Problems on downhill grade

[Traintown]

I got my second P2K GP38-2 this morning and installed the couplers and broke it in. I hooked the two locos (They are idenical twins) together and they pull great on flat ground and going up my 2.5% grade. Now my problem...going down hill the locos will speed up then slow down to nearly a dead stop then pick up speed then slow down again. This happens until the locos reach the bottom of the incline.
I am running DC in HO scale pulling 22 freight cars, Going down hill the voltage is running about 7,5 volts and when the locos slow the voltage drops to about 5 volts.
I know that it is not in the track, I have a metal Model Power F7 that does not do it at all. Pulling one of the P2K's will do it some but not as bad as when I double head them. I am running an Athearn power pack that came a train set, it has been a good power pack. It seems as though the cars are trying to push the locos until the locos slow down the cars then will speed up again.
Any suggestions as to what is causing it to do this and what to do to correct the problem? Thanks, Mike

[Cjcrescent]

Given the differences in the voltages you describe, it sounds like the power pack.

Is there anyway you can put another power pack on the track and see if it does the same thing?

[Cjcrescent]

Also meant to add, it could also be "gear slop" and "motor slop" caused by too much play in the mechanism. This can be easily corrected by the use of some thrust washers placed on the gears. If you've ever worked on an Athearn mech, you can work on a P2K mech.

[jbaakko]

I'd say drop the DC! I also think it sounds like a problem of power coming from the controller.

[SP4439]

I would also agree that it is the power pack I have a friend that has a layout with at the steepest a 3.5 precent grade and two P2K GP38-2's haul about the same size maybe more without the surging downhill they operate like champs no gear or motor slop so check the power supply.

[CBCNSfan]

I am running an Athearn power pack that came a train set,

Hi Mike, I agree with the above members, these trainset power packs really have little control of the current in the event of varying loads (power requirements). Basically they are composed of a transformer two rectifiers to change the 18Volts AC to a DC for the engines. The speed control is just a rheostat (variable resistor) to set your speed. When the train requires more power like on a grade more voltage will be dropped across the resistor and that much less power for the engine. It's a little more in depth than that but thats the general idea. If you are going to operate on grades or double headed then you should get a decent throttle, solid state circuitry preferred.
Cheers

[Larry]

I agree w/all of the above about the powerpack. When U run the cheap engines w/only a pickup on 1 end of the loco it doesn't take much power to operate a loco. When you get into the better quality engines you need a quality powerpack. I use #9500 MRC digital w/amp & volt meter. I also have other throttles for diff. main lines. I never have trouble w/any power surges.
I can run 6 to 8 engines & 50 to 75 cars w/my controllers w/out any problems. Have U checked for dirty track, loose joints, dirt on wheels of engines, etc.?
Larry

[kjd]

On my layout I had in college, I had a grade that was about 80 feet long and maxed out about 3%. I ran Athearn locos exclusively because I couldn't afford anything else. Anyway, I had several locos that surged like described. I used a Hogger Blue and a MRC 1400 and then Digitrax DCC. I never got it to go away totally but removing nearly all the play in the worm gear with NWSL washers made it better.

Paul