water pump problem - please help diagnose -

[patsdad]

85 Minnie Winnie. Problem : water pump works when on 110 ( either plugged in or off the gennie ), but not on 12v. It still runs on 12v, but fails to prime ( or something ), and never gets pressure in the system.  On 110, it runs for perhaps 1 minute, and then gets pressure, and the pressure switch shuts it off, and so it's fine.  On 12v, it runs and runs, but never gets pressure in the system.
I don't know if it's likely to be a bad pump impeller, or more likely some kind of air-in-the-lines problem.  I also don't know if they sell impellers seperately, or if you have to buy the pump and motor as one unit.  The systen holds pressure fine - no spigots leak, and the pump doesn't cycle once you've got pressure in it. 
I don't think it's the electric motor, because you can hear it running at a normal clip, and then ( on 110 ) you can hear the motor load up when it gets pressure in the system, right before it turns off.    I have run it for several minutes at a time without achieving pressure, and it that's 'dry' running, I guess it's possible I damage the impeller.  The manual said it wouldn't hurt it, which is why I wasn't paranoid about shutting it off at the first sign of trouble.

I'll be dry camping this weekend, and hope to have water, and any help is much appreciated.  Any advice on the most logical way to proceed , given the tight time frame ( and not unlimited budget ) ?

 

[King]

It sounds like you have an air leak into the suction side of the pump.  Either that or you're pulling the water a long way from the tank.
Art

 

[Carl L]

With a 1985 unit, I am just guessing.  If it were a later unit I would say, check your battery for charge and ability to hold a charge.  The water pump in common with a lot of other accessories runs off the battery so it can operate when boondocking.  When you are on 120VAC, your converter is carrying the load.  Off it, the battery is.  Since your water pump works OK on 120VAC and not off it, I would suspect that first.

 

[Gary RV_Wizard]

Since it works on shore power but not on battery power, you have an electrical problem rather than a pump problem.

The pump is 12V powered regardless of the Rvs power source. When on 110 shore power, your converter is providing solid 12V power, actually more like 13.6 volts in most RVs. Without shore power, you are relying on the batery to provide the 12V and it sounds as though it is not up to the job.  A battery in good condition provides 12.6V, whereas a poor one may be in the 11.5-12.0V range, especially with a load appplied.  The pump will struggle to reach pressure if the voltage is too low and will keep on running. Most pumps are factory set to pump until they reach about 48 psi and they will keep running as long as the pressure is below that.

So check your battery voltage, with some load applied to it (e.g. several lights on or the pump running) and no 110V power in the RV - it should be in the 12.3-12.6 range with the load applied and a minimum of 12.6 without it.  Or, bring your car along side and run jumper cables from the car battery to the Rv battery and see if that solves the problem. I fit does, buy a new battery.

 

[John From Detroit]

Don't like to disagree with you Romer, but I'm thinking it's a pump problem  I will explain why

When running on batteries it's being fed perhaps 12-13 volts, it runs a bit slower than when running on the converter which outputs 13-14 volts.

He specifically said it runs, but does not build up pressure so it runs and runs and runs and runs.

Possible problems are 1: Dirty or corroded connections further lowering voltage (Take measurements at both battery and pump when it is running, no more than a single volt drop and I'd like less)

2: Worn pump that just needs that extra "Little bit" to get going

 

[patsdad]

Thanks for the input, guys.  The speed of the pump sounds about the same whether on battery or shore power.  Since the only thing that's been touched since when it was working right is the ( really cheap and cheesy ) drain valve for the water tank, I think I'll start there, and make sure that's not leaking air, and also clean the motor connections, and with luck, one of those will give me some results.

The coach battery is new, and making correct voltage, so that's not it.  I really appreciate the help. Hope you all had a good holiday.

 

[Gary RV_Wizard]

The only difference in pump operation between shore power and battery power is the line voltage, so it stands to reason the problem is somehow related to that. An air leak or other plumbing problem would be the same in either situation, so I doubt if that's it.
Corroded contacts, a weak pump motor, etc. are all electrical-reasons why the pump may not be able to build up enough pressure to cauuase the pressure sensing switch to shut off.

That's assuming somebody didn't just happen to open the tank fill valve a the same time you switched from shore power to battery.  An open fill valve is the #1 cause of the pump running continuously and it's a mistake that  every Rvers makes from time to time.

 

[patsdad]

Thanks Roamer. I got it working, and the culprit seems to be that drain valve leaking air. I probably wasn't as precise as I could have been - on 12v it fails to prime.  Once it's primed, it pumps OK on 12v.  But the drain valve was apprently leaking just a bit of air ( it's in line between the tank and the pump ), and it was just enough to keep the pump from priming , unless it had that little extra juice. 

Works OK now ( and I can recreate the failure by playing with that valve, so that's where the problem is ).  THAT piece of junk will be replaced at first opportunity;  but I'm ready to go see the Vintage races at Watkins Glen now  ( if you haven't been, go ), and I'm set to dry camp at the track for the whole weekend.

Thanks for the help.

 

[Gary RV_Wizard]

Good that you found it. Now go have fun at the Glen.