail and with the shell off, start by poking the other probe on each of the motor leads, the frame itself, the wires coming up from the trucks (if any) then try to get between the bolster and the wheels.

Then do the same with the right rail. Then poke around from wheel-to-wheel to see if any of the axel ends are touching. If you have two of the same locomotives, place both of them on the rails (just a short, isolated piece of track) and see if you have continuity between both the frames. If so, you will need to make sure current can not pass from one frame to the other.

With observation you can "map out" how the current is collected from the rails and eventually gets to the motor so that when it comes to installing your decoder you have a better understanding of what to wire where.

Your meter is your friend! I like to use one that has an audio beep for continuity so I don't have to see the meter when I'm checking circuits.

Good Luck, Ed

[Anonymous]

peahrens

 

gmpullman

You can always place the locomotive (s) on a length of track and use your meter to check that the frame is isolated from the rails.

 

 

Ed, this is what I was looking for.  I think I'll check this way when uncertain.

You can also put the locomotive on the track with kadee #5/#148/other metal coupler and perform a continuity check to see if coulper is shorted to the rail.

This thread made me go back and check my Athearn RTRs for this short.