A problem going on two years now!

[CPbuff]

To make a short story long here's the problem:

I run an intermountain F7A and F7B together pulling 8 passenger cars. Each equipped with Digitrax DH163IP decoders. (Plug-ins).

The problem is that some days (not always) the F7A bucks or stops completely sometimes. Never on the same portion of track and never the F7B. Yes! The wheels have been cleaned and all the wiring checked and rechecked. I thought I had it solved when I ran a volt meter hooked up between the A and B to their couplers and found .3volts moving between units. So to solve this I one again ripped both units apart checked for grounding issues etc. So to solve the problem I changed out the metal couplers to plastic ones so the voltage would not have any other path. Both units seem to work well for the past 3 months and now the F7A is back to it's old games again.

I add insult to injury I was able to obtain a new decoder and a new F7A from the good folks at Digitrax and Intermountain (on warranty).  I put them all together and hooked the F7A up to the F7B and guess what happened again. So I swapped the decoders between the two units and again the same problem.

The units seem to run better without the passengers cars (no load) but the F7A still develops this problem. When I separate the the A& B or just run the F7A it doesn't happen as often.

My only thought left is that because both units are running the same decoder make and style that the signatures used during the development of both the decoders are so closely matched that one decoder doesn't know if the command received (ie. to increase speed) is for itself or the other decoder causing decoder confusion or signal interuption.

I think my next step is to try a different decoder manufacturer and see if that solves the problem by replacing the decoder in one unit.

I am really embarassed as I have an electronics background and I am STUMPED!

However the good news is I still have my hair!  For now!

Any thoughts would be greatly appreciated!

 

 

 

 

 

[selector]

Does each unit have its own decoder?  If so, could thermal shut-down be the problem in the A unit?

If not, is there a connector between the two?  You have changed the A unit, but not the B, and you still get the same behaviour.  I would suggest that there might be a problem with the B side of things, maybe in the connector?    I am just trying to guess....don't know if it is of any use to you.

-Crandell

[retsignalmtr]

you've cleaned the wheels, checked the wiring and replaced the decoders to no avail. i don't think it is because they are using the same model of decoder. i have run 12 locos mu'ed with digitrax dn163ao's with no problems. do the lights go off when the loco stops? does it happen when it is being run on dc? since you are using a plug in decoder it could be something with the locos original circuit board and not the decoder.

[Texas Zepher]

CPbuff

the F7A bucks or stops completely sometimes. Never on the same portion of track and never the F7B.

Since you seem to have eliminated all other options, I would be interested if it makes a difference if the F7A is run backwards?  What happens when the B unit is run backwords.  What happens when the B unit is run on the point?  Maybe there is something in the mechanism that doesn't like to be pushed (by the B).   Are the two units running really close to the same speed?  Could it be the A unit is running just enough faster that it is trying to drag the B unit and overheating?

Have you considered physically MUing them together.  That is connect the power between the units so both units get power picked up from all 16 wheels?   It is still troubling that the stray 0.3V was not able to be isolated.   That would indicate to me that somehow one unit is 0.3V power depraved as opposed to its running mate.

My only thought left is that because both units are running the same decoder make and style that the signatures used during the development of both the decoders are so closely matched that one decoder doesn't know if the command received (ie. to increase speed) is for itself or the other decoder causing decoder confusion or signal interuption.

While not out of the realm of possability that they are talking via stray radio frequencies, I would tend to discount this theory. I've run many multiple units with the same brands of decoders and never had this sort of thing happen.   The fact that the "new" decoder behaves the same way also seems to point away from this.  Two decoders that had the same amount of RF leakage?  I doubt it.

[Hamltnblue]

Have you run each engine alone?? If not give it a try.

[CPbuff]

Thanks for all the replies and suggestions. In response:

Yes!. It has been run by itself and it seems to work better than when it's under load but still has the problem intermittently.

Each unit does have it's own decoder and in my testing trials I have run both with the shells off. Also this some times has happened 10 feet down the track from a cold start. There is no electrical connection between the two just the coupler.

Yes. The lights go on and off without a problem after the loco stops dead!  I only run DCC. I tend to agree the loco's original circuit board could be the problem with a compatability issue with this type of decoder so I think my next step is to try a diffrent make!

The units are so closely matched that I can run the A and B around the 18' by 12' oval and they maintain a 1" separation.

I have never thought of running the A in reverse with the B so I will try all combinations and see if that works. I will then try a different make decoder in the A and see if that works and if not I will try MUing them together.

Thanks to all for your suggestions and assistance!

 

[cacole]

 Something else to check, which I have experienced as a problem with Broadway Limited, Athearn, and Precision Craft locomotives:

Don't use Kadee metal couplers on both units -- they can short out through the couplers.  Put a plastic coupler on either one of the two locomotives and see if that solves the problem.

 

[mikebo]

 Have you tried turning off BEMF?  It can cause locos to act erratic when in a consist, espeecially under a significant load.

[CPbuff]

Yes! I have changed both couplers to plastic as I found .3v running between the units when separated but still the problem!

[CPbuff]

I will check as I haven't got that far into experimenting with the CV's until I have this problem solved. This problem also happens after resetting the decoder to factory default. CV =08

[BRVRR]

You didn't mention which DCC system you are using, though I don't think it makes too much difference.

I have a Digitrax Zephyr and the few times I have had glitches similar to those you describe, I have the problem loco selected on more than one throttle. I have MRC Railpower 1300s attached to the jump ports on the Zephyr and a DT400.

To cure the problem when it occurs, I dispatch the loco from one of the throttles.

In a couple of extreme cases I have reset the Zephyr using Option Switch 36 which clears the memory of all addresses and consist information.

When dispatching doesn't work, for whatever reason, resetting Option Switch 36 has always cured the problem for me.

I don't know about other systems, but most must have a similar "reset" procedure.

Hope this helps, Good luck.

[CPbuff]

Thanks. I am running  MRC but as I only have a small layout I am running only one throttle. I generally clear the unneeded or unused addresses as I go, so I don't think that is the problem. All my other consists and locomotives all work fine and do not have even a hint of this problem.